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[gnu-emacs] / lispintro / emacs-lisp-intro.txt

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* Emacs Lisp Intro: (eintr).
                        A simple introduction to Emacs Lisp programming.
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Short Contents
**************

An Introduction to Programming in Emacs Lisp
Preface
List Processing
Practicing Evaluation
How To Write Function Definitions
A Few Buffer-Related Functions
A Few More Complex Functions
Narrowing and Widening
`car', `cdr', `cons': Fundamental Functions
Cutting and Storing Text
How Lists are Implemented
Yanking Text Back
Loops and Recursion
Regular Expression Searches
Counting: Repetition and Regexps
Counting Words in a `defun'
Readying a Graph
Your `.emacs' File
Debugging
Conclusion
The `the-the' Function
Handling the Kill Ring
A Graph with Labelled Axes
GNU Free Documentation License
Index
About the Author


Table of Contents
*****************


An Introduction to Programming in Emacs Lisp

Preface
  Why Study Emacs Lisp?
  On Reading this Text
  For Whom This is Written
  Lisp History
  A Note for Novices
  Thank You

List Processing
  Lisp Lists
    Numbers, Lists inside of Lists
    Lisp Atoms
    Whitespace in Lists
    GNU Emacs Helps You Type Lists
  Run a Program
  Generate an Error Message
  Symbol Names and Function Definitions
  The Lisp Interpreter
    Complications
    Byte Compiling
  Evaluation
    Evaluating Inner Lists
  Variables
    `fill-column', an Example Variable
    Error Message for a Symbol Without a Function
    Error Message for a Symbol Without a Value
  Arguments
    Arguments' Data Types
    An Argument as the Value of a Variable or List
    Variable Number of Arguments
    Using the Wrong Type Object as an Argument
    The `message' Function
  Setting the Value of a Variable
    Using `set'
    Using `setq'
    Counting
  Summary
  Exercises

Practicing Evaluation
  How to Evaluate
  Buffer Names
  Getting Buffers
  Switching Buffers
  Buffer Size and the Location of Point
  Exercise

How To Write Function Definitions
  An Aside about Primitive Functions
  The `defun' Special Form
  Install a Function Definition
    The effect of installation
    Change a Function Definition
  Make a Function Interactive
    An Interactive `multiply-by-seven', An Overview
    An Interactive `multiply-by-seven'
  Different Options for `interactive'
  Install Code Permanently
  `let'
    `let' Prevents Confusion
    The Parts of a `let' Expression
    Sample `let' Expression
    Uninitialized Variables in a `let' Statement
  The `if' Special Form
    `if' in more detail
    The `type-of-animal' Function in Detail
  If-then-else Expressions
  Truth and Falsehood in Emacs Lisp
    An explanation of `nil'
  `save-excursion'
    Point and Mark
    Template for a `save-excursion' Expression
  Review
  Exercises

A Few Buffer-Related Functions
  Finding More Information
  A Simplified `beginning-of-buffer' Definition
  The Definition of `mark-whole-buffer'
    An overview of `mark-whole-buffer'
    Body of `mark-whole-buffer'
  The Definition of `append-to-buffer'
    An Overview of `append-to-buffer'
    The `append-to-buffer' Interactive Expression
    The Body of `append-to-buffer'
    `save-excursion' in `append-to-buffer'
  Review
  Exercises

A Few More Complex Functions
  The Definition of `copy-to-buffer'
  The Definition of `insert-buffer'
    The Code for `insert-buffer'
    The Interactive Expression in `insert-buffer'
      A Read-only Buffer
      `b' in an Interactive Expression
    The Body of the `insert-buffer' Function
    `insert-buffer' With an `if' Instead of an `or'
    The `or' in the Body
    The `let' Expression in `insert-buffer'
  Complete Definition of `beginning-of-buffer'
    Optional Arguments
    `beginning-of-buffer' with an Argument
      Disentangle `beginning-of-buffer'
      What happens in a large buffer
      What happens in a small buffer
    The Complete `beginning-of-buffer'
  Review
  `optional' Argument Exercise

Narrowing and Widening
  The Advantages of Narrowing
  The `save-restriction' Special Form
  `what-line'
  Exercise with Narrowing

`car', `cdr', `cons': Fundamental Functions
  Strange Names
  `car' and `cdr'
  `cons'
    Build a list
    Find the Length of a List: `length'
  `nthcdr'
  `nth'
  `setcar'
  `setcdr'
  Exercise

Cutting and Storing Text
  Storing Text in a List
  `zap-to-char'
    The Complete `zap-to-char' Implementation
    The `interactive' Expression
    The Body of `zap-to-char'
    The `search-forward' Function
    The `progn' Special Form
    Summing up `zap-to-char'
  `kill-region'
    The Complete `kill-region' Definition
    `condition-case'
    `delete-and-extract-region'
  Digression into C
  Initializing a Variable with `defvar'
    Seeing the Current Value of a Variable
    `defvar' and an asterisk
  `copy-region-as-kill'
    The complete `copy-region-as-kill' function definition
    The Body of `copy-region-as-kill'
      `last-command' and `this-command'
      The `kill-append' function
      The `kill-new' function
  Review
  Searching Exercises

How Lists are Implemented
  Lists diagrammed
  Symbols as a Chest of Drawers
  Exercise

Yanking Text Back
  Kill Ring Overview
  The `kill-ring-yank-pointer' Variable
  Exercises with `yank' and `nthcdr'

Loops and Recursion
  `while'
    Looping with `while'
    A `while' Loop and a List
    An Example: `print-elements-of-list'
    A Loop with an Incrementing Counter
      Example with incrementing counter
      The parts of the function definition
      Putting the function definition together
    Loop with a Decrementing Counter
      Example with decrementing counter
      The parts of the function definition
      Putting the function definition together
  Save your time: `dolist' and `dotimes'
      The `dolist' Macro
      The `dotimes' Macro
  Recursion
    Building Robots: Extending the Metaphor
    The Parts of a Recursive Definition
    Recursion with a List
    Recursion in Place of a Counter
      An argument of 1 or 2
      An argument of 3 or 4
    Recursion Example Using `cond'
    Recursive Patterns
      Recursive Pattern: _every_
      Recursive Pattern: _accumulate_
      Recursive Pattern: _keep_
    Recursion without Deferments
    No Deferment Solution
  Looping Exercise

Regular Expression Searches
  The Regular Expression for `sentence-end'
  The `re-search-forward' Function
  `forward-sentence'
    Complete `forward-sentence' function definition
    The `while' loops
    The regular expression search
  `forward-paragraph': a Goldmine of Functions
    Shortened `forward-paragraph' function definition
    The `let*' expression
    The forward motion `while' loop
    Between paragraphs
    Within paragraphs
    No fill prefix
    With a fill prefix
    Summary
  Create Your Own `TAGS' File
  Review
  Exercises with `re-search-forward'

Counting: Repetition and Regexps
  Counting words
  The `count-words-region' Function
    Designing `count-words-region'
    The Whitespace Bug in `count-words-region'
  Count Words Recursively
  Exercise: Counting Punctuation

Counting Words in a `defun'
  Divide and Conquer
  What to Count?
  What Constitutes a Word or Symbol?
  The `count-words-in-defun' Function
  Count Several `defuns' Within a File
  Find a File
  `lengths-list-file' in Detail
  Count Words in `defuns' in Different Files
    Determine the lengths of `defuns'
    The `append' Function
  Recursively Count Words in Different Files
  Prepare the Data for Display in a Graph
    Sorting Lists
    Making a List of Files
    Counting function definitions

Readying a Graph
  Printing the Columns of a Graph
  The `graph-body-print' Function
  The `recursive-graph-body-print' Function
  Need for Printed Axes
  Exercise

Your `.emacs' File
  Emacs' Default Configuration
  Site-wide Initialization Files
  Specifying Variables using `defcustom'
  Beginning a `.emacs' File
  Text and Auto Fill Mode
  Mail Aliases
  Indent Tabs Mode
  Some Keybindings
  Keymaps
  Loading Files
  Autoloading
  A Simple Extension: `line-to-top-of-window'
  X11 Colors
  Miscellaneous Settings for a `.emacs' File
  A Modified Mode Line

Debugging
  `debug'
  `debug-on-entry'
  `debug-on-quit' and `(debug)'
  The `edebug' Source Level Debugger
  Debugging Exercises

Conclusion

The `the-the' Function

Handling the Kill Ring
  The `rotate-yank-pointer' Function
    `rotate-yank-pointer' in Outline
    The Body of `rotate-yank-pointer'
      Digression about the word `error'
      The else-part of the `if' expression
      The `%' remainder function
      Using `%' in `rotate-yank-pointer'
      Pointing to the last element
  `yank'
      Passing the argument
      Passing a negative argument
  `yank-pop'

A Graph with Labelled Axes
  Labelled Example Graph
  The `print-graph' Varlist
  The `print-Y-axis' Function
    What height should the label be?
    Side Trip: Compute a Remainder
    Construct a Y Axis Element
    Create a Y Axis Column
    The Not Quite Final Version of `print-Y-axis'
  The `print-X-axis' Function
    Similarities and differences
    X Axis Tic Marks
  Printing the Whole Graph
    Changes for the Final Version
    Testing `print-graph'
    Graphing Numbers of Words and Symbols
    A `lambda' Expression: Useful Anonymity
    The `mapcar' Function
    Another Bug ... Most Insidious
    The Printed Graph

GNU Free Documentation License

Index

About the Author


An Introduction to Programming in Emacs Lisp
********************************************

This is an introduction to `Programming in Emacs Lisp', for people
who are not programmers.

Edition 2.07, 2002 Aug 23

Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1997, 2001, 2002
Free Software Foundation, Inc.


Published by the Free Software Foundation, Inc.
59 Temple Place, Suite 330
Boston, MA 02111-1307 USA
Edition 2.07, 2002 Aug 23

ISBN 1-882114-43-4

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; there
being no Invariant Section, with the Front-Cover Texts being "A GNU
Manual", and with the Back-Cover Texts as in (a) below.  A copy of
the license is included in the section entitled "GNU Free
Documentation License".

(a) The FSF's Back-Cover Text is: "You have freedom to copy and
modify this GNU Manual, like GNU software.  Copies published by the
Free Software Foundation raise funds for GNU development."
This master menu first lists each chapter and index; then it lists
every node in every chapter.

Preface
*******

Most of the GNU Emacs integrated environment is written in the
programming language called Emacs Lisp.  The code written in this
programming language is the software--the sets of instructions--that
tell the computer what to do when you give it commands.  Emacs is
designed so that you can write new code in Emacs Lisp and easily
install it as an extension to the editor.

(GNU Emacs is sometimes called an "extensible editor", but it does
much more than provide editing capabilities.  It is better to refer to
Emacs as an "extensible computing environment".  However, that phrase
is quite a mouthful.  It is easier to refer to Emacs simply as an
editor.  Moreover, everything you do in Emacs--find the Mayan date
and phases of the moon, simplify polynomials, debug code, manage
files, read letters, write books--all these activities are kinds of
editing in the most general sense of the word.)

Why Study Emacs Lisp?
=====================

Although Emacs Lisp is usually thought of in association only with
Emacs, it is a full computer programming language.  You can use Emacs
Lisp as you would any other programming language.

Perhaps you want to understand programming; perhaps you want to extend
Emacs; or perhaps you want to become a programmer.  This introduction
to Emacs Lisp is designed to get you started: to guide you in
learning the fundamentals of programming, and more importantly, to
show you how you can teach yourself to go further.

On Reading this Text
====================

All through this document, you will see little sample programs you can
run inside of Emacs.  If you read this document in Info inside of GNU
Emacs, you can run the programs as they appear.  (This is easy to do
and is explained when the examples are presented.)  Alternatively,
you can read this introduction as a printed book while sitting beside
a computer running Emacs.  (This is what I like to do; I like printed
books.)  If you don't have a running Emacs beside you, you can still
read this book, but in this case, it is best to treat it as a novel
or as a travel guide to a country not yet visited: interesting, but
not the same as being there.

Much of this introduction is dedicated to walk-throughs or guided
tours of code used in GNU Emacs.  These tours are designed for two
purposes: first, to give you familiarity with real, working code
(code you use every day); and, second, to give you familiarity with
the way Emacs works.  It is interesting to see how a working
environment is implemented.  Also, I hope that you will pick up the
habit of browsing through source code.  You can learn from it and
mine it for ideas.  Having GNU Emacs is like having a dragon's cave
of treasures.

In addition to learning about Emacs as an editor and Emacs Lisp as a
programming language, the examples and guided tours will give you an
opportunity to get acquainted with Emacs as a Lisp programming
environment.  GNU Emacs supports programming and provides tools that
you will want to become comfortable using, such as `M-.' (the key
which invokes the `find-tag' command).  You will also learn about
buffers and other objects that are part of the environment.  Learning
about these features of Emacs is like learning new routes around your
home town.

Finally, I hope to convey some of the skills for using Emacs to learn
aspects of programming that you don't know.  You can often use Emacs
to help you understand what puzzles you or to find out how to do
something new.  This self-reliance is not only a pleasure, but an
advantage.

For Whom This is Written
========================

This text is written as an elementary introduction for people who are
not programmers.  If you are a programmer, you may not be satisfied
with this primer.  The reason is that you may have become expert at
reading reference manuals and be put off by the way this text is
organized.

An expert programmer who reviewed this text said to me:

     I prefer to learn from reference manuals.  I "dive into" each
     paragraph, and "come up for air" between paragraphs.

     When I get to the end of a paragraph, I assume that that subject
     is done, finished, that I know everything I need (with the
     possible exception of the case when the next paragraph starts
     talking about it in more detail).  I expect that a well written
     reference manual will not have a lot of redundancy, and that it
     will have excellent pointers to the (one) place where the
     information I want is.

This introduction is not written for this person!

Firstly, I try to say everything at least three times: first, to
introduce it; second, to show it in context; and third, to show it in
a different context, or to review it.

Secondly, I hardly ever put all the information about a subject in one
place, much less in one paragraph.  To my way of thinking, that
imposes too heavy a burden on the reader.  Instead I try to explain
only what you need to know at the time.  (Sometimes I include a
little extra information so you won't be surprised later when the
additional information is formally introduced.)

When you read this text, you are not expected to learn everything the
first time.  Frequently, you need only make, as it were, a `nodding
acquaintance' with some of the items mentioned.  My hope is that I
have structured the text and given you enough hints that you will be
alert to what is important, and concentrate on it.

You will need to "dive into" some paragraphs; there is no other way
to read them.  But I have tried to keep down the number of such
paragraphs.  This book is intended as an approachable hill, rather
than as a daunting mountain.

This introduction to `Programming in Emacs Lisp' has a companion
document, *Note The GNU Emacs Lisp Reference Manual: (elisp)Top.  The
reference manual has more detail than this introduction.  In the
reference manual, all the information about one topic is concentrated
in one place.  You should turn to it if you are like the programmer
quoted above.  And, of course, after you have read this
`Introduction', you will find the `Reference Manual' useful when you
are writing your own programs.

Lisp History
============

Lisp was first developed in the late 1950s at the Massachusetts
Institute of Technology for research in artificial intelligence.  The
great power of the Lisp language makes it superior for other purposes
as well, such as writing editor commands and integrated environments.

GNU Emacs Lisp is largely inspired by Maclisp, which was written at
MIT in the 1960s.  It is somewhat inspired by Common Lisp, which
became a standard in the 1980s.  However, Emacs Lisp is much simpler
than Common Lisp.  (The standard Emacs distribution contains an
optional extensions file, `cl.el', that adds many Common Lisp
features to Emacs Lisp.)

A Note for Novices
==================

If you don't know GNU Emacs, you can still read this document
profitably.  However, I recommend you learn Emacs, if only to learn to
move around your computer screen.  You can teach yourself how to use
Emacs with the on-line tutorial.  To use it, type `C-h t'.  (This
means you press and release the <CTRL> key and the `h' at the same
time, and then press and release `t'.)

Also, I often refer to one of Emacs' standard commands by listing the
keys which you press to invoke the command and then giving the name of
the command in parentheses, like this: `M-C-\' (`indent-region').
What this means is that the `indent-region' command is customarily
invoked by typing `M-C-\'.  (You can, if you wish, change the keys
that are typed to invoke the command; this is called "rebinding".
*Note Keymaps: Keymaps.)  The abbreviation `M-C-\' means that you
type your <META> key, <CTRL> key and <\> key all at the same time.
(On many modern keyboards the <META> key is labelled <ALT>.)
Sometimes a combination like this is called a keychord, since it is
similar to the way you play a chord on a piano.  If your keyboard does
not have a <META> key, the <ESC> key prefix is used in place of it.
In this case, `M-C-\' means that you press and release your <ESC> key
and then type the <CTRL> key and the <\> key at the same time.  But
usually `M-C-\' means press the <CTRL> key along with the key that is
labelled <ALT> and, at the same time, press the <\> key.

In addition to typing a lone keychord, you can prefix what you type
with `C-u', which is called the `universal argument'.  The `C-u'
keychord passes an argument to the subsequent command.  Thus, to
indent a region of plain text by 6 spaces, mark the region, and then
type `C-u 6 M-C-\'.  (If you do not specify a number, Emacs either
passes the number 4 to the command or otherwise runs the command
differently than it would otherwise.)  *Note Numeric Arguments:
(emacs)Arguments.

If you are reading this in Info using GNU Emacs, you can read through
this whole document just by pressing the space bar, <SPC>.  (To learn
about Info, type `C-h i' and then select Info.)

A note on terminology:  when I use the word Lisp alone, I often am
referring to the various dialects of Lisp in general, but when I speak
of Emacs Lisp, I am referring to GNU Emacs Lisp in particular.

Thank You
=========

My thanks to all who helped me with this book.  My especial thanks to
Jim Blandy, Noah Friedman, Jim Kingdon, Roland McGrath, Frank Ritter,
Randy Smith, Richard M. Stallman, and Melissa Weisshaus.  My thanks
also go to both Philip Johnson and David Stampe for their patient
encouragement.  My mistakes are my own.

                                                   Robert J. Chassell

List Processing
***************

To the untutored eye, Lisp is a strange programming language.  In Lisp
code there are parentheses everywhere.  Some people even claim that
the name stands for `Lots of Isolated Silly Parentheses'.  But the
claim is unwarranted.  Lisp stands for LISt Processing, and the
programming language handles _lists_ (and lists of lists) by putting
them between parentheses.  The parentheses mark the boundaries of the
list.  Sometimes a list is preceded by a single apostrophe or
quotation mark, `''.  Lists are the basis of Lisp.

Lisp Lists
==========

In Lisp, a list looks like this: `'(rose violet daisy buttercup)'.
This list is preceded by a single apostrophe.  It could just as well
be written as follows, which looks more like the kind of list you are
likely to be familiar with:

     '(rose
       violet
       daisy
       buttercup)

The elements of this list are the names of the four different flowers,
separated from each other by whitespace and surrounded by parentheses,
like flowers in a field with a stone wall around them.

Numbers, Lists inside of Lists
------------------------------

Lists can also have numbers in them, as in this list: `(+ 2 2)'.
This list has a plus-sign, `+', followed by two `2's, each separated
by whitespace.

In Lisp, both data and programs are represented the same way; that is,
they are both lists of words, numbers, or other lists, separated by
whitespace and surrounded by parentheses.  (Since a program looks like
data, one program may easily serve as data for another; this is a very
powerful feature of Lisp.)  (Incidentally, these two parenthetical
remarks are _not_ Lisp lists, because they contain `;' and `.' as
punctuation marks.)

Here is another list, this time with a list inside of it:

     '(this list has (a list inside of it))

The components of this list are the words `this', `list', `has', and
the list `(a list inside of it)'.  The interior list is made up of
the words `a', `list', `inside', `of', `it'.

Lisp Atoms
----------

In Lisp, what we have been calling words are called "atoms".  This
term comes from the historical meaning of the word atom, which means
`indivisible'.  As far as Lisp is concerned, the words we have been
using in the lists cannot be divided into any smaller parts and still
mean the same thing as part of a program; likewise with numbers and
single character symbols like `+'.  On the other hand, unlike an
atom, a list can be split into parts.  (*Note `car' `cdr' & `cons'
Fundamental Functions: car cdr & cons.)

In a list, atoms are separated from each other by whitespace.  They
can be right next to a parenthesis.

Technically speaking, a list in Lisp consists of parentheses
surrounding atoms separated by whitespace or surrounding other lists
or surrounding both atoms and other lists.  A list can have just one
atom in it or have nothing in it at all.  A list with nothing in it
looks like this: `()', and is called the "empty list".  Unlike
anything else, an empty list is considered both an atom and a list at
the same time.

The printed representation of both atoms and lists are called
"symbolic expressions" or, more concisely, "s-expressions".  The word
"expression" by itself can refer to either the printed
representation, or to the atom or list as it is held internally in the
computer.  Often, people use the term "expression" indiscriminately.
(Also, in many texts, the word "form" is used as a synonym for
expression.)

Incidentally, the atoms that make up our universe were named such when
they were thought to be indivisible; but it has been found that
physical atoms are not indivisible.  Parts can split off an atom or
it can fission into two parts of roughly equal size.  Physical atoms
were named prematurely, before their truer nature was found.  In
Lisp, certain kinds of atom, such as an array, can be separated into
parts; but the mechanism for doing this is different from the
mechanism for splitting a list.  As far as list operations are
concerned, the atoms of a list are unsplittable.

As in English, the meanings of the component letters of a Lisp atom
are different from the meaning the letters make as a word.  For
example, the word for the South American sloth, the `ai', is
completely different from the two words, `a', and `i'.

There are many kinds of atom in nature but only a few in Lisp: for
example, "numbers", such as 37, 511, or 1729, and "symbols", such as
`+', `foo', or `forward-line'.  The words we have listed in the
examples above are all symbols.  In everyday Lisp conversation, the
word "atom" is not often used, because programmers usually try to be
more specific about what kind of atom they are dealing with.  Lisp
programming is mostly about symbols (and sometimes numbers) within
lists.  (Incidentally, the preceding three word parenthetical remark
is a proper list in Lisp, since it consists of atoms, which in this
case are symbols, separated by whitespace and enclosed by
parentheses, without any non-Lisp punctuation.)

In addition, text between double quotation marks--even sentences or
paragraphs--is an atom.  Here is an example:

     '(this list includes "text between quotation marks.")

In Lisp, all of the quoted text including the punctuation mark and the
blank spaces is a single atom.  This kind of atom is called a
"string" (for `string of characters') and is the sort of thing that
is used for messages that a computer can print for a human to read.
Strings are a different kind of atom than numbers or symbols and are
used differently.

Whitespace in Lists
-------------------

The amount of whitespace in a list does not matter.  From the point
of view of the Lisp language,

     '(this list
        looks like this)

is exactly the same as this:

     '(this list looks like this)

Both examples show what to Lisp is the same list, the list made up of
the symbols `this', `list', `looks', `like', and `this' in that order.

Extra whitespace and newlines are designed to make a list more
readable by humans.  When Lisp reads the expression, it gets rid of
all the extra whitespace (but it needs to have at least one space
between atoms in order to tell them apart.)

Odd as it seems, the examples we have seen cover almost all of what
Lisp lists look like!  Every other list in Lisp looks more or less
like one of these examples, except that the list may be longer and
more complex.  In brief, a list is between parentheses, a string is
between quotation marks, a symbol looks like a word, and a number
looks like a number.  (For certain situations, square brackets, dots
and a few other special characters may be used; however, we will go
quite far without them.)

GNU Emacs Helps You Type Lists
------------------------------

When you type a Lisp expression in GNU Emacs using either Lisp
Interaction mode or Emacs Lisp mode, you have available to you several
commands to format the Lisp expression so it is easy to read.  For
example, pressing the <TAB> key automatically indents the line the
cursor is on by the right amount.  A command to properly indent the
code in a region is customarily bound to `M-C-\'.  Indentation is
designed so that you can see which elements of a list belong to which
list--elements of a sub-list are indented more than the elements of
the enclosing list.

In addition, when you type a closing parenthesis, Emacs momentarily
jumps the cursor back to the matching opening parenthesis, so you can
see which one it is.  This is very useful, since every list you type
in Lisp must have its closing parenthesis match its opening
parenthesis.  (*Note Major Modes: (emacs)Major Modes, for more
information about Emacs' modes.)

Run a Program
=============

A list in Lisp--any list--is a program ready to run.  If you run it
(for which the Lisp jargon is "evaluate"), the computer will do one
of three things: do nothing except return to you the list itself; send
you an error message; or, treat the first symbol in the list as a
command to do something.  (Usually, of course, it is the last of these
three things that you really want!)

The single apostrophe, `'', that I put in front of some of the
example lists in preceding sections is called a "quote"; when it
precedes a list, it tells Lisp to do nothing with the list, other than
take it as it is written.  But if there is no quote preceding a list,
the first item of the list is special: it is a command for the
computer to obey.  (In Lisp, these commands are called _functions_.)
The list `(+ 2 2)' shown above did not have a quote in front of it,
so Lisp understands that the `+' is an instruction to do something
with the rest of the list: add the numbers that follow.

If you are reading this inside of GNU Emacs in Info, here is how you
can evaluate such a list:  place your cursor immediately after the
right hand parenthesis of the following list and then type `C-x C-e':

     (+ 2 2)

You will see the number `4' appear in the echo area.  (In the jargon,
what you have just done is "evaluate the list."  The echo area is the
line at the bottom of the screen that displays or "echoes" text.)
Now try the same thing with a quoted list:  place the cursor right
after the following list and type `C-x C-e':

     '(this is a quoted list)

You will see `(this is a quoted list)' appear in the echo area.

In both cases, what you are doing is giving a command to the program
inside of GNU Emacs called the "Lisp interpreter"--giving the
interpreter a command to evaluate the expression.  The name of the
Lisp interpreter comes from the word for the task done by a human who
comes up with the meaning of an expression--who "interprets" it.

You can also evaluate an atom that is not part of a list--one that is
not surrounded by parentheses; again, the Lisp interpreter translates
from the humanly readable expression to the language of the computer.
But before discussing this (*note Variables::), we will discuss what
the Lisp interpreter does when you make an error.

Generate an Error Message
=========================

Partly so you won't worry if you do it accidentally, we will now give
a command to the Lisp interpreter that generates an error message.
This is a harmless activity; and indeed, we will often try to generate
error messages intentionally.  Once you understand the jargon, error
messages can be informative.  Instead of being called "error"
messages, they should be called "help" messages.  They are like
signposts to a traveller in a strange country; deciphering them can be
hard, but once understood, they can point the way.

The error message is generated by a built-in GNU Emacs debugger.  We
will `enter the debugger'.  You get out of the debugger by typing `q'.

What we will do is evaluate a list that is not quoted and does not
have a meaningful command as its first element.  Here is a list almost
exactly the same as the one we just used, but without the single-quote
in front of it.  Position the cursor right after it and type `C-x
C-e':

     (this is an unquoted list)

What you see depends on which version of Emacs you are running.  GNU
Emacs version 21 provides more information than version 20 and before.
First, the more recent result of generating an error; then the
earlier, version 20 result.

In GNU Emacs version 21, a `*Backtrace*' window will open up and you
will see the following in it:

     ---------- Buffer: *Backtrace* ----------
     Debugger entered--Lisp error: (void-function this)
       (this is an unquoted list)
       eval((this is an unquoted list))
       eval-last-sexp-1(nil)
       eval-last-sexp(nil)
       call-interactively(eval-last-sexp)
     ---------- Buffer: *Backtrace* ----------

Your cursor will be in this window (you may have to wait a few seconds
before it becomes visible).  To quit the debugger and make the
debugger window go away, type:

     q

Please type `q' right now, so you become confident that you can get
out of the debugger.  Then, type `C-x C-e' again to re-enter it.

Based on what we already know, we can almost read this error message.

You read the `*Backtrace*' buffer from the bottom up; it tells you
what Emacs did.  When you typed `C-x C-e', you made an interactive
call to the command `eval-last-sexp'.  `eval' is an abbreviation for
`evaluate' and `sexp' is an abbreviation for `symbolic expression'.
The command means `evaluate last symbolic expression', which is the
expression just before your cursor.

Each line above tells you what the Lisp interpreter evaluated next.
The most recent action is at the top.  The buffer is called the
`*Backtrace*' buffer because it enables you to track Emacs backwards.

At the top of the `*Backtrace*' buffer, you see the line:

     Debugger entered--Lisp error: (void-function this)

The Lisp interpreter tried to evaluate the first atom of the list, the
word `this'.  It is this action that generated the error message
`void-function this'.

The message contains the words `void-function' and `this'.

The word `function' was mentioned once before.  It is a very
important word.  For our purposes, we can define it by saying that a
"function" is a set of instructions to the computer that tell the
computer to do something.

Now we can begin to understand the error message: `void-function
this'.  The function (that is, the word `this') does not have a
definition of any set of instructions for the computer to carry out.

The slightly odd word, `void-function', is designed to cover the way
Emacs Lisp is implemented, which is that when a symbol does not have
a function definition attached to it, the place that should contain
the instructions is `void'.

On the other hand, since we were able to add 2 plus 2 successfully, by
evaluating `(+ 2 2)', we can infer that the symbol `+' must have a
set of instructions for the computer to obey and those instructions
must be to add the numbers that follow the `+'.

In GNU Emacs version 20, and in earlier versions, you will see only
one line of error message; it will appear in the echo area and look
like this:

     Symbol's function definition is void: this

(Also, your terminal may beep at you--some do, some don't; and others
blink.  This is just a device to get your attention.)  The message
goes away as soon as you type another key, even just to move the
cursor.

We know the meaning of the word `Symbol'.  It refers to the first
atom of the list, the word `this'.  The word `function' refers to the
instructions that tell the computer what to do.  (Technically, the
symbol tells the computer where to find the instructions, but this is
a complication we can ignore for the moment.)

The error message can be understood: `Symbol's function definition is
void: this'.  The symbol (that is, the word `this') lacks
instructions for the computer to carry out.

Symbol Names and Function Definitions
=====================================

We can articulate another characteristic of Lisp based on what we have
discussed so far--an important characteristic: a symbol, like `+', is
not itself the set of instructions for the computer to carry out.
Instead, the symbol is used, perhaps temporarily, as a way of
locating the definition or set of instructions.  What we see is the
name through which the instructions can be found.  Names of people
work the same way.  I can be referred to as `Bob'; however, I am not
the letters `B', `o', `b' but am the consciousness consistently
associated with a particular life-form.  The name is not me, but it
can be used to refer to me.

In Lisp, one set of instructions can be attached to several names.
For example, the computer instructions for adding numbers can be
linked to the symbol `plus' as well as to the symbol `+' (and are in
some dialects of Lisp).  Among humans, I can be referred to as
`Robert' as well as `Bob' and by other words as well.

On the other hand, a symbol can have only one function definition
attached to it at a time.  Otherwise, the computer would be confused
as to which definition to use.  If this were the case among people,
only one person in the world could be named `Bob'.  However, the
function definition to which the name refers can be changed readily.
(*Note Install a Function Definition: Install.)

Since Emacs Lisp is large, it is customary to name symbols in a way
that identifies the part of Emacs to which the function belongs.
Thus, all the names for functions that deal with Texinfo start with
`texinfo-' and those for functions that deal with reading mail start
with `rmail-'.

The Lisp Interpreter
====================

Based on what we have seen, we can now start to figure out what the
Lisp interpreter does when we command it to evaluate a list.  First,
it looks to see whether there is a quote before the list; if there
is, the interpreter just gives us the list.  On the other hand, if
there is no quote, the interpreter looks at the first element in the
list and sees whether it has a function definition.  If it does, the
interpreter carries out the instructions in the function definition.
Otherwise, the interpreter prints an error message.

This is how Lisp works.  Simple.  There are added complications which
we will get to in a minute, but these are the fundamentals.  Of
course, to write Lisp programs, you need to know how to write
function definitions and attach them to names, and how to do this
without confusing either yourself or the computer.

Complications
-------------

Now, for the first complication.  In addition to lists, the Lisp
interpreter can evaluate a symbol that is not quoted and does not have
parentheses around it.  The Lisp interpreter will attempt to determine
the symbol's value as a "variable".  This situation is described in
the section on variables.  (*Note Variables::.)

The second complication occurs because some functions are unusual and
do not work in the usual manner.  Those that don't are called "special
forms".  They are used for special jobs, like defining a function, and
there are not many of them.  In the next few chapters, you will be
introduced to several of the more important special forms.

The third and final complication is this: if the function that the
Lisp interpreter is looking at is not a special form, and if it is
part of a list, the Lisp interpreter looks to see whether the list
has a list inside of it.  If there is an inner list, the Lisp
interpreter first figures out what it should do with the inside list,
and then it works on the outside list.  If there is yet another list
embedded inside the inner list, it works on that one first, and so
on.  It always works on the innermost list first.  The interpreter
works on the innermost list first, to evaluate the result of that
list.  The result may be used by the enclosing expression.

Otherwise, the interpreter works left to right, from one expression to
the next.

Byte Compiling
--------------

One other aspect of interpreting: the Lisp interpreter is able to
interpret two kinds of entity: humanly readable code, on which we will
focus exclusively, and specially processed code, called "byte
compiled" code, which is not humanly readable.  Byte compiled code
runs faster than humanly readable code.

You can transform humanly readable code into byte compiled code by
running one of the compile commands such as `byte-compile-file'.
Byte compiled code is usually stored in a file that ends with a
`.elc' extension rather than a `.el' extension.  You will see both
kinds of file in the `emacs/lisp' directory; the files to read are
those with `.el' extensions.

As a practical matter, for most things you might do to customize or
extend Emacs, you do not need to byte compile; and I will not discuss
the topic here.  *Note Byte Compilation: (elisp)Byte Compilation, for
a full description of byte compilation.

Evaluation
==========

When the Lisp interpreter works on an expression, the term for the
activity is called "evaluation".  We say that the interpreter
`evaluates the expression'.  I've used this term several times before.
The word comes from its use in everyday language, `to ascertain the
value or amount of; to appraise', according to `Webster's New
Collegiate Dictionary'.

After evaluating an expression, the Lisp interpreter will most likely
"return" the value that the computer produces by carrying out the
instructions it found in the function definition, or perhaps it will
give up on that function and produce an error message.  (The
interpreter may also find itself tossed, so to speak, to a different
function or it may attempt to repeat continually what it is doing for
ever and ever in what is called an `infinite loop'.  These actions
are less common; and we can ignore them.)  Most frequently, the
interpreter returns a value.

At the same time the interpreter returns a value, it may do something
else as well, such as move a cursor or copy a file; this other kind of
action is called a "side effect".  Actions that we humans think are
important, such as printing results, are often "side effects" to the
Lisp interpreter.  The jargon can sound peculiar, but it turns out
that it is fairly easy to learn to use side effects.

In summary, evaluating a symbolic expression most commonly causes the
Lisp interpreter to return a value and perhaps carry out a side
effect; or else produce an error.

Evaluating Inner Lists
----------------------

If evaluation applies to a list that is inside another list, the outer
list may use the value returned by the first evaluation as information
when the outer list is evaluated.  This explains why inner expressions
are evaluated first: the values they return are used by the outer
expressions.

We can investigate this process by evaluating another addition
example.  Place your cursor after the following expression and type
`C-x C-e':

     (+ 2 (+ 3 3))

The number 8 will appear in the echo area.

What happens is that the Lisp interpreter first evaluates the inner
expression, `(+ 3 3)', for which the value 6 is returned; then it
evaluates the outer expression as if it were written `(+ 2 6)', which
returns the value 8.  Since there are no more enclosing expressions to
evaluate, the interpreter prints that value in the echo area.

Now it is easy to understand the name of the command invoked by the
keystrokes `C-x C-e': the name is `eval-last-sexp'.  The letters
`sexp' are an abbreviation for `symbolic expression', and `eval' is
an abbreviation for `evaluate'.  The command means `evaluate last
symbolic expression'.

As an experiment, you can try evaluating the expression by putting the
cursor at the beginning of the next line immediately following the
expression, or inside the expression.

Here is another copy of the expression:

     (+ 2 (+ 3 3))

If you place the cursor at the beginning of the blank line that
immediately follows the expression and type `C-x C-e', you will still
get the value 8 printed in the echo area.  Now try putting the cursor
inside the expression.  If you put it right after the next to last
parenthesis (so it appears to sit on top of the last parenthesis),
you will get a 6 printed in the echo area!  This is because the
command evaluates the expression `(+ 3 3)'.

Now put the cursor immediately after a number.  Type `C-x C-e' and
you will get the number itself.  In Lisp, if you evaluate a number,
you get the number itself--this is how numbers differ from symbols.
If you evaluate a list starting with a symbol like `+', you will get a
value returned that is the result of the computer carrying out the
instructions in the function definition attached to that name.  If a
symbol by itself is evaluated, something different happens, as we will
see in the next section.

Variables
=========

In Emacs Lisp, a symbol can have a value attached to it just as it can
have a function definition attached to it.  The two are different.
The function definition is a set of instructions that a computer will
obey.  A value, on the other hand, is something, such as number or a
name, that can vary (which is why such a symbol is called a variable).
The value of a symbol can be any expression in Lisp, such as a symbol,
number, list, or string.  A symbol that has a value is often called a
"variable".

A symbol can have both a function definition and a value attached to
it at the same time.  Or it can have just one or the other.  The two
are separate.  This is somewhat similar to the way the name Cambridge
can refer to the city in Massachusetts and have some information
attached to the name as well, such as "great programming center".

Another way to think about this is to imagine a symbol as being a
chest of drawers.  The function definition is put in one drawer, the
value in another, and so on.  What is put in the drawer holding the
value can be changed without affecting the contents of the drawer
holding the function definition, and vice-versa.

`fill-column', an Example Variable
----------------------------------

The variable `fill-column' illustrates a symbol with a value attached
to it: in every GNU Emacs buffer, this symbol is set to some value,
usually 72 or 70, but sometimes to some other value.  To find the
value of this symbol, evaluate it by itself.  If you are reading this
in Info inside of GNU Emacs, you can do this by putting the cursor
after the symbol and typing `C-x C-e':

     fill-column

After I typed `C-x C-e', Emacs printed the number 72 in my echo area.
This is the value for which `fill-column' is set for me as I write
this.  It may be different for you in your Info buffer.  Notice that
the value returned as a variable is printed in exactly the same way
as the value returned by a function carrying out its instructions.
From the point of view of the Lisp interpreter, a value returned is a
value returned.  What kind of expression it came from ceases to
matter once the value is known.

A symbol can have any value attached to it or, to use the jargon, we
can "bind" the variable to a value: to a number, such as 72; to a
string, `"such as this"'; to a list, such as `(spruce pine oak)'; we
can even bind a variable to a function definition.

A symbol can be bound to a value in several ways.  *Note Setting the
Value of a Variable: set & setq, for information about one way to do
this.

Error Message for a Symbol Without a Function
---------------------------------------------

When we evaluated `fill-column' to find its value as a variable, we
did not place parentheses around the word.  This is because we did
not intend to use it as a function name.

If `fill-column' were the first or only element of a list, the Lisp
interpreter would attempt to find the function definition attached to
it.  But `fill-column' has no function definition.  Try evaluating
this:

     (fill-column)

In GNU Emacs version 21, you will create a `*Backtrace*' buffer that
says:

     ---------- Buffer: *Backtrace* ----------
     Debugger entered--Lisp error: (void-function fill-column)
       (fill-column)
       eval((fill-column))
       eval-last-sexp-1(nil)
       eval-last-sexp(nil)
       call-interactively(eval-last-sexp)
     ---------- Buffer: *Backtrace* ----------

(Remember, to quit the debugger and make the debugger window go away,
type `q' in the `*Backtrace*' buffer.)

In GNU Emacs 20 and before, you will produce an error message that
says:

     Symbol's function definition is void: fill-column

(The message will go away away as soon as you move the cursor or type
another key.)

Error Message for a Symbol Without a Value
------------------------------------------

If you attempt to evaluate a symbol that does not have a value bound
to it, you will receive an error message.  You can see this by
experimenting with our 2 plus 2 addition.  In the following
expression, put your cursor right after the `+', before the first
number 2, type `C-x C-e':

     (+ 2 2)

In GNU Emacs 21, you will create a `*Backtrace*' buffer that says:

     ---------- Buffer: *Backtrace* ----------
     Debugger entered--Lisp error: (void-variable +)
       eval(+)
       eval-last-sexp-1(nil)
       eval-last-sexp(nil)
       call-interactively(eval-last-sexp)
     ---------- Buffer: *Backtrace* ----------

(As with the other times we entered the debugger, you can quit by
typing `q' in the `*Backtrace*' buffer.)

This backtrace is different from the very first error message we saw,
which said, `Debugger entered--Lisp error: (void-function this)'.  In
this case, the function does not have a value as a variable; while in
the other error message, the function (the word `this') did not have
a definition.

In this experiment with the `+', what we did was cause the Lisp
interpreter to evaluate the `+' and look for the value of the
variable instead of the function definition.  We did this by placing
the cursor right after the symbol rather than after the parenthesis
of the enclosing list as we did before.  As a consequence, the Lisp
interpreter evaluated the preceding s-expression, which in this case
was the `+' by itself.

Since `+' does not have a value bound to it, just the function
definition, the error message reported that the symbol's value as a
variable was void.

In GNU Emacs version 20 and before, your error message will say:

     Symbol's value as variable is void: +

The meaning is the same as in GNU Emacs 21.

Arguments
=========

To see how information is passed to functions, let's look again at
our old standby, the addition of two plus two.  In Lisp, this is
written as follows:

     (+ 2 2)

If you evaluate this expression, the number 4 will appear in your echo
area.  What the Lisp interpreter does is add the numbers that follow
the `+'.

The numbers added by `+' are called the "arguments" of the function
`+'.  These numbers are the information that is given to or "passed"
to the function.

The word `argument' comes from the way it is used in mathematics and
does not refer to a disputation between two people; instead it refers
to the information presented to the function, in this case, to the
`+'.  In Lisp, the arguments to a function are the atoms or lists
that follow the function.  The values returned by the evaluation of
these atoms or lists are passed to the function.  Different functions
require different numbers of arguments; some functions require none at
all.(1)

---------- Footnotes ----------

(1) It is curious to track the path by which the word `argument' came
to have two different meanings, one in mathematics and the other in
everyday English.  According to the `Oxford English Dictionary', the
word derives from the Latin for `to make clear, prove'; thus it came
to mean, by one thread of derivation, `the evidence offered as
proof', which is to say, `the information offered', which led to its
meaning in Lisp.  But in the other thread of derivation, it came to
mean `to assert in a manner against which others may make counter
assertions', which led to the meaning of the word as a disputation.
(Note here that the English word has two different definitions
attached to it at the same time.  By contrast, in Emacs Lisp, a
symbol cannot have two different function definitions at the same
time.)

Arguments' Data Types
---------------------

The type of data that should be passed to a function depends on what
kind of information it uses.  The arguments to a function such as `+'
must have values that are numbers, since `+' adds numbers.  Other
functions use different kinds of data for their arguments.

For example, the `concat' function links together or unites two or
more strings of text to produce a string.  The arguments are strings.
Concatenating the two character strings `abc', `def' produces the
single string `abcdef'.  This can be seen by evaluating the following:

     (concat "abc" "def")

The value produced by evaluating this expression is `"abcdef"'.

A function such as `substring' uses both a string and numbers as
arguments.  The function returns a part of the string, a substring of
the first argument.  This function takes three arguments.  Its first
argument is the string of characters, the second and third arguments
are numbers that indicate the beginning and end of the substring.  The
numbers are a count of the number of characters (including spaces and
punctuations) from the beginning of the string.

For example, if you evaluate the following:

     (substring "The quick brown fox jumped." 16 19)

you will see `"fox"' appear in the echo area.  The arguments are the
string and the two numbers.

Note that the string passed to `substring' is a single atom even
though it is made up of several words separated by spaces.  Lisp
counts everything between the two quotation marks as part of the
string, including the spaces.  You can think of the `substring'
function as a kind of `atom smasher' since it takes an otherwise
indivisible atom and extracts a part.  However, `substring' is only
able to extract a substring from an argument that is a string, not
from another type of atom such as a number or symbol.

An Argument as the Value of a Variable or List
----------------------------------------------

An argument can be a symbol that returns a value when it is evaluated.
For example, when the symbol `fill-column' by itself is evaluated, it
returns a number.  This number can be used in an addition.

Position the cursor after the following expression and type `C-x C-e':

     (+ 2 fill-column)

The value will be a number two more than what you get by evaluating
`fill-column' alone.  For me, this is 74, because the value of
`fill-column' is 72.

As we have just seen, an argument can be a symbol that returns a value
when evaluated.  In addition, an argument can be a list that returns a
value when it is evaluated.  For example, in the following expression,
the arguments to the function `concat' are the strings `"The "' and
`" red foxes."' and the list `(number-to-string (+ 2 fill-column))'.

     (concat "The " (number-to-string (+ 2 fill-column)) " red foxes.")

If you evaluate this expression--and if, as with my Emacs,
`fill-column' evaluates to 72--`"The 74 red foxes."' will appear in
the echo area.  (Note that you must put spaces after the word `The'
and before the word `red' so they will appear in the final string.
The function `number-to-string' converts the integer that the
addition function returns to a string.  `number-to-string' is also
known as `int-to-string'.)

Variable Number of Arguments
----------------------------

Some functions, such as `concat', `+' or `*', take any number of
arguments.  (The `*' is the symbol for multiplication.)  This can be
seen by evaluating each of the following expressions in the usual
way.  What you will see in the echo area is printed in this text
after `=>', which you may read as `evaluates to'.

In the first set, the functions have no arguments:

     (+)       => 0
     
     (*)       => 1

In this set, the functions have one argument each:

     (+ 3)     => 3
     
     (* 3)     => 3

In this set, the functions have three arguments each:

     (+ 3 4 5) => 12
     
     (* 3 4 5) => 60

Using the Wrong Type Object as an Argument
------------------------------------------

When a function is passed an argument of the wrong type, the Lisp
interpreter produces an error message.  For example, the `+' function
expects the values of its arguments to be numbers.  As an experiment
we can pass it the quoted symbol `hello' instead of a number.
Position the cursor after the following expression and type `C-x C-e':

     (+ 2 'hello)

When you do this you will generate an error message.  What has
happened is that `+' has tried to add the 2 to the value returned by
`'hello', but the value returned by `'hello' is the symbol `hello',
not a number.  Only numbers can be added.  So `+' could not carry out
its addition.

In GNU Emacs version 21, you will create and enter a `*Backtrace*'
buffer that says:

     ---------- Buffer: *Backtrace* ----------
     Debugger entered--Lisp error:
              (wrong-type-argument number-or-marker-p hello)
       +(2 hello)
       eval((+ 2 (quote hello)))
       eval-last-sexp-1(nil)
       eval-last-sexp(nil)
       call-interactively(eval-last-sexp)
     ---------- Buffer: *Backtrace* ----------

As usual, the error message tries to be helpful and makes sense after
you learn how to read it.

The first part of the error message is straightforward; it says
`wrong type argument'.  Next comes the mysterious jargon word
`number-or-marker-p'.  This word is trying to tell you what kind of
argument the `+' expected.

The symbol `number-or-marker-p' says that the Lisp interpreter is
trying to determine whether the information presented it (the value of
the argument) is a number or a marker (a special object representing a
buffer position).  What it does is test to see whether the `+' is
being given numbers to add.  It also tests to see whether the
argument is something called a marker, which is a specific feature of
Emacs Lisp.  (In Emacs, locations in a buffer are recorded as markers.
When the mark is set with the `C-@' or `C-<SPC>' command, its
position is kept as a marker.  The mark can be considered a
number--the number of characters the location is from the beginning
of the buffer.)  In Emacs Lisp, `+' can be used to add the numeric
value of marker positions as numbers.

The `p' of `number-or-marker-p' is the embodiment of a practice
started in the early days of Lisp programming.  The `p' stands for
`predicate'.  In the jargon used by the early Lisp researchers, a
predicate refers to a function to determine whether some property is
true or false.  So the `p' tells us that `number-or-marker-p' is the
name of a function that determines whether it is true or false that
the argument supplied is a number or a marker.  Other Lisp symbols
that end in `p' include `zerop', a function that tests whether its
argument has the value of zero, and `listp', a function that tests
whether its argument is a list.

Finally, the last part of the error message is the symbol `hello'.
This is the value of the argument that was passed to `+'.  If the
addition had been passed the correct type of object, the value passed
would have been a number, such as 37, rather than a symbol like
`hello'.  But then you would not have got the error message.

In GNU Emacs version 20 and before, the echo area displays an error
message that says:

     Wrong type argument: number-or-marker-p, hello

This says, in different words, the same as the top line of the
`*Backtrace*' buffer.

The `message' Function
----------------------

Like `+', the `message' function takes a variable number of
arguments.  It is used to send messages to the user and is so useful
that we will describe it here.

A message is printed in the echo area.  For example, you can print a
message in your echo area by evaluating the following list:

     (message "This message appears in the echo area!")

The whole string between double quotation marks is a single argument
and is printed in toto.  (Note that in this example, the message
itself will appear in the echo area within double quotes; that is
because you see the value returned by the `message' function.  In
most uses of `message' in programs that you write, the text will be
printed in the echo area as a side-effect, without the quotes.  *Note
`multiply-by-seven' in detail: multiply-by-seven in detail, for an
example of this.)

However, if there is a `%s' in the quoted string of characters, the
`message' function does not print the `%s' as such, but looks to the
argument that follows the string.  It evaluates the second argument
and prints the value at the location in the string where the `%s' is.

You can see this by positioning the cursor after the following
expression and typing `C-x C-e':

     (message "The name of this buffer is: %s." (buffer-name))

In Info, `"The name of this buffer is: *info*."' will appear in the
echo area.  The function `buffer-name' returns the name of the buffer
as a string, which the `message' function inserts in place of `%s'.

To print a value as an integer, use `%d' in the same way as `%s'.
For example, to print a message in the echo area that states the
value of the `fill-column', evaluate the following:

     (message "The value of fill-column is %d." fill-column)

On my system, when I evaluate this list, `"The value of fill-column
is 72."' appears in my echo area(1).

If there is more than one `%s' in the quoted string, the value of the
first argument following the quoted string is printed at the location
of the first `%s' and the value of the second argument is printed at
the location of the second `%s', and so on.

For example, if you evaluate the following,

     (message "There are %d %s in the office!"
              (- fill-column 14) "pink elephants")

a rather whimsical message will appear in your echo area.  On my
system it says, `"There are 58 pink elephants in the office!"'.

The expression `(- fill-column 14)' is evaluated and the resulting
number is inserted in place of the `%d'; and the string in double
quotes, `"pink elephants"', is treated as a single argument and
inserted in place of the `%s'.  (That is to say, a string between
double quotes evaluates to itself, like a number.)

Finally, here is a somewhat complex example that not only illustrates
the computation of a number, but also shows how you can use an
expression within an expression to generate the text that is
substituted for `%s':

     (message "He saw %d %s"
              (- fill-column 34)
              (concat "red "
                      (substring
                       "The quick brown foxes jumped." 16 21)
                      " leaping."))

In this example, `message' has three arguments: the string, `"He saw
%d %s"', the expression, `(- fill-column 32)', and the expression
beginning with the function `concat'.  The value resulting from the
evaluation of `(- fill-column 32)' is inserted in place of the `%d';
and the value returned by the expression beginning with `concat' is
inserted in place of the `%s'.

When I evaluate the expression, the message `"He saw 38 red foxes
leaping."' appears in my echo area.

---------- Footnotes ----------

(1) Actually, you can use `%s' to print a number.  It is
non-specific.  `%d' prints only the part of a number left of a
decimal point, and not anything that is not a number.

Setting the Value of a Variable
===============================

There are several ways by which a variable can be given a value.  One
of the ways is to use either the function `set' or the function
`setq'.  Another way is to use `let' (*note let::).  (The jargon for
this process is to "bind" a variable to a value.)

The following sections not only describe how `set' and `setq' work
but also illustrate how arguments are passed.

Using `set'
-----------

To set the value of the symbol `flowers' to the list `'(rose violet
daisy buttercup)', evaluate the following expression by positioning
the cursor after the expression and typing `C-x C-e'.

     (set 'flowers '(rose violet daisy buttercup))

The list `(rose violet daisy buttercup)' will appear in the echo
area.  This is what is _returned_ by the `set' function.  As a side
effect, the symbol `flowers' is bound to the list ; that is, the
symbol `flowers', which can be viewed as a variable, is given the
list as its value.  (This process, by the way, illustrates how a side
effect to the Lisp interpreter, setting the value, can be the primary
effect that we humans are interested in.  This is because every Lisp
function must return a value if it does not get an error, but it will
only have a side effect if it is designed to have one.)

After evaluating the `set' expression, you can evaluate the symbol
`flowers' and it will return the value you just set.  Here is the
symbol.  Place your cursor after it and type `C-x C-e'.

     flowers

When you evaluate `flowers', the list `(rose violet daisy buttercup)'
appears in the echo area.

Incidentally, if you evaluate `'flowers', the variable with a quote
in front of it, what you will see in the echo area is the symbol
itself, `flowers'.  Here is the quoted symbol, so you can try this:

     'flowers

Note also, that when you use `set', you need to quote both arguments
to `set', unless you want them evaluated.  Since we do not want
either argument evaluated, neither the variable `flowers' nor the
list `(rose violet daisy buttercup)', both are quoted.  (When you use
`set' without quoting its first argument, the first argument is
evaluated before anything else is done.  If you did this and
`flowers' did not have a value already, you would get an error
message that the `Symbol's value as variable is void'; on the other
hand, if `flowers' did return a value after it was evaluated, the
`set' would attempt to set the value that was returned.  There are
situations where this is the right thing for the function to do; but
such situations are rare.)

Using `setq'
------------

As a practical matter, you almost always quote the first argument to
`set'.  The combination of `set' and a quoted first argument is so
common that it has its own name: the special form `setq'.  This
special form is just like `set' except that the first argument is
quoted automatically, so you don't need to type the quote mark
yourself.  Also, as an added convenience, `setq' permits you to set
several different variables to different values, all in one
expression.

To set the value of the variable `carnivores' to the list `'(lion
tiger leopard)' using `setq', the following expression is used:

     (setq carnivores '(lion tiger leopard))

This is exactly the same as using `set' except the first argument is
automatically quoted by `setq'.  (The `q' in `setq' means `quote'.)

With `set', the expression would look like this:

     (set 'carnivores '(lion tiger leopard))

Also, `setq' can be used to assign different values to different
variables.  The first argument is bound to the value of the second
argument, the third argument is bound to the value of the fourth
argument, and so on.  For example, you could use the following to
assign a list of trees to the symbol `trees' and a list of herbivores
to the symbol `herbivores':

     (setq trees '(pine fir oak maple)
           herbivores '(gazelle antelope zebra))

(The expression could just as well have been on one line, but it might
not have fit on a page; and humans find it easier to read nicely
formatted lists.)

Although I have been using the term `assign', there is another way of
thinking about the workings of `set' and `setq'; and that is to say
that `set' and `setq' make the symbol _point_ to the list.  This
latter way of thinking is very common and in forthcoming chapters we
shall come upon at least one symbol that has `pointer' as part of its
name.  The name is chosen because the symbol has a value,
specifically a list, attached to it; or, expressed another way, the
symbol is set to "point" to the list.

Counting
--------

Here is an example that shows how to use `setq' in a counter.  You
might use this to count how many times a part of your program repeats
itself.  First set a variable to zero; then add one to the number each
time the program repeats itself.  To do this, you need a variable that
serves as a counter, and two expressions: an initial `setq'
expression that sets the counter variable to zero; and a second
`setq' expression that increments the counter each time it is
evaluated.

     (setq counter 0)                ; Let's call this the initializer.
     
     (setq counter (+ counter 1))    ; This is the incrementer.
     
     counter                         ; This is the counter.

(The text following the `;' are comments.  *Note Change a Function
Definition: Change a defun.)

If you evaluate the first of these expressions, the initializer,
`(setq counter 0)', and then evaluate the third expression,
`counter', the number `0' will appear in the echo area.  If you then
evaluate the second expression, the incrementer, `(setq counter (+
counter 1))', the counter will get the value 1.  So if you again
evaluate `counter', the number `1' will appear in the echo area.
Each time you evaluate the second expression, the value of the
counter will be incremented.

When you evaluate the incrementer, `(setq counter (+ counter 1))',
the Lisp interpreter first evaluates the innermost list; this is the
addition.  In order to evaluate this list, it must evaluate the
variable `counter' and the number `1'.  When it evaluates the variable
`counter', it receives its current value.  It passes this value and
the number `1' to the `+' which adds them together.  The sum is then
returned as the value of the inner list and passed to the `setq'
which sets the variable `counter' to this new value.  Thus, the value
of the variable, `counter', is changed.

Summary
=======

Learning Lisp is like climbing a hill in which the first part is the
steepest.  You have now climbed the most difficult part; what remains
becomes easier as you progress onwards.

In summary,

   * Lisp programs are made up of expressions, which are lists or
     single atoms.

   * Lists are made up of zero or more atoms or inner lists,
     separated by whitespace and surrounded by parentheses.  A list
     can be empty.

   * Atoms are multi-character symbols, like `forward-paragraph',
     single character symbols like `+', strings of characters between
     double quotation marks, or numbers.

   * A number evaluates to itself.

   * A string between double quotes also evaluates to itself.

   * When you evaluate a symbol by itself, its value is returned.

   * When you evaluate a list, the Lisp interpreter looks at the
     first symbol in the list and then at the function definition
     bound to that symbol.  Then the instructions in the function
     definition are carried out.

   * A single-quote, `'', tells the Lisp interpreter that it should
     return the following expression as written, and not evaluate it
     as it would if the quote were not there.

   * Arguments are the information passed to a function.  The
     arguments to a function are computed by evaluating the rest of
     the elements of the list of which the function is the first
     element.

   * A function always returns a value when it is evaluated (unless
     it gets an error); in addition, it may also carry out some
     action called a "side effect".  In many cases, a function's
     primary purpose is to create a side effect.

Exercises
=========

A few simple exercises:

   * Generate an error message by evaluating an appropriate symbol
     that is not within parentheses.

   * Generate an error message by evaluating an appropriate symbol
     that is between parentheses.

   * Create a counter that increments by two rather than one.

   * Write an expression that prints a message in the echo area when
     evaluated.

Practicing Evaluation
*********************

Before learning how to write a function definition in Emacs Lisp, it
is useful to spend a little time evaluating various expressions that
have already been written.  These expressions will be lists with the
functions as their first (and often only) element.  Since some of the
functions associated with buffers are both simple and interesting, we
will start with those.  In this section, we will evaluate a few of
these.  In another section, we will study the code of several other
buffer-related functions, to see how they were written.

How to Evaluate
===============

Whenever you give an editing command to Emacs Lisp, such as the
command to move the cursor or to scroll the screen, you are evaluating
an expression, the first element of which is a function.  This is how
Emacs works.

When you type keys, you cause the Lisp interpreter to evaluate an
expression and that is how you get your results.  Even typing plain
text involves evaluating an Emacs Lisp function, in this case, one
that uses `self-insert-command', which simply inserts the character
you typed.  The functions you evaluate by typing keystrokes are called
"interactive" functions, or "commands"; how you make a function
interactive will be illustrated in the chapter on how to write
function definitions.  *Note Making a Function Interactive:
Interactive.

In addition to typing keyboard commands, we have seen a second way to
evaluate an expression: by positioning the cursor after a list and
typing `C-x C-e'.  This is what we will do in the rest of this
section.  There are other ways to evaluate an expression as well;
these will be described as we come to them.

Besides being used for practicing evaluation, the functions shown in
the next few sections are important in their own right.  A study of
these functions makes clear the distinction between buffers and
files, how to switch to a buffer, and how to determine a location
within it.

Buffer Names
============

The two functions, `buffer-name' and `buffer-file-name', show the
difference between a file and a buffer.  When you evaluate the
following expression, `(buffer-name)', the name of the buffer appears
in the echo area.  When you evaluate `(buffer-file-name)', the name
of the file to which the buffer refers appears in the echo area.
Usually, the name returned by `(buffer-name)' is the same as the name
of the file to which it refers, and the name returned by
`(buffer-file-name)' is the full path-name of the file.

A file and a buffer are two different entities.  A file is information
recorded permanently in the computer (unless you delete it).  A
buffer, on the other hand, is information inside of Emacs that will
vanish at the end of the editing session (or when you kill the
buffer).  Usually, a buffer contains information that you have copied
from a file; we say the buffer is "visiting" that file.  This copy is
what you work on and modify.  Changes to the buffer do not change the
file, until you save the buffer.  When you save the buffer, the
buffer is copied to the file and is thus saved permanently.

If you are reading this in Info inside of GNU Emacs, you can evaluate
each of the following expressions by positioning the cursor after it
and typing `C-x C-e'.

     (buffer-name)
     
     (buffer-file-name)

When I do this, `"introduction.texinfo"' is the value returned by
evaluating `(buffer-name)', and
`"/gnu/work/intro/introduction.texinfo"' is the value returned by
evaluating `(buffer-file-name)'.  The former is the name of the
buffer and the latter is the name of the file.  (In the expressions,
the parentheses tell the Lisp interpreter to treat `buffer-name' and
`buffer-file-name' as functions; without the parentheses, the
interpreter would attempt to evaluate the symbols as variables.
*Note Variables::.)

In spite of the distinction between files and buffers, you will often
find that people refer to a file when they mean a buffer and
vice-versa.  Indeed, most people say, "I am editing a file," rather
than saying, "I am editing a buffer which I will soon save to a
file."  It is almost always clear from context what people mean.
When dealing with computer programs, however, it is important to keep
the distinction in mind, since the computer is not as smart as a
person.

The word `buffer', by the way, comes from the meaning of the word as a
cushion that deadens the force of a collision.  In early computers, a
buffer cushioned the interaction between files and the computer's
central processing unit.  The drums or tapes that held a file and the
central processing unit were pieces of equipment that were very
different from each other, working at their own speeds, in spurts.
The buffer made it possible for them to work together effectively.
Eventually, the buffer grew from being an intermediary, a temporary
holding place, to being the place where work is done.  This
transformation is rather like that of a small seaport that grew into a
great city: once it was merely the place where cargo was warehoused
temporarily before being loaded onto ships; then it became a business
and cultural center in its own right.

Not all buffers are associated with files.  For example, when you
start an Emacs session by typing the command `emacs' alone, without
naming any files, Emacs will start with the `*scratch*' buffer on the
screen.  This buffer is not visiting any file.  Similarly, a `*Help*'
buffer is not associated with any file.

If you switch to the `*scratch*' buffer, type `(buffer-name)',
position the cursor after it, and type `C-x C-e' to evaluate the
expression, the name `"*scratch*"' is returned and will appear in the
echo area.  `"*scratch*"' is the name of the buffer.  However, if you
type `(buffer-file-name)' in the `*scratch*' buffer and evaluate
that, `nil' will appear in the echo area.  `nil' is from the Latin
word for `nothing'; in this case, it means that the `*scratch*'
buffer is not associated with any file.  (In Lisp, `nil' is also used
to mean `false' and is a synonym for the empty list, `()'.)

Incidentally, if you are in the `*scratch*' buffer and want the value
returned by an expression to appear in the `*scratch*' buffer itself
rather than in the echo area, type `C-u C-x C-e' instead of `C-x
C-e'.  This causes the value returned to appear after the expression.
The buffer will look like this:

     (buffer-name)"*scratch*"

You cannot do this in Info since Info is read-only and it will not
allow you to change the contents of the buffer.  But you can do this
in any buffer you can edit; and when you write code or documentation
(such as this book), this feature is very useful.

Getting Buffers
===============

The `buffer-name' function returns the _name_ of the buffer; to get
the buffer _itself_, a different function is needed: the
`current-buffer' function.  If you use this function in code, what
you get is the buffer itself.

A name and the object or entity to which the name refers are different
from each other.  You are not your name.  You are a person to whom
others refer by name.  If you ask to speak to George and someone
hands you a card with the letters `G', `e', `o', `r', `g', and `e'
written on it, you might be amused, but you would not be satisfied.
You do not want to speak to the name, but to the person to whom the
name refers.  A buffer is similar: the name of the scratch buffer is
`*scratch*', but the name is not the buffer.  To get a buffer itself,
you need to use a function such as `current-buffer'.

However, there is a slight complication: if you evaluate
`current-buffer' in an expression on its own, as we will do here,
what you see is a printed representation of the name of the buffer
without the contents of the buffer.  Emacs works this way for two
reasons: the buffer may be thousands of lines long--too long to be
conveniently displayed; and, another buffer may have the same contents
but a different name, and it is important to distinguish between them.

Here is an expression containing the function:

     (current-buffer)

If you evaluate the expression in the usual way, `#<buffer *info*>'
appears in the echo area.  The special format indicates that the
buffer itself is being returned, rather than just its name.

Incidentally, while you can type a number or symbol into a program,
you cannot do that with the printed representation of a buffer: the
only way to get a buffer itself is with a function such as
`current-buffer'.

A related function is `other-buffer'.  This returns the most recently
selected buffer other than the one you are in currently.  If you have
recently switched back and forth from the `*scratch*' buffer,
`other-buffer' will return that buffer.

You can see this by evaluating the expression:

     (other-buffer)

You should see `#<buffer *scratch*>' appear in the echo area, or the
name of whatever other buffer you switched back from most recently(1).

---------- Footnotes ----------

(1) Actually, by default, if the buffer from which you just switched
is visible to you in another window, `other-buffer' will choose the
most recent buffer that you cannot see; this is a subtlety that I
often forget.

Switching Buffers
=================

The `other-buffer' function actually provides a buffer when it is
used as an argument to a function that requires one.  We can see this
by using `other-buffer' and `switch-to-buffer' to switch to a
different buffer.

But first, a brief introduction to the `switch-to-buffer' function.
When you switched back and forth from Info to the `*scratch*' buffer
to evaluate `(buffer-name)', you most likely typed `C-x b' and then
typed `*scratch*'(1) when prompted in the minibuffer for the name of
the buffer to which you wanted to switch.  The keystrokes, `C-x b',
cause the Lisp interpreter to evaluate the interactive function
`switch-to-buffer'.  As we said before, this is how Emacs works:
different keystrokes call or run different functions.  For example,
`C-f' calls `forward-char', `M-e' calls `forward-sentence', and so on.

By writing `switch-to-buffer' in an expression, and giving it a
buffer to switch to, we can switch buffers just the way `C-x b' does.

Here is the Lisp expression:

     (switch-to-buffer (other-buffer))

The symbol `switch-to-buffer' is the first element of the list, so
the Lisp interpreter will treat it as a function and carry out the
instructions that are attached to it.  But before doing that, the
interpreter will note that `other-buffer' is inside parentheses and
work on that symbol first.  `other-buffer' is the first (and in this
case, the only) element of this list, so the Lisp interpreter calls
or runs the function.  It returns another buffer.  Next, the
interpreter runs `switch-to-buffer', passing to it, as an argument,
the other buffer, which is what Emacs will switch to.  If you are
reading this in Info, try this now.  Evaluate the expression.  (To
get back, type `C-x b <RET>'.)(2)

In the programming examples in later sections of this document, you
will see the function `set-buffer' more often than
`switch-to-buffer'.  This is because of a difference between computer
programs and humans: humans have eyes and expect to see the buffer on
which they are working on their computer terminals.  This is so
obvious, it almost goes without saying.  However, programs do not
have eyes.  When a computer program works on a buffer, that buffer
does not need to be visible on the screen.

`switch-to-buffer' is designed for humans and does two different
things: it switches the buffer to which Emacs' attention is directed;
and it switches the buffer displayed in the window to the new buffer.
`set-buffer', on the other hand, does only one thing: it switches the
attention of the computer program to a different buffer.  The buffer
on the screen remains unchanged (of course, normally nothing happens
there until the command finishes running).

Also, we have just introduced another jargon term, the word "call".
When you evaluate a list in which the first symbol is a function, you
are calling that function.  The use of the term comes from the notion
of the function as an entity that can do something for you if you
`call' it--just as a plumber is an entity who can fix a leak if you
call him or her.

---------- Footnotes ----------

(1) Or rather, to save typing, you probably typed just part of the
name, such as `*sc', and then pressed your `TAB' key to cause it to
expand to the full name; and then typed your `RET' key.

(2) Remember, this expression will move you to your most recent other
buffer that you cannot see.  If you really want to go to your most
recently selected buffer, even if you can still see it, you need to
evaluate the following more complex expression:

     (switch-to-buffer (other-buffer (current-buffer) t))

In this case, the first argument to `other-buffer' tells it which
buffer to skip--the current one--and the second argument tells
`other-buffer' it is OK to switch to a visible buffer.  In regular
use, `switch-to-buffer' takes you to an invisible window since you
would most likely use `C-x o' (`other-window') to go to another
visible buffer.

Buffer Size and the Location of Point
=====================================

Finally, let's look at several rather simple functions,
`buffer-size', `point', `point-min', and `point-max'.  These give
information about the size of a buffer and the location of point
within it.

The function `buffer-size' tells you the size of the current buffer;
that is, the function returns a count of the number of characters in
the buffer.

     (buffer-size)

You can evaluate this in the usual way, by positioning the cursor
after the expression and typing `C-x C-e'.

In Emacs, the current  position of the cursor is called "point".  The
expression `(point)' returns a number that tells you where the cursor
is located as a count of the number of characters from the beginning
of the buffer up to point.

You can see the character count for point in this buffer by evaluating
the following expression in the usual way:

     (point)

As I write this, the value of `point' is 65724.  The `point' function
is frequently used in some of the examples later in this book.

The value of point depends, of course, on its location within the
buffer.  If you evaluate point in this spot, the number will be
larger:

     (point)

For me, the value of point in this location is 66043, which means that
there are 319 characters (including spaces) between the two
expressions.

The function `point-min' is somewhat similar to `point', but it
returns the value of the minimum permissible value of point in the
current buffer.  This is the number 1 unless "narrowing" is in
effect.  (Narrowing is a mechanism whereby you can restrict yourself,
or a program, to operations on just a part of a buffer.  *Note
Narrowing and Widening: Narrowing & Widening.)  Likewise, the
function `point-max' returns the value of the maximum permissible
value of point in the current buffer.

Exercise
========

Find a file with which you are working and move towards its middle.
Find its buffer name, file name, length, and your position in the
file.

How To Write Function Definitions
*********************************

When the Lisp interpreter evaluates a list, it looks to see whether
the first symbol on the list has a function definition attached to
it; or, put another way, whether the symbol points to a function
definition.  If it does, the computer carries out the instructions in
the definition.  A symbol that has a function definition is called,
simply, a function (although, properly speaking, the definition is
the function and the symbol refers to it.)

An Aside about Primitive Functions
==================================

All functions are defined in terms of other functions, except for a
few "primitive" functions that are written in the C programming
language.  When you write functions' definitions, you will write them
in Emacs Lisp and use other functions as your building blocks.  Some
of the functions you will use will themselves be written in Emacs
Lisp (perhaps by you) and some will be primitives written in C.  The
primitive functions are used exactly like those written in Emacs Lisp
and behave like them.  They are written in C so we can easily run GNU
Emacs on any computer that has sufficient power and can run C.

Let me re-emphasize this: when you write code in Emacs Lisp, you do
not distinguish between the use of functions written in C and the use
of functions written in Emacs Lisp.  The difference is irrelevant.  I
mention the distinction only because it is interesting to know.
Indeed, unless you investigate, you won't know whether an
already-written function is written in Emacs Lisp or C.

The `defun' Special Form
========================

In Lisp, a symbol such as `mark-whole-buffer' has code attached to it
that tells the computer what to do when the function is called.  This
code is called the "function definition" and is created by evaluating
a Lisp expression that starts with the symbol `defun' (which is an
abbreviation for _define function_).  Because `defun' does not
evaluate its arguments in the usual way, it is called a "special
form".

In subsequent sections, we will look at function definitions from the
Emacs source code, such as `mark-whole-buffer'.  In this section, we
will describe a simple function definition so you can see how it
looks.  This function definition uses arithmetic because it makes for
a simple example.  Some people dislike examples using arithmetic;
however, if you are such a person, do not despair.  Hardly any of the
code we will study in the remainder of this introduction involves
arithmetic or mathematics.  The examples mostly involve text in one
way or another.

A function definition has up to five parts following the word `defun':

  1. The name of the symbol to which the function definition should be
     attached.

  2. A list of the arguments that will be passed to the function.  If
     no arguments will be passed to the function, this is an empty
     list, `()'.

  3. Documentation describing the function.  (Technically optional,
     but strongly recommended.)

  4. Optionally, an expression to make the function interactive so
     you can use it by typing `M-x' and then the name of the
     function; or by typing an appropriate key or keychord.

  5. The code that instructs the computer what to do: the "body" of
     the function definition.

It is helpful to think of the five parts of a function definition as
being organized in a template, with slots for each part:

     (defun FUNCTION-NAME (ARGUMENTS...)
       "OPTIONAL-DOCUMENTATION..."
       (interactive ARGUMENT-PASSING-INFO)     ; optional
       BODY...)

As an example, here is the code for a function that multiplies its
argument by 7.  (This example is not interactive.  *Note Making a
Function Interactive: Interactive, for that information.)

     (defun multiply-by-seven (number)
       "Multiply NUMBER by seven."
       (* 7 number))

This definition begins with a parenthesis and the symbol `defun',
followed by the name of the function.

The name of the function is followed by a list that contains the
arguments that will be passed to the function.  This list is called
the "argument list".  In this example, the list has only one element,
the symbol, `number'.  When the function is used, the symbol will be
bound to the value that is used as the argument to the function.

Instead of choosing the word `number' for the name of the argument, I
could have picked any other name.  For example, I could have chosen
the word `multiplicand'.  I picked the word `number' because it tells
what kind of value is intended for this slot; but I could just as
well have chosen the word `multiplicand' to indicate the role that the
value placed in this slot will play in the workings of the function.
I could have called it `foogle', but that would have been a bad
choice because it would not tell humans what it means.  The choice of
name is up to the programmer and should be chosen to make the meaning
of the function clear.

Indeed, you can choose any name you wish for a symbol in an argument
list, even the name of a symbol used in some other function: the name
you use in an argument list is private to that particular definition.
In that definition, the name refers to a different entity than any use
of the same name outside the function definition.  Suppose you have a
nick-name `Shorty' in your family; when your family members refer to
`Shorty', they mean you.  But outside your family, in a movie, for
example, the name `Shorty' refers to someone else.  Because a name in
an argument list is private to the function definition, you can
change the value of such a symbol inside the body of a function
without changing its value outside the function.  The effect is
similar to that produced by a `let' expression.  (*Note `let': let.)

The argument list is followed by the documentation string that
describes the function.  This is what you see when you type `C-h f'
and the name of a function.  Incidentally, when you write a
documentation string like this, you should make the first line a
complete sentence since some commands, such as `apropos', print only
the first line of a multi-line documentation string.  Also, you
should not indent the second line of a documentation string, if you
have one, because that looks odd when you use `C-h f'
(`describe-function').  The documentation string is optional, but it
is so useful, it should be included in almost every function you
write.

The third line of the example consists of the body of the function
definition.  (Most functions' definitions, of course, are longer than
this.)  In this function, the body is the list, `(* 7 number)', which
says to multiply the value of NUMBER by 7.  (In Emacs Lisp, `*' is
the function for multiplication, just as `+' is the function for
addition.)

When you use the `multiply-by-seven' function, the argument `number'
evaluates to the actual number you want used.  Here is an example
that shows how `multiply-by-seven' is used; but don't try to evaluate
this yet!

     (multiply-by-seven 3)

The symbol `number', specified in the function definition in the next
section, is given or "bound to" the value 3 in the actual use of the
function.  Note that although `number' was inside parentheses in the
function definition, the argument passed to the `multiply-by-seven'
function is not in parentheses.  The parentheses are written in the
function definition so the computer can figure out where the argument
list ends and the rest of the function definition begins.

If you evaluate this example, you are likely to get an error message.
(Go ahead, try it!)  This is because we have written the function
definition, but not yet told the computer about the definition--we
have not yet installed (or `loaded') the function definition in Emacs.
Installing a function is the process that tells the Lisp interpreter
the definition of the function.  Installation is described in the next
section.

Install a Function Definition
=============================

If you are reading this inside of Info in Emacs, you can try out the
`multiply-by-seven' function by first evaluating the function
definition and then evaluating `(multiply-by-seven 3)'.  A copy of
the function definition follows.  Place the cursor after the last
parenthesis of the function definition and type `C-x C-e'.  When you
do this, `multiply-by-seven' will appear in the echo area.  (What
this means is that when a function definition is evaluated, the value
it returns is the name of the defined function.)  At the same time,
this action installs the function definition.

     (defun multiply-by-seven (number)
       "Multiply NUMBER by seven."
       (* 7 number))

By evaluating this `defun', you have just installed
`multiply-by-seven' in Emacs.  The function is now just as much a
part of Emacs as `forward-word' or any other editing function you
use.  (`multiply-by-seven' will stay installed until you quit Emacs.
To reload code automatically whenever you start Emacs, see *Note
Installing Code Permanently: Permanent Installation.)

The effect of installation
--------------------------

You can see the effect of installing `multiply-by-seven' by
evaluating the following sample.  Place the cursor after the following
expression and type `C-x C-e'.  The number 21 will appear in the echo
area.

     (multiply-by-seven 3)

If you wish, you can read the documentation for the function by typing
`C-h f' (`describe-function') and then the name of the function,
`multiply-by-seven'.  When you do this, a `*Help*' window will appear
on your screen that says:

     multiply-by-seven:
     Multiply NUMBER by seven.

(To return to a single window on your screen, type `C-x 1'.)

Change a Function Definition
----------------------------

If you want to change the code in `multiply-by-seven', just rewrite
it.  To install the new version in place of the old one, evaluate the
function definition again.  This is how you modify code in Emacs.  It
is very simple.

As an example, you can change the `multiply-by-seven' function to add
the number to itself seven times instead of multiplying the number by
seven.  It produces the same answer, but by a different path.  At the
same time, we will add a comment to the code; a comment is text that
the Lisp interpreter ignores, but that a human reader may find useful
or enlightening.  The comment is that this is the "second version".

     (defun multiply-by-seven (number)       ; Second version.
       "Multiply NUMBER by seven."
       (+ number number number number number number number))

The comment follows a semicolon, `;'.  In Lisp, everything on a line
that follows a semicolon is a comment.  The end of the line is the
end of the comment.  To stretch a comment over two or more lines,
begin each line with a semicolon.

*Note Beginning a `.emacs' File: Beginning a .emacs File, and *Note
Comments: (elisp)Comments, for more about comments.

You can install this version of the `multiply-by-seven' function by
evaluating it in the same way you evaluated the first function: place
the cursor after the last parenthesis and type `C-x C-e'.

In summary, this is how you write code in Emacs Lisp: you write a
function; install it; test it; and then make fixes or enhancements and
install it again.

Make a Function Interactive
===========================

You make a function interactive by placing a list that begins with
the special form `interactive' immediately after the documentation.
A user can invoke an interactive function by typing `M-x' and then
the name of the function; or by typing the keys to which it is bound,
for example, by typing `C-n' for `next-line' or `C-x h' for
`mark-whole-buffer'.

Interestingly, when you call an interactive function interactively,
the value returned is not automatically displayed in the echo area.
This is because you often call an interactive function for its side
effects, such as moving forward by a word or line, and not for the
value returned.  If the returned value were displayed in the echo area
each time you typed a key, it would be very distracting.

An Interactive `multiply-by-seven', An Overview
-----------------------------------------------

Both the use of the special form `interactive' and one way to display
a value in the echo area can be illustrated by creating an
interactive version of `multiply-by-seven'.

Here is the code:

     (defun multiply-by-seven (number)       ; Interactive version.
       "Multiply NUMBER by seven."
       (interactive "p")
       (message "The result is %d" (* 7 number)))

You can install this code by placing your cursor after it and typing
`C-x C-e'.  The name of the function will appear in your echo area.
Then, you can use this code by typing `C-u' and a number and then
typing `M-x multiply-by-seven' and pressing <RET>.  The phrase `The
result is ...' followed by the product will appear in the echo area.

Speaking more generally, you invoke a function like this in either of
two ways:

  1. By typing a prefix argument that contains the number to be
     passed, and then typing `M-x' and the name of the function, as
     with `C-u 3 M-x forward-sentence'; or,

  2. By typing whatever key or keychord the function is bound to, as
     with `C-u 3 M-e'.

Both the examples just mentioned work identically to move point
forward three sentences.  (Since `multiply-by-seven' is not bound to
a key, it could not be used as an example of key binding.)

(*Note Some Keybindings: Keybindings, to learn how to bind a command
to a key.)

A prefix argument is passed to an interactive function by typing the
<META> key followed by a number, for example, `M-3 M-e', or by typing
`C-u' and then a number, for example, `C-u 3 M-e' (if you type `C-u'
without a number, it defaults to 4).

An Interactive `multiply-by-seven'
----------------------------------

Let's look at the use of the special form `interactive' and then at
the function `message' in the interactive version of
`multiply-by-seven'.  You will recall that the function definition
looks like this:

     (defun multiply-by-seven (number)       ; Interactive version.
       "Multiply NUMBER by seven."
       (interactive "p")
       (message "The result is %d" (* 7 number)))

In this function, the expression, `(interactive "p")', is a list of
two elements.  The `"p"' tells Emacs to pass the prefix argument to
the function and use its value for the argument of the function.

The argument will be a number.  This means that the symbol `number'
will be bound to a number in the line:

     (message "The result is %d" (* 7 number))

For example, if your prefix argument is 5, the Lisp interpreter will
evaluate the line as if it were:

     (message "The result is %d" (* 7 5))

(If you are reading this in GNU Emacs, you can evaluate this
expression yourself.)  First, the interpreter will evaluate the inner
list, which is `(* 7 5)'.  This returns a value of 35.  Next, it will
evaluate the outer list, passing the values of the second and
subsequent elements of the list to the function `message'.

As we have seen, `message' is an Emacs Lisp function especially
designed for sending a one line message to a user.  (*Note The
`message' function: message.)  In summary, the `message' function
prints its first argument in the echo area as is, except for
occurrences of `%d', `%s', or `%c'.  When it sees one of these
control sequences, the function looks to the second and subsequent
arguments and prints the value of the argument in the location in the
string where the control sequence is located.

In the interactive `multiply-by-seven' function, the control string
is `%d', which requires a number, and the value returned by
evaluating `(* 7 5)' is the number 35.  Consequently, the number 35
is printed in place of the `%d' and the message is `The result is 35'.

(Note that when you call the function `multiply-by-seven', the
message is printed without quotes, but when you call `message', the
text is printed in double quotes.  This is because the value returned
by `message' is what appears in the echo area when you evaluate an
expression whose first element is `message'; but when embedded in a
function, `message' prints the text as a side effect without quotes.)

Different Options for `interactive'
===================================

In the example, `multiply-by-seven' used `"p"' as the argument to
`interactive'.  This argument told Emacs to interpret your typing
either `C-u' followed by a number or <META> followed by a number as a
command to pass that number to the function as its argument.  Emacs
has more than twenty characters predefined for use with
`interactive'.  In almost every case, one of these options will
enable you to pass the right information interactively to a function.
(*Note Code Characters for `interactive': (elisp)Interactive Codes.)

For example, the character `r' causes Emacs to pass the beginning and
end of the region (the current values of point and mark) to the
function as two separate arguments.  It is used as follows:

     (interactive "r")

On the other hand, a `B' tells Emacs to ask for the name of a buffer
that will be passed to the function.  When it sees a `B', Emacs will
ask for the name by prompting the user in the minibuffer, using a
string that follows the `B', as in `"BAppend to buffer: "'.  Not only
will Emacs prompt for the name, but Emacs will complete the name if
you type enough of it and press <TAB>.

A function with two or more arguments can have information passed to
each argument by adding parts to the string that follows
`interactive'.  When you do this, the information is passed to each
argument in the same order it is specified in the `interactive' list.
In the string, each part is separated from the next part by a `\n',
which is a newline.  For example, you could follow `"BAppend to
buffer: "' with a `\n') and an `r'.  This would cause Emacs to pass
the values of point and mark to the function as well as prompt you
for the buffer--three arguments in all.

In this case, the function definition would look like the following,
where `buffer', `start', and `end' are the symbols to which
`interactive' binds the buffer and the current values of the
beginning and ending of the region:

     (defun NAME-OF-FUNCTION (buffer start end)
       "DOCUMENTATION..."
       (interactive "BAppend to buffer: \nr")
       BODY-OF-FUNCTION...)

(The space after the colon in the prompt makes it look better when you
are prompted.  The `append-to-buffer' function looks exactly like
this.  *Note The Definition of `append-to-buffer': append-to-buffer.)

If a function does not have arguments, then `interactive' does not
require any.  Such a function contains the simple expression
`(interactive)'.  The `mark-whole-buffer' function is like this.

Alternatively, if the special letter-codes are not right for your
application, you can pass your own arguments to `interactive' as a
list.  *Note Using `Interactive': (elisp)interactive, for more
information about this advanced technique.

Install Code Permanently
========================

When you install a function definition by evaluating it, it will stay
installed until you quit Emacs.  The next time you start a new session
of Emacs, the function will not be installed unless you evaluate the
function definition again.

At some point, you may want to have code installed automatically
whenever you start a new session of Emacs.  There are several ways of
doing this:

   * If you have code that is just for yourself, you can put the code
     for the function definition in your `.emacs' initialization
     file.  When you start Emacs, your `.emacs' file is automatically
     evaluated and all the function definitions within it are
     installed.  *Note Your `.emacs' File: Emacs Initialization.

   * Alternatively, you can put the function definitions that you want
     installed in one or more files of their own and use the `load'
     function to cause Emacs to evaluate and thereby install each of
     the functions in the files.  *Note Loading Files: Loading Files.

   * On the other hand, if you have code that your whole site will
     use, it is usual to put it in a file called `site-init.el' that
     is loaded when Emacs is built.  This makes the code available to
     everyone who uses your machine.  (See the `INSTALL' file that is
     part of the Emacs distribution.)

Finally, if you have code that everyone who uses Emacs may want, you
can post it on a computer network or send a copy to the Free Software
Foundation.  (When you do this, please license the code and its
documentation under a license that permits other people to run, copy,
study, modify, and redistribute the code and which protects you from
having your work taken from you.)  If you send a copy of your code to
the Free Software Foundation, and properly protect yourself and
others, it may be included in the next release of Emacs.  In large
part, this is how Emacs has grown over the past years, by donations.

`let'
=====

The `let' expression is a special form in Lisp that you will need to
use in most function definitions.

`let' is used to attach or bind a symbol to a value in such a way
that the Lisp interpreter will not confuse the variable with a
variable of the same name that is not part of the function.

To understand why the `let' special form is necessary, consider the
situation in which you own a home that you generally refer to as `the
house', as in the sentence, "The house needs painting."  If you are
visiting a friend and your host refers to `the house', he is likely
to be referring to _his_ house, not yours, that is, to a different
house.

If your friend is referring to his house and you think he is referring
to your house, you may be in for some confusion.  The same thing could
happen in Lisp if a variable that is used inside of one function has
the same name as a variable that is used inside of another function,
and the two are not intended to refer to the same value.  The `let'
special form prevents this kind of confusion.

`let' Prevents Confusion
------------------------

The `let' special form prevents confusion.  `let' creates a name for
a "local variable" that overshadows any use of the same name outside
the `let' expression.  This is like understanding that whenever your
host refers to `the house', he means his house, not yours.  (Symbols
used in argument lists work the same way.  *Note The `defun' Special
Form: defun.)

Local variables created by a `let' expression retain their value
_only_ within the `let' expression itself (and within expressions
called within the `let' expression); the local variables have no
effect outside the `let' expression.

Another way to think about `let' is that it is like a `setq' that is
temporary and local.  The values set by `let' are automatically
undone when the `let' is finished.  The setting only affects
expressions that are inside the bounds of the `let' expression.  In
computer science jargon, we would say "the binding of a symbol is
visible only in functions called in the `let' form; in Emacs Lisp,
scoping is dynamic, not lexical."

`let' can create more than one variable at once.  Also, `let' gives
each variable it creates an initial value, either a value specified
by you, or `nil'.  (In the jargon, this is called `binding the
variable to the value'.)  After `let' has created and bound the
variables, it executes the code in the body of the `let', and returns
the value of the last expression in the body, as the value of the
whole `let' expression.  (`Execute' is a jargon term that means to
evaluate a list; it comes from the use of the word meaning `to give
practical effect to' (`Oxford English Dictionary').  Since you
evaluate an expression to perform an action, `execute' has evolved as
a synonym to `evaluate'.)

The Parts of a `let' Expression
-------------------------------

A `let' expression is a list of three parts.  The first part is the
symbol `let'.  The second part is a list, called a "varlist", each
element of which is either a symbol by itself or a two-element list,
the first element of which is a symbol.  The third part of the `let'
expression is the body of the `let'.  The body usually consists of
one or more lists.

A template for a `let' expression looks like this:

     (let VARLIST BODY...)

The symbols in the varlist are the variables that are given initial
values by the `let' special form.  Symbols by themselves are given
the initial value of `nil'; and each symbol that is the first element
of a two-element list is bound to the value that is returned when the
Lisp interpreter evaluates the second element.

Thus, a varlist might look like this: `(thread (needles 3))'.  In
this case, in a `let' expression, Emacs binds the symbol `thread' to
an initial value of `nil', and binds the symbol `needles' to an
initial value of 3.

When you write a `let' expression, what you do is put the appropriate
expressions in the slots of the `let' expression template.

If the varlist is composed of two-element lists, as is often the case,
the template for the `let' expression looks like this:

     (let ((VARIABLE VALUE)
           (VARIABLE VALUE)
           ...)
       BODY...)

Sample `let' Expression
-----------------------

The following expression creates and gives initial values to the two
variables `zebra' and `tiger'.  The body of the `let' expression is a
list which calls the `message' function.

     (let ((zebra 'stripes)
           (tiger 'fierce))
       (message "One kind of animal has %s and another is %s."
                zebra tiger))

Here, the varlist is `((zebra 'stripes) (tiger 'fierce))'.

The two variables are `zebra' and `tiger'.  Each variable is the
first element of a two-element list and each value is the second
element of its two-element list.  In the varlist, Emacs binds the
variable `zebra' to the value `stripes', and binds the variable
`tiger' to the value `fierce'.  In this example, both values are
symbols preceded by a quote.  The values could just as well have been
another list or a string.  The body of the `let' follows after the
list holding the variables.  In this example, the body is a list that
uses the `message' function to print a string in the echo area.

You may evaluate the example in the usual fashion, by placing the
cursor after the last parenthesis and typing `C-x C-e'.  When you do
this, the following will appear in the echo area:

     "One kind of animal has stripes and another is fierce."

As we have seen before, the `message' function prints its first
argument, except for `%s'.  In this example, the value of the variable
`zebra' is printed at the location of the first `%s' and the value of
the variable `tiger' is printed at the location of the second `%s'.

Uninitialized Variables in a `let' Statement
--------------------------------------------

If you do not bind the variables in a `let' statement to specific
initial values, they will automatically be bound to an initial value
of `nil', as in the following expression:

     (let ((birch 3)
           pine
           fir
           (oak 'some))
       (message
        "Here are %d variables with %s, %s, and %s value."
        birch pine fir oak))

Here, the varlist is `((birch 3) pine fir (oak 'some))'.

If you evaluate this expression in the usual way, the following will
appear in your echo area:

     "Here are 3 variables with nil, nil, and some value."

In this example, Emacs binds the symbol `birch' to the number 3,
binds the symbols `pine' and `fir' to `nil', and binds the symbol
`oak' to the value `some'.

Note that in the first part of the `let', the variables `pine' and
`fir' stand alone as atoms that are not surrounded by parentheses;
this is because they are being bound to `nil', the empty list.  But
`oak' is bound to `some' and so is a part of the list `(oak 'some)'.
Similarly, `birch' is bound to the number 3 and so is in a list with
that number.  (Since a number evaluates to itself, the number does
not need to be quoted.  Also, the number is printed in the message
using a `%d' rather than a `%s'.)  The four variables as a group are
put into a list to delimit them from the body of the `let'.

The `if' Special Form
=====================

A third special form, in addition to `defun' and `let', is the
conditional `if'.  This form is used to instruct the computer to make
decisions.  You can write function definitions without using `if',
but it is used often enough, and is important enough, to be included
here.  It is used, for example, in the code for the function
`beginning-of-buffer'.

The basic idea behind an `if', is that "_if_ a test is true, _then_
an expression is evaluated."  If the test is not true, the expression
is not evaluated.  For example, you might make a decision such as,
"if it is warm and sunny, then go to the beach!"

`if' in more detail
-------------------

An `if' expression written in Lisp does not use the word `then'; the
test and the action are the second and third elements of the list
whose first element is `if'.  Nonetheless, the test part of an `if'
expression is often called the "if-part" and the second argument is
often called the "then-part".

Also, when an `if' expression is written, the true-or-false-test is
usually written on the same line as the symbol `if', but the action
to carry out if the test is true, the "then-part", is written on the
second and subsequent lines.  This makes the `if' expression easier
to read.

     (if TRUE-OR-FALSE-TEST
         ACTION-TO-CARRY-OUT-IF-TEST-IS-TRUE)

The true-or-false-test will be an expression that is evaluated by the
Lisp interpreter.

Here is an example that you can evaluate in the usual manner.  The
test is whether the number 5 is greater than the number 4.  Since it
is, the message `5 is greater than 4!' will be printed.

     (if (> 5 4)                             ; if-part
         (message "5 is greater than 4!"))   ; then-part

(The function `>' tests whether its first argument is greater than
its second argument and returns true if it is.)

Of course, in actual use, the test in an `if' expression will not be
fixed for all time as it is by the expression `(> 5 4)'.  Instead, at
least one of the variables used in the test will be bound to a value
that is not known ahead of time.  (If the value were known ahead of
time, we would not need to run the test!)

For example, the value may be bound to an argument of a function
definition.  In the following function definition, the character of
the animal is a value that is passed to the function.  If the value
bound to `characteristic' is `fierce', then the message, `It's a
tiger!' will be printed; otherwise, `nil' will be returned.

     (defun type-of-animal (characteristic)
       "Print message in echo area depending on CHARACTERISTIC.
     If the CHARACTERISTIC is the symbol `fierce',
     then warn of a tiger."
       (if (equal characteristic 'fierce)
           (message "It's a tiger!")))

If you are reading this inside of GNU Emacs, you can evaluate the
function definition in the usual way to install it in Emacs, and then
you can evaluate the following two expressions to see the results:

     (type-of-animal 'fierce)
     
     (type-of-animal 'zebra)

When you evaluate `(type-of-animal 'fierce)', you will see the
following message printed in the echo area: `"It's a tiger!"'; and
when you evaluate `(type-of-animal 'zebra)' you will see `nil'
printed in the echo area.

The `type-of-animal' Function in Detail
---------------------------------------

Let's look at the `type-of-animal' function in detail.

The function definition for `type-of-animal' was written by filling
the slots of two templates, one for a function definition as a whole,
and a second for an `if' expression.

The template for every function that is not interactive is:

     (defun NAME-OF-FUNCTION (ARGUMENT-LIST)
       "DOCUMENTATION..."
       BODY...)

The parts of the function that match this template look like this:

     (defun type-of-animal (characteristic)
       "Print message in echo area depending on CHARACTERISTIC.
     If the CHARACTERISTIC is the symbol `fierce',
     then warn of a tiger."
       BODY: THE `if' EXPRESSION)

The name of function is `type-of-animal'; it is passed the value of
one argument.  The argument list is followed by a multi-line
documentation string.  The documentation string is included in the
example because it is a good habit to write documentation string for
every function definition.  The body of the function definition
consists of the `if' expression.

The template for an `if' expression looks like this:

     (if TRUE-OR-FALSE-TEST
         ACTION-TO-CARRY-OUT-IF-THE-TEST-RETURNS-TRUE)

In the `type-of-animal' function, the code for the `if' looks like
this:

     (if (equal characteristic 'fierce)
         (message "It's a tiger!")))

Here, the true-or-false-test is the expression:

     (equal characteristic 'fierce)

In Lisp, `equal' is a function that determines whether its first
argument is equal to its second argument.  The second argument is the
quoted symbol `'fierce' and the first argument is the value of the
symbol `characteristic'--in other words, the argument passed to this
function.

In the first exercise of `type-of-animal', the argument `fierce' is
passed to `type-of-animal'.  Since `fierce' is equal to `fierce', the
expression, `(equal characteristic 'fierce)', returns a value of
true.  When this happens, the `if' evaluates the second argument or
then-part of the `if': `(message "It's tiger!")'.

On the other hand, in the second exercise of `type-of-animal', the
argument `zebra' is passed to `type-of-animal'.  `zebra' is not equal
to `fierce', so the then-part is not evaluated and `nil' is returned
by the `if' expression.

If-then-else Expressions
========================

An `if' expression may have an optional third argument, called the
"else-part", for the case when the true-or-false-test returns false.
When this happens, the second argument or then-part of the overall
`if' expression is _not_ evaluated, but the third or else-part _is_
evaluated.  You might think of this as the cloudy day alternative for
the decision `if it is warm and sunny, then go to the beach, else
read a book!".

The word "else" is not written in the Lisp code; the else-part of an
`if' expression comes after the then-part.  In the written Lisp, the
else-part is usually written to start on a line of its own and is
indented less than the then-part:

     (if TRUE-OR-FALSE-TEST
         ACTION-TO-CARRY-OUT-IF-THE-TEST-RETURNS-TRUE
       ACTION-TO-CARRY-OUT-IF-THE-TEST-RETURNS-FALSE)

For example, the following `if' expression prints the message `4 is
not greater than 5!' when you evaluate it in the usual way:

     (if (> 4 5)                             ; if-part
         (message "5 is greater than 4!")    ; then-part
       (message "4 is not greater than 5!")) ; else-part

Note that the different levels of indentation make it easy to
distinguish the then-part from the else-part.  (GNU Emacs has several
commands that automatically indent `if' expressions correctly.  *Note
GNU Emacs Helps You Type Lists: Typing Lists.)

We can extend the `type-of-animal' function to include an else-part
by simply incorporating an additional part to the `if' expression.

You can see the consequences of doing this if you evaluate the
following version of the `type-of-animal' function definition to
install it and then evaluate the two subsequent expressions to pass
different arguments to the function.

     (defun type-of-animal (characteristic)  ; Second version.
       "Print message in echo area depending on CHARACTERISTIC.
     If the CHARACTERISTIC is the symbol `fierce',
     then warn of a tiger;
     else say it's not fierce."
       (if (equal characteristic 'fierce)
           (message "It's a tiger!")
         (message "It's not fierce!")))


     (type-of-animal 'fierce)
     
     (type-of-animal 'zebra)

When you evaluate `(type-of-animal 'fierce)', you will see the
following message printed in the echo area: `"It's a tiger!"'; but
when you evaluate `(type-of-animal 'zebra)', you will see `"It's not
fierce!"'.

(Of course, if the CHARACTERISTIC were `ferocious', the message
`"It's not fierce!"' would be printed; and it would be misleading!
When you write code, you need to take into account the possibility
that some such argument will be tested by the `if' and write your
program accordingly.)

Truth and Falsehood in Emacs Lisp
=================================

There is an important aspect to the truth test in an `if' expression.
So far, we have spoken of `true' and `false' as values of predicates
as if they were new kinds of Emacs Lisp objects.  In fact, `false' is
just our old friend `nil'.  Anything else--anything at all--is `true'.

The expression that tests for truth is interpreted as "true" if the
result of evaluating it is a value that is not `nil'.  In other
words, the result of the test is considered true if the value
returned is a number such as 47, a string such as `"hello"', or a
symbol (other than `nil') such as `flowers', or a list, or even a
buffer!

An explanation of `nil'
-----------------------

Before illustrating a test for truth, we need an explanation of `nil'.

In Emacs Lisp, the symbol `nil' has two meanings.  First, it means the
empty list.  Second, it means false and is the value returned when a
true-or-false-test tests false.  `nil' can be written as an empty
list, `()', or as `nil'.  As far as the Lisp interpreter is
concerned, `()' and `nil' are the same.  Humans, however, tend to use
`nil' for false and `()' for the empty list.

In Emacs Lisp, any value that is not `nil'--is not the empty list--is
considered true.  This means that if an evaluation returns something
that is not an empty list, an `if' expression will test true.  For
example, if a number is put in the slot for the test, it will be
evaluated and will return itself, since that is what numbers do when
evaluated.  In this conditional, the `if' expression will test true.
The expression tests false only when `nil', an empty list, is
returned by evaluating the expression.

You can see this by evaluating the two expressions in the following
examples.

In the first example, the number 4 is evaluated as the test in the
`if' expression and returns itself; consequently, the then-part of
the expression is evaluated and returned: `true' appears in the echo
area.  In the second example, the `nil' indicates false;
consequently, the else-part of the expression is evaluated and
returned: `false' appears in the echo area.

     (if 4
         'true
       'false)
     
     (if nil
         'true
       'false)

Incidentally, if some other useful value is not available for a test
that returns true, then the Lisp interpreter will return the symbol
`t' for true.  For example, the expression `(> 5 4)' returns `t' when
evaluated, as you can see by evaluating it in the usual way:

     (> 5 4)

On the other hand, this function returns `nil' if the test is false.

     (> 4 5)

`save-excursion'
================

The `save-excursion' function is the fourth and final special form
that we will discuss in this chapter.

In Emacs Lisp programs used for editing, the `save-excursion'
function is very common.  It saves the location of point and mark,
executes the body of the function, and then restores point and mark to
their previous positions if their locations were changed.  Its primary
purpose is to keep the user from being surprised and disturbed by
unexpected movement of point or mark.

Point and Mark
--------------

Before discussing `save-excursion', however, it may be useful first
to review what point and mark are in GNU Emacs.  "Point" is the
current location of the cursor.  Wherever the cursor is, that is
point.  More precisely, on terminals where the cursor appears to be
on top of a character, point is immediately before the character.  In
Emacs Lisp, point is an integer.  The first character in a buffer is
number one, the second is number two, and so on.  The function
`point' returns the current position of the cursor as a number.  Each
buffer has its own value for point.

The "mark" is another position in the buffer; its value can be set
with a command such as `C-<SPC>' (`set-mark-command').  If a mark has
been set, you can use the command `C-x C-x'
(`exchange-point-and-mark') to cause the cursor to jump to the mark
and set the mark to be the previous position of point.  In addition,
if you set another mark, the position of the previous mark is saved
in the mark ring.  Many mark positions can be saved this way.  You
can jump the cursor to a saved mark by typing `C-u C-<SPC>' one or
more times.

The part of the buffer between point and mark is called "the region".
Numerous commands work on the region, including `center-region',
`count-lines-region', `kill-region', and `print-region'.

The `save-excursion' special form saves the locations of point and
mark and restores those positions after the code within the body of
the special form is evaluated by the Lisp interpreter.  Thus, if
point were in the beginning of a piece of text and some code moved
point to the end of the buffer, the `save-excursion' would put point
back to where it was before, after the expressions in the body of the
function were evaluated.

In Emacs, a function frequently moves point as part of its internal
workings even though a user would not expect this.  For example,
`count-lines-region' moves point.  To prevent the user from being
bothered by jumps that are both unexpected and (from the user's point
of view) unnecessary, `save-excursion' is often used to keep point and
mark in the location expected by the user.  The use of
`save-excursion' is good housekeeping.

To make sure the house stays clean, `save-excursion' restores the
values of point and mark even if something goes wrong in the code
inside of it (or, to be more precise and to use the proper jargon,
"in case of abnormal exit").  This feature is very helpful.

In addition to recording the values of point and mark,
`save-excursion' keeps track of the current buffer, and restores it,
too.  This means you can write code that will change the buffer and
have `save-excursion' switch you back to the original buffer.  This
is how `save-excursion' is used in `append-to-buffer'.  (*Note The
Definition of `append-to-buffer': append-to-buffer.)

Template for a `save-excursion' Expression
------------------------------------------

The template for code using `save-excursion' is simple:

     (save-excursion
       BODY...)

The body of the function is one or more expressions that will be
evaluated in sequence by the Lisp interpreter.  If there is more than
one expression in the body, the value of the last one will be returned
as the value of the `save-excursion' function.  The other expressions
in the body are evaluated only for their side effects; and
`save-excursion' itself is used only for its side effect (which is
restoring the positions of point and mark).

In more detail, the template for a `save-excursion' expression looks
like this:

     (save-excursion
       FIRST-EXPRESSION-IN-BODY
       SECOND-EXPRESSION-IN-BODY
       THIRD-EXPRESSION-IN-BODY
        ...
       LAST-EXPRESSION-IN-BODY)

An expression, of course, may be a symbol on its own or a list.

In Emacs Lisp code, a `save-excursion' expression often occurs within
the body of a `let' expression.  It looks like this:

     (let VARLIST
       (save-excursion
         BODY...))

Review
======

In the last few chapters we have introduced a fair number of functions
and special forms.  Here they are described in brief, along with a few
similar functions that have not been mentioned yet.

`eval-last-sexp'
     Evaluate the last symbolic expression before the current
     location of point.  The value is printed in the echo area unless
     the function is invoked with an argument; in that case, the
     output is printed in the current buffer.  This command is
     normally bound to `C-x C-e'.

`defun'
     Define function.  This special form has up to five parts: the
     name, a template for the arguments that will be passed to the
     function, documentation, an optional interactive declaration,
     and the body of the definition.

     For example:

          (defun back-to-indentation ()
            "Move point to first visible character on line."
            (interactive)
            (beginning-of-line 1)
            (skip-chars-forward " \t"))

`interactive'
     Declare to the interpreter that the function can be used
     interactively.  This special form may be followed by a string
     with one or more parts that pass the information to the
     arguments of the function, in sequence.  These parts may also
     tell the interpreter to prompt for information.  Parts of the
     string are separated by newlines, `\n'.

     Common code characters are:

    `b'
          The name of an existing buffer.

    `f'
          The name of an existing file.

    `p'
          The numeric prefix argument.  (Note that this `p' is lower
          case.)

    `r'
          Point and the mark, as two numeric arguments, smallest
          first.  This is the only code letter that specifies two
          successive arguments rather than one.

     *Note Code Characters for `interactive': (elisp)Interactive
     Codes, for a complete list of code characters.

`let'
     Declare that a list of variables is for use within the body of
     the `let' and give them an initial value, either `nil' or a
     specified value; then evaluate the rest of the expressions in
     the body of the `let' and return the value of the last one.
     Inside the body of the `let', the Lisp interpreter does not see
     the values of the variables of the same names that are bound
     outside of the `let'.

     For example,

          (let ((foo (buffer-name))
                (bar (buffer-size)))
            (message
             "This buffer is %s and has %d characters."
             foo bar))

`save-excursion'
     Record the values of point and mark and the current buffer before
     evaluating the body of this special form.  Restore the values of
     point and mark and buffer afterward.

     For example,

          (message "We are %d characters into this buffer."
                   (- (point)
                      (save-excursion
                        (goto-char (point-min)) (point))))

`if'
     Evaluate the first argument to the function; if it is true,
     evaluate the second argument; else evaluate the third argument,
     if there is one.

     The `if' special form is called a "conditional".  There are
     other conditionals in Emacs Lisp, but `if' is perhaps the most
     commonly used.

     For example,

          (if (string-equal
               (number-to-string 21)
               (substring (emacs-version) 10 12))
              (message "This is version 21 Emacs")
            (message "This is not version 21 Emacs"))

`equal'
`eq'
     Test whether two objects are the same.  `equal' uses one meaning
     of the word `same' and `eq' uses another:  `equal' returns true
     if the two objects have a similar structure and contents, such as
     two copies of the same book.  On the other hand, `eq', returns
     true if both arguments are actually the same object.

`<'
`>'
`<='
`>='
     The `<' function tests whether its first argument is smaller than
     its second argument.  A corresponding function, `>', tests
     whether the first argument is greater than the second.
     Likewise, `<=' tests whether the first argument is less than or
     equal to the second and `>=' tests whether the first argument is
     greater than or equal to the second.  In all cases, both
     arguments must be numbers or markers (markers indicate positions
     in buffers).

`string<'
`string-lessp'
`string='
`string-equal'
     The `string-lessp' function tests whether its first argument is
     smaller than the second argument.  A shorter, alternative name
     for the same function (a `defalias') is `string<'.

     The arguments to `string-lessp' must be strings or symbols; the
     ordering is lexicographic, so case is significant.  The print
     names of symbols are used instead of the symbols themselves.

     An empty string, `""', a string with no characters in it, is
     smaller than any string of characters.

     `string-equal' provides the corresponding test for equality.  Its
     shorter, alternative name is `string='.  There are no string test
     functions that correspond to >, `>=', or `<='.

`message'
     Print a message in the echo area. The first argument is a string
     that can contain `%s', `%d', or `%c' to print the value of
     arguments that follow the string.  The argument used by `%s' must
     be a string or a symbol; the argument used by `%d' must be a
     number.  The argument used by `%c' must be an ascii code number;
     it will be printed as the character with that ASCII code.

`setq'
`set'
     The `setq' function sets the value of its first argument to the
     value of the second argument.  The first argument is
     automatically quoted by `setq'.  It does the same for succeeding
     pairs of arguments.  Another function, `set', takes only two
     arguments and evaluates both of them before setting the value
     returned by its first argument to the value returned by its
     second argument.

`buffer-name'
     Without an argument, return the name of the buffer, as a string.

`buffer-file-name'
     Without an argument, return the name of the file the buffer is
     visiting.

`current-buffer'
     Return the buffer in which Emacs is active; it may not be the
     buffer that is visible on the screen.

`other-buffer'
     Return the most recently selected buffer (other than the buffer
     passed to `other-buffer' as an argument and other than the
     current buffer).

`switch-to-buffer'
     Select a buffer for Emacs to be active in and display it in the
     current window so users can look at it.  Usually bound to `C-x
     b'.

`set-buffer'
     Switch Emacs' attention to a buffer on which programs will run.
     Don't alter what the window is showing.

`buffer-size'
     Return the number of characters in the current buffer.

`point'
     Return the value of the current position of the cursor, as an
     integer counting the number of characters from the beginning of
     the buffer.

`point-min'
     Return the minimum permissible value of point in the current
     buffer.  This is 1, unless narrowing is in effect.

`point-max'
     Return the value of the maximum permissible value of point in the
     current buffer.  This is the end of the buffer, unless narrowing
     is in effect.

Exercises
=========

   * Write a non-interactive function that doubles the value of its
     argument, a number.  Make that function interactive.

   * Write a function that tests whether the current value of
     `fill-column' is greater than the argument passed to the
     function, and if so, prints an appropriate message.

A Few Buffer-Related Functions
******************************

In this chapter we study in detail several of the functions used in
GNU Emacs.  This is called a "walk-through".  These functions are
used as examples of Lisp code, but are not imaginary examples; with
the exception of the first, simplified function definition, these
functions show the actual code used in GNU Emacs.  You can learn a
great deal from these definitions.  The functions described here are
all related to buffers.  Later, we will study other functions.

Finding More Information
========================

In this walk-through, I will describe each new function as we come to
it, sometimes in detail and sometimes briefly.  If you are interested,
you can get the full documentation of any Emacs Lisp function at any
time by typing `C-h f' and then the name of the function (and then
<RET>).  Similarly, you can get the full documentation for a variable
by typing `C-h v' and then the name of the variable (and then <RET>).

In versions 20 and higher, when a function is written in Emacs Lisp,
`describe-function' will also tell you the location of the function
definition.  If you move point over the file name and press the <RET>
key, which is this case means `help-follow' rather than `return' or
`enter', Emacs will take you directly to the function definition.

More generally, if you want to see a function in its original source
file, you can use the `find-tags' function to jump to it.
`find-tags' works with a wide variety of languages, not just Lisp,
and C, and it works with non-programming text as well.  For example,
`find-tags' will jump to the various nodes in the Texinfo source file
of this document.

The `find-tags' function depends on `tags tables' that record the
locations of the functions, variables, and other items to which
`find-tags' jumps.

To use the `find-tags' command, type `M-.'  (i.e., type the <META>
key and the period key at the same time, or else type the <ESC> key
and then type the period key), and then, at the prompt, type in the
name of the function whose source code you want to see, such as
`mark-whole-buffer', and then type <RET>.  Emacs will switch buffers
and display the source code for the function on your screen.  To
switch back to your current buffer, type `C-x b <RET>'.  (On some
keyboards, the <META> key is labelled <ALT>.)

Depending on how the initial default values of your copy of Emacs are
set, you may also need to specify the location of your `tags table',
which is a file called `TAGS'.  For example, if you are interested in
Emacs sources, the tags table you will most likely want, if it has
already been created for you, will be in a subdirectory of the
`/usr/local/share/emacs/' directory; thus you would use the `M-x
visit-tags-table' command and specify a pathname such as
`/usr/local/share/emacs/21.0.100/lisp/TAGS' or
`/usr/local/src/emacs/lisp/TAGS'.  If the tags table has not already
been created, you will have to create it yourself.

To create a `TAGS' file in a specific directory, switch to that
directory in Emacs using `M-x cd' command, or list the directory with
`C-x d' (`dired').  Then run the compile command, with `etags *.el'
as the command to execute

     M-x compile RET etags *.el RET

For more information, see *Note Create Your Own `TAGS' File: etags.

After you become more familiar with Emacs Lisp, you will find that
you will frequently use `find-tags' to navigate your way around
source code; and you will create your own `TAGS' tables.

Incidentally, the files that contain Lisp code are conventionally
called "libraries".  The metaphor is derived from that of a
specialized library, such as a law library or an engineering library,
rather than a general library.  Each library, or file, contains
functions that relate to a particular topic or activity, such as
`abbrev.el' for handling abbreviations and other typing shortcuts,
and `help.el' for on-line help.  (Sometimes several libraries provide
code for a single activity, as the various `rmail...' files provide
code for reading electronic mail.)  In `The GNU Emacs Manual', you
will see sentences such as "The `C-h p' command lets you search the
standard Emacs Lisp libraries by topic keywords."

A Simplified `beginning-of-buffer' Definition
=============================================

The `beginning-of-buffer' command is a good function to start with
since you are likely to be familiar with it and it is easy to
understand.  Used as an interactive command, `beginning-of-buffer'
moves the cursor to the beginning of the buffer, leaving the mark at
the previous position.  It is generally bound to `M-<'.

In this section, we will discuss a shortened version of the function
that shows how it is most frequently used.  This shortened function
works as written, but it does not contain the code for a complex
option.  In another section, we will describe the entire function.
(*Note Complete Definition of `beginning-of-buffer':
beginning-of-buffer.)

Before looking at the code, let's consider what the function
definition has to contain: it must include an expression that makes
the function interactive so it can be called by typing `M-x
beginning-of-buffer' or by typing a keychord such as `M-<'; it must
include code to leave a mark at the original position in the buffer;
and it must include code to move the cursor to the beginning of the
buffer.

Here is the complete text of the shortened version of the function:

     (defun simplified-beginning-of-buffer ()
       "Move point to the beginning of the buffer;
     leave mark at previous position."
       (interactive)
       (push-mark)
       (goto-char (point-min)))

Like all function definitions, this definition has five parts
following the special form `defun':

  1. The name: in this example, `simplified-beginning-of-buffer'.

  2. A list of the arguments: in this example, an empty list, `()',

  3. The documentation string.

  4. The interactive expression.

  5. The body.

In this function definition, the argument list is empty; this means
that this function does not require any arguments.  (When we look at
the definition for the complete function, we will see that it may be
passed an optional argument.)

The interactive expression tells Emacs that the function is intended
to be used interactively.  In this example, `interactive' does not
have an argument because `simplified-beginning-of-buffer' does not
require one.

The body of the function consists of the two lines:

     (push-mark)
     (goto-char (point-min))

The first of these lines is the expression, `(push-mark)'.  When this
expression is evaluated by the Lisp interpreter, it sets a mark at
the current position of the cursor, wherever that may be.  The
position of this mark is saved in the mark ring.

The next line is `(goto-char (point-min))'.  This expression jumps
the cursor to the minimum point in the buffer, that is, to the
beginning of the buffer (or to the beginning of the accessible portion
of the buffer if it is narrowed.  *Note Narrowing and Widening:
Narrowing & Widening.)

The `push-mark' command sets a mark at the place where the cursor was
located before it was moved to the beginning of the buffer by the
`(goto-char (point-min))' expression.  Consequently, you can, if you
wish, go back to where you were originally by typing `C-x C-x'.

That is all there is to the function definition!

When you are reading code such as this and come upon an unfamiliar
function, such as `goto-char', you can find out what it does by using
the `describe-function' command.  To use this command, type `C-h f'
and then type in the name of the function and press <RET>.  The
`describe-function' command will print the function's documentation
string in a `*Help*' window.  For example, the documentation for
`goto-char' is:

     One arg, a number.  Set point to that number.
     Beginning of buffer is position (point-min),
     end is (point-max).

(The prompt for `describe-function' will offer you the symbol under
or preceding the cursor, so you can save typing by positioning the
cursor right over or after the function and then typing `C-h f
<RET>'.)

The `end-of-buffer' function definition is written in the same way as
the `beginning-of-buffer' definition except that the body of the
function contains the expression `(goto-char (point-max))' in place
of `(goto-char (point-min))'.

The Definition of `mark-whole-buffer'
=====================================

The `mark-whole-buffer' function is no harder to understand than the
`simplified-beginning-of-buffer' function.  In this case, however, we
will look at the complete function, not a shortened version.

The `mark-whole-buffer' function is not as commonly used as the
`beginning-of-buffer' function, but is useful nonetheless: it marks a
whole buffer as a region by putting point at the beginning and a mark
at the end of the buffer.  It is generally bound to `C-x h'.

An overview of `mark-whole-buffer'
----------------------------------

In GNU Emacs 20, the code for the complete function looks like this:

     (defun mark-whole-buffer ()
       "Put point at beginning and mark at end of buffer."
       (interactive)
       (push-mark (point))
       (push-mark (point-max))
       (goto-char (point-min)))

Like all other functions, the `mark-whole-buffer' function fits into
the template for a function definition.  The template looks like this:

     (defun NAME-OF-FUNCTION (ARGUMENT-LIST)
       "DOCUMENTATION..."
       (INTERACTIVE-EXPRESSION...)
       BODY...)

Here is how the function works: the name of the function is
`mark-whole-buffer'; it is followed by an empty argument list, `()',
which means that the function does not require arguments.  The
documentation comes next.

The next line is an `(interactive)' expression that tells Emacs that
the function will be used interactively.  These details are similar
to the `simplified-beginning-of-buffer' function described in the
previous section.

Body of `mark-whole-buffer'
---------------------------

The body of the `mark-whole-buffer' function consists of three lines
of code:

     (push-mark (point))
     (push-mark (point-max))
     (goto-char (point-min))

The first of these lines is the expression, `(push-mark (point))'.

This line does exactly the same job as the first line of the body of
the `simplified-beginning-of-buffer' function, which is written
`(push-mark)'.  In both cases, the Lisp interpreter sets a mark at
the current position of the cursor.

I don't know why the expression in `mark-whole-buffer' is written
`(push-mark (point))' and the expression in `beginning-of-buffer' is
written `(push-mark)'.  Perhaps whoever wrote the code did not know
that the arguments for `push-mark' are optional and that if
`push-mark' is not passed an argument, the function automatically
sets mark at the location of point by default.  Or perhaps the
expression was written so as to parallel the structure of the next
line.  In any case, the line causes Emacs to determine the position
of point and set a mark there.

The next line of `mark-whole-buffer' is `(push-mark (point-max)'.
This expression sets a mark at the point in the buffer that has the
highest number.  This will be the end of the buffer (or, if the
buffer is narrowed, the end of the accessible portion of the buffer.
*Note Narrowing and Widening: Narrowing & Widening, for more about
narrowing.)  After this mark has been set, the previous mark, the one
set at point, is no longer set, but Emacs remembers its position,
just as all other recent marks are always remembered.  This means
that you can, if you wish, go back to that position by typing `C-u
C-<SPC>' twice.

(In GNU Emacs 21, the `(push-mark (point-max)' is slightly more
complicated than shown here.  The line reads

     (push-mark (point-max) nil t)

(The expression works nearly the same as before.  It sets a mark at
the highest numbered place in the buffer that it can.  However, in
this version, `push-mark' has two additional arguments.  The second
argument to `push-mark' is `nil'.  This tells the function it
_should_ display a message that says `Mark set' when it pushes the
mark.  The third argument is `t'.  This tells `push-mark' to activate
the mark when Transient Mark mode is turned on.  Transient Mark mode
highlights the currently active region.  It is usually turned off.)

Finally, the last line of the function is `(goto-char (point-min)))'.
This is written exactly the same way as it is written in
`beginning-of-buffer'.  The expression moves the cursor to the
minimum point in the buffer, that is, to the beginning of the buffer
(or to the beginning of the accessible portion of the buffer).  As a
result of this, point is placed at the beginning of the buffer and
mark is set at the end of the buffer.  The whole buffer is,
therefore, the region.

The Definition of `append-to-buffer'
====================================

The `append-to-buffer' command is very nearly as simple as the
`mark-whole-buffer' command.  What it does is copy the region (that
is, the part of the buffer between point and mark) from the current
buffer to a specified buffer.

An Overview of `append-to-buffer'
---------------------------------

The `append-to-buffer' command uses the `insert-buffer-substring'
function to copy the region.  `insert-buffer-substring' is described
by its name: it takes a string of characters from part of a buffer, a
"substring", and inserts them into another buffer.  Most of
`append-to-buffer' is concerned with setting up the conditions for
`insert-buffer-substring' to work: the code must specify both the
buffer to which the text will go and the region that will be copied.
Here is the complete text of the function:

     (defun append-to-buffer (buffer start end)
       "Append to specified buffer the text of the region.
     It is inserted into that buffer before its point.
     
     When calling from a program, give three arguments:
     a buffer or the name of one, and two character numbers
     specifying the portion of the current buffer to be copied."
       (interactive "BAppend to buffer: \nr")
       (let ((oldbuf (current-buffer)))
         (save-excursion
           (set-buffer (get-buffer-create buffer))
           (insert-buffer-substring oldbuf start end))))

The function can be understood by looking at it as a series of
filled-in templates.

The outermost template is for the function definition.  In this
function, it looks like this (with several slots filled in):

     (defun append-to-buffer (buffer start end)
       "DOCUMENTATION..."
       (interactive "BAppend to buffer: \nr")
       BODY...)

The first line of the function includes its name and three arguments.
The arguments are the `buffer' to which the text will be copied, and
the `start' and `end' of the region in the current buffer that will
be copied.

The next part of the function is the documentation, which is clear and
complete.

The `append-to-buffer' Interactive Expression
---------------------------------------------

Since the `append-to-buffer' function will be used interactively, the
function must have an `interactive' expression.  (For a review of
`interactive', see *Note Making a Function Interactive: Interactive.)
The expression reads as follows:

     (interactive "BAppend to buffer: \nr")

This expression has an argument inside of quotation marks and that
argument has two parts, separated by `\n'.

The first part is `BAppend to buffer: '.  Here, the `B' tells Emacs
to ask for the name of the buffer that will be passed to the
function.  Emacs will ask for the name by prompting the user in the
minibuffer, using the string following the `B', which is the string
`Append to buffer: '.  Emacs then binds the variable `buffer' in the
function's argument list to the specified buffer.

The newline, `\n', separates the first part of the argument from the
second part.  It is followed by an `r' that tells Emacs to bind the
two arguments that follow the symbol `buffer' in the function's
argument list (that is, `start' and `end') to the values of point and
mark.

The Body of `append-to-buffer'
------------------------------

The body of the `append-to-buffer' function begins with `let'.

As we have seen before (*note `let': let.), the purpose of a `let'
expression is to create and give initial values to one or more
variables that will only be used within the body of the `let'.  This
means that such a variable will not be confused with any variable of
the same name outside the `let' expression.

We can see how the `let' expression fits into the function as a whole
by showing a template for `append-to-buffer' with the `let'
expression in outline:

     (defun append-to-buffer (buffer start end)
       "DOCUMENTATION..."
       (interactive "BAppend to buffer: \nr")
       (let ((VARIABLE VALUE))
             BODY...)

The `let' expression has three elements:

  1. The symbol `let';

  2. A varlist containing, in this case, a single two-element list,
     `(VARIABLE VALUE)';

  3. The body of the `let' expression.

In the `append-to-buffer' function, the varlist looks like this:

     (oldbuf (current-buffer))

In this part of the `let' expression, the one variable, `oldbuf', is
bound to the value returned by the `(current-buffer)' expression.
The variable, `oldbuf', is used to keep track of the buffer in which
you are working and from which you will copy.

The element or elements of a varlist are surrounded by a set of
parentheses so the Lisp interpreter can distinguish the varlist from
the body of the `let'.  As a consequence, the two-element list within
the varlist is surrounded by a circumscribing set of parentheses.
The line looks like this:

     (let ((oldbuf (current-buffer)))
       ... )

The two parentheses before `oldbuf' might surprise you if you did not
realize that the first parenthesis before `oldbuf' marks the boundary
of the varlist and the second parenthesis marks the beginning of the
two-element list, `(oldbuf (current-buffer))'.

`save-excursion' in `append-to-buffer'
--------------------------------------

The body of the `let' expression in `append-to-buffer' consists of a
`save-excursion' expression.

The `save-excursion' function saves the locations of point and mark,
and restores them to those positions after the expressions in the
body of the `save-excursion' complete execution.  In addition,
`save-excursion' keeps track of the original buffer, and restores it.
This is how `save-excursion' is used in `append-to-buffer'.

Incidentally, it is worth noting here that a Lisp function is normally
formatted so that everything that is enclosed in a multi-line spread
is indented more to the right than the first symbol.  In this function
definition, the `let' is indented more than the `defun', and the
`save-excursion' is indented more than the `let', like this:

     (defun ...
       ...
       ...
       (let...
         (save-excursion
           ...

This formatting convention makes it easy to see that the two lines in
the body of the `save-excursion' are enclosed by the parentheses
associated with `save-excursion', just as the `save-excursion' itself
is enclosed by the parentheses associated with the `let':

     (let ((oldbuf (current-buffer)))
       (save-excursion
         (set-buffer (get-buffer-create buffer))
         (insert-buffer-substring oldbuf start end))))

The use of the `save-excursion' function can be viewed as a process
of filling in the slots of a template:

     (save-excursion
       FIRST-EXPRESSION-IN-BODY
       SECOND-EXPRESSION-IN-BODY
        ...
       LAST-EXPRESSION-IN-BODY)

In this function, the body of the `save-excursion' contains only two
expressions.  The body looks like this:

     (set-buffer (get-buffer-create buffer))
     (insert-buffer-substring oldbuf start end)

When the `append-to-buffer' function is evaluated, the two
expressions in the body of the `save-excursion' are evaluated in
sequence.  The value of the last expression is returned as the value
of the `save-excursion' function; the other expression is evaluated
only for its side effects.

The first line in the body of the `save-excursion' uses the
`set-buffer' function to change the current buffer to the one
specified in the first argument to `append-to-buffer'.  (Changing the
buffer is the side effect; as we have said before, in Lisp, a side
effect is often the primary thing we want.)  The second line does the
primary work of the function.

The `set-buffer' function changes Emacs' attention to the buffer to
which the text will be copied and from which `save-excursion' will
return.

The line looks like this:

     (set-buffer (get-buffer-create buffer))

The innermost expression of this list is `(get-buffer-create
buffer)'.  This expression uses the `get-buffer-create' function,
which either gets the named buffer, or if it does not exist, creates
one with the given name.  This means you can use `append-to-buffer' to
put text into a buffer that did not previously exist.

`get-buffer-create' also keeps `set-buffer' from getting an
unnecessary error: `set-buffer' needs a buffer to go to; if you were
to specify a buffer that does not exist, Emacs would baulk.  Since
`get-buffer-create' will create a buffer if none exists, `set-buffer'
is always provided with a buffer.

The last line of `append-to-buffer' does the work of appending the
text:

     (insert-buffer-substring oldbuf start end)

The `insert-buffer-substring' function copies a string _from_ the
buffer specified as its first argument and inserts the string into
the present buffer.  In this case, the argument to
`insert-buffer-substring' is the value of the variable created and
bound by the `let', namely the value of `oldbuf', which was the
current buffer when you gave the `append-to-buffer' command.

After `insert-buffer-substring' has done its work, `save-excursion'
will restore the action to the original buffer and `append-to-buffer'
will have done its job.

Written in skeletal form, the workings of the body look like this:

     (let (BIND-`oldbuf'-TO-VALUE-OF-`current-buffer')
       (save-excursion                       ; Keep track of buffer.
         CHANGE-BUFFER
         INSERT-SUBSTRING-FROM-`oldbuf'-INTO-BUFFER)
     
       CHANGE-BACK-TO-ORIGINAL-BUFFER-WHEN-FINISHED
     LET-THE-LOCAL-MEANING-OF-`oldbuf'-DISAPPEAR-WHEN-FINISHED

In summary, `append-to-buffer' works as follows: it saves the value
of the current buffer in the variable called `oldbuf'.  It gets the
new buffer, creating one if need be, and switches Emacs to it.  Using
the value of `oldbuf', it inserts the region of text from the old
buffer into the new buffer; and then using `save-excursion', it
brings you back to your original buffer.

In looking at `append-to-buffer', you have explored a fairly complex
function.  It shows how to use `let' and `save-excursion', and how to
change to and come back from another buffer.  Many function
definitions use `let', `save-excursion', and `set-buffer' this way.

Review
======

Here is a brief summary of the various functions discussed in this
chapter.

`describe-function'
`describe-variable'
     Print the documentation for a function or variable.
     Conventionally bound to `C-h f' and `C-h v'.

`find-tag'
     Find the file containing the source for a function or variable
     and switch buffers to it, positioning point at the beginning of
     the item.  Conventionally bound to `M-.' (that's a period
     following the <META> key).

`save-excursion'
     Save the location of point and mark and restore their values
     after the arguments to `save-excursion' have been evaluated.
     Also, remember the current buffer and return to it.

`push-mark'
     Set mark at a location and record the value of the previous mark
     on the mark ring.  The mark is a location in the buffer that
     will keep its relative position even if text is added to or
     removed from the buffer.

`goto-char'
     Set point to the location specified by the value of the
     argument, which can be a number, a marker,  or an expression
     that returns the number of a position, such as `(point-min)'.

`insert-buffer-substring'
     Copy a region of text from a buffer that is passed to the
     function as an argument and insert the region into the current
     buffer.

`mark-whole-buffer'
     Mark the whole buffer as a region.  Normally bound to `C-x h'.

`set-buffer'
     Switch the attention of Emacs to another buffer, but do not
     change the window being displayed.  Used when the program rather
     than a human is to work on a different buffer.

`get-buffer-create'
`get-buffer'
     Find a named buffer or create one if a buffer of that name does
     not exist.  The `get-buffer' function returns `nil' if the named
     buffer does not exist.

Exercises
=========

   * Write your own `simplified-end-of-buffer' function definition;
     then test it to see whether it works.

   * Use `if' and `get-buffer' to write a function that prints a
     message telling you whether a buffer exists.

   * Using `find-tag', find the source for the `copy-to-buffer'
     function.

A Few More Complex Functions
****************************

In this chapter, we build on what we have learned in previous chapters
by looking at more complex functions.  The `copy-to-buffer' function
illustrates use of two `save-excursion' expressions in one
definition, while the `insert-buffer' function illustrates use of an
asterisk in an `interactive' expression, use of `or', and the
important distinction between a name and the object to which the name
refers.

The Definition of `copy-to-buffer'
==================================

After understanding how `append-to-buffer' works, it is easy to
understand `copy-to-buffer'.  This function copies text into a
buffer, but instead of adding to the second buffer, it replaces the
previous text in the second buffer.  The code for the
`copy-to-buffer' function is almost the same as the code for
`append-to-buffer', except that `erase-buffer' and a second
`save-excursion' are used.  (*Note The Definition of
`append-to-buffer': append-to-buffer, for the description of
`append-to-buffer'.)

The body of `copy-to-buffer' looks like this

     ...
     (interactive "BCopy to buffer: \nr")
       (let ((oldbuf (current-buffer)))
         (save-excursion
           (set-buffer (get-buffer-create buffer))
           (erase-buffer)
           (save-excursion
             (insert-buffer-substring oldbuf start end)))))

This code is similar to the code in `append-to-buffer': it is only
after changing to the buffer to which the text will be copied that
the definition for this function diverges from the definition for
`append-to-buffer': the `copy-to-buffer' function erases the buffer's
former contents.  (This is what is meant by `replacement'; to replace
text, Emacs erases the previous text and then inserts new text.)
After erasing the previous contents of the buffer, `save-excursion'
is used for a second time and the new text is inserted.

Why is `save-excursion' used twice?  Consider again what the function
does.

In outline, the body of `copy-to-buffer' looks like this:

     (let (BIND-`oldbuf'-TO-VALUE-OF-`current-buffer')
       (save-excursion         ; First use of `save-excursion'.
         CHANGE-BUFFER
           (erase-buffer)
           (save-excursion     ; Second use of `save-excursion'.
             INSERT-SUBSTRING-FROM-`oldbuf'-INTO-BUFFER)))

The first use of `save-excursion' returns Emacs to the buffer from
which the text is being copied.  That is clear, and is just like its
use in `append-to-buffer'.  Why the second use?  The reason is that
`insert-buffer-substring' always leaves point at the _end_ of the
region being inserted.  The second `save-excursion' causes Emacs to
leave point at the beginning of the text being inserted.  In most
circumstances, users prefer to find point at the beginning of
inserted text.  (Of course, the `copy-to-buffer' function returns the
user to the original buffer when done--but if the user _then_
switches to the copied-to buffer, point will go to the beginning of
the text.  Thus, this use of a second `save-excursion' is a little
nicety.)

The Definition of `insert-buffer'
=================================

`insert-buffer' is yet another buffer-related function.  This command
copies another buffer _into_ the current buffer.  It is the reverse
of `append-to-buffer' or `copy-to-buffer', since they copy a region
of text _from_ the current buffer to another buffer.

In addition, this code illustrates the use of `interactive' with a
buffer that might be "read-only" and the important distinction
between the name of an object and the object actually referred to.

The Code for `insert-buffer'
----------------------------

Here is the code:

     (defun insert-buffer (buffer)
       "Insert after point the contents of BUFFER.
     Puts mark after the inserted text.
     BUFFER may be a buffer or a buffer name."
       (interactive "*bInsert buffer: ")
       (or (bufferp buffer)
           (setq buffer (get-buffer buffer)))
       (let (start end newmark)
         (save-excursion
           (save-excursion
             (set-buffer buffer)
             (setq start (point-min) end (point-max)))
           (insert-buffer-substring buffer start end)
           (setq newmark (point)))
         (push-mark newmark)))

As with other function definitions, you can use a template to see an
outline of the function:

     (defun insert-buffer (buffer)
       "DOCUMENTATION..."
       (interactive "*bInsert buffer: ")
       BODY...)

The Interactive Expression in `insert-buffer'
---------------------------------------------

In `insert-buffer', the argument to the `interactive' declaration has
two parts, an asterisk, `*', and `bInsert buffer: '.

A Read-only Buffer
..................

The asterisk is for the situation when the current buffer is a
read-only buffer--a buffer that cannot be modified.  If
`insert-buffer' is called when the current buffer is read-only, a
message to this effect is printed in the echo area and the terminal
may beep or blink at you; you will not be permitted to insert anything
into current buffer.  The asterisk does not need to be followed by a
newline to separate it from the next argument.

`b' in an Interactive Expression
................................

The next argument in the interactive expression starts with a lower
case `b'.  (This is different from the code for `append-to-buffer',
which uses an upper-case `B'.  *Note The Definition of
`append-to-buffer': append-to-buffer.)  The lower-case `b' tells the
Lisp interpreter that the argument for `insert-buffer' should be an
existing buffer or else its name.  (The upper-case `B' option
provides for the possibility that the buffer does not exist.)  Emacs
will prompt you for the name of the buffer, offering you a default
buffer, with name completion enabled.  If the buffer does not exist,
you receive a message that says "No match"; your terminal may beep at
you as well.

The Body of the `insert-buffer' Function
----------------------------------------

The body of the `insert-buffer' function has two major parts: an `or'
expression and a `let' expression.  The purpose of the `or'
expression is to ensure that the argument `buffer' is bound to a
buffer and not just the name of a buffer.  The body of the `let'
expression contains the code which copies the other buffer into the
current buffer.

In outline, the two expressions fit into the `insert-buffer' function
like this:

     (defun insert-buffer (buffer)
       "DOCUMENTATION..."
       (interactive "*bInsert buffer: ")
       (or ...
           ...
       (let (VARLIST)
           BODY-OF-`let'... )

To understand how the `or' expression ensures that the argument
`buffer' is bound to a buffer and not to the name of a buffer, it is
first necessary to understand the `or' function.

Before doing this, let me rewrite this part of the function using
`if' so that you can see what is done in a manner that will be
familiar.

`insert-buffer' With an `if' Instead of an `or'
-----------------------------------------------

The job to be done is to make sure the value of `buffer' is a buffer
itself and not the name of a buffer.  If the value is the name, then
the buffer itself must be got.

You can imagine yourself at a conference where an usher is wandering
around holding a list with your name on it and looking for you: the
usher is "bound" to your name, not to you; but when the usher finds
you and takes your arm, the usher becomes "bound" to you.

In Lisp, you might describe this situation like this:

     (if (not (holding-on-to-guest))
         (find-and-take-arm-of-guest))

We want to do the same thing with a buffer--if we do not have the
buffer itself, we want to get it.

Using a predicate called `bufferp' that tells us whether we have a
buffer (rather than its name), we can write the code like this:

     (if (not (bufferp buffer))              ; if-part
         (setq buffer (get-buffer buffer)))  ; then-part

Here, the true-or-false-test of the `if' expression is
`(not (bufferp buffer))'; and the then-part is the expression
`(setq buffer (get-buffer buffer))'.

In the test, the function `bufferp' returns true if its argument is a
buffer--but false if its argument is the name of the buffer.  (The
last character of the function name `bufferp' is the character `p';
as we saw earlier, such use of `p' is a convention that indicates
that the function is a predicate, which is a term that means that the
function will determine whether some property is true or false.
*Note Using the Wrong Type Object as an Argument: Wrong Type of
Argument.)

The function `not' precedes the expression `(bufferp buffer)', so the
true-or-false-test looks like this:

     (not (bufferp buffer))

`not' is a function that returns true if its argument is false and
false if its argument is true.  So if `(bufferp buffer)' returns
true, the `not' expression returns false and vice-versa: what is "not
true" is false and what is "not false" is true.

Using this test, the `if' expression works as follows: when the value
of the variable `buffer' is actually a buffer rather then its name,
the true-or-false-test returns false and the `if' expression does not
evaluate the then-part.  This is fine, since we do not need to do
anything to the variable `buffer' if it really is a buffer.

On the other hand, when the value of `buffer' is not a buffer itself,
but the name of a buffer, the true-or-false-test returns true and the
then-part of the expression is evaluated.  In this case, the
then-part is `(setq buffer (get-buffer buffer))'.  This expression
uses the `get-buffer' function to return an actual buffer itself,
given its name.  The `setq' then sets the variable `buffer' to the
value of the buffer itself, replacing its previous value (which was
the name of the buffer).

The `or' in the Body
--------------------

The purpose of the `or' expression in the `insert-buffer' function is
to ensure that the argument `buffer' is bound to a buffer and not
just to the name of a buffer.  The previous section shows how the job
could have been done using an `if' expression.  However, the
`insert-buffer' function actually uses `or'.  To understand this, it
is necessary to understand how `or' works.

An `or' function can have any number of arguments.  It evaluates each
argument in turn and returns the value of the first of its arguments
that is not `nil'.  Also, and this is a crucial feature of `or', it
does not evaluate any subsequent arguments after returning the first
non-`nil' value.

The `or' expression looks like this:

     (or (bufferp buffer)
         (setq buffer (get-buffer buffer)))

The first argument to `or' is the expression `(bufferp buffer)'.
This expression returns true (a non-`nil' value) if the buffer is
actually a buffer, and not just the name of a buffer.  In the `or'
expression, if this is the case, the `or' expression returns this
true value and does not evaluate the next expression--and this is fine
with us, since we do not want to do anything to the value of `buffer'
if it really is a buffer.

On the other hand, if the value of `(bufferp buffer)' is `nil', which
it will be if the value of `buffer' is the name of a buffer, the Lisp
interpreter evaluates the next element of the `or' expression.  This
is the expression `(setq buffer (get-buffer buffer))'.  This
expression returns a non-`nil' value, which is the value to which it
sets the variable `buffer'--and this value is a buffer itself, not
the name of a buffer.

The result of all this is that the symbol `buffer' is always bound to
a buffer itself rather than to the name of a buffer.  All this is
necessary because the `set-buffer' function in a following line only
works with a buffer itself, not with the name to a buffer.

Incidentally, using `or', the situation with the usher would be
written like this:

     (or (holding-on-to-guest) (find-and-take-arm-of-guest))

The `let' Expression in `insert-buffer'
---------------------------------------

After ensuring that the variable `buffer' refers to a buffer itself
and not just to the name of a buffer, the `insert-buffer function'
continues with a `let' expression.  This specifies three local
variables, `start', `end', and `newmark' and binds them to the
initial value `nil'.  These variables are used inside the remainder
of the `let' and temporarily hide any other occurrence of variables
of the same name in Emacs until the end of the `let'.

The body of the `let' contains two `save-excursion' expressions.
First, we will look at the inner `save-excursion' expression in
detail.  The expression looks like this:

     (save-excursion
       (set-buffer buffer)
       (setq start (point-min) end (point-max)))

The expression `(set-buffer buffer)' changes Emacs' attention from
the current buffer to the one from which the text will copied.  In
that buffer, the variables `start' and `end' are set to the beginning
and end of the buffer, using the commands `point-min' and
`point-max'.  Note that we have here an illustration of how `setq' is
able to set two variables in the same expression.  The first argument
of `setq' is set to the value of its second, and its third argument
is set to the value of its fourth.

After the body of the inner `save-excursion' is evaluated, the
`save-excursion' restores the original buffer, but `start' and `end'
remain set to the values of the beginning and end of the buffer from
which the text will be copied.

The outer `save-excursion' expression looks like this:

     (save-excursion
       (INNER-`save-excursion'-EXPRESSION
          (GO-TO-NEW-BUFFER-AND-SET-`start'-AND-`end')
       (insert-buffer-substring buffer start end)
       (setq newmark (point)))

The `insert-buffer-substring' function copies the text _into_ the
current buffer _from_ the region indicated by `start' and `end' in
`buffer'.  Since the whole of the second buffer lies between `start'
and `end', the whole of the second buffer is copied into the buffer
you are editing.  Next, the value of point, which will be at the end
of the inserted text, is recorded in the variable `newmark'.

After the body of the outer `save-excursion' is evaluated, point and
mark are relocated to their original places.

However, it is convenient to locate a mark at the end of the newly
inserted text and locate point at its beginning.  The `newmark'
variable records the end of the inserted text.  In the last line of
the `let' expression, the `(push-mark newmark)' expression function
sets a mark to this location.  (The previous location of the mark is
still accessible; it is recorded on the mark ring and you can go back
to it with `C-u C-<SPC>'.)  Meanwhile, point is located at the
beginning of the inserted text, which is where it was before you
called the insert function.

The whole `let' expression looks like this:

     (let (start end newmark)
       (save-excursion
         (save-excursion
           (set-buffer buffer)
           (setq start (point-min) end (point-max)))
         (insert-buffer-substring buffer start end)
         (setq newmark (point)))
       (push-mark newmark))

Like the `append-to-buffer' function, the `insert-buffer' function
uses `let', `save-excursion', and `set-buffer'.  In addition, the
function illustrates one way to use `or'.  All these functions are
building blocks that we will find and use again and again.

Complete Definition of `beginning-of-buffer'
============================================

The basic structure of the `beginning-of-buffer' function has already
been discussed.  (*Note A Simplified `beginning-of-buffer'
Definition: simplified-beginning-of-buffer.)  This section describes
the complex part of the definition.

As previously described, when invoked without an argument,
`beginning-of-buffer' moves the cursor to the beginning of the
buffer, leaving the mark at the previous position.  However, when the
command is invoked with a number between one and ten, the function
considers that number to be a fraction of the length of the buffer,
measured in tenths, and Emacs moves the cursor that fraction of the
way from the beginning of the buffer.  Thus, you can either call this
function with the key command `M-<', which will move the cursor to
the beginning of the buffer, or with a key command such as `C-u 7
M-<' which will move the cursor to a point 70% of the way through the
buffer.  If a number bigger than ten is used for the argument, it
moves to the end of the buffer.

The `beginning-of-buffer' function can be called with or without an
argument.  The use of the argument is optional.

Optional Arguments
------------------

Unless told otherwise, Lisp expects that a function with an argument
in its function definition will be called with a value for that
argument.  If that does not happen, you get an error and a message
that says `Wrong number of arguments'.

However, optional arguments are a feature of Lisp: a "keyword" may be
used to tell the Lisp interpreter that an argument is optional.  The
keyword is `&optional'.  (The `&' in front of `optional' is part of
the keyword.)  In a function definition, if an argument follows the
keyword `&optional', a value does not need to be passed to that
argument when the function is called.

The first line of the function definition of `beginning-of-buffer'
therefore looks like this:

     (defun beginning-of-buffer (&optional arg)

In outline, the whole function looks like this:

     (defun beginning-of-buffer (&optional arg)
       "DOCUMENTATION..."
       (interactive "P")
       (push-mark)
       (goto-char
         (IF-THERE-IS-AN-ARGUMENT
             FIGURE-OUT-WHERE-TO-GO
           ELSE-GO-TO
           (point-min))))

The function is similar to the `simplified-beginning-of-buffer'
function except that the `interactive' expression has `"P"' as an
argument and the `goto-char' function is followed by an if-then-else
expression that figures out where to put the cursor if there is an
argument.

The `"P"' in the `interactive' expression tells Emacs to pass a
prefix argument, if there is one, to the function.  A prefix argument
is made by typing the <META> key followed by a number, or by typing
`C-u' and then a number (if you don't type a number, `C-u' defaults
to 4).

The true-or-false-test of the `if' expression is simple: it is simply
the argument `arg'.  If `arg' has a value that is not `nil', which
will be the case if `beginning-of-buffer' is called with an argument,
then this true-or-false-test will return true and the then-part of
the `if' expression will be evaluated.  On the other hand, if
`beginning-of-buffer' is not called with an argument, the value of
`arg' will be `nil' and the else-part of the `if' expression will be
evaluated.  The else-part is simply `point-min', and when this is the
outcome, the whole `goto-char' expression is `(goto-char
(point-min))', which is how we saw the `beginning-of-buffer' function
in its simplified form.

`beginning-of-buffer' with an Argument
--------------------------------------

When `beginning-of-buffer' is called with an argument, an expression
is evaluated which calculates what value to pass to `goto-char'.
This expression is rather complicated at first sight.  It includes an
inner `if' expression and much arithmetic.  It looks like this:

     (if (> (buffer-size) 10000)
         ;; Avoid overflow for large buffer sizes!
         (* (prefix-numeric-value arg) (/ (buffer-size) 10))
       (/
        (+ 10
           (*
            (buffer-size) (prefix-numeric-value arg))) 10))

Disentangle `beginning-of-buffer'
.................................

Like other complex-looking expressions, the conditional expression
within `beginning-of-buffer' can be disentangled by looking at it as
parts of a template, in this case, the template for an if-then-else
expression.  In skeletal form, the expression looks like this:

     (if (BUFFER-IS-LARGE
         DIVIDE-BUFFER-SIZE-BY-10-AND-MULTIPLY-BY-ARG
       ELSE-USE-ALTERNATE-CALCULATION

The true-or-false-test of this inner `if' expression checks the size
of the buffer.  The reason for this is that the old Version 18 Emacs
used numbers that are no bigger than eight million or so and in the
computation that followed, the programmer feared that Emacs might try
to use over-large numbers if the buffer were large.  The term
`overflow', mentioned in the comment, means numbers that are over
large.  Version 21 Emacs uses larger numbers, but this code has not
been touched, if only because people now look at buffers that are far,
far larger than ever before.

There are two cases:  if the buffer is large and if it is not.

What happens in a large buffer
..............................

In `beginning-of-buffer', the inner `if' expression tests whether the
size of the buffer is greater than 10,000 characters.  To do this, it
uses the `>' function and the `buffer-size' function.

The line looks like this:

     (if (> (buffer-size) 10000)

When the buffer is large, the then-part of the `if' expression is
evaluated.  It reads like this (after formatting for easy reading):

     (*
       (prefix-numeric-value arg)
       (/ (buffer-size) 10))

This expression is a multiplication, with two arguments to the
function `*'.

The first argument is `(prefix-numeric-value arg)'.  When `"P"' is
used as the argument for `interactive', the value passed to the
function as its argument is passed a "raw prefix argument", and not a
number.  (It is a number in a list.)  To perform the arithmetic, a
conversion is necessary, and `prefix-numeric-value' does the job.

The second argument is `(/ (buffer-size) 10)'.  This expression
divides the numeric value of the buffer by ten.  This produces a
number that tells how many characters make up one tenth of the buffer
size.  (In Lisp, `/' is used for division, just as `*' is used for
multiplication.)

In the multiplication expression as a whole, this amount is multiplied
by the value of the prefix argument--the multiplication looks like
this:

     (* NUMERIC-VALUE-OF-PREFIX-ARG
        NUMBER-OF-CHARACTERS-IN-ONE-TENTH-OF-THE-BUFFER)

If, for example, the prefix argument is `7', the one-tenth value will
be multiplied by 7 to give a position 70% of the way through the
buffer.

The result of all this is that if the buffer is large, the
`goto-char' expression reads like this:

     (goto-char (* (prefix-numeric-value arg)
                   (/ (buffer-size) 10)))

This puts the cursor where we want it.

What happens in a small buffer
..............................

If the buffer contains fewer than 10,000 characters, a slightly
different computation is performed.  You might think this is not
necessary, since the first computation could do the job.  However, in
a small buffer, the first method may not put the cursor on exactly the
desired line; the second method does a better job.

The code looks like this:

     (/ (+ 10 (* (buffer-size) (prefix-numeric-value arg))) 10))

This is code in which you figure out what happens by discovering how
the functions are embedded in parentheses.  It is easier to read if
you reformat it with each expression indented more deeply than its
enclosing expression:

       (/
        (+ 10
           (*
            (buffer-size)
            (prefix-numeric-value arg)))
        10))

Looking at parentheses, we see that the innermost operation is
`(prefix-numeric-value arg)', which converts the raw argument to a
number.  This number is multiplied by the buffer size in the following
expression:

     (* (buffer-size) (prefix-numeric-value arg)

This multiplication creates a number that may be larger than the size
of the buffer--seven times larger if the argument is 7, for example.
Ten is then added to this number and finally the large number is
divided by ten to provide a value that is one character larger than
the percentage position in the buffer.

The number that results from all this is passed to `goto-char' and
the cursor is moved to that point.

The Complete `beginning-of-buffer'
----------------------------------

Here is the complete text of the `beginning-of-buffer' function:

     (defun beginning-of-buffer (&optional arg)
       "Move point to the beginning of the buffer;
     leave mark at previous position.
     With arg N, put point N/10 of the way
     from the true beginning.
     Don't use this in Lisp programs!
     \(goto-char (point-min)) is faster
     and does not set the mark."
       (interactive "P")
       (push-mark)
       (goto-char
        (if arg
            (if (> (buffer-size) 10000)
                ;; Avoid overflow for large buffer sizes!
                (* (prefix-numeric-value arg)
                   (/ (buffer-size) 10))
              (/ (+ 10 (* (buffer-size)
                          (prefix-numeric-value arg)))
                 10))
          (point-min)))
       (if arg (forward-line 1)))

Except for two small points, the previous discussion shows how this
function works.  The first point deals with a detail in the
documentation string, and the second point concerns the last line of
the function.

In the documentation string, there is reference to an expression:

     \(goto-char (point-min))

A `\' is used before the first parenthesis of this expression.  This
`\' tells the Lisp interpreter that the expression should be printed
as shown in the documentation rather than evaluated as a symbolic
expression, which is what it looks like.

Finally, the last line of the `beginning-of-buffer' command says to
move point to the beginning of the next line if the command is
invoked with an argument:

     (if arg (forward-line 1)))

This puts the cursor at the beginning of the first line after the
appropriate tenths position in the buffer.  This is a flourish that
means that the cursor is always located _at least_ the requested
tenths of the way through the buffer, which is a nicety that is,
perhaps, not necessary, but which, if it did not occur, would be sure
to draw complaints.

Review
======

Here is a brief summary of some of the topics covered in this chapter.

`or'
     Evaluate each argument in sequence, and return the value of the
     first argument that is not `nil'; if none return a value that is
     not `nil', return `nil'.  In brief, return the first true value
     of the arguments; return a true value if one _or_ any of the
     other are true.

`and'
     Evaluate each argument in sequence, and if any are `nil', return
     `nil'; if none are `nil', return the value of the last argument.
     In brief, return a true value only if all the arguments are
     true; return a true value if one _and_ each of the others is
     true.

`&optional'
     A keyword used to indicate that an argument to a function
     definition is optional; this means that the function can be
     evaluated without the argument, if desired.

`prefix-numeric-value'
     Convert the `raw prefix argument' produced by `(interactive
     "P")' to a numeric value.

`forward-line'
     Move point forward to the beginning of the next line, or if the
     argument is greater than one, forward that many lines.  If it
     can't move as far forward as it is supposed to, `forward-line'
     goes forward as far as it can and then returns a count of the
     number of additional lines it was supposed to move but couldn't.

`erase-buffer'
     Delete the entire contents of the current buffer.

`bufferp'
     Return `t' if its argument is a buffer; otherwise return `nil'.

`optional' Argument Exercise
============================

Write an interactive function with an optional argument that tests
whether its argument, a number, is greater or less than the value of
`fill-column', and tells you which, in a message.  However, if you do
not pass an argument to the function, use 56 as a default value.

Narrowing and Widening
**********************

Narrowing is a feature of Emacs that makes it possible for you to
focus on a specific part of a buffer, and work without accidentally
changing other parts.  Narrowing is normally disabled since it can
confuse novices.

The Advantages of Narrowing
===========================

With narrowing, the rest of a buffer is made invisible, as if it
weren't there.  This is an advantage if, for example, you want to
replace a word in one part of a buffer but not in another: you narrow
to the part you want and the replacement is carried out only in that
section, not in the rest of the buffer.  Searches will only work
within a narrowed region, not outside of one, so if you are fixing a
part of a document, you can keep yourself from accidentally finding
parts you do not need to fix by narrowing just to the region you want.
(The key binding for `narrow-to-region' is `C-x n n'.)

However, narrowing does make the rest of the buffer invisible, which
can scare people who inadvertently invoke narrowing and think they
have deleted a part of their file.  Moreover, the `undo' command
(which is usually bound to `C-x u') does not turn off narrowing (nor
should it), so people can become quite desperate if they do not know
that they can return the rest of a buffer to visibility with the
`widen' command.  (The key binding for `widen' is `C-x n w'.)

Narrowing is just as useful to the Lisp interpreter as to a human.
Often, an Emacs Lisp function is designed to work on just part of a
buffer; or conversely, an Emacs Lisp function needs to work on all of
a buffer that has been narrowed.  The `what-line' function, for
example, removes the narrowing from a buffer, if it has any narrowing
and when it has finished its job, restores the narrowing to what it
was.  On the other hand, the `count-lines' function, which is called
by `what-line', uses narrowing to restrict itself to just that portion
of the buffer in which it is interested and then restores the previous
situation.

The `save-restriction' Special Form
===================================

In Emacs Lisp, you can use the `save-restriction' special form to
keep track of whatever narrowing is in effect, if any.  When the Lisp
interpreter meets with `save-restriction', it executes the code in
the body of the `save-restriction' expression, and then undoes any
changes to narrowing that the code caused.  If, for example, the
buffer is narrowed and the code that follows `save-restriction' gets
rid of the narrowing, `save-restriction' returns the buffer to its
narrowed region afterwards.  In the `what-line' command, any
narrowing the buffer may have is undone by the `widen' command that
immediately follows the `save-restriction' command.  Any original
narrowing is restored just before the completion of the function.

The template for a `save-restriction' expression is simple:

     (save-restriction
       BODY... )

The body of the `save-restriction' is one or more expressions that
will be evaluated in sequence by the Lisp interpreter.

Finally, a point to note: when you use both `save-excursion' and
`save-restriction', one right after the other, you should use
`save-excursion' outermost.  If you write them in reverse order, you
may fail to record narrowing in the buffer to which Emacs switches
after calling `save-excursion'.  Thus, when written together,
`save-excursion' and `save-restriction' should be written like this:

     (save-excursion
       (save-restriction
         BODY...))

In other circumstances, when not written together, the
`save-excursion' and `save-restriction' special forms must be written
in the order appropriate to the function.

For example,

       (save-restriction
         (widen)
         (save-excursion
         BODY...))

`what-line'
===========

The `what-line' command tells you the number of the line in which the
cursor is located.  The function illustrates the use of the
`save-restriction' and `save-excursion' commands.  Here is the text
of the function in full:

     (defun what-line ()
       "Print the current line number (in the buffer) of point."
       (interactive)
       (save-restriction
         (widen)
         (save-excursion
           (beginning-of-line)
           (message "Line %d"
                    (1+ (count-lines 1 (point)))))))

The function has a documentation line and is interactive, as you would
expect.  The next two lines use the functions `save-restriction' and
`widen'.

The `save-restriction' special form notes whatever narrowing is in
effect, if any, in the current buffer and restores that narrowing
after the code in the body of the `save-restriction' has been
evaluated.

The `save-restriction' special form is followed by `widen'.  This
function undoes any narrowing the current buffer may have had when
`what-line' was called.  (The narrowing that was there is the
narrowing that `save-restriction' remembers.)  This widening makes it
possible for the line counting commands to count from the beginning
of the buffer.  Otherwise, they would have been limited to counting
within the accessible region.  Any original narrowing is restored
just before the completion of the function by the `save-restriction'
special form.

The call to `widen' is followed by `save-excursion', which saves the
location of the cursor (i.e., of point) and of the mark, and restores
them after the code in the body of the `save-excursion' uses the
`beginning-of-line' function to move point.

(Note that the `(widen)' expression comes between the
`save-restriction' and `save-excursion' special forms.  When you
write the two `save- ...' expressions in sequence, write
`save-excursion' outermost.)

The last two lines of the `what-line' function are functions to count
the number of lines in the buffer and then print the number in the
echo area.

     (message "Line %d"
              (1+ (count-lines 1 (point)))))))

The `message' function prints a one-line message at the bottom of the
Emacs screen.  The first argument is inside of quotation marks and is
printed as a string of characters.  However, it may contain `%d',
`%s', or `%c' to print arguments that follow the string.  `%d' prints
the argument as a decimal, so the message will say something such as
`Line 243'.

The number that is printed in place of the `%d' is computed by the
last line of the function:

     (1+ (count-lines 1 (point)))

What this does is count the lines from the first position of the
buffer, indicated by the `1', up to `(point)', and then add one to
that number.  (The `1+' function adds one to its argument.)  We add
one to it because line 2 has only one line before it, and
`count-lines' counts only the lines _before_ the current line.

After `count-lines' has done its job, and the message has been
printed in the echo area, the `save-excursion' restores point and
mark to their original positions; and `save-restriction' restores the
original narrowing, if any.

Exercise with Narrowing
=======================

Write a function that will display the first 60 characters of the
current buffer, even if you have narrowed the buffer to its latter
half so that the first line is inaccessible.  Restore point, mark,
and narrowing.  For this exercise, you need to use
`save-restriction', `widen', `goto-char', `point-min',
`buffer-substring', `message', and other functions, a whole potpourri.

`car', `cdr', `cons': Fundamental Functions
*******************************************

In Lisp, `car', `cdr', and `cons' are fundamental functions.  The
`cons' function is used to construct lists, and the `car' and `cdr'
functions are used to take them apart.

In the walk through of the `copy-region-as-kill' function, we will
see `cons' as well as two variants on `cdr', namely, `setcdr' and
`nthcdr'.  (*Note copy-region-as-kill::.)

Strange Names
=============

The name of the `cons' function is not unreasonable: it is an
abbreviation of the word `construct'.  The origins of the names for
`car' and `cdr', on the other hand, are esoteric: `car' is an acronym
from the phrase `Contents of the Address part of the Register'; and
`cdr' (pronounced `could-er') is an acronym from the phrase `Contents
of the Decrement part of the Register'.  These phrases refer to
specific pieces of hardware on the very early computer on which the
original Lisp was developed.  Besides being obsolete, the phrases
have been completely irrelevant for more than 25 years to anyone
thinking about Lisp.  Nonetheless, although a few brave scholars have
begun to use more reasonable names for these functions, the old terms
are still in use.  In particular, since the terms are used in the
Emacs Lisp source code, we will use them in this introduction.

`car' and `cdr'
===============

The CAR of a list is, quite simply, the first item in the list.  Thus
the CAR of the list `(rose violet daisy buttercup)' is `rose'.

If you are reading this in Info in GNU Emacs, you can see this by
evaluating the following:

     (car '(rose violet daisy buttercup))

After evaluating the expression, `rose' will appear in the echo area.

Clearly, a more reasonable name for the `car' function would be
`first' and this is often suggested.

`car' does not remove the first item from the list; it only reports
what it is.  After `car' has been applied to a list, the list is
still the same as it was.  In the jargon, `car' is `non-destructive'.
This feature turns out to be important.

The CDR of a list is the rest of the list, that is, the `cdr'
function returns the part of the list that follows the first item.
Thus, while the CAR of the list `'(rose violet daisy buttercup)' is
`rose', the rest of the list, the value returned by the `cdr'
function, is `(violet daisy buttercup)'.

You can see this by evaluating the following in the usual way:

     (cdr '(rose violet daisy buttercup))

When you evaluate this, `(violet daisy buttercup)' will appear in the
echo area.

Like `car', `cdr' does not remove any elements from the list--it just
returns a report of what the second and subsequent elements are.

Incidentally, in the example, the list of flowers is quoted.  If it
were not, the Lisp interpreter would try to evaluate the list by
calling `rose' as a function.  In this example, we do not want to do
that.

Clearly, a more reasonable name for `cdr' would be `rest'.

(There is a lesson here: when you name new functions, consider very
carefully what you are doing, since you may be stuck with the names
for far longer than you expect.  The reason this document perpetuates
these names is that the Emacs Lisp source code uses them, and if I did
not use them, you would have a hard time reading the code; but do,
please, try to avoid using these terms yourself.  The people who come
after you will be grateful to you.)

When `car' and `cdr' are applied to a list made up of symbols, such
as the list `(pine fir oak maple)', the element of the list returned
by the function `car' is the symbol `pine' without any parentheses
around it.  `pine' is the first element in the list.  However, the
CDR of the list is a list itself, `(fir oak maple)', as you can see
by evaluating the following expressions in the usual way:

     (car '(pine fir oak maple))
     
     (cdr '(pine fir oak maple))

On the other hand, in a list of lists, the first element is itself a
list.  `car' returns this first element as a list.  For example, the
following list contains three sub-lists, a list of carnivores, a list
of herbivores and a list of sea mammals:

     (car '((lion tiger cheetah)
            (gazelle antelope zebra)
            (whale dolphin seal)))

In this example, the first element or CAR of the list is the list of
carnivores, `(lion tiger cheetah)', and the rest of the list is
`((gazelle antelope zebra) (whale dolphin seal))'.

     (cdr '((lion tiger cheetah)
            (gazelle antelope zebra)
            (whale dolphin seal)))

It is worth saying again that `car' and `cdr' are
non-destructive--that is, they do not modify or change lists to which
they are applied.  This is very important for how they are used.

Also, in the first chapter, in the discussion about atoms, I said that
in Lisp, "certain kinds of atom, such as an array, can be separated
into parts; but the mechanism for doing this is different from the
mechanism for splitting a list.  As far as Lisp is concerned, the
atoms of a list are unsplittable."  (*Note Lisp Atoms::.)  The `car'
and `cdr' functions are used for splitting lists and are considered
fundamental to Lisp.  Since they cannot split or gain access to the
parts of an array, an array is considered an atom.  Conversely, the
other fundamental function, `cons', can put together or construct a
list, but not an array.  (Arrays are handled by array-specific
functions.  *Note Arrays: (elisp)Arrays.)

`cons'
======

The `cons' function constructs lists; it is the inverse of `car' and
`cdr'.  For example, `cons' can be used to make a four element list
from the three element list, `(fir oak maple)':

     (cons 'pine '(fir oak maple))

After evaluating this list, you will see

     (pine fir oak maple)

appear in the echo area.  `cons' causes the creation of a new list in
which the element is followed by the elements of the original list.

We often say that ``cons' puts a new element at the beginning of a
list; it attaches or pushes elements onto the list', but this
phrasing can be misleading, since `cons' does not change an existing
list, but creates a new one.

Like `car' and `cdr', `cons' is non-destructive.

Build a list
------------

`cons' must have a list to attach to.(1)  You cannot start from
absolutely nothing.  If you are building a list, you need to provide
at least an empty list at the beginning.  Here is a series of `cons'
expressions that build up a list of flowers.  If you are reading this
in Info in GNU Emacs, you can evaluate each of the expressions in the
usual way; the value is printed in this text after `=>', which you
may read as `evaluates to'.

     (cons 'buttercup ())
          => (buttercup)
     
     (cons 'daisy '(buttercup))
          => (daisy buttercup)
     
     (cons 'violet '(daisy buttercup))
          => (violet daisy buttercup)
     
     (cons 'rose '(violet daisy buttercup))
          => (rose violet daisy buttercup)

In the first example, the empty list is shown as `()' and a list made
up of `buttercup' followed by the empty list is constructed.  As you
can see, the empty list is not shown in the list that was
constructed.  All that you see is `(buttercup)'.  The empty list is
not counted as an element of a list because there is nothing in an
empty list.  Generally speaking, an empty list is invisible.

The second example, `(cons 'daisy '(buttercup))' constructs a new,
two element list by putting `daisy' in front of `buttercup'; and the
third example constructs a three element list by putting `violet' in
front of `daisy' and `buttercup'.

---------- Footnotes ----------

(1) Actually, you can `cons' an element to an atom to produce a
dotted pair.  Dotted pairs are not discussed here; see *Note Dotted
Pair Notation: (elisp)Dotted Pair Notation.

Find the Length of a List: `length'
-----------------------------------

You can find out how many elements there are in a list by using the
Lisp function `length', as in the following examples:

     (length '(buttercup))
          => 1
     
     (length '(daisy buttercup))
          => 2
     
     (length (cons 'violet '(daisy buttercup)))
          => 3

In the third example, the `cons' function is used to construct a
three element list which is then passed to the `length' function as
its argument.

We can also use `length' to count the number of elements in an empty
list:

     (length ())
          => 0

As you would expect, the number of elements in an empty list is zero.

An interesting experiment is to find out what happens if you try to
find the length of no list at all; that is, if you try to call
`length' without giving it an argument, not even an empty list:

     (length )

What you see, if you evaluate this, is the error message

     Wrong number of arguments: #<subr length>, 0

This means that the function receives the wrong number of arguments,
zero, when it expects some other number of arguments.  In this case,
one argument is expected, the argument being a list whose length the
function is measuring.  (Note that _one_ list is _one_ argument, even
if the list has many elements inside it.)

The part of the error message that says `#<subr length>' is the name
of the function.  This is written with a special notation, `#<subr',
that indicates that the function `length' is one of the primitive
functions written in C rather than in Emacs Lisp.  (`subr' is an
abbreviation for `subroutine'.)  *Note What Is a Function?:
(elisp)What Is a Function, for more about subroutines.

`nthcdr'
========

The `nthcdr' function is associated with the `cdr' function.  What it
does is take the CDR of a list repeatedly.

If you take the CDR of the list `(pine fir oak maple)', you will be
returned the list `(fir oak maple)'.  If you repeat this on what was
returned, you will be returned the list `(oak maple)'.  (Of course,
repeated CDRing on the original list will just give you the original
CDR since the function does not change the list.  You need to
evaluate the CDR of the CDR and so on.)  If you continue this,
eventually you will be returned an empty list, which in this case,
instead of being shown as `()' is shown as `nil'.

For review, here is a series of repeated CDRs, the text following the
`=>' shows what is returned.

     (cdr '(pine fir oak maple))
          =>(fir oak maple)
     
     (cdr '(fir oak maple))
          => (oak maple)
     
     (cdr '(oak maple))
          =>(maple)
     
     (cdr '(maple))
          => nil
     
     (cdr 'nil)
          => nil
     
     (cdr ())
          => nil

You can also do several CDRs without printing the values in between,
like this:

     (cdr (cdr '(pine fir oak maple)))
          => (oak maple)

In this example, the Lisp interpreter evaluates the innermost list
first.  The innermost list is quoted, so it just passes the list as
it is to the innermost `cdr'.  This `cdr' passes a list made up of the
second and subsequent elements of the list to the outermost `cdr',
which produces a list composed of the third and subsequent elements of
the original list.  In this example, the `cdr' function is repeated
and returns a list that consists of the original list without its
first two elements.

The `nthcdr' function does the same as repeating the call to `cdr'.
In the following example, the argument 2 is passed to the function
`nthcdr', along with the list, and the value returned is the list
without its first two items, which is exactly the same as repeating
`cdr' twice on the list:

     (nthcdr 2 '(pine fir oak maple))
          => (oak maple)

Using the original four element list, we can see what happens when
various numeric arguments are passed to `nthcdr', including 0, 1, and
5:

     ;; Leave the list as it was.
     (nthcdr 0 '(pine fir oak maple))
          => (pine fir oak maple)
     
     ;; Return a copy without the first element.
     (nthcdr 1 '(pine fir oak maple))
          => (fir oak maple)
     
     ;; Return a copy of the list without three elements.
     (nthcdr 3 '(pine fir oak maple))
          => (maple)
     
     ;; Return a copy lacking all four elements.
     (nthcdr 4 '(pine fir oak maple))
          => nil
     
     ;; Return a copy lacking all elements.
     (nthcdr 5 '(pine fir oak maple))
          => nil

`nth'
=====

The `nthcdr' function takes the CDR of a list repeatedly.  The `nth'
function takes the CAR of the result returned by `nthcdr'.  It
returns the Nth element of the list.

Thus, if it were not defined in C for speed, the definition of `nth'
would be:

     (defun nth (n list)
       "Returns the Nth element of LIST.
     N counts from zero.  If LIST is not that long, nil is returned."
       (car (nthcdr n list)))

(Originally, `nth' was defined in Emacs Lisp in `subr.el', but its
definition was redone in C in the 1980s.)

The `nth' function returns a single element of a list.  This can be
very convenient.

Note that the elements are numbered from zero, not one.  That is to
say, the first element of a list, its CAR is the zeroth element.
This is called `zero-based' counting and often bothers people who are
accustomed to the first element in a list being number one, which is
`one-based'.

For example:

     (nth 0 '("one" "two" "three"))
         => "one"
     
     (nth 1 '("one" "two" "three"))
         => "two"

It is worth mentioning that `nth', like `nthcdr' and `cdr', does not
change the original list--the function is non-destructive.  This is
in sharp contrast to the `setcar' and `setcdr' functions.

`setcar'
========

As you might guess from their names, the `setcar' and `setcdr'
functions set the CAR or the CDR of a list to a new value.  They
actually change the original list, unlike `car' and `cdr' which leave
the original list as it was.  One way to find out how this works is
to experiment.  We will start with the `setcar' function.

First, we can make a list and then set the value of a variable to the
list, using the `setq' function.  Here is a list of animals:

     (setq animals '(antelope giraffe lion tiger))

If you are reading this in Info inside of GNU Emacs, you can evaluate
this expression in the usual fashion, by positioning the cursor after
the expression and typing `C-x C-e'.  (I'm doing this right here as I
write this.  This is one of the advantages of having the interpreter
built into the computing environment.)

When we evaluate the variable `animals', we see that it is bound to
the list `(antelope giraffe lion tiger)':

     animals
          => (antelope giraffe lion tiger)

Put another way, the variable `animals' points to the list `(antelope
giraffe lion tiger)'.

Next, evaluate the function `setcar' while passing it two arguments,
the variable `animals' and the quoted symbol `hippopotamus'; this is
done by writing the three element list `(setcar animals
'hippopotamus)' and then evaluating it in the usual fashion:

     (setcar animals 'hippopotamus)

After evaluating this expression, evaluate the variable `animals'
again.  You will see that the list of animals has changed:

     animals
          => (hippopotamus giraffe lion tiger)

The first element on the list, `antelope' is replaced by
`hippopotamus'.

So we can see that `setcar' did not add a new element to the list as
`cons' would have; it replaced `giraffe' with `hippopotamus'; it
_changed_ the list.

`setcdr'
========

The `setcdr' function is similar to the `setcar' function, except
that the function replaces the second and subsequent elements of a
list rather than the first element.

To see how this works, set the value of the variable to a list of
domesticated animals by evaluating the following expression:

     (setq domesticated-animals '(horse cow sheep goat))

If you now evaluate the list, you will be returned the list `(horse
cow sheep goat)':

     domesticated-animals
          => (horse cow sheep goat)

Next, evaluate `setcdr' with two arguments, the name of the variable
which has a list as its value, and the list to which the CDR of the
first list will be set;

     (setcdr domesticated-animals '(cat dog))

If you evaluate this expression, the list `(cat dog)' will appear in
the echo area.  This is the value returned by the function.  The
result we are interested in is the "side effect", which we can see by
evaluating the variable `domesticated-animals':

     domesticated-animals
          => (horse cat dog)

Indeed, the list is changed from `(horse cow sheep goat)' to `(horse
cat dog)'.  The CDR of the list is changed from `(cow sheep goat)' to
`(cat dog)'.

Exercise
========

Construct a list of four birds by evaluating several expressions with
`cons'.  Find out what happens when you `cons' a list onto itself.
Replace the first element of the list of four birds with a fish.
Replace the rest of that list with a list of other fish.

Cutting and Storing Text
************************

Whenever you cut or clip text out of a buffer with a `kill' command in
GNU Emacs, it is stored in a list and you can bring it back with a
`yank' command.

(The use of the word `kill' in Emacs for processes which specifically
_do not_ destroy the values of the entities is an unfortunate
historical accident.  A much more appropriate word would be `clip'
since that is what the kill commands do; they clip text out of a
buffer and put it into storage from which it can be brought back.  I
have often been tempted to replace globally all occurrences of `kill'
in the Emacs sources with `clip' and all occurrences of `killed' with
`clipped'.)

Storing Text in a List
======================

When text is cut out of a buffer, it is stored on a list.  Successive
pieces of text are stored on the list successively, so the list might
look like this:

     ("a piece of text" "previous piece")

The function `cons' can be used to to create a new list from a piece
of text (an `atom', to use the jargon) and an existing list, like
this:

     (cons "another piece"
           '("a piece of text" "previous piece"))

If you evaluate this expression, a list of three elements will appear
in the echo area:

     ("another piece" "a piece of text" "previous piece")

With the `car' and `nthcdr' functions, you can retrieve whichever
piece of text you want.  For example, in the following code, `nthcdr
1 ...' returns the list with the first item removed; and the `car'
returns the first element of that remainder--the second element of
the original list:

     (car (nthcdr 1 '("another piece"
                      "a piece of text"
                      "previous piece")))
          => "a piece of text"

The actual functions in Emacs are more complex than this, of course.
The code for cutting and retrieving text has to be written so that
Emacs can figure out which element in the list you want--the first,
second, third, or whatever.  In addition, when you get to the end of
the list, Emacs should give you the first element of the list, rather
than nothing at all.

The list that holds the pieces of text is called the "kill ring".
This chapter leads up to a description of the kill ring and how it is
used by first tracing how the `zap-to-char' function works.  This
function uses (or `calls') a function that invokes a function that
manipulates the kill ring.  Thus, before reaching the mountains, we
climb the foothills.

A subsequent chapter describes how text that is cut from the buffer is
retrieved.  *Note Yanking Text Back: Yanking.

`zap-to-char'
=============

The `zap-to-char' function barely changed between GNU Emacs version
19 and GNU Emacs version 21.  However, `zap-to-char' calls another
function, `kill-region', which enjoyed a major rewrite on the way to
version 21.

The `kill-region' function in Emacs 19 is complex, but does not use
code that is important at this time.  We will skip it.

The `kill-region' function in Emacs 21 is easier to read than the
same function in Emacs 19 and introduces a very important concept,
that of error handling.  We will walk through the function.

But first, let us look at the interactive `zap-to-char' function.

The Complete `zap-to-char' Implementation
-----------------------------------------

The GNU Emacs version 19 and version 21 implementations of the
`zap-to-char' function are nearly identical in form, and they work
alike.  The function removes the text in the region between the
location of the cursor (i.e., of point) up to and including the next
occurrence of a specified character.  The text that `zap-to-char'
removes is put in the kill ring; and it can be retrieved from the kill
ring by typing `C-y' (`yank').  If the command is given an argument,
it removes text through that number of occurrences.  Thus, if the
cursor were at the beginning of this sentence and the character were
`s', `Thus' would be removed.  If the argument were two, `Thus, if
the curs' would be removed, up to and including the `s' in `cursor'.

If the specified character is not found, `zap-to-char' will say
"Search failed", tell you the character you typed, and not remove any
text.

In order to determine how much text to remove, `zap-to-char' uses a
search function.  Searches are used extensively in code that
manipulates text, and we will focus attention on them as well as on
the deletion command.

Here is the complete text of the version 19 implementation of the
function:

     (defun zap-to-char (arg char)  ; version 19 implementation
       "Kill up to and including ARG'th occurrence of CHAR.
     Goes backward if ARG is negative; error if CHAR not found."
       (interactive "*p\ncZap to char: ")
       (kill-region (point)
                    (progn
                      (search-forward
                       (char-to-string char) nil nil arg)
                      (point))))

The `interactive' Expression
----------------------------

The interactive expression in the `zap-to-char' command looks like
this:

     (interactive "*p\ncZap to char: ")

The part within quotation marks, `"*p\ncZap to char: "', specifies
three different things.  First, and most simply, the asterisk, `*',
causes an error to be signalled if the buffer is read-only.  This
means that if you try `zap-to-char' in a read-only buffer you will
not be able to remove text, and you will receive a message that says
"Buffer is read-only"; your terminal may beep at you as well.

The version 21 implementation does not have the asterisk, `*'.  The
function works the same as in version 19: in both cases, it cannot
remove text from a read-only buffer but the function does copy the
text that would have been removed to the kill ring.  Also, in both
cases, you see an error message.

However, the version 19 implementation copies text from a read-only
buffer only because of a mistake in the implementation of
`interactive'.  According to the documentation for `interactive', the
asterisk, `*', should prevent the `zap-to-char' function from doing
anything at all when the buffer is read only.  The function should
not copy the text to the kill ring.  It is a bug that it does.

In version 21, `interactive' is implemented correctly.  So the
asterisk, `*', had to be removed from the interactive specification.
If you insert an `*' and evaluate the function definition, then the
next time you run the `zap-to-char' function on a read-only buffer,
you will not copy any text.

That change aside, and a change to the documentation, the two versions
of the  `zap-to-char' function are identical.

Let us continue with the interactive specification.

The second part of `"*p\ncZap to char: "' is the `p'.  This part is
separated from the next part by a newline, `\n'.  The `p' means that
the first argument to the function will be passed the value of a
`processed prefix'.  The prefix argument is passed by typing `C-u'
and a number, or `M-' and a number.  If the function is called
interactively without a prefix, 1 is passed to this argument.

The third part of `"*p\ncZap to char: "' is `cZap to char: '.  In
this part, the lower case `c' indicates that `interactive' expects a
prompt and that the argument will be a character.  The prompt follows
the `c' and is the string `Zap to char: ' (with a space after the
colon to make it look good).

What all this does is prepare the arguments to `zap-to-char' so they
are of the right type, and give the user a prompt.

The Body of `zap-to-char'
-------------------------

The body of the `zap-to-char' function contains the code that kills
(that is, removes) the text in the region from the current position
of the cursor up to and including the specified character.  The first
part of the code looks like this:

     (kill-region (point) ...

`(point)' is the current position of the cursor.

The next part of the code is an expression using `progn'.  The body
of the `progn' consists of calls to `search-forward' and `point'.

It is easier to understand how `progn' works after learning about
`search-forward', so we will look at `search-forward' and then at
`progn'.

The `search-forward' Function
-----------------------------

The `search-forward' function is used to locate the
zapped-for-character in `zap-to-char'.  If the search is successful,
`search-forward' leaves point immediately after the last character in
the target string.  (In `zap-to-char', the target string is just one
character long.)  If the search is backwards, `search-forward' leaves
point just before the first character in the target.  Also,
`search-forward' returns `t' for true.  (Moving point is therefore a
`side effect'.)

In `zap-to-char', the `search-forward' function looks like this:

     (search-forward (char-to-string char) nil nil arg)

The `search-forward' function takes four arguments:

  1. The first argument is the target, what is searched for.  This
     must be a string, such as `"z"'.

     As it happens, the argument passed to `zap-to-char' is a single
     character.  Because of the way computers are built, the Lisp
     interpreter may treat a single character as being different from
     a string of characters.  Inside the computer, a single character
     has a different electronic format than a string of one
     character.  (A single character can often be recorded in the
     computer using exactly one byte; but a string may be longer, and
     the computer needs to be ready for this.)  Since the
     `search-forward' function searches for a string, the character
     that the `zap-to-char' function receives as its argument must be
     converted inside the computer from one format to the other;
     otherwise the `search-forward' function will fail.  The
     `char-to-string' function is used to make this conversion.

  2. The second argument bounds the search; it is specified as a
     position in the buffer.  In this case, the search can go to the
     end of the buffer, so no bound is set and the second argument is
     `nil'.

  3. The third argument tells the function what it should do if the
     search fails--it can signal an error (and print a message) or it
     can return `nil'.  A `nil' as the third argument causes the
     function to signal an error when the search fails.

  4. The fourth argument to `search-forward' is the repeat count--how
     many occurrences of the string to look for.  This argument is
     optional and if the function is called without a repeat count,
     this argument is passed the value 1.  If this argument is
     negative, the search goes backwards.

In template form, a `search-forward' expression looks like this:

     (search-forward "TARGET-STRING"
                     LIMIT-OF-SEARCH
                     WHAT-TO-DO-IF-SEARCH-FAILS
                     REPEAT-COUNT)

We will look at `progn' next.

The `progn' Special Form
------------------------

`progn' is a special form that causes each of its arguments to be
evaluated in sequence and then returns the value of the last one.  The
preceding expressions are evaluated only for the side effects they
perform.  The values produced by them are discarded.

The template for a `progn' expression is very simple:

     (progn
       BODY...)

In `zap-to-char', the `progn' expression has to do two things: put
point in exactly the right position; and return the location of point
so that `kill-region' will know how far to kill to.

The first argument to the `progn' is `search-forward'.  When
`search-forward' finds the string, the function leaves point
immediately after the last character in the target string.  (In this
case the target string is just one character long.)  If the search is
backwards, `search-forward' leaves point just before the first
character in the target.  The movement of point is a side effect.

The second and last argument to `progn' is the expression `(point)'.
This expression returns the value of point, which in this case will
be the location to which it has been moved by `search-forward'.  This
value is returned by the `progn' expression and is passed to
`kill-region' as `kill-region''s second argument.

Summing up `zap-to-char'
------------------------

Now that we have seen how `search-forward' and `progn' work, we can
see how the `zap-to-char' function works as a whole.

The first argument to `kill-region' is the position of the cursor
when the `zap-to-char' command is given--the value of point at that
time.  Within the `progn', the search function then moves point to
just after the zapped-to-character and `point' returns the value of
this location.  The `kill-region' function puts together these two
values of point, the first one as the beginning of the region and the
second one as the end of the region, and removes the region.

The `progn' special form is necessary because the `kill-region'
command takes two arguments; and it would fail if `search-forward'
and `point' expressions were  written in sequence as two additional
arguments.  The `progn' expression is a single argument to
`kill-region' and returns the one value that `kill-region' needs for
its second argument.

`kill-region'
=============

The `zap-to-char' function uses the `kill-region' function.  This
function clips text from a region and copies that text to the kill
ring, from which it may be retrieved.

The Emacs 21 version of that function uses `condition-case' and
`copy-region-as-kill', both of which we will explain.
`condition-case' is an important special form.

In essence, the `kill-region' function calls `condition-case', which
takes three arguments.  In this function, the first argument does
nothing.  The second argument contains the code that does the work
when all goes well.  The third argument contains the code that is
called in the event of an error.

The Complete `kill-region' Definition
-------------------------------------

We will go through the `condition-case' code in a moment.  First, let
us look at the complete definition of `kill-region', with comments
added:

     (defun kill-region (beg end)
       "Kill between point and mark.
     The text is deleted but saved in the kill ring."
       (interactive "r")
     
       ;; 1. `condition-case' takes three arguments.
       ;;    If the first argument is nil, as it is here,
       ;;    information about the error signal is not
       ;;    stored for use by another function.
       (condition-case nil
     
           ;; 2. The second argument to `condition-case'
           ;;    tells the Lisp interpreter what to do when all goes well.
     
           ;;    The `delete-and-extract-region' function usually does the
           ;;    work.  If the beginning and ending of the region are both
           ;;    the same, then the variable `string' will be empty, or nil
           (let ((string (delete-and-extract-region beg end)))
     
             ;; `when' is an `if' clause that cannot take an `else-part'.
             ;; Emacs normally sets the value of `last-command' to the
             ;; previous command.
             ;; `kill-append' concatenates the new string and the old.
             ;; `kill-new' inserts text into a new item in the kill ring.
             (when string
               (if (eq last-command 'kill-region)
                   ;; if true, prepend string
                   (kill-append string (< end beg))
                 (kill-new string)))
             (setq this-command 'kill-region))
     
         ;; 3. The third argument to `condition-case' tells the interpreter
         ;;    what to do with an error.
         ;;    The third argument has a conditions part and a body part.
         ;;    If the conditions are met (in this case,
         ;;             if text or buffer is read-only)
         ;;    then the body is executed.
         ((buffer-read-only text-read-only) ;; this is the if-part
          ;; then...
          (copy-region-as-kill beg end)
          (if kill-read-only-ok            ;; usually this variable is nil
              (message "Read only text copied to kill ring")
            ;; or else, signal an error if the buffer is read-only;
            (barf-if-buffer-read-only)
            ;; and, in any case, signal that the text is read-only.
            (signal 'text-read-only (list (current-buffer)))))))

`condition-case'
----------------

As we have seen earlier (*note Generate an Error Message: Making
Errors.), when the Emacs Lisp interpreter has trouble evaluating an
expression, it provides you with help; in the jargon, this is called
"signaling an error".  Usually, the computer stops the program and
shows you a message.

However, some programs undertake complicated actions.  They should not
simply stop on an error.  In the `kill-region' function, the most
likely error is that you will try to kill text that is read-only and
cannot be removed.  So the `kill-region' function contains code to
handle this circumstance.  This code, which makes up the body of the
`kill-region' function, is inside of a `condition-case' special form.

The template for `condition-case' looks like this:

     (condition-case
       VAR
       BODYFORM
       ERROR-HANDLER...)

The second argument, BODYFORM, is straightforward.  The
`condition-case' special form causes the Lisp interpreter to evaluate
the code in BODYFORM.  If no error occurs, the special form returns
the code's value and produces the side-effects, if any.

In short, the BODYFORM part of a `condition-case' expression
determines what should happen when everything works correctly.

However, if an error occurs, among its other actions, the function
generating the error signal will define one or more error condition
names.

An error handler is the third argument to `condition case'.  An error
handler has two parts, a CONDITION-NAME and a BODY.  If the
CONDITION-NAME part of an error handler matches a condition name
generated by an error, then the BODY part of the error handler is run.

As you will expect, the CONDITION-NAME part of an error handler may
be either a single condition name or a list of condition names.

Also, a complete `condition-case' expression may contain more than
one error handler.  When an error occurs, the first applicable
handler is run.

Lastly, the first argument to the `condition-case' expression, the
VAR argument, is sometimes bound to a variable that contains
information about the error.  However, if that argument is nil, as is
the case in `kill-region', that information is discarded.

In brief, in the `kill-region' function, the code `condition-case'
works like this:

     IF NO ERRORS, RUN ONLY THIS CODE
         BUT, IF ERRORS, RUN THIS OTHER CODE.

`delete-and-extract-region'
---------------------------

A `condition-case' expression has two parts, a part that is evaluated
in the expectation that all will go well, but which may generate an
error; and a part that is evaluated when there is an error.

First, let us look at the code in `kill-region' that is run in the
expectation that all goes well.  This is the core of the function.
The code looks like this:

     (let ((string (delete-and-extract-region beg end)))
       (when string
         (if (eq last-command 'kill-region)
             (kill-append string (< end beg))
           (kill-new string)))
       (setq this-command 'kill-region))

It looks complicated because we have the new functions
`delete-and-extract-region', `kill-append', and `kill-new' as well as
the new variables, `last-command' and `this-command'.

The `delete-and-extract-region' function is straightforward.  It is a
built-in function that deletes the text in a region (a side effect)
and also returns that text.  This is the function that actually
removes the text.  (And if it cannot do that, it signals the error.)

In this `let' expression, the text that `delete-and-extract-region'
returns is placed in the local variable called `string'.  This is the
text that is removed from the buffer.  (To be more precise, the
variable is set to point to the address of the extracted text; to say
it is `placed in' the variable is simply a shorthand.)

If the variable `string' does point to text, that text is added to
the kill ring.  The variable will have a `nil' value if no text was
removed.

The code uses `when' to determine whether the variable `string'
points to text.  A `when' statement is simply a programmers'
convenience.  A `when' statement is an `if' statement without the
possibility of an else clause.  In your mind, you can replace `when'
with `if' and understand what goes on.  That is what the Lisp
interpreter does.

Technically speaking, `when' is a Lisp macro.  A Lisp "macro" enables
you to define new control constructs and other language features.  It
tells the interpreter how to compute another Lisp expression which
will in turn compute the value.  In this case, the `other expression'
is an `if' expression.  For more about Lisp macros, see *Note Macros:
(elisp)Macros.  The C programming language also provides macros.
These are different, but also useful.  We will briefly look at C
macros in *Note Digression into C::.

If the string has content, then another conditional expression is
executed.  This is an `if' with both a then-part and an else-part.

     (if (eq last-command 'kill-region)
         (kill-append string (< end beg))
       (kill-new string)))

The then-part is evaluated if the previous command was another call to
`kill-region'; if not, the else-part is evaluated.

`last-command' is a variable that comes with Emacs that we have not
seen before.  Normally, whenever a function is executed, Emacs sets
the value of `last-command' to the previous command.

In this segment of the definition, the `if' expression checks whether
the previous command was `kill-region'.  If it was,

     (kill-append string (< end beg))

concatenates a copy of the newly clipped text to the just previously
clipped text in the kill ring.  (If the `(< end beg))' expression is
true, `kill-append' prepends the string to the just previously
clipped text.  For a detailed discussion, see *Note The `kill-append'
function: kill-append function.)

If you then yank back the text, i.e., `paste' it, you get both pieces
of text at once.  That way, if you delete two words in a row, and
then yank them back, you get both words, in their proper order, with
one yank.  (The `(< end beg))' expression makes sure the order is
correct.)

On the other hand, if the previous command is not `kill-region', then
the `kill-new' function is called, which adds the text to the kill
ring as the latest item, and sets the `kill-ring-yank-pointer'
variable to point to it.

Digression into C
=================

The `zap-to-char' command uses the `delete-and-extract-region'
function, which in turn uses two other functions,
`copy-region-as-kill' and `del_range_1'.  The `copy-region-as-kill'
function will be described in a following section; it puts a copy of
the region in the kill ring so it can be yanked back.  (*Note
`copy-region-as-kill': copy-region-as-kill.)

The `delete-and-extract-region' function removes the contents of a
region and you cannot get them back.

Unlike the other code discussed here, `delete-and-extract-region' is
not written in Emacs Lisp; it is written in C and is one of the
primitives of the GNU Emacs system.  Since it is very simple, I will
digress briefly from Lisp and describe it here.

Like many of the other Emacs primitives, `delete-and-extract-region'
is written as an instance of a C macro, a macro being a template for
code.  The complete macro looks like this:

     DEFUN ("delete-and-extract-region", Fdelete_and_extract_region,
            Sdelete_and_extract_region, 2, 2, 0,
       "Delete the text between START and END and return it.")
       (start, end)
          Lisp_Object start, end;
     {
       validate_region (&start, &end);
       return del_range_1 (XINT (start), XINT (end), 1, 1);
     }

Without going into the details of the macro writing process, let me
point out that this macro starts with the word `DEFUN'.  The word
`DEFUN' was chosen since the code serves the same purpose as `defun'
does in Lisp.  The word `DEFUN' is followed by seven parts inside of
parentheses:

   * The first part is the name given to the function in Lisp,
     `delete-and-extract-region'.

   * The second part is the name of the function in C,
     `Fdelete_and_extract_region'.  By convention, it starts with
     `F'.  Since C does not use hyphens in names, underscores are used
     instead.

   * The third part is the name for the C constant structure that
     records information on this function for internal use.  It is
     the name of the function in C but begins with an `S' instead of
     an `F'.

   * The fourth and fifth parts specify the minimum and maximum
     number of arguments the function can have.  This function
     demands exactly 2 arguments.

   * The sixth part is nearly like the argument that follows the
     `interactive' declaration in a function written in Lisp: a letter
     followed, perhaps, by a prompt.  The only difference from the
     Lisp is when the macro is called with no arguments.  Then you
     write a `0' (which is a `null string'), as in this macro.

     If you were to specify arguments, you would place them between
     quotation marks.  The C macro for `goto-char' includes `"NGoto
     char: "' in this position to indicate that the function expects
     a raw prefix, in this case, a numerical location in a buffer,
     and provides a prompt.

   * The seventh part is a documentation string, just like the one
     for a function written in Emacs Lisp, except that every newline
     must be written explicitly as `\n' followed by a backslash and
     carriage return.

     Thus, the first two lines of documentation for  `goto-char' are
     written like this:

            "Set point to POSITION, a number or marker.\n\
          Beginning of buffer is position (point-min), end is (point-max).

In a C macro, the formal parameters come next, with a statement of
what kind of object they are, followed by what might be called the
`body' of the macro.  For `delete-and-extract-region' the `body'
consists of the following two lines:

     validate_region (&start, &end);
     return del_range_1 (XINT (start), XINT (end), 1, 1);

The first function, `validate_region' checks whether the values
passed as the beginning and end of the region are the proper type and
are within range.  The second function, `del_range_1', actually
deletes the text.

`del_range_1' is a complex function we will not look into.  It
updates the buffer and does other things.

However, it is worth looking at the two arguments passed to
`del_range'.  These are `XINT (start)' and `XINT (end)'.

As far as the C language is concerned, `start' and `end' are two
integers that mark the beginning and end of the region to be
deleted(1).

In early versions of Emacs, these two numbers were thirty-two bits
long, but the code is slowly being generalized to handle other
lengths.  Three of the available bits are used to specify the type of
information and a fourth bit is used for handling the computer's
memory; the remaining bits are used as `content'.

`XINT' is a C macro that extracts the relevant number from the longer
collection of bits; the four other bits are discarded.

The command in `delete-and-extract-region' looks like this:

     del_range_1 (XINT (start), XINT (end), 1, 1);

It deletes the region between the beginning position, `start', and
the ending position, `end'.

From the point of view of the person writing Lisp, Emacs is all very
simple; but hidden underneath is a great deal of complexity to make it
all work.

---------- Footnotes ----------

(1) More precisely, and requiring more expert knowledge to
understand, the two integers are of type `Lisp_Object', which can
also be a C union instead of an integer type.

Initializing a Variable with `defvar'
=====================================

Unlike the `delete-and-extract-region' function, the
`copy-region-as-kill' function is written in Emacs Lisp.  Two
functions within it, `kill-append' and `kill-new', copy a region in a
buffer and save it in a variable called the `kill-ring'.  This
section describes how the `kill-ring' variable is created and
initialized using the `defvar' special form.

(Again we note that the term `kill-ring' is a misnomer.  The text
that is clipped out of the buffer can be brought back; it is not a
ring of corpses, but a ring of resurrectable text.)

In Emacs Lisp, a variable such as the `kill-ring' is created and
given an initial value by using the `defvar' special form.  The name
comes from "define variable".

The `defvar' special form is similar to `setq' in that it sets the
value of a variable.  It is unlike `setq' in two ways: first, it only
sets the value of the variable if the variable does not already have
a value.  If the variable already has a value, `defvar' does not
override the existing value.  Second, `defvar' has a documentation
string.

(Another special form, `defcustom', is designed for variables that
people customize.  It has more features than `defvar'.  (*Note
Setting Variables with `defcustom': defcustom.)

Seeing the Current Value of a Variable
--------------------------------------

You can see the current value of a variable, any variable, by using
the `describe-variable' function, which is usually invoked by typing
`C-h v'.  If you type `C-h v' and then `kill-ring' (followed by
<RET>) when prompted, you will see what is in your current kill
ring--this may be quite a lot!  Conversely, if you have been doing
nothing this Emacs session except read this document, you may have
nothing in it.  Also, you will see the documentation for `kill-ring':

     Documentation:
     List of killed text sequences.
     Since the kill ring is supposed to interact nicely with cut-and-paste
     facilities offered by window systems, use of this variable should
     interact nicely with `interprogram-cut-function' and
     `interprogram-paste-function'.  The functions `kill-new',
     `kill-append', and `current-kill' are supposed to implement this
     interaction; you may want to use them instead of manipulating the kill
     ring directly.

The kill ring is defined by a `defvar' in the following way:

     (defvar kill-ring nil
       "List of killed text sequences.
     ...")

In this variable definition, the variable is given an initial value of
`nil', which makes sense, since if you have saved nothing, you want
nothing back if you give a `yank' command.  The documentation string
is written just like the documentation string of a `defun'.  As with
the documentation string of the `defun', the first line of the
documentation should be a complete sentence, since some commands,
like `apropos', print only the first line of documentation.
Succeeding lines should not be indented; otherwise they look odd when
you use `C-h v' (`describe-variable').

`defvar' and an asterisk
------------------------

In the past, Emacs used the `defvar' special form both for internal
variables that you would not expect a user to change and for
variables that you do expect a user to change.  Although you can still
use `defvar' for user customizable variables, please use `defcustom'
instead, since that special form provides a path into the
Customization commands.  (*Note Setting Variables with `defcustom':
defcustom.)

When you specified a variable using the `defvar' special form, you
could distinguish a readily settable variable from others by typing
an asterisk, `*', in the first column of its documentation string.
For example:

     (defvar shell-command-default-error-buffer nil
       "*Buffer name for `shell-command' ... error output.
     ... ")

This means that you could (and still can) use the `edit-options'
command to change the value of `shell-command-default-error-buffer'
temporarily.

However, options set using `edit-options' are set only for the
duration of your editing session.  The new values are not saved
between sessions.  Each time Emacs starts, it reads the original
value, unless you change the value within your `.emacs' file, either
by setting it manually or by using `customize'.  *Note Your `.emacs'
File: Emacs Initialization.

For me, the major use of the `edit-options' command is to suggest
variables that I might want to set in my `.emacs' file.  I urge you
to look through the list.  (*Note Editing Variable Values:
(emacs)Edit Options.)

`copy-region-as-kill'
=====================

The `copy-region-as-kill' function copies a region of text from a
buffer and (via either `kill-append' or `kill-new') saves it in the
`kill-ring'.

If you call `copy-region-as-kill' immediately after a `kill-region'
command, Emacs appends the newly copied text to the previously copied
text.  This means that if you yank back the text, you get it all,
from both this and the previous operation.  On the other hand, if
some other command precedes the `copy-region-as-kill', the function
copies the text into a separate entry in the kill ring.

The complete `copy-region-as-kill' function definition
------------------------------------------------------

Here is the complete text of the version 21 `copy-region-as-kill'
function:

     (defun copy-region-as-kill (beg end)
       "Save the region as if killed, but don't kill it.
     In Transient Mark mode, deactivate the mark.
     If `interprogram-cut-function' is non-nil, also save
     the text for a window system cut and paste."
       (interactive "r")
       (if (eq last-command 'kill-region)
           (kill-append (buffer-substring beg end) (< end beg))
         (kill-new (buffer-substring beg end)))
       (if transient-mark-mode
           (setq deactivate-mark t))
       nil)

As usual, this function can be divided into its component parts:

     (defun copy-region-as-kill (ARGUMENT-LIST)
       "DOCUMENTATION..."
       (interactive "r")
       BODY...)

The arguments are `beg' and `end' and the function is interactive
with `"r"', so the two arguments must refer to the beginning and end
of the region.  If you have been reading though this document from
the beginning, understanding these parts of a function is almost
becoming routine.

The documentation is somewhat confusing unless you remember that the
word `kill' has a meaning different from its usual meaning.  The
`Transient Mark' and `interprogram-cut-function' comments explain
certain side-effects.

After you once set a mark, a buffer always contains a region.  If you
wish, you can use Transient Mark mode to highlight the region
temporarily.  (No one wants to highlight the region all the time, so
Transient Mark mode highlights it only at appropriate times.  Many
people turn off Transient Mark mode, so the region is never
highlighted.)

Also, a windowing system allows you to copy, cut, and paste among
different programs.  In the X windowing system, for example, the
`interprogram-cut-function' function is `x-select-text', which works
with the windowing system's equivalent of the Emacs kill ring.

The body of the `copy-region-as-kill' function starts with an `if'
clause.  What this clause does is distinguish between two different
situations: whether or not this command is executed immediately after
a previous `kill-region' command.  In the first case, the new region
is appended to the previously copied text.  Otherwise, it is inserted
into the beginning of the kill ring as a separate piece of text from
the previous piece.

The last two lines of the function prevent the region from lighting up
if Transient Mark mode is turned on.

The body of `copy-region-as-kill' merits discussion in detail.

The Body of `copy-region-as-kill'
---------------------------------

The `copy-region-as-kill' function works in much the same way as the
`kill-region' function (*note `kill-region': kill-region.).  Both are
written so that two or more kills in a row combine their text into a
single entry.  If you yank back the text from the kill ring, you get
it all in one piece.  Moreover, kills that kill forward from the
current position of the cursor are added to the end of the previously
copied text and commands that copy text backwards add it to the
beginning of the previously copied text.  This way, the words in the
text stay in the proper order.

Like `kill-region', the `copy-region-as-kill' function makes use of
the `last-command' variable that keeps track of the previous Emacs
command.

`last-command' and `this-command'
.................................

Normally, whenever a function is executed, Emacs sets the value of
`this-command' to the function being executed (which in this case
would be `copy-region-as-kill').  At the same time, Emacs sets the
value of `last-command' to the previous value of `this-command'.

In the first part of the body of the `copy-region-as-kill' function,
an `if' expression determines whether the value of `last-command' is
`kill-region'.  If so, the then-part of the `if' expression is
evaluated; it uses the `kill-append' function to concatenate the text
copied at this call to the function with the text already in the
first element (the CAR) of the kill ring.  On the other hand, if the
value of `last-command' is not `kill-region', then the
`copy-region-as-kill' function attaches a new element to the kill
ring using the `kill-new' function.

The `if' expression reads as follows; it uses `eq', which is a
function we have not yet seen:

       (if (eq last-command 'kill-region)
           ;; then-part
           (kill-append (buffer-substring beg end) (< end beg))
         ;; else-part
         (kill-new (buffer-substring beg end)))

The `eq' function tests whether its first argument is the same Lisp
object as its second argument.  The `eq' function is similar to the
`equal' function in that it is used to test for equality, but differs
in that it determines whether two representations are actually the
same object inside the computer, but with different names.  `equal'
determines whether the structure and contents of two expressions are
the same.

If the previous command was `kill-region', then the Emacs Lisp
interpreter calls the `kill-append' function

The `kill-append' function
..........................

The `kill-append' function looks like this:

     (defun kill-append (string before-p)
       "Append STRING to the end of the latest kill in the kill ring.
     If BEFORE-P is non-nil, prepend STRING to the kill.
     If `interprogram-cut-function' is set, pass the resulting kill to
     it."
       (kill-new (if before-p
                     (concat string (car kill-ring))
                   (concat (car kill-ring) string))
                 t))

The `kill-append' function is fairly straightforward.  It uses the
`kill-new' function, which we will discuss in more detail in a moment.

First, let us look at the conditional that is one of the two arguments
to `kill-new'.  It uses `concat' to concatenate the new text to the
CAR of the kill ring.  Whether it prepends or appends the text
depends on the results of an `if' expression:

     (if before-p                            ; if-part
         (concat string (car kill-ring))     ; then-part
       (concat (car kill-ring) string))      ; else-part

If the region being killed is before the region that was killed in the
last command, then it should be prepended before the material that was
saved in the previous kill; and conversely, if the killed text follows
what was just killed, it should be appended after the previous text.
The `if' expression depends on the predicate `before-p' to decide
whether the newly saved text should be put before or after the
previously saved text.

The symbol `before-p' is the name of one of the arguments to
`kill-append'.  When the `kill-append' function is evaluated, it is
bound to the value returned by evaluating the actual argument.  In
this case, this is the expression `(< end beg)'.  This expression
does not directly determine whether the killed text in this command
is located before or after the kill text of the last command; what is
does is determine whether the value of the variable `end' is less
than the value of the variable `beg'.  If it is, it means that the
user is most likely heading towards the beginning of the buffer.
Also, the result of evaluating the predicate expression, `(< end
beg)', will be true and the text will be prepended before the
previous text.  On the other hand, if the value of the variable `end'
is greater than the value of the variable `beg', the text will be
appended after the previous text.

When the newly saved text will be prepended, then the string with the
new text will be concatenated before the old text:

     (concat string (car kill-ring))

But if the text will be appended, it will be concatenated after the
old text:

     (concat (car kill-ring) string))

To understand how this works, we first need to review the `concat'
function.  The `concat' function links together or unites two strings
of text.  The result is a string.  For example:

     (concat "abc" "def")
          => "abcdef"
     
     (concat "new "
             (car '("first element" "second element")))
          => "new first element"
     
     (concat (car
             '("first element" "second element")) " modified")
          => "first element modified"

We can now make sense of `kill-append': it modifies the contents of
the kill ring.  The kill ring is a list, each element of which is
saved text.  The `kill-append' function uses the `kill-new' function
which in turn uses the `setcar' function.

The `kill-new' function
.......................

The `kill-new' function looks like this:

     (defun kill-new (string &optional replace)
       "Make STRING the latest kill in the kill ring.
     Set the kill-ring-yank pointer to point to it.
     If `interprogram-cut-function' is non-nil, apply it to STRING.
     Optional second argument REPLACE non-nil means that STRING will replace
     the front of the kill ring, rather than being added to the list."
       (and (fboundp 'menu-bar-update-yank-menu)
            (menu-bar-update-yank-menu string (and replace (car kill-ring))))
       (if (and replace kill-ring)
           (setcar kill-ring string)
         (setq kill-ring (cons string kill-ring))
         (if (> (length kill-ring) kill-ring-max)
             (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
       (setq kill-ring-yank-pointer kill-ring)
       (if interprogram-cut-function
           (funcall interprogram-cut-function string (not replace))))

As usual, we can look at this function in parts.

The first line of the documentation makes sense:

     Make STRING the latest kill in the kill ring.

Let's skip over the rest of the documentation for the moment.

Also, let's skip over the first two lines of code, those involving
`menu-bar-update-yank-menu'.  We will explain them below.

The critical lines are these:

       (if (and replace kill-ring)
           ;; then
           (setcar kill-ring string)
         ;; else
         (setq kill-ring (cons string kill-ring))
         (if (> (length kill-ring) kill-ring-max)
             ;; avoid overly long kill ring
             (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
       (setq kill-ring-yank-pointer kill-ring)
       (if interprogram-cut-function
           (funcall interprogram-cut-function string (not replace))))

The conditional test is `(and replace kill-ring)'.  This will be true
when two conditions are met:  the kill ring has something in it, and
the `replace' variable is true.

The `kill-append' function sets `replace' to be true; then, when the
kill ring has at least one item in it, the `setcar' expression is
executed:

     (setcar kill-ring string)

The `setcar' function actually changes the first element of the
`kill-ring' list to the value of `string'.  It replaces the first
element.

On the other hand, if the kill ring is empty, or replace is false, the
else-part of the condition is executed:

     (setq kill-ring (cons string kill-ring))
     (if (> (length kill-ring) kill-ring-max)
         (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil))

This expression first constructs a new version of the kill ring by
prepending `string' to the existing kill ring as a new element.  Then
it executes a second `if' clause.  This second `if' clause keeps the
kill ring from growing too long.

Let's look at these two expressions in order.

The `setq' line of the else-part sets the new value of the kill ring
to what results from adding the string being killed to the old kill
ring.

We can see how this works with an example:

     (setq example-list '("here is a clause" "another clause"))

After evaluating this expression with `C-x C-e', you can evaluate
`example-list' and see what it returns:

     example-list
          => ("here is a clause" "another clause")

Now, we can add a new element on to this list by evaluating the
following expression:

     (setq example-list (cons "a third clause" example-list))

When we evaluate `example-list', we find its value is:

     example-list
          => ("a third clause" "here is a clause" "another clause")

Thus, the third clause was added to the list by `cons'.

This is exactly similar to what the `setq' and `cons' do in the
function.  Here is the line again:

     (setq kill-ring (cons string kill-ring))

Now for the second part of the `if' clause.  This expression keeps
the kill ring from growing too long.  It looks like this:

     (if (> (length kill-ring) kill-ring-max)
         (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil))

The code checks whether the length of the kill ring is greater than
the maximum permitted length.  This is the value of `kill-ring-max'
(which is 60, by default).  If the length of the kill ring is too
long, then this code sets the last element of the kill ring to `nil'.
It does this by using two functions, `nthcdr' and `setcdr'.

We looked at `setcdr' earlier (*note `setcdr': setcdr.).  It sets the
CDR of a list, just as `setcar' sets the CAR of a list.  In this
case, however, `setcdr' will not be setting the CDR of the whole kill
ring; the `nthcdr' function is used to cause it to set the CDR of the
next to last element of the kill ring--this means that since the CDR
of the next to last element is the last element of the kill ring, it
will set the last element of the kill ring.

The `nthcdr' function works by repeatedly taking the CDR of a
list--it takes the CDR of the CDR of the CDR ...  It does this N
times and returns the results.

Thus, if we had a four element list that was supposed to be three
elements long, we could set the CDR of the next to last element to
`nil', and thereby shorten the list.

You can see this by evaluating the following three expressions in
turn.  First set the value of `trees' to `(maple oak pine birch)',
then set the CDR of its second CDR to `nil' and then find the value
of `trees':

     (setq trees '(maple oak pine birch))
          => (maple oak pine birch)
     
     (setcdr (nthcdr 2 trees) nil)
          => nil
     
     trees
          => (maple oak pine)

(The value returned by the `setcdr' expression is `nil' since that is
what the CDR is set to.)

To repeat, in `kill-new', the `nthcdr' function takes the CDR a
number of times that is one less than the maximum permitted size of
the kill ring and sets the CDR of that element (which will be the
rest of the elements in the kill ring) to `nil'.  This prevents the
kill ring from growing too long.

The next to last expression in the `kill-new' function is

     (setq kill-ring-yank-pointer kill-ring)

The `kill-ring-yank-pointer' is a global variable that is set to be
the `kill-ring'.

Even though the `kill-ring-yank-pointer' is called a `pointer', it is
a variable just like the kill ring.  However, the name has been
chosen to help humans understand how the variable is used.  The
variable is used in functions such as `yank' and `yank-pop' (*note
Yanking Text Back: Yanking.).

Now, to return to the first two lines in the body of the function:

       (and (fboundp 'menu-bar-update-yank-menu)
            (menu-bar-update-yank-menu string (and replace (car kill-ring))))

This is an expression whose first element is the function `and'.

The `and' special form evaluates each of its arguments until one of
the arguments returns a value of `nil', in which case the `and'
expression returns `nil'; however, if none of the arguments returns a
value of `nil', the value resulting from evaluating the last argument
is returned.  (Since such a value is not `nil', it is considered true
in Emacs Lisp.)  In other words, an `and' expression returns a true
value only if all its arguments are true.

In this case, the expression tests first to see whether
`menu-bar-update-yank-menu' exists as a function, and if so, calls
it.  The `fboundp' function returns true if the symbol it is testing
has a function definition that `is not void'.  If the symbol's
function definition were void, we would receive an error message, as
we did when we created errors intentionally (*note Generate an Error
Message: Making Errors.).

Essentially, the `and' is an `if' expression that reads like this:

     if THE-MENU-BAR-FUNCTION-EXISTS
       then EXECUTE-IT

`menu-bar-update-yank-menu' is one of the functions that make it
possible to use the `Select and Paste' menu in the Edit item of a menu
bar; using a mouse, you can look at the various pieces of text you
have saved and select one piece to paste.

Finally, the last expression in the `kill-new' function adds the
newly copied string to whatever facility exists for copying and
pasting among different programs running in a windowing system.  In
the X Windowing system, for example, the `x-select-text' function
takes the string and stores it in memory operated by X.  You can paste
the string in another program, such as an Xterm.

The expression looks like this:

       (if interprogram-cut-function
           (funcall interprogram-cut-function string (not replace))))

If an `interprogram-cut-function' exists, then Emacs executes
`funcall', which in turn calls its first argument as a function and
passes the remaining arguments to it.  (Incidentally, as far as I can
see, this `if' expression could be replaced by an `and' expression
similar to the one in the first part of the function.)

We are not going to discuss windowing systems and other programs
further, but merely note that this is a mechanism that enables GNU
Emacs to work easily and well with other programs.

This code for placing text in the kill ring, either concatenated with
an existing element or as a new element, leads us to the code for
bringing back text that has been cut out of the buffer--the yank
commands.  However, before discussing the yank commands, it is better
to learn how lists are implemented in a computer.  This will make
clear such mysteries as the use of the term `pointer'.

Review
======

Here is a brief summary of some recently introduced functions.

`car'
`cdr'
     `car' returns the first element of a list; `cdr' returns the
     second and subsequent elements of a list.

     For example:

          (car '(1 2 3 4 5 6 7))
               => 1
          (cdr '(1 2 3 4 5 6 7))
               => (2 3 4 5 6 7)

`cons'
     `cons' constructs a list by prepending its first argument to its
     second argument.

     For example:

          (cons 1 '(2 3 4))
               => (1 2 3 4)

`nthcdr'
     Return the result of taking CDR `n' times on a list.  The `rest
     of the rest', as it were.

     For example:

          (nthcdr 3 '(1 2 3 4 5 6 7))
               => (4 5 6 7)

`setcar'
`setcdr'
     `setcar' changes the first element of a list; `setcdr' changes
     the second and subsequent elements of a list.

     For example:

          (setq triple '(1 2 3))
          
          (setcar triple '37)
          
          triple
               => (37 2 3)
          
          (setcdr triple '("foo" "bar"))
          
          triple
               => (37 "foo" "bar")

`progn'
     Evaluate each argument in sequence and then return the value of
     the last.

     For example:

          (progn 1 2 3 4)
               => 4

`save-restriction'
     Record whatever narrowing is in effect in the current buffer, if
     any, and restore that narrowing after evaluating the arguments.

`search-forward'
     Search for a string, and if the string is found, move point.

     Takes four arguments:

       1. The string to search for.

       2. Optionally, the limit of the search.

       3. Optionally, what to do if the search fails, return `nil' or
          an error message.

       4. Optionally, how many times to repeat the search; if
          negative, the search goes backwards.

`kill-region'
`delete-region'
`copy-region-as-kill'
     `kill-region' cuts the text between point and mark from the
     buffer and stores that text in the kill ring, so you can get it
     back by yanking.

     `delete-and-extract-region' removes the text between point and
     mark from the buffer and throws it away.  You cannot get it back.

     `copy-region-as-kill' copies the text between point and mark into
     the kill ring, from which you can get it by yanking.  The
     function does not cut or remove the text from the buffer.

Searching Exercises
===================

   * Write an interactive function that searches for a string.  If the
     search finds the string, leave point after it and display a
     message that says "Found!".  (Do not use `search-forward' for
     the name of this function; if you do, you will overwrite the
     existing version of `search-forward' that comes with Emacs.  Use
     a name such as `test-search' instead.)

   * Write a function that prints the third element of the kill ring
     in the echo area, if any; if the kill ring does not contain a
     third element, print an appropriate message.

How Lists are Implemented
*************************

In Lisp, atoms are recorded in a straightforward fashion; if the
implementation is not straightforward in practice, it is, nonetheless,
straightforward in theory.  The atom `rose', for example, is recorded
as the four contiguous letters `r', `o', `s', `e'.  A list, on the
other hand, is kept differently.  The mechanism is equally simple,
but it takes a moment to get used to the idea.  A list is kept using
a series of pairs of pointers.  In the series, the first pointer in
each pair points to an atom or to another list, and the second
pointer in each pair points to the next pair, or to the symbol `nil',
which marks the end of the list.

A pointer itself is quite simply the electronic address of what is
pointed to.  Hence, a list is kept as a series of electronic
addresses.

Lists diagrammed
================

For example, the list `(rose violet buttercup)' has three elements,
`rose', `violet', and `buttercup'.  In the computer, the electronic
address of `rose' is recorded in a segment of computer memory along
with the address that gives the electronic address of where the atom
`violet' is located; and that address (the one that tells where
`violet' is located) is kept along with an address that tells where
the address for the atom `buttercup' is located.

This sounds more complicated than it is and is easier seen in a
diagram:

         ___ ___      ___ ___      ___ ___
        |___|___|--> |___|___|--> |___|___|--> nil
          |            |            |
          |            |            |
           --> rose     --> violet   --> buttercup



In the diagram, each box represents a word of computer memory that
holds a Lisp object, usually in the form of a memory address.  The
boxes, i.e. the addresses, are in pairs.  Each arrow points to what
the address is the address of, either an atom or another pair of
addresses.  The first box is the electronic address of `rose' and the
arrow points to `rose'; the second box is the address of the next
pair of boxes, the first part of which is the address of `violet' and
the second part of which is the address of the next pair.  The very
last box points to the symbol `nil', which marks the end of the list.

When a variable is set to a list with a function such as `setq', it
stores the address of the first box in the variable.  Thus,
evaluation of the expression

     (setq bouquet '(rose violet buttercup))

creates a situation like this:

     bouquet
          |
          |     ___ ___      ___ ___      ___ ___
           --> |___|___|--> |___|___|--> |___|___|--> nil
                 |            |            |
                 |            |            |
                  --> rose     --> violet   --> buttercup



In this example, the symbol `bouquet' holds the address of the first
pair of boxes.

This same list can be illustrated in a different sort of box notation
like this:

     bouquet
      |
      |    --------------       ---------------       ----------------
      |   | car   | cdr  |     | car    | cdr  |     | car     | cdr  |
       -->| rose  |   o------->| violet |   o------->| butter- |  nil |
          |       |      |     |        |      |     | cup     |      |
           --------------       ---------------       ----------------



(Symbols consist of more than pairs of addresses, but the structure of
a symbol is made up of addresses.  Indeed, the symbol `bouquet'
consists of a group of address-boxes, one of which is the address of
the printed word `bouquet', a second of which is the address of a
function definition attached to the symbol, if any, a third of which
is the address of the first pair of address-boxes for the list `(rose
violet buttercup)', and so on.  Here we are showing that the symbol's
third address-box points to the first pair of address-boxes for the
list.)

If a symbol is set to the CDR of a list, the list itself is not
changed; the symbol simply has an address further down the list.  (In
the jargon, CAR and CDR are `non-destructive'.)  Thus, evaluation of
the following expression

     (setq flowers (cdr bouquet))

produces this:


     bouquet        flowers
       |              |
       |     ___ ___  |     ___ ___      ___ ___
        --> |   |   |  --> |   |   |    |   |   |
            |___|___|----> |___|___|--> |___|___|--> nil
              |              |            |
              |              |            |
               --> rose       --> violet   --> buttercup




The value of `flowers' is `(violet buttercup)', which is to say, the
symbol `flowers' holds the address of the pair of address-boxes, the
first of which holds the address of `violet', and the second of which
holds the address of `buttercup'.

A pair of address-boxes is called a "cons cell" or "dotted pair".
*Note List Type: (elisp)List Type, and *Note Dotted Pair Notation:
(elisp)Dotted Pair Notation, for more information about cons cells
and dotted pairs.

The function `cons' adds a new pair of addresses to the front of a
series of addresses like that shown above.  For example, evaluating
the expression

     (setq bouquet (cons 'lily bouquet))

produces:


     bouquet                       flowers
       |                             |
       |     ___ ___        ___ ___  |     ___ ___       ___ ___
        --> |   |   |      |   |   |  --> |   |   |     |   |   |
            |___|___|----> |___|___|----> |___|___|---->|___|___|--> nil
              |              |              |             |
              |              |              |             |
               --> lily      --> rose       --> violet    --> buttercup




However, this does not change the value of the symbol `flowers', as
you can see by evaluating the following,

     (eq (cdr (cdr bouquet)) flowers)

which returns `t' for true.

Until it is reset, `flowers' still has the value `(violet
buttercup)'; that is, it has the address of the cons cell whose first
address is of `violet'.  Also, this does not alter any of the
pre-existing cons cells; they are all still there.

Thus, in Lisp, to get the CDR of a list, you just get the address of
the next cons cell in the series; to get the CAR of a list, you get
the address of the first element of the list; to `cons' a new element
on a list, you add a new cons cell to the front of the list.  That is
all there is to it!  The underlying structure of Lisp is brilliantly
simple!

And what does the last address in a series of cons cells refer to?  It
is the address of the empty list, of `nil'.

In summary, when a Lisp variable is set to a value, it is provided
with the address of the list to which the variable refers.

Symbols as a Chest of Drawers
=============================

In an earlier section, I suggested that you might imagine a symbol as
being a chest of drawers.  The function definition is put in one
drawer, the value in another, and so on.  What is put in the drawer
holding the value can be changed without affecting the contents of the
drawer holding the function definition, and vice-versa.

Actually, what is put in each drawer is the address of the value or
function definition.  It is as if you found an old chest in the attic,
and in one of its drawers you found a map giving you directions to
where the buried treasure lies.

(In addition to its name, symbol definition, and variable value, a
symbol has a `drawer' for a "property list" which can be used to
record other information.  Property lists are not discussed here; see
*Note Property Lists: (elisp)Property Lists.)

Here is a fanciful representation:


                 Chest of Drawers            Contents of Drawers
     
                 __   o0O0o   __
               /                 \
              ---------------------
             |    directions to    |            [map to]
             |     symbol name     |             bouquet
             |                     |
             +---------------------+
             |    directions to    |
             |  symbol definition  |             [none]
             |                     |
             +---------------------+
             |    directions to    |            [map to]
             |    variable value   |             (rose violet buttercup)
             |                     |
             +---------------------+
             |    directions to    |
             |    property list    |             [not described here]
             |                     |
             +---------------------+
             |/                   \|




Exercise
========

Set `flowers' to `violet' and `buttercup'.  Cons two more flowers on
to this list and set this new list to `more-flowers'.  Set the CAR of
`flowers' to a fish.  What does the `more-flowers' list now contain?

Yanking Text Back
*****************

Whenever you cut text out of a buffer with a `kill' command in GNU
Emacs, you can bring it back with a `yank' command.  The text that is
cut out of the buffer is put in the kill ring and the yank commands
insert the appropriate contents of the kill ring back into a buffer
(not necessarily the original buffer).

A simple `C-y' (`yank') command inserts the first item from the kill
ring into the current buffer.  If the `C-y' command is followed
immediately by `M-y', the first element is replaced by the second
element.  Successive `M-y' commands replace the second element with
the third, fourth, or fifth element, and so on.  When the last
element in the kill ring is reached, it is replaced by the first
element and the cycle is repeated.  (Thus the kill ring is called a
`ring' rather than just a `list'.  However, the actual data structure
that holds the text is a list.  *Note Handling the Kill Ring: Kill
Ring, for the details of how the list is handled as a ring.)

Kill Ring Overview
==================

The kill ring is a list of textual strings.  This is what it looks
like:

     ("some text" "a different piece of text" "yet more text")

If this were the contents of my kill ring and I pressed `C-y', the
string of characters saying `some text' would be inserted in this
buffer where my cursor is located.

The `yank' command is also used for duplicating text by copying it.
The copied text is not cut from the buffer, but a copy of it is put
on the kill ring and is inserted by yanking it back.

Three functions are used for bringing text back from the kill ring:
`yank', which is usually bound to `C-y'; `yank-pop', which is usually
bound to `M-y'; and `rotate-yank-pointer', which is used by the two
other functions.

These functions refer to the kill ring through a variable called the
`kill-ring-yank-pointer'.  Indeed, the insertion code for both the
`yank' and `yank-pop' functions is:

     (insert (car kill-ring-yank-pointer))

To begin to understand how `yank' and `yank-pop' work, it is first
necessary to look at the `kill-ring-yank-pointer' variable and the
`rotate-yank-pointer' function.

The `kill-ring-yank-pointer' Variable
=====================================

`kill-ring-yank-pointer' is a variable, just as `kill-ring' is a
variable.  It points to something by being bound to the value of what
it points to, like any other Lisp variable.

Thus, if the value of the kill ring is:

     ("some text" "a different piece of text" "yet more text")

and the `kill-ring-yank-pointer' points to the second clause, the
value of `kill-ring-yank-pointer' is:

     ("a different piece of text" "yet more text")

As explained in the previous chapter (*note List Implementation::),
the computer does not keep two different copies of the text being
pointed to by both the `kill-ring' and the `kill-ring-yank-pointer'.
The words "a different piece of text" and "yet more text" are not
duplicated.  Instead, the two Lisp variables point to the same pieces
of text.  Here is a diagram:

     kill-ring     kill-ring-yank-pointer
         |               |
         |      ___ ___  |     ___ ___      ___ ___
          ---> |   |   |  --> |   |   |    |   |   |
               |___|___|----> |___|___|--> |___|___|--> nil
                 |              |            |
                 |              |            |
                 |              |             --> "yet more text"
                 |              |
                 |               --> "a different piece of text
                 |
                  --> "some text"




Both the variable `kill-ring' and the variable
`kill-ring-yank-pointer' are pointers.  But the kill ring itself is
usually described as if it were actually what it is composed of.  The
`kill-ring' is spoken of as if it were the list rather than that it
points to the list.  Conversely, the `kill-ring-yank-pointer' is
spoken of as pointing to a list.

These two ways of talking about the same thing sound confusing at
first but make sense on reflection.  The kill ring is generally
thought of as the complete structure of data that holds the
information of what has recently been cut out of the Emacs buffers.
The `kill-ring-yank-pointer' on the other hand, serves to
indicate--that is, to `point to'--that part of the kill ring of which
the first element (the CAR) will be inserted.

The `rotate-yank-pointer' function changes the element in the kill
ring to which the `kill-ring-yank-pointer' points; when the pointer
is set to point to the next element beyond the end of the kill ring,
it automatically sets it to point to the first element of the kill
ring.  This is how the list is transformed into a ring.  The
`rotate-yank-pointer' function itself is not difficult, but contains
many details.  It and the much simpler `yank' and `yank-pop'
functions are described in an appendix.  *Note Handling the Kill
Ring: Kill Ring.

Exercises with `yank' and `nthcdr'
==================================

   * Using `C-h v' (`describe-variable'), look at the value of your
     kill ring.  Add several items to your kill ring; look at its
     value again.  Using `M-y' (`yank-pop)', move all the way around
     the kill ring.  How many items were in your kill ring?  Find the
     value of `kill-ring-max'.  Was your kill ring full, or could you
     have kept more blocks of text within it?

   * Using `nthcdr' and `car', construct a series of expressions to
     return the first, second, third, and fourth elements of a list.

Loops and Recursion
*******************

Emacs Lisp has two primary ways to cause an expression, or a series of
expressions, to be evaluated repeatedly: one uses a `while' loop, and
the other uses "recursion".

Repetition can be very valuable.  For example, to move forward four
sentences, you need only write a program that will move forward one
sentence and then repeat the process four times.  Since a computer
does not get bored or tired, such repetitive action does not have the
deleterious effects that excessive or the wrong kinds of repetition
can have on humans.

People mostly write Emacs Lisp functions using `while' loops and
their kin; but you can use recursion, which provides a very powerful
way to think about and then to solve problems(1).

---------- Footnotes ----------

(1) You can write recursive functions to be frugal or wasteful of
mental or computer resources; as it happens, methods that people find
easy--that are frugal of `mental resources'--sometimes use
considerable computer resources.  Emacs was designed to run on
machines that we now consider limited and its default settings are
conservative.  You may want to increase the values of
`max-specpdl-size' and `max-lisp-eval-depth'.  In my `.emacs' file, I
set them to 15 and 30 times their default value.

`while'
=======

The `while' special form tests whether the value returned by
evaluating its first argument is true or false.  This is similar to
what the Lisp interpreter does with an `if'; what the interpreter does
next, however, is different.

In a `while' expression, if the value returned by evaluating the
first argument is false, the Lisp interpreter skips the rest of the
expression (the "body" of the expression) and does not evaluate it.
However, if the value is true, the Lisp interpreter evaluates the body
of the expression and then again tests whether the first argument to
`while' is true or false.  If the value returned by evaluating the
first argument is again true, the Lisp interpreter again evaluates the
body of the expression.

The template for a `while' expression looks like this:

     (while TRUE-OR-FALSE-TEST
       BODY...)

Looping with `while'
--------------------

So long as the true-or-false-test of the `while' expression returns a
true value when it is evaluated, the body is repeatedly evaluated.
This process is called a loop since the Lisp interpreter repeats the
same thing again and again, like an airplane doing a loop.  When the
result of evaluating the true-or-false-test is false, the Lisp
interpreter does not evaluate the rest of the `while' expression and
`exits the loop'.

Clearly, if the value returned by evaluating the first argument to
`while' is always true, the body following will be evaluated again
and again ... and again ... forever.  Conversely, if the value
returned is never true, the expressions in the body will never be
evaluated.  The craft of writing a `while' loop consists of choosing
a mechanism such that the true-or-false-test returns true just the
number of times that you want the subsequent expressions to be
evaluated, and then have the test return false.

The value returned by evaluating a `while' is the value of the
true-or-false-test.  An interesting consequence of this is that a
`while' loop that evaluates without error will return `nil' or false
regardless of whether it has looped 1 or 100 times or none at all.  A
`while' expression that evaluates successfully never returns a true
value!  What this means is that `while' is always evaluated for its
side effects, which is to say, the consequences of evaluating the
expressions within the body of the `while' loop.  This makes sense.
It is not the mere act of looping that is desired, but the
consequences of what happens when the expressions in the loop are
repeatedly evaluated.

A `while' Loop and a List
-------------------------

A common way to control a `while' loop is to test whether a list has
any elements.  If it does, the loop is repeated; but if it does not,
the repetition is ended.  Since this is an important technique, we
will create a short example to illustrate it.

A simple way to test whether a list has elements is to evaluate the
list: if it has no elements, it is an empty list and will return the
empty list, `()', which is a synonym for `nil' or false.  On the
other hand, a list with elements will return those elements when it
is evaluated.  Since Emacs Lisp considers as true any value that is
not `nil', a list that returns elements will test true in a `while'
loop.

For example, you can set the variable `empty-list' to `nil' by
evaluating the following `setq' expression:

     (setq empty-list ())

After evaluating the `setq' expression, you can evaluate the variable
`empty-list' in the usual way, by placing the cursor after the symbol
and typing `C-x C-e'; `nil' will appear in your echo area:

     empty-list

On the other hand, if you set a variable to be a list with elements,
the list will appear when you evaluate the variable, as you can see by
evaluating the following two expressions:

     (setq animals '(gazelle giraffe lion tiger))
     
     animals

Thus, to create a `while' loop that tests whether there are any items
in the list `animals', the first part of the loop will be written
like this:

     (while animals
            ...

When the `while' tests its first argument, the variable `animals' is
evaluated.  It returns a list.  So long as the list has elements, the
`while' considers the results of the test to be true; but when the
list is empty, it considers the results of the test to be false.

To prevent the `while' loop from running forever, some mechanism
needs to be provided to empty the list eventually.  An oft-used
technique is to have one of the subsequent forms in the `while'
expression set the value of the list to be the CDR of the list.  Each
time the `cdr' function is evaluated, the list will be made shorter,
until eventually only the empty list will be left.  At this point,
the test of the `while' loop will return false, and the arguments to
the `while' will no longer be evaluated.

For example, the list of animals bound to the variable `animals' can
be set to be the CDR of the original list with the following
expression:

     (setq animals (cdr animals))

If you have evaluated the previous expressions and then evaluate this
expression, you will see `(giraffe lion tiger)' appear in the echo
area.  If you evaluate the expression again, `(lion tiger)' will
appear in the echo area.  If you evaluate it again and yet again,
`(tiger)' appears and then the empty list, shown by `nil'.

A template for a `while' loop that uses the `cdr' function repeatedly
to cause the true-or-false-test eventually to test false looks like
this:

     (while TEST-WHETHER-LIST-IS-EMPTY
       BODY...
       SET-LIST-TO-CDR-OF-LIST)

This test and use of `cdr' can be put together in a function that
goes through a list and prints each element of the list on a line of
its own.

An Example: `print-elements-of-list'
------------------------------------

The `print-elements-of-list' function illustrates a `while' loop with
a list.

The function requires several lines for its output.  If you are
reading this in Emacs 21 or a later version, you can evaluate the
following expression inside of Info, as usual.

If you are using an earlier version of Emacs, you need to copy the
necessary expressions to your `*scratch*' buffer and evaluate them
there.  This is because the echo area had only one line in the
earlier versions.

You can copy the expressions by marking the beginning of the region
with `C-<SPC>' (`set-mark-command'), moving the cursor to the end of
the region and then copying the region using `M-w'
(`copy-region-as-kill').  In the `*scratch*' buffer, you can yank the
expressions back by typing `C-y' (`yank').

After you have copied the expressions to the `*scratch*' buffer,
evaluate each expression in turn.  Be sure to evaluate the last
expression, `(print-elements-of-list animals)', by typing `C-u C-x
C-e', that is, by giving an argument to `eval-last-sexp'.  This will
cause the result of the evaluation to be printed in the `*scratch*'
buffer instead of being printed in the echo area.  (Otherwise you
will see something like this in your echo area:
`^Jgazelle^J^Jgiraffe^J^Jlion^J^Jtiger^Jnil', in which each `^J'
stands for a `newline'.)

If you are using Emacs 21 or later, you can evaluate these expressions
directly in the Info buffer, and the echo area will grow to show the
results.

     (setq animals '(gazelle giraffe lion tiger))
     
     (defun print-elements-of-list (list)
       "Print each element of LIST on a line of its own."
       (while list
         (print (car list))
         (setq list (cdr list))))
     
     (print-elements-of-list animals)

When you evaluate the three expressions in sequence, you will see
this:

     gazelle
     
     giraffe
     
     lion
     
     tiger
     nil

Each element of the list is printed on a line of its own (that is what
the function `print' does) and then the value returned by the
function is printed.  Since the last expression in the function is the
`while' loop, and since `while' loops always return `nil', a `nil' is
printed after the last element of the list.

A Loop with an Incrementing Counter
-----------------------------------

A loop is not useful unless it stops when it ought.  Besides
controlling a loop with a list, a common way of stopping a loop is to
write the first argument as a test that returns false when the correct
number of repetitions are complete.  This means that the loop must
have a counter--an expression that counts how many times the loop
repeats itself.

The test can be an expression such as `(< count desired-number)'
which returns `t' for true if the value of `count' is less than the
`desired-number' of repetitions and `nil' for false if the value of
`count' is equal to or is greater than the `desired-number'.  The
expression that increments the count can be a simple `setq' such as
`(setq count (1+ count))', where `1+' is a built-in function in Emacs
Lisp that adds 1 to its argument.  (The expression `(1+ count)' has
the same result as `(+ count 1)', but is easier for a human to read.)

The template for a `while' loop controlled by an incrementing counter
looks like this:

     SET-COUNT-TO-INITIAL-VALUE
     (while (< count desired-number)         ; true-or-false-test
       BODY...
       (setq count (1+ count)))              ; incrementer

Note that you need to set the initial value of `count'; usually it is
set to 1.

Example with incrementing counter
.................................

Suppose you are playing on the beach and decide to make a triangle of
pebbles, putting one pebble in the first row, two in the second row,
three in the third row and so on, like this:


                    *
                   * *
                  * * *
                 * * * *


(About 2500 years ago, Pythagoras and others developed the beginnings
of number theory by considering questions such as this.)

Suppose you want to know how many pebbles you will need to make a
triangle with 7 rows?

Clearly, what you need to do is add up the numbers from 1 to 7.  There
are two ways to do this; start with the smallest number, one, and add
up the list in sequence, 1, 2, 3, 4 and so on; or start with the
largest number and add the list going down: 7, 6, 5, 4 and so on.
Because both mechanisms illustrate common ways of writing `while'
loops, we will create two examples, one counting up and the other
counting down.  In this first example, we will start with 1 and add
2, 3, 4 and so on.

If you are just adding up a short list of numbers, the easiest way to
do it is to add up all the numbers at once.  However, if you do not
know ahead of time how many numbers your list will have, or if you
want to be prepared for a very long list, then you need to design
your addition so that what you do is repeat a simple process many
times instead of doing a more complex process once.

For example, instead of adding up all the pebbles all at once, what
you can do is add the number of pebbles in the first row, 1, to the
number in the second row, 2, and then add the total of those two rows
to the third row, 3.  Then you can add the number in the fourth row,
4, to the total of the first three rows; and so on.

The critical characteristic of the process is that each repetitive
action is simple.  In this case, at each step we add only two numbers,
the number of pebbles in the row and the total already found.  This
process of adding two numbers is repeated again and again until the
last row has been added to the total of all the preceding rows.  In a
more complex loop the repetitive action might not be so simple, but
it will be simpler than doing everything all at once.

The parts of the function definition
....................................

The preceding analysis gives us the bones of our function definition:
first, we will need a variable that we can call `total' that will be
the total number of pebbles.  This will be the value returned by the
function.

Second, we know that the function will require an argument: this
argument will be the total number of rows in the triangle.  It can be
called `number-of-rows'.

Finally, we need a variable to use as a counter.  We could call this
variable `counter', but a better name is `row-number'.  That is
because what the counter does is count rows, and a program should be
written to be as understandable as possible.

When the Lisp interpreter first starts evaluating the expressions in
the function, the value of `total' should be set to zero, since we
have not added anything to it.  Then the function should add the
number of pebbles in the first row to the total, and then add the
number of pebbles in the second to the total, and then add the number
of pebbles in the third row to the total, and so on, until there are
no more rows left to add.

Both `total' and `row-number' are used only inside the function, so
they can be declared as local variables with `let' and given initial
values.  Clearly, the initial value for `total' should be 0.  The
initial value of `row-number' should be 1, since we start with the
first row.  This means that the `let' statement will look like this:

       (let ((total 0)
             (row-number 1))
         BODY...)

After the internal variables are declared and bound to their initial
values, we can begin the `while' loop.  The expression that serves as
the test should return a value of `t' for true so long as the
`row-number' is less than or equal to the `number-of-rows'.  (If the
expression tests true only so long as the row number is less than the
number of rows in the triangle, the last row will never be added to
the total; hence the row number has to be either less than or equal
to the number of rows.)

Lisp provides the `<=' function that returns true if the value of its
first argument is less than or equal to the value of its second
argument and false otherwise.  So the expression that the `while'
will evaluate as its test should look like this:

     (<= row-number number-of-rows)

The total number of pebbles can be found by repeatedly adding the
number of pebbles in a row to the total already found.  Since the
number of pebbles in the row is equal to the row number, the total
can be found by adding the row number to the total.  (Clearly, in a
more complex situation, the number of pebbles in the row might be
related to the row number in a more complicated way; if this were the
case, the row number would be replaced by the appropriate expression.)

     (setq total (+ total row-number))

What this does is set the new value of `total' to be equal to the sum
of adding the number of pebbles in the row to the previous total.

After setting the value of `total', the conditions need to be
established for the next repetition of the loop, if there is one.
This is done by incrementing the value of the `row-number' variable,
which serves as a counter.  After the `row-number' variable has been
incremented, the true-or-false-test at the beginning of the `while'
loop tests whether its value is still less than or equal to the value
of the `number-of-rows' and if it is, adds the new value of the
`row-number' variable to the `total' of the previous repetition of
the loop.

The built-in Emacs Lisp function `1+' adds 1 to a number, so the
`row-number' variable can be incremented with this expression:

     (setq row-number (1+ row-number))

Putting the function definition together
........................................

We have created the parts for the function definition; now we need to
put them together.

First, the contents of the `while' expression:

     (while (<= row-number number-of-rows)   ; true-or-false-test
       (setq total (+ total row-number))
       (setq row-number (1+ row-number)))    ; incrementer

Along with the `let' expression varlist, this very nearly completes
the body of the function definition.  However, it requires one final
element, the need for which is somewhat subtle.

The final touch is to place the variable `total' on a line by itself
after the `while' expression.  Otherwise, the value returned by the
whole function is the value of the last expression that is evaluated
in the body of the `let', and this is the value returned by the
`while', which is always `nil'.

This may not be evident at first sight.  It almost looks as if the
incrementing expression is the last expression of the whole function.
But that expression is part of the body of the `while'; it is the
last element of the list that starts with the symbol `while'.
Moreover, the whole of the `while' loop is a list within the body of
the `let'.

In outline, the function will look like this:

     (defun NAME-OF-FUNCTION (ARGUMENT-LIST)
       "DOCUMENTATION..."
       (let (VARLIST)
         (while (TRUE-OR-FALSE-TEST)
           BODY-OF-WHILE... )
         ... )                     ; Need final expression here.

The result of evaluating the `let' is what is going to be returned by
the `defun' since the `let' is not embedded within any containing
list, except for the `defun' as a whole.  However, if the `while' is
the last element of the `let' expression, the function will always
return `nil'.  This is not what we want!  Instead, what we want is
the value of the variable `total'.  This is returned by simply
placing the symbol as the last element of the list starting with
`let'.  It gets evaluated after the preceding elements of the list
are evaluated, which means it gets evaluated after it has been
assigned the correct value for the total.

It may be easier to see this by printing the list starting with `let'
all on one line.  This format makes it evident that the VARLIST and
`while' expressions are the second and third elements of the list
starting with `let', and the `total' is the last element:

     (let (VARLIST) (while (TRUE-OR-FALSE-TEST) BODY-OF-WHILE... ) total)

Putting everything together, the `triangle' function definition looks
like this:

     (defun triangle (number-of-rows)    ; Version with
                                         ;   incrementing counter.
       "Add up the number of pebbles in a triangle.
     The first row has one pebble, the second row two pebbles,
     the third row three pebbles, and so on.
     The argument is NUMBER-OF-ROWS."
       (let ((total 0)
             (row-number 1))
         (while (<= row-number number-of-rows)
           (setq total (+ total row-number))
           (setq row-number (1+ row-number)))
         total))

After you have installed `triangle' by evaluating the function, you
can try it out.  Here are two examples:

     (triangle 4)
     
     (triangle 7)

The sum of the first four numbers is 10 and the sum of the first seven
numbers is 28.

Loop with a Decrementing Counter
--------------------------------

Another common way to write a `while' loop is to write the test so
that it determines whether a counter is greater than zero.  So long
as the counter is greater than zero, the loop is repeated.  But when
the counter is equal to or less than zero, the loop is stopped.  For
this to work, the counter has to start out greater than zero and then
be made smaller and smaller by a form that is evaluated repeatedly.

The test will be an expression such as `(> counter 0)' which returns
`t' for true if the value of `counter' is greater than zero, and
`nil' for false if the value of `counter' is equal to or less than
zero.  The expression that makes the number smaller and smaller can
be a simple `setq' such as `(setq counter (1- counter))', where `1-'
is a built-in function in Emacs Lisp that subtracts 1 from its
argument.

The template for a decrementing `while' loop looks like this:

     (while (> counter 0)                    ; true-or-false-test
       BODY...
       (setq counter (1- counter)))          ; decrementer

Example with decrementing counter
.................................

To illustrate a loop with a decrementing counter, we will rewrite the
`triangle' function so the counter decreases to zero.

This is the reverse of the earlier version of the function.  In this
case, to find out how many pebbles are needed to make a triangle with
3 rows, add the number of pebbles in the third row, 3, to the number
in the preceding row, 2, and then add the total of those two rows to
the row that precedes them, which is 1.

Likewise, to find the number of pebbles in a triangle with 7 rows, add
the number of pebbles in the seventh row, 7, to the number in the
preceding row, which is 6, and then add the total of those two rows to
the row that precedes them, which is 5, and so on.  As in the previous
example, each addition only involves adding two numbers, the total of
the rows already added up and the number of pebbles in the row that is
being added to the total.  This process of adding two numbers is
repeated again and again until there are no more pebbles to add.

We know how many pebbles to start with: the number of pebbles in the
last row is equal to the number of rows.  If the triangle has seven
rows, the number of pebbles in the last row is 7.  Likewise, we know
how many pebbles are in the preceding row: it is one less than the
number in the row.

The parts of the function definition
....................................

We start with three variables: the total number of rows in the
triangle; the number of pebbles in a row; and the total number of
pebbles, which is what we want to calculate.  These variables can be
named `number-of-rows', `number-of-pebbles-in-row', and `total',
respectively.

Both `total' and `number-of-pebbles-in-row' are used only inside the
function and are declared with `let'.  The initial value of `total'
should, of course, be zero.  However, the initial value of
`number-of-pebbles-in-row' should be equal to the number of rows in
the triangle, since the addition will start with the longest row.

This means that the beginning of the `let' expression will look like
this:

     (let ((total 0)
           (number-of-pebbles-in-row number-of-rows))
       BODY...)

The total number of pebbles can be found by repeatedly adding the
number of pebbles in a row to the total already found, that is, by
repeatedly evaluating the following expression:

     (setq total (+ total number-of-pebbles-in-row))

After the `number-of-pebbles-in-row' is added to the `total', the
`number-of-pebbles-in-row' should be decremented by one, since the
next time the loop repeats, the preceding row will be added to the
total.

The number of pebbles in a preceding row is one less than the number
of pebbles in a row, so the built-in Emacs Lisp function `1-' can be
used to compute the number of pebbles in the preceding row.  This can
be done with the following expression:

     (setq number-of-pebbles-in-row
           (1- number-of-pebbles-in-row))

Finally, we know that the `while' loop should stop making repeated
additions when there are no pebbles in a row.  So the test for the
`while' loop is simply:

     (while (> number-of-pebbles-in-row 0)

Putting the function definition together
........................................

We can put these expressions together to create a function definition
that works.  However, on examination, we find that one of the local
variables is unneeded!

The function definition looks like this:

     ;;; First subtractive version.
     (defun triangle (number-of-rows)
       "Add up the number of pebbles in a triangle."
       (let ((total 0)
             (number-of-pebbles-in-row number-of-rows))
         (while (> number-of-pebbles-in-row 0)
           (setq total (+ total number-of-pebbles-in-row))
           (setq number-of-pebbles-in-row
                 (1- number-of-pebbles-in-row)))
         total))

As written, this function works.

However, we do not need `number-of-pebbles-in-row'.

When the `triangle' function is evaluated, the symbol
`number-of-rows' will be bound to a number, giving it an initial
value.  That number can be changed in the body of the function as if
it were a local variable, without any fear that such a change will
effect the value of the variable outside of the function.  This is a
very useful characteristic of Lisp; it means that the variable
`number-of-rows' can be used anywhere in the function where
`number-of-pebbles-in-row' is used.

Here is a second version of the function written a bit more cleanly:

     (defun triangle (number)                ; Second version.
       "Return sum of numbers 1 through NUMBER inclusive."
       (let ((total 0))
         (while (> number 0)
           (setq total (+ total number))
           (setq number (1- number)))
         total))

In brief, a properly written `while' loop will consist of three parts:

  1. A test that will return false after the loop has repeated itself
     the correct number of times.

  2. An expression the evaluation of which will return the value
     desired after being repeatedly evaluated.

  3. An expression to change the value passed to the
     true-or-false-test so that the test returns false after the loop
     has repeated itself the right number of times.

Save your time: `dolist' and `dotimes'
======================================

In addition to `while', both `dolist' and `dotimes' provide for
looping.  Sometimes these are quicker to write than the equivalent
`while' loop.  Both are Lisp macros.  (*Note Macros: (elisp)Macros. )

`dolist' works like a `while' loop that `CDRs down a list':  `dolist'
automatically shortens the list each time it loops--takes the CDR of
the list--and binds the CAR of each shorter version of the list to
the first of its arguments.

`dotimes' loops a specific number of times: you specify the number.

The `dolist' Macro
..................

Suppose, for example, you want to reverse a list, so that "first"
"second" "third" becomes "third" "second" "first".

In practice, you would use the `reverse' function, like this:

     (setq animals '(gazelle giraffe lion tiger))
     
     (reverse animals)

Here is how you could reverse the list using a `while' loop:

     (setq animals '(gazelle giraffe lion tiger))
     
     (defun reverse-list-with-while (list)
       "Using while, reverse the order of LIST."
       (let (value)  ; make sure list starts empty
         (while list
           (setq value (cons (car list) value))
           (setq list (cdr list)))
         value))
     
     (reverse-list-with-while animals)

And here is how you could use the `dolist' macro:

     (setq animals '(gazelle giraffe lion tiger))
     
     (defun reverse-list-with-dolist (list)
       "Using dolist, reverse the order of LIST."
       (let (value)  ; make sure list starts empty
         (dolist (element list value)
           (setq value (cons element value)))))
     
     (reverse-list-with-dolist animals)

In Info, you can place your cursor after the closing parenthesis of
each expression and type `C-x C-e'; in each case, you should see

     (tiger lion giraffe gazelle)

in the echo area.

For this example, the existing `reverse' function is obviously best.
The `while' loop is just like our first example (*note A `while' Loop
and a List: Loop Example.).  The `while' first checks whether the
list has elements; if so, it constructs a new list by adding the
first element of the list to the existing list (which in the first
iteration of the loop is `nil').  Since the second element is
prepended in front of the first element, and the third element is
prepended in front of the second element, the list is reversed.

In the expression using a `while' loop, the `(setq list (cdr list))'
expression shortens the list, so the `while' loop eventually stops.
In addition, it provides the `cons' expression with a new first
element by creating a new and shorter list at each repetition of the
loop.

The `dolist' expression does very much the same as the `while'
expression, except that the `dolist' macro does some of the work you
have to do when writing a `while' expression.

Like a `while' loop, a `dolist' loops.  What is different is that it
automatically shortens the list each time it loops -- it `CDRs down
the list' on its own -- and it automatically binds the CAR of each
shorter version of the list to the first of its arguments.

In the example, the CAR of each shorter version of the list is
referred to using the symbol `element', the list itself is called
`list', and the value returned is called `value'.  The remainder of
the `dolist' expression is the body.

The `dolist' expression binds the CAR of each shorter version of the
list to `element' and then evaluates the body of the expression; and
repeats the loop.  The result is returned in `value'.

The `dotimes' Macro
...................

The `dotimes' macro is similar to `dolist', except that it loops a
specific number of times.

The first argument to `dotimes' is assigned the numbers 0, 1, 2 and
so forth each time around the loop, and the value of the third
argument is returned.  You need to provide the value of the second
argument, which is how many times the macro loops.

For example, the following binds the numbers from 0 up to, but not
including, the number 3 to the first argument, NUMBER, and then
constructs a list of the three numbers.  (The first number is 0, the
second number is 1, and the third number is 2; this makes a total of
three numbers in all, starting with zero as the first number.)

     (let (value)      ; otherwise a value is a void variable
       (dotimes (number 3 value)
         (setq value (cons number value))))
     
     => (2 1 0)

`dotimes' returns `value', so the way to use `dotimes' is to operate
on some expression NUMBER number of times and then return the result,
either as a list or an atom.

Here is an example of a `defun' that uses `dotimes' to add up the
number of pebbles in a triangle.

     (defun triangle-using-dotimes (number-of-rows)
       "Using dotimes, add up the number of pebbles in a triangle."
     (let ((total 0))  ; otherwise a total is a void variable
       (dotimes (number number-of-rows total)
         (setq total (+ total (1+ number))))))
     
     (triangle-using-dotimes 4)

Recursion
=========

A recursive function contains code that tells the Lisp interpreter to
call a program that runs exactly like itself, but with slightly
different arguments.  The code runs exactly the same because it has
the same name.  However, even though the program has the same name, it
is not the same entity.  It is different.  In the jargon, it is a
different `instance'.

Eventually, if the program is written correctly, the `slightly
different arguments' will become sufficiently different from the first
arguments that the final instance will stop.

Building Robots: Extending the Metaphor
---------------------------------------

It is sometimes helpful to think of a running program as a robot that
does a job.  In doing its job, a recursive function calls on a second
robot to help it.  The second robot is identical to the first in every
way, except that the second robot helps the first and has been passed
different arguments than the first.

In a recursive function, the second robot may call a third; and the
third may call a fourth, and so on.  Each of these is a different
entity; but all are clones.

Since each robot has slightly different instructions--the arguments
will differ from one robot to the next--the last robot should know
when to stop.

Let's expand on the metaphor in which a computer program is a robot.

A function definition provides the blueprints for a robot.  When you
install a function definition, that is, when you evaluate a `defun'
special form, you install the necessary equipment to build robots.
It is as if you were in a factory, setting up an assembly line.
Robots with the same name are built according to the same blueprints.
So they have, as it were, the same `model number', but a different
`serial number'.

We often say that a recursive function `calls itself'.  What we mean
is that the instructions in a recursive function cause the Lisp
interpreter to run a different function that has the same name and
does the same job as the first, but with different arguments.

It is important that the arguments differ from one instance to the
next; otherwise, the process will never stop.

The Parts of a Recursive Definition
-----------------------------------

A recursive function typically contains a conditional expression which
has three parts:

  1. A true-or-false-test that determines whether the function is
     called again, here called the "do-again-test".

  2. The name of the function.  When this name is called, a new
     instance of the function--a new robot, as it were--is created
     and told what to do.

  3. An expression that returns a different value each time the
     function is called, here called the "next-step-expression".
     Consequently, the argument (or arguments) passed to the new
     instance of the function will be different from that passed to
     the previous instance.  This causes the conditional expression,
     the "do-again-test", to test false after the correct number of
     repetitions.

Recursive functions can be much simpler than any other kind of
function.  Indeed, when people first start to use them, they often
look so mysteriously simple as to be incomprehensible.  Like riding a
bicycle, reading a recursive function definition takes a certain knack
which is hard at first but then seems simple.

There are several different common recursive patterns.  A very simple
pattern looks like this:

     (defun NAME-OF-RECURSIVE-FUNCTION (ARGUMENT-LIST)
       "DOCUMENTATION..."
       (if DO-AGAIN-TEST
         BODY...
         (NAME-OF-RECURSIVE-FUNCTION
              NEXT-STEP-EXPRESSION)))

Each time a recursive function is evaluated, a new instance of it is
created and told what to do.  The arguments tell the instance what to
do.

An argument is bound to the value of the next-step-expression.  Each
instance runs with a different value of the next-step-expression.

The value in the next-step-expression is used in the do-again-test.

The value returned by the next-step-expression is passed to the new
instance of the function, which evaluates it (or some
transmogrification of it) to determine whether to continue or stop.
The next-step-expression is designed so that the do-again-test returns
false when the function should no longer be repeated.

The do-again-test is sometimes called the "stop condition", since it
stops the repetitions when it tests false.

Recursion with a List
---------------------

The example of a `while' loop that printed the elements of a list of
numbers can be written recursively.  Here is the code, including an
expression to set the value of the variable `animals' to a list.

If you are using Emacs 20 or before, this example must be copied to
the `*scratch*' buffer and each expression must be evaluated there.
Use `C-u C-x C-e' to evaluate the `(print-elements-recursively
animals)' expression so that the results are printed in the buffer;
otherwise the Lisp interpreter will try to squeeze the results into
the one line of the echo area.

Also, place your cursor immediately after the last closing parenthesis
of the `print-elements-recursively' function, before the comment.
Otherwise, the Lisp interpreter will try to evaluate the comment.

If you are using Emacs 21 or later, you can evaluate this expression
directly in Info.

     (setq animals '(gazelle giraffe lion tiger))
     
     (defun print-elements-recursively (list)
       "Print each element of LIST on a line of its own.
     Uses recursion."
       (if list                              ; do-again-test
           (progn
             (print (car list))              ; body
             (print-elements-recursively     ; recursive call
              (cdr list)))))                 ; next-step-expression
     
     (print-elements-recursively animals)

The `print-elements-recursively' function first tests whether there
is any content in the list; if there is, the function prints the
first element of the list, the CAR of the list.  Then the function
`invokes itself', but gives itself as its argument, not the whole
list, but the second and subsequent elements of the list, the CDR of
the list.

Put another way, if the list is not empty, the function invokes
another instance of code that is similar to the initial code, but is a
different thread of execution, with different arguments than the first
instance.

Put in yet another way, if the list is not empty, the first robot
assemblies a second robot and tells it what to do; the second robot is
a different individual from the first, but is the same model.

When the second evaluation occurs, the `if' expression is evaluated
and if true, prints the first element of the list it receives as its
argument (which is the second element of the original list).  Then
the function `calls itself' with the CDR of the list it is invoked
with, which (the second time around) is the CDR of the CDR of the
original list.

Note that although we say that the function `calls itself', what we
mean is that the Lisp interpreter assembles and instructs a new
instance of the program.  The new instance is a clone of the first,
but is a separate individual.

Each time the function `invokes itself', it invokes itself on a
shorter version of the original list.  It creates a new instance that
works on a shorter list.

Eventually, the function invokes itself on an empty list.  It creates
a new instance whose argument is `nil'.  The conditional expression
tests the value of `list'.  Since the value of `list' is `nil', the
`if' expression tests false so the then-part is not evaluated.  The
function as a whole then returns `nil'.

When you evaluate `(print-elements-recursively animals)' in the
`*scratch*' buffer, you see this result:

     gazelle
     
     giraffe
     
     lion
     
     tiger
     nil

Recursion in Place of a Counter
-------------------------------

The `triangle' function described in a previous section can also be
written recursively.  It looks like this:

     (defun triangle-recursively (number)
       "Return the sum of the numbers 1 through NUMBER inclusive.
     Uses recursion."
       (if (= number 1)                    ; do-again-test
           1                               ; then-part
         (+ number                         ; else-part
            (triangle-recursively          ; recursive call
             (1- number)))))               ; next-step-expression
     
     (triangle-recursively 7)

You can install this function by evaluating it and then try it by
evaluating `(triangle-recursively 7)'.  (Remember to put your cursor
immediately after the last parenthesis of the function definition,
before the comment.)  The function evaluates to 28.

To understand how this function works, let's consider what happens in
the various cases when the function is passed 1, 2, 3, or 4 as the
value of its argument.

An argument of 1 or 2
.....................

First, what happens if the value of the argument is 1?

The function has an `if' expression after the documentation string.
It tests whether the value of `number' is equal to 1; if so, Emacs
evaluates the then-part of the `if' expression, which returns the
number 1 as the value of the function.  (A triangle with one row has
one pebble in it.)

Suppose, however, that the value of the argument is 2.  In this case,
Emacs evaluates the else-part of the `if' expression.

The else-part consists of an addition, the recursive call to
`triangle-recursively' and a decrementing action; and it looks like
this:

     (+ number (triangle-recursively (1- number)))

When Emacs evaluates this expression, the innermost expression is
evaluated first; then the other parts in sequence.  Here are the steps
in detail:

Step 1    Evaluate the innermost expression.
     The innermost expression is `(1- number)' so Emacs decrements the
     value of `number' from 2 to 1.

Step 2    Evaluate the `triangle-recursively' function.
     The Lisp interpreter creates an individual instance of
     `triangle-recursively'.  It does not matter that this function is
     contained within itself.  Emacs passes the result Step 1 as the
     argument used by this instance of the `triangle-recursively'
     function

     In this case, Emacs evaluates `triangle-recursively' with an
     argument of 1.  This means that this evaluation of
     `triangle-recursively' returns 1.

Step 3    Evaluate the value of `number'.
     The variable `number' is the second element of the list that
     starts with `+'; its value is 2.

Step 4    Evaluate the `+' expression.
     The `+' expression receives two arguments, the first from the
     evaluation of `number' (Step 3) and the second from the
     evaluation of `triangle-recursively' (Step 2).

     The result of the addition is the sum of 2 plus 1, and the
     number 3 is returned, which is correct.  A triangle with two
     rows has three pebbles in it.

An argument of 3 or 4
.....................

Suppose that `triangle-recursively' is called with an argument of 3.

Step 1    Evaluate the do-again-test.
     The `if' expression is evaluated first.  This is the do-again
     test and returns false, so the else-part of the `if' expression
     is evaluated.  (Note that in this example, the do-again-test
     causes the function to call itself when it tests false, not when
     it tests true.)

Step 2    Evaluate the innermost expression of the else-part.
     The innermost expression of the else-part is evaluated, which
     decrements 3 to 2.  This is the next-step-expression.

Step 3    Evaluate the `triangle-recursively' function.
     The number 2 is passed to the `triangle-recursively' function.

     We know what happens when Emacs evaluates `triangle-recursively'
     with an argument of 2.  After going through the sequence of
     actions described earlier, it returns a value of 3.  So that is
     what will happen here.

Step 4    Evaluate the addition.
     3 will be passed as an argument to the addition and will be
     added to the number with which the function was called, which is
     3.

The value returned by the function as a whole will be 6.

Now that we know what will happen when `triangle-recursively' is
called with an argument of 3, it is evident what will happen if it is
called with an argument of 4:

     In the recursive call, the evaluation of

          (triangle-recursively (1- 4))

     will return the value of evaluating

          (triangle-recursively 3)

     which is 6 and this value will be added to 4 by the addition in
     the third line.

The value returned by the function as a whole will be 10.

Each time `triangle-recursively' is evaluated, it evaluates a version
of itself--a different instance of itself--with a smaller argument,
until the argument is small enough so that it does not evaluate
itself.

Note that this particular design for a recursive function requires
that operations be deferred.

Before `(triangle-recursively 7)' can calculate its answer, it must
call `(triangle-recursively 6)'; and before `(triangle-recursively
6)' can calculate its answer, it must call `(triangle-recursively
5)'; and so on.  That is to say, the calculation that
`(triangle-recursively 7)' makes must be deferred until
`(triangle-recursively 6)' makes its calculation; and
`(triangle-recursively 6)' must defer until `(triangle-recursively
5)' completes; and so on.

If each of these instances of `triangle-recursively' are thought of
as different robots, the first robot must wait for the second to
complete its job, which must wait until the third completes, and so
on.

There is a way around this kind of waiting, which we will discuss in
*Note Recursion without Deferments: No Deferment.

Recursion Example Using `cond'
------------------------------

The version of `triangle-recursively' described earlier is written
with the `if' special form.  It can also be written using another
special form called `cond'.  The name of the special form `cond' is
an abbreviation of the word `conditional'.

Although the `cond' special form is not used as often in the Emacs
Lisp sources as `if', it is used often enough to justify explaining
it.

The template for a `cond' expression looks like this:

     (cond
      BODY...)

where the BODY is a series of lists.

Written out more fully, the template looks like this:

     (cond
      (FIRST-TRUE-OR-FALSE-TEST FIRST-CONSEQUENT)
      (SECOND-TRUE-OR-FALSE-TEST SECOND-CONSEQUENT)
      (THIRD-TRUE-OR-FALSE-TEST THIRD-CONSEQUENT)
       ...)

When the Lisp interpreter evaluates the `cond' expression, it
evaluates the first element (the CAR or true-or-false-test) of the
first expression in a series of expressions within the body of the
`cond'.

If the true-or-false-test returns `nil' the rest of that expression,
the consequent, is skipped and  the true-or-false-test of the next
expression is evaluated.  When an expression is found whose
true-or-false-test returns a value that is not `nil', the consequent
of that expression is evaluated.  The consequent can be one or more
expressions.  If the consequent consists of more than one expression,
the expressions are evaluated in sequence and the value of the last
one is returned.  If the expression does not have a consequent, the
value of the true-or-false-test is returned.

If none of the true-or-false-tests test true, the `cond' expression
returns `nil'.

Written using `cond', the `triangle' function looks like this:

     (defun triangle-using-cond (number)
       (cond ((<= number 0) 0)
             ((= number 1) 1)
             ((> number 1)
              (+ number (triangle-using-cond (1- number))))))

In this example, the `cond' returns 0 if the number is less than or
equal to 0, it returns 1 if the number is 1 and it evaluates `(+
number (triangle-using-cond (1- number)))' if the number is greater
than 1.

Recursive Patterns
------------------

Here are three common recursive patterns.  Each involves a list.
Recursion does not need to involve lists, but Lisp is designed for
lists and this provides a sense of its primal capabilities.

Recursive Pattern: _every_
..........................

In the `every' recursive pattern, an action is performed on every
element of a list.

The basic pattern is:

   * If a list be empty, return `nil'.

   * Else, act on the beginning of the list (the CAR of the list)
        -     through a recursive call by the function on the rest
          (the     CDR) of the list,

        -     and, optionally, combine the acted-on element, using
          `cons',     with the results of acting on the rest.

Here is example:

     (defun square-each (numbers-list)
       "Square each of a NUMBERS LIST, recursively."
       (if (not numbers-list)                ; do-again-test
           nil
         (cons
          (* (car numbers-list) (car numbers-list))
          (square-each (cdr numbers-list))))) ; next-step-expression
     
     (square-each '(1 2 3))
         => (1 4 9)

If `numbers-list' is empty, do nothing.  But if it has content,
construct a list combining the square of the first number in the list
with the result of the recursive call.

(The example follows the pattern exactly: `nil' is returned if the
numbers' list is empty.  In practice, you would write the conditional
so it carries out the action when the numbers' list is not empty.)

The `print-elements-recursively' function (*note Recursion with a
List: Recursion with list.) is another example of an `every' pattern,
except in this case, rather than bring the results together using
`cons', we print each element of output.

The `print-elements-recursively' function looks like this:

     (setq animals '(gazelle giraffe lion tiger))
     
     (defun print-elements-recursively (list)
       "Print each element of LIST on a line of its own.
     Uses recursion."
       (if list                              ; do-again-test
           (progn
             (print (car list))              ; body
             (print-elements-recursively     ; recursive call
              (cdr list)))))                 ; next-step-expression
     
     (print-elements-recursively animals)

The pattern for `print-elements-recursively' is:

   * If the list be empty, do nothing.

   * But if the list has at least one element,
        -     act on the beginning of the list (the CAR of the list),

        -     and make a recursive call on the rest (the CDR) of the
          list.

Recursive Pattern: _accumulate_
...............................

Another recursive pattern is called the `accumulate' pattern.  In the
`accumulate' recursive pattern, an action is performed on every
element of a list and the result of that action is accumulated with
the results of performing the action on the other elements.

This is very like the `every' pattern using `cons', except that
`cons' is not used, but some other combiner.

The pattern is:

   * If a list be empty, return zero or some other constant.

   * Else, act on the beginning of the list (the CAR of the list),
        -     and combine that acted-on element, using `+' or
          some other combining function, with

        -     a recursive call by the function on the rest (the CDR)
          of the list.

Here is an example:

     (defun add-elements (numbers-list)
       "Add the elements of NUMBERS-LIST together."
       (if (not numbers-list)
           0
         (+ (car numbers-list) (add-elements (cdr numbers-list)))))
     
     (add-elements '(1 2 3 4))
         => 10

*Note Making a List of Files: Files List, for an example of the
accumulate pattern.

Recursive Pattern: _keep_
.........................

A third recursive pattern is called the `keep' pattern.  In the
`keep' recursive pattern, each element of a list is tested; the
element is acted on and the results are kept only if the element
meets a criterion.

Again, this is very like the `every' pattern, except the element is
skipped unless it meets a criterion.

The pattern has three parts:

   * If a list be empty, return `nil'.

   * Else, if the beginning of the list (the CAR of the list) passes
           a test
        -     act on that element and combine it, using `cons' with

        -     a recursive call by the function on the rest (the CDR)
          of the list.

   * Otherwise, if the beginning of the list (the CAR of the list)
     fails the test
        -     skip on that element,

        -     and, recursively call the function on the rest (the
          CDR) of the list.

Here is an example that uses `cond':

     (defun keep-three-letter-words (word-list)
       "Keep three letter words in WORD-LIST."
       (cond
        ;; First do-again-test: stop-condition
        ((not word-list) nil)
     
        ;; Second do-again-test: when to act
        ((eq 3 (length (symbol-name (car word-list))))
         ;; combine acted-on element with recursive call on shorter list
         (cons (car word-list) (keep-three-letter-words (cdr word-list))))
     
        ;; Third do-again-test: when to skip element;
        ;;   recursively call shorter list with next-step expression
        (t  (keep-three-letter-words (cdr word-list)))))
     
     (keep-three-letter-words '(one two three four five six))
         => (one two six)

It goes without saying that you need not use `nil' as the test for
when to stop; and you can, of course, combine these patterns.

Recursion without Deferments
----------------------------

Let's consider again what happens with the `triangle-recursively'
function.  We will find that the intermediate calculations are
deferred until all can be done.

Here is the function definition:

     (defun triangle-recursively (number)
       "Return the sum of the numbers 1 through NUMBER inclusive.
     Uses recursion."
       (if (= number 1)                    ; do-again-test
           1                               ; then-part
         (+ number                         ; else-part
            (triangle-recursively          ; recursive call
             (1- number)))))               ; next-step-expression

What happens when we call this function with a argument of 7?

The first instance of the `triangle-recursively' function adds the
number 7 to the value returned by a second instance of
`triangle-recursively', an instance that has been passed an argument
of 6.  That is to say, the first calculation is:

     (+ 7 (triangle-recursively 6))

The first instance of `triangle-recursively'--you may want to think
of it as a little robot--cannot complete its job.  It must hand off
the calculation for `(triangle-recursively 6)' to a second instance
of the program, to a second robot.  This second individual is
completely different from the first one; it is, in the jargon, a
`different instantiation'.  Or, put another way, it is a different
robot.  It is the same model as the first; it calculates triangle
numbers recursively; but it has a different serial number.

And what does `(triangle-recursively 6)' return?  It returns the
number 6 added to the value returned by evaluating
`triangle-recursively' with an argument of 5.  Using the robot
metaphor, it asks yet another robot to help it.

Now the total is:

     (+ 7 6 (triangle-recursively 5))

And what happens next?

     (+ 7 6 5 (triangle-recursively 4))

Each time `triangle-recursively' is called, except for the last time,
it creates another instance of the program--another robot--and asks
it to make a calculation.

Eventually, the full addition is set up and performed:

     (+ 7 6 5 4 3 2 1)

This design for the function defers the calculation of the first step
until the second can be done, and defers that until the third can be
done, and so on.  Each deferment means the computer must remember what
is being waited on.  This is not a problem when there are only a few
steps, as in this example.  But it can be a problem when there are
more steps.

No Deferment Solution
---------------------

The solution to the problem of deferred operations is to write in a
manner that does not defer operations(1).  This requires writing to a
different pattern, often one that involves writing two function
definitions, an `initialization' function and a `helper' function.

The `initialization' function sets up the job; the `helper' function
does the work.

Here are the two function definitions for adding up numbers.  They are
so simple, I find them hard to understand.

     (defun triangle-initialization (number)
       "Return the sum of the numbers 1 through NUMBER inclusive.
     This is the `initialization' component of a two function
     duo that uses recursion."
       (triangle-recursive-helper 0 0 number))

     (defun triangle-recursive-helper (sum counter number)
       "Return SUM, using COUNTER, through NUMBER inclusive.
     This is the `helper' component of a two function duo
     that uses recursion."
       (if (> counter number)
           sum
         (triangle-recursive-helper (+ sum counter)  ; sum
                                    (1+ counter)     ; counter
                                    number)))        ; number

Install both function definitions by evaluating them, then call
`triangle-initialization' with 2 rows:

     (triangle-initialization 2)
         => 3

The `initialization' function calls the first instance of the `helper'
function with three arguments: zero, zero, and a number which is the
number of rows in the triangle.

The first two arguments passed to the `helper' function are
initialization values.  These values are changed when
`triangle-recursive-helper' invokes new instances.(2)

Let's see what happens when we have a triangle that has one row.
(This triangle will have one pebble in it!)

`triangle-initialization' will call its helper with the arguments
`0 0 1'.  That function will run the conditional test whether `(>
counter number)':

     (> 0 1)

and find that the result is false, so it will invoke the then-part of
the `if' clause:

         (triangle-recursive-helper
          (+ sum counter)  ; sum plus counter => sum
          (1+ counter)     ; increment counter => counter
          number)          ; number stays the same

which will first compute:

     (triangle-recursive-helper (+ 0 0)  ; sum
                                (1+ 0)   ; counter
                                1)       ; number
which is:

     (triangle-recursive-helper 0 1 1)

Again, `(> counter number)' will be false, so again, the Lisp
interpreter will evaluate `triangle-recursive-helper', creating a new
instance with new arguments.

This new instance will be;

         (triangle-recursive-helper
          (+ sum counter)  ; sum plus counter => sum
          (1+ counter)     ; increment counter => counter
          number)          ; number stays the same
     
which is:

     (triangle-recursive-helper 1 2 1)

In this case, the `(> counter number)' test will be true!  So the
instance will return the value of the sum, which will be 1, as
expected.

Now, let's pass `triangle-initialization' an argument of 2, to find
out how many pebbles there are in a triangle with two rows.

That function calls `(triangle-recursive-helper 0 0 2)'.

In stages, the instances called will be:

                               sum counter number
     (triangle-recursive-helper 0    1       2)
     
     (triangle-recursive-helper 1    2       2)
     
     (triangle-recursive-helper 3    3       2)

When the last instance is called, the `(> counter number)' test will
be true, so the instance will return the value of `sum', which will
be 3.

This kind of pattern helps when you are writing functions that can use
many resources in a computer.

---------- Footnotes ----------

(1) The phrase "tail recursive" is used to describe such a process,
one that uses `constant space'.

(2) The jargon is mildly confusing:  `triangle-recursive-helper' uses
a process that is iterative in a procedure that is recursive.  The
process is called iterative because the computer need only record the
three values, `sum', `counter', and `number'; the procedure is
recursive because the function `calls itself'.  On the other hand,
both the process and the procedure used by `triangle-recursively' are
called recursive.  The word `recursive' has different meanings in the
two contexts.

Looping Exercise
================

   * Write a function similar to `triangle' in which each row has a
     value which is the square of the row number.  Use a `while' loop.

   * Write a function similar to `triangle' that multiplies instead of
     adds the values.

   * Rewrite these two functions recursively.  Rewrite these functions
     using `cond'.

   * Write a function for Texinfo mode that creates an index entry at
     the beginning of a paragraph for every `@dfn' within the
     paragraph.  (In a Texinfo file, `@dfn' marks a definition.  For
     more information, see *Note Indicating Definitions:
     (texinfo)Indicating.)

Regular Expression Searches
***************************

Regular expression searches are used extensively in GNU Emacs.  The
two functions, `forward-sentence' and `forward-paragraph', illustrate
these searches well.  They use regular expressions to find where to
move point.  The phrase `regular expression' is often written as
`regexp'.

Regular expression searches are described in *Note Regular Expression
Search: (emacs)Regexp Search, as well as in *Note Regular
Expressions: (elisp)Regular Expressions.  In writing this chapter, I
am presuming that you have at least a mild acquaintance with them.
The major point to remember is that regular expressions permit you to
search for patterns as well as for literal strings of characters.
For example, the code in `forward-sentence' searches for the pattern
of possible characters that could mark the end of a sentence, and
moves point to that spot.

Before looking at the code for the `forward-sentence' function, it is
worth considering what the pattern that marks the end of a sentence
must be.  The pattern is discussed in the next section; following that
is a description of the regular expression search function,
`re-search-forward'.  The `forward-sentence' function is described in
the section following.  Finally, the `forward-paragraph' function is
described in the last section of this chapter.  `forward-paragraph'
is a complex function that introduces several new features.

The Regular Expression for `sentence-end'
=========================================

The symbol `sentence-end' is bound to the pattern that marks the end
of a sentence.  What should this regular expression be?

Clearly, a sentence may be ended by a period, a question mark, or an
exclamation mark.  Indeed, only clauses that end with one of those
three characters should be considered the end of a sentence.  This
means that the pattern should include the character set:

     [.?!]

However, we do not want `forward-sentence' merely to jump to a
period, a question mark, or an exclamation mark, because such a
character might be used in the middle of a sentence.  A period, for
example, is used after abbreviations.  So other information is needed.

According to convention, you type two spaces after every sentence, but
only one space after a period, a question mark, or an exclamation
mark in the body of a sentence.  So a period, a question mark, or an
exclamation mark followed by two spaces is a good indicator of an end
of sentence.  However, in a file, the two spaces may instead be a tab
or the end of a line.  This means that the regular expression should
include these three items as alternatives.

This group of alternatives will look like this:

     \\($\\| \\|  \\)
            ^   ^^
           TAB  SPC

Here, `$' indicates the end of the line, and I have pointed out where
the tab and two spaces are inserted in the expression.  Both are
inserted by putting the actual characters into the expression.

Two backslashes, `\\', are required before the parentheses and
vertical bars: the first backslash quotes the following backslash in
Emacs; and the second indicates that the following character, the
parenthesis or the vertical bar, is special.

Also, a sentence may be followed by one or more carriage returns, like
this:

     [
     ]*

Like tabs and spaces, a carriage return is inserted into a regular
expression by inserting it literally.  The asterisk indicates that the
<RET> is repeated zero or more times.

But a sentence end does not consist only of a period, a question mark
or an exclamation mark followed by appropriate space: a closing
quotation mark or a closing brace of some kind may precede the space.
Indeed more than one such mark or brace may precede the space.
These require a expression that looks like this:

     []\"')}]*

In this expression, the first `]' is the first character in the
expression; the second character is `"', which is preceded by a `\'
to tell Emacs the `"' is _not_ special.  The last three characters
are `'', `)', and `}'.

All this suggests what the regular expression pattern for matching the
end of a sentence should be; and, indeed, if we evaluate
`sentence-end' we find that it returns the following value:

     sentence-end
          => "[.?!][]\"')}]*\\($\\|     \\|  \\)[
     ]*"

The `re-search-forward' Function
================================

The `re-search-forward' function is very like the `search-forward'
function.  (*Note The `search-forward' Function: search-forward.)

`re-search-forward' searches for a regular expression.  If the search
is successful, it leaves point immediately after the last character
in the target.  If the search is backwards, it leaves point just
before the first character in the target.  You may tell
`re-search-forward' to return `t' for true.  (Moving point is
therefore a `side effect'.)

Like `search-forward', the `re-search-forward' function takes four
arguments:

  1. The first argument is the regular expression that the function
     searches for.  The regular expression will be a string between
     quotations marks.

  2. The optional second argument limits how far the function will
     search; it is a bound, which is specified as a position in the
     buffer.

  3. The optional third argument specifies how the function responds
     to failure: `nil' as the third argument causes the function to
     signal an error (and print a message) when the search fails; any
     other value causes it to return `nil' if the search fails and `t'
     if the search succeeds.

  4. The optional fourth argument is the repeat count.  A negative
     repeat count causes `re-search-forward' to search backwards.

The template for `re-search-forward' looks like this:

     (re-search-forward "REGULAR-EXPRESSION"
                     LIMIT-OF-SEARCH
                     WHAT-TO-DO-IF-SEARCH-FAILS
                     REPEAT-COUNT)

The second, third, and fourth arguments are optional.  However, if you
want to pass a value to either or both of the last two arguments, you
must also pass a value to all the preceding arguments.  Otherwise, the
Lisp interpreter will mistake which argument you are passing the value
to.

In the `forward-sentence' function, the regular expression will be
the value of the variable `sentence-end', namely:

     "[.?!][]\"')}]*\\($\\|  \\|  \\)[
     ]*"

The limit of the search will be the end of the paragraph (since a
sentence cannot go beyond a paragraph).  If the search fails, the
function will return `nil'; and the repeat count will be provided by
the argument to the `forward-sentence' function.

`forward-sentence'
==================

The command to move the cursor forward a sentence is a straightforward
illustration of how to use regular expression searches in Emacs Lisp.
Indeed, the function looks longer and more complicated than it is;
this is because the function is designed to go backwards as well as
forwards; and, optionally, over more than one sentence.  The function
is usually bound to the key command `M-e'.

Complete `forward-sentence' function definition
-----------------------------------------------

Here is the code for `forward-sentence':

     (defun forward-sentence (&optional arg)
       "Move forward to next sentence-end.  With argument, repeat.
     With negative argument, move backward repeatedly to sentence-beginning.
     Sentence ends are identified by the value of sentence-end
     treated as a regular expression.  Also, every paragraph boundary
     terminates sentences as well."
       (interactive "p")
       (or arg (setq arg 1))
       (while (< arg 0)
         (let ((par-beg
                (save-excursion (start-of-paragraph-text) (point))))
           (if (re-search-backward
                (concat sentence-end "[^ \t\n]") par-beg t)
               (goto-char (1- (match-end 0)))
             (goto-char par-beg)))
         (setq arg (1+ arg)))
       (while (> arg 0)
         (let ((par-end
                (save-excursion (end-of-paragraph-text) (point))))
           (if (re-search-forward sentence-end par-end t)
               (skip-chars-backward " \t\n")
             (goto-char par-end)))
         (setq arg (1- arg))))

The function looks long at first sight and it is best to look at its
skeleton first, and then its muscle.  The way to see the skeleton is
to look at the expressions that start in the left-most columns:

     (defun forward-sentence (&optional arg)
       "DOCUMENTATION..."
       (interactive "p")
       (or arg (setq arg 1))
       (while (< arg 0)
         BODY-OF-WHILE-LOOP
       (while (> arg 0)
         BODY-OF-WHILE-LOOP

This looks much simpler!  The function definition consists of
documentation, an `interactive' expression, an `or' expression, and
`while' loops.

Let's look at each of these parts in turn.

We note that the documentation is thorough and understandable.

The function has an `interactive "p"' declaration.  This means that
the processed prefix argument, if any, is passed to the function as
its argument.  (This will be a number.)  If the function is not
passed an argument (it is optional) then the argument `arg' will be
bound to 1.  When `forward-sentence' is called non-interactively
without an argument, `arg' is bound to `nil'.

The `or' expression handles the prefix argument.  What it does is
either leave the value of `arg' as it is, but only if `arg' is bound
to a value; or it sets the value of `arg' to 1, in the case when
`arg' is bound to `nil'.

The `while' loops
-----------------

Two `while' loops follow the `or' expression.  The first `while' has
a true-or-false-test that tests true if the prefix argument for
`forward-sentence' is a negative number.  This is for going
backwards.  The body of this loop is similar to the body of the
second `while' clause, but it is not exactly the same.  We will skip
this `while' loop and concentrate on the second `while' loop.

The second `while' loop is for moving point forward.  Its skeleton
looks like this:

     (while (> arg 0)            ; true-or-false-test
       (let VARLIST
         (if (TRUE-OR-FALSE-TEST)
             THEN-PART
           ELSE-PART
       (setq arg (1- arg))))     ; `while' loop decrementer

The `while' loop is of the decrementing kind.  (*Note A Loop with a
Decrementing Counter: Decrementing Loop.)  It has a
true-or-false-test that tests true so long as the counter (in this
case, the variable `arg') is greater than zero; and it has a
decrementer that subtracts 1 from the value of the counter every time
the loop repeats.

If no prefix argument is given to `forward-sentence', which is the
most common way the command is used, this `while' loop will run once,
since the value of `arg' will be 1.

The body of the `while' loop consists of a `let' expression, which
creates and binds a local variable, and has, as its body, an `if'
expression.

The body of the `while' loop looks like this:

     (let ((par-end
            (save-excursion (end-of-paragraph-text) (point))))
       (if (re-search-forward sentence-end par-end t)
           (skip-chars-backward " \t\n")
         (goto-char par-end)))

The `let' expression creates and binds the local variable `par-end'.
As we shall see, this local variable is designed to provide a bound
or limit to the regular expression search.  If the search fails to
find a proper sentence ending in the paragraph, it will stop on
reaching the end of the paragraph.

But first, let us examine how `par-end' is bound to the value of the
end of the paragraph.  What happens is that the `let' sets the value
of `par-end' to the value returned when the Lisp interpreter
evaluates the expression

     (save-excursion (end-of-paragraph-text) (point))

In this expression, `(end-of-paragraph-text)' moves point to the end
of the paragraph, `(point)' returns the value of point, and then
`save-excursion' restores point to its original position.  Thus, the
`let' binds `par-end' to the value returned by the `save-excursion'
expression, which is the position of the end of the paragraph.  (The
`(end-of-paragraph-text)' function uses `forward-paragraph', which we
will discuss shortly.)

Emacs next evaluates the body of the `let', which is an `if'
expression that looks like this:

     (if (re-search-forward sentence-end par-end t) ; if-part
         (skip-chars-backward " \t\n")              ; then-part
       (goto-char par-end)))                        ; else-part

The `if' tests whether its first argument is true and if so,
evaluates its then-part; otherwise, the Emacs Lisp interpreter
evaluates the else-part.  The true-or-false-test of the `if'
expression is the regular expression search.

It may seem odd to have what looks like the `real work' of the
`forward-sentence' function buried here, but this is a common way
this kind of operation is carried out in Lisp.

The regular expression search
-----------------------------

The `re-search-forward' function searches for the end of the
sentence, that is, for the pattern defined by the `sentence-end'
regular expression.  If the pattern is found--if the end of the
sentence is found--then the `re-search-forward' function does two
things:

  1. The `re-search-forward' function carries out a side effect, which
     is to move point to the end of the occurrence found.

  2. The `re-search-forward' function returns a value of true.  This
     is the value received by the `if', and means that the search was
     successful.

The side effect, the movement of point, is completed before the `if'
function is handed the value returned by the successful conclusion of
the search.

When the `if' function receives the value of true from a successful
call to `re-search-forward', the `if' evaluates the then-part, which
is the expression `(skip-chars-backward " \t\n")'.  This expression
moves backwards over any blank spaces, tabs or carriage returns until
a printed character is found and then leaves point after the
character.  Since point has already been moved to the end of the
pattern that marks the end of the sentence, this action leaves point
right after the closing printed character of the sentence, which is
usually a period.

On the other hand, if the `re-search-forward' function fails to find
a pattern marking the end of the sentence, the function returns
false.  The false then causes the `if' to evaluate its third
argument, which is `(goto-char par-end)':  it moves point to the end
of the paragraph.

Regular expression searches are exceptionally useful and the pattern
illustrated by `re-search-forward', in which the search is the test
of an `if' expression, is handy.  You will see or write code
incorporating this pattern often.

`forward-paragraph': a Goldmine of Functions
============================================

The `forward-paragraph' function moves point forward to the end of
the paragraph.  It is usually bound to `M-}' and makes use of a
number of functions that are important in themselves, including
`let*', `match-beginning', and `looking-at'.

The function definition for `forward-paragraph' is considerably
longer than the function definition for `forward-sentence' because it
works with a paragraph, each line of which may begin with a fill
prefix.

A fill prefix consists of a string of characters that are repeated at
the beginning of each line.  For example, in Lisp code, it is a
convention to start each line of a paragraph-long comment with `;;;
'.  In Text mode, four blank spaces make up another common fill
prefix, creating an indented paragraph.  (*Note Fill Prefix:
(emacs)Fill Prefix, for more information about fill prefixes.)

The existence of a fill prefix means that in addition to being able to
find the end of a paragraph whose lines begin on the left-most
column, the `forward-paragraph' function must be able to find the end
of a paragraph when all or many of the lines in the buffer begin with
the fill prefix.

Moreover, it is sometimes practical to ignore a fill prefix that
exists, especially when blank lines separate paragraphs.  This is an
added complication.

Shortened `forward-paragraph' function definition
-------------------------------------------------

Rather than print all of the `forward-paragraph' function, we will
only print parts of it.  Read without preparation, the function can
be daunting!

In outline, the function looks like this:

     (defun forward-paragraph (&optional arg)
       "DOCUMENTATION..."
       (interactive "p")
       (or arg (setq arg 1))
       (let*
           VARLIST
         (while (< arg 0)        ; backward-moving-code
           ...
           (setq arg (1+ arg)))
         (while (> arg 0)        ; forward-moving-code
           ...
           (setq arg (1- arg)))))

The first parts of the function are routine: the function's argument
list consists of one optional argument.  Documentation follows.

The lower case `p' in the `interactive' declaration means that the
processed prefix argument, if any, is passed to the function.  This
will be a number, and is the repeat count of how many paragraphs
point will move.  The `or' expression in the next line handles the
common case when no argument is passed to the function, which occurs
if the function is called from other code rather than interactively.
This case was described earlier.  (*Note The `forward-sentence'
function: forward-sentence.)  Now we reach the end of the familiar
part of this function.

The `let*' expression
---------------------

The next line of the `forward-paragraph' function begins a `let*'
expression.  This is a different kind of expression than we have seen
so far.  The symbol is `let*' not `let'.

The `let*' special form is like `let' except that Emacs sets each
variable in sequence, one after another, and variables in the latter
part of the varlist can make use of the values to which Emacs set
variables in the earlier part of the varlist.

In the `let*' expression in this function, Emacs binds two variables:
`fill-prefix-regexp' and `paragraph-separate'.  The value to which
`paragraph-separate' is bound depends on the value of
`fill-prefix-regexp'.

Let's look at each in turn.  The symbol `fill-prefix-regexp' is set
to the value returned by evaluating the following list:

     (and fill-prefix
          (not (equal fill-prefix ""))
          (not paragraph-ignore-fill-prefix)
          (regexp-quote fill-prefix))

This is an expression whose first element is the `and' special form.

As we learned earlier (*note The `kill-new' function: kill-new
function.), the `and' special form evaluates each of its arguments
until one of the arguments returns a value of `nil', in which case
the `and' expression returns `nil'; however, if none of the arguments
returns a value of `nil', the value resulting from evaluating the
last argument is returned.  (Since such a value is not `nil', it is
considered true in Lisp.)  In other words, an `and' expression
returns a true value only if all its arguments are true.

In this case, the variable `fill-prefix-regexp' is bound to a
non-`nil' value only if the following four expressions produce a true
(i.e., a non-`nil') value when they are evaluated; otherwise,
`fill-prefix-regexp' is bound to `nil'.

`fill-prefix'
     When this variable is evaluated, the value of the fill prefix,
     if any, is returned.  If there is no fill prefix, this variable
     returns `nil'.

`(not (equal fill-prefix "")'
     This expression checks whether an existing fill prefix is an
     empty string, that is, a string with no characters in it.  An
     empty string is not a useful fill prefix.

`(not paragraph-ignore-fill-prefix)'
     This expression returns `nil' if the variable
     `paragraph-ignore-fill-prefix' has been turned on by being set
     to a true value such as `t'.

`(regexp-quote fill-prefix)'
     This is the last argument to the `and' special form.  If all the
     arguments to the `and' are true, the value resulting from
     evaluating this expression will be returned by the `and'
     expression and bound to the variable `fill-prefix-regexp',

The result of evaluating this `and' expression successfully is that
`fill-prefix-regexp' will be bound to the value of `fill-prefix' as
modified by the `regexp-quote' function.  What `regexp-quote' does is
read a string and return a regular expression that will exactly match
the string and match nothing else.  This means that
`fill-prefix-regexp' will be set to a value that will exactly match
the fill prefix if the fill prefix exists.  Otherwise, the variable
will be set to `nil'.

The second local variable in the `let*' expression is
`paragraph-separate'.  It is bound to the value returned by
evaluating the expression:

     (if fill-prefix-regexp
         (concat paragraph-separate
                 "\\|^" fill-prefix-regexp "[ \t]*$")
       paragraph-separate)))

This expression shows why `let*' rather than `let' was used.  The
true-or-false-test for the `if' depends on whether the variable
`fill-prefix-regexp' evaluates to `nil' or some other value.

If `fill-prefix-regexp' does not have a value, Emacs evaluates the
else-part of the `if' expression and binds `paragraph-separate' to
its local value.  (`paragraph-separate' is a regular expression that
matches what separates paragraphs.)

But if `fill-prefix-regexp' does have a value, Emacs evaluates the
then-part of the `if' expression and binds `paragraph-separate' to a
regular expression that includes the `fill-prefix-regexp' as part of
the pattern.

Specifically, `paragraph-separate' is set to the original value of
the paragraph separate regular expression concatenated with an
alternative expression that consists of the `fill-prefix-regexp'
followed by a blank line.  The `^' indicates that the
`fill-prefix-regexp' must begin a line, and the optional whitespace
to the end of the line is defined by `"[ \t]*$"'.)  The `\\|' defines
this portion of the regexp as an alternative to `paragraph-separate'.

Now we get into the body of the `let*'.  The first part of the body
of the `let*' deals with the case when the function is given a
negative argument and is therefore moving backwards.  We will skip
this section.

The forward motion `while' loop
-------------------------------

The second part of the body of the `let*' deals with forward motion.
It is a `while' loop that repeats itself so long as the value of
`arg' is greater than zero.  In the most common use of the function,
the value of the argument is 1, so the body of the `while' loop is
evaluated exactly once, and the cursor moves forward one paragraph.

This part handles three situations: when point is between paragraphs,
when point is within a paragraph and there is a fill prefix, and when
point is within a paragraph and there is no fill prefix.

The `while' loop looks like this:

     (while (> arg 0)
       (beginning-of-line)
     
       ;; between paragraphs
       (while (prog1 (and (not (eobp))
                          (looking-at paragraph-separate))
                (forward-line 1)))
     
       ;; within paragraphs, with a fill prefix
       (if fill-prefix-regexp
           ;; There is a fill prefix; it overrides paragraph-start.
           (while (and (not (eobp))
                       (not (looking-at paragraph-separate))
                       (looking-at fill-prefix-regexp))
             (forward-line 1))
     
         ;; within paragraphs, no fill prefix
         (if (re-search-forward paragraph-start nil t)
             (goto-char (match-beginning 0))
           (goto-char (point-max))))
     
       (setq arg (1- arg)))

We can see immediately that this is a decrementing counter `while'
loop, using the expression `(setq arg (1- arg))' as the decrementer.

The body of the loop consists of three expressions:

     ;; between paragraphs
     (beginning-of-line)
     (while
         BODY-OF-WHILE)
     
     ;; within paragraphs, with fill prefix
     (if TRUE-OR-FALSE-TEST
         THEN-PART
     
     ;; within paragraphs, no fill prefix
       ELSE-PART

When the Emacs Lisp interpreter evaluates the body of the `while'
loop, the first thing it does is evaluate the `(beginning-of-line)'
expression and move point to the beginning of the line.  Then there
is an inner `while' loop.  This `while' loop is designed to move the
cursor out of the blank space between paragraphs, if it should happen
to be there.  Finally, there is an `if' expression that actually
moves point to the end of the paragraph.

Between paragraphs
------------------

First, let us look at the inner `while' loop.  This loop handles the
case when point is between paragraphs; it uses three functions that
are new to us: `prog1', `eobp' and `looking-at'.

   * `prog1' is similar to the `progn' special form, except that
     `prog1' evaluates its arguments in sequence and then returns the
     value of its first argument as the value of the whole
     expression.  (`progn' returns the value of its last argument as
     the value of the expression.) The second and subsequent
     arguments to `prog1' are evaluated only for their side effects.

   * `eobp' is an abbreviation of `End Of Buffer P' and is a function
     that returns true if point is at the end of the buffer.

   * `looking-at' is a function that returns true if the text
     following point matches the regular expression passed
     `looking-at' as its argument.

The `while' loop we are studying looks like this:

     (while (prog1 (and (not (eobp))
                        (looking-at paragraph-separate))
                   (forward-line 1)))

This is a `while' loop with no body!  The true-or-false-test of the
loop is the expression:

     (prog1 (and (not (eobp))
                 (looking-at paragraph-separate))
            (forward-line 1))

The first argument to the `prog1' is the `and' expression.  It has
within in it a test of whether point is at the end of the buffer and
also a test of whether the pattern following point matches the regular
expression for separating paragraphs.

If the cursor is not at the end of the buffer and if the characters
following the cursor mark the separation between two paragraphs, then
the `and' expression is true.  After evaluating the `and' expression,
the Lisp interpreter evaluates the second argument to `prog1', which
is `forward-line'.  This moves point forward one line.  The value
returned by the `prog1' however, is the value of its first argument,
so the `while' loop continues so long as point is not at the end of
the buffer and is between paragraphs.  When, finally, point is moved
to a paragraph, the `and' expression tests false.  Note however, that
the `forward-line' command is carried out anyhow.  This means that
when point is moved from between paragraphs to a paragraph, it is left
at the beginning of the second line of the paragraph.

Within paragraphs
-----------------

The next expression in the outer `while' loop is an `if' expression.
The Lisp interpreter evaluates the then-part of the `if' when the
`fill-prefix-regexp' variable has a value other than `nil', and it
evaluates the else-part when the value of `if fill-prefix-regexp' is
`nil', that is, when there is no fill prefix.

No fill prefix
--------------

It is simplest to look at the code for the case when there is no fill
prefix first.  This code consists of yet another inner `if'
expression, and reads as follows:

     (if (re-search-forward paragraph-start nil t)
         (goto-char (match-beginning 0))
       (goto-char (point-max)))

This expression actually does the work that most people think of as
the primary purpose of the `forward-paragraph' command: it causes a
regular expression search to occur that searches forward to the start
of the next paragraph and if it is found, moves point there; but if
the start of another paragraph if not found, it moves point to the
end of the accessible region of the buffer.

The only unfamiliar part of this is the use of `match-beginning'.
This is another function that is new to us.  The `match-beginning'
function returns a number specifying the location of the start of the
text that was matched by the last regular expression search.

The `match-beginning' function is used here because of a
characteristic of a forward search: a successful forward search,
regardless of whether it is a plain search or a regular expression
search, will move point to the end of the text that is found.  In this
case, a successful search will move point to the end of the pattern
for `paragraph-start', which will be the beginning of the next
paragraph rather than the end of the current one.

However, we want to put point at the end of the current paragraph,
not at the beginning of the next one.  The two positions may be
different, because there may be several blank lines between
paragraphs.

When given an argument of 0, `match-beginning' returns the position
that is the start of the text that the most recent regular expression
search matched.  In this case, the most recent regular expression
search is the one looking for `paragraph-start', so `match-beginning'
returns the beginning position of the pattern, rather than the end of
the pattern.  The beginning position is the end of the paragraph.

(Incidentally, when passed a positive number as an argument, the
`match-beginning' function will place point at that parenthesized
expression in the last regular expression.  It is a useful function.)

With a fill prefix
------------------

The inner `if' expression just discussed is the else-part of an
enclosing `if' expression which tests whether there is a fill prefix.
If there is a fill prefix, the then-part of this `if' is evaluated.
It looks like this:

     (while (and (not (eobp))
                 (not (looking-at paragraph-separate))
                 (looking-at fill-prefix-regexp))
       (forward-line 1))

What this expression does is move point forward line by line so long
as three conditions are true:

  1. Point is not at the end of the buffer.

  2. The text following point does not separate paragraphs.

  3. The pattern following point is the fill prefix regular
     expression.

The last condition may be puzzling, until you remember that point was
moved to the beginning of the line early in the `forward-paragraph'
function.  This means that if the text has a fill prefix, the
`looking-at' function will see it.

Summary
-------

In summary, when moving forward, the `forward-paragraph' function
does the following:

   * Move point to the beginning of the line.

   * Skip over lines between paragraphs.

   * Check whether there is a fill prefix, and if there is:

        -- Go forward line by line so long as the line is not a
          paragraph separating line.

   * But if there is no fill prefix,

        -- Search for the next paragraph start pattern.

        -- Go to the beginning of the paragraph start pattern, which
          will be the end of the previous paragraph.

        -- Or else go to the end of the accessible portion of the
          buffer.

For review, here is the code we have just been discussing, formatted
for clarity:

     (interactive "p")
     (or arg (setq arg 1))
     (let* (
            (fill-prefix-regexp
             (and fill-prefix (not (equal fill-prefix ""))
                  (not paragraph-ignore-fill-prefix)
                  (regexp-quote fill-prefix)))
     
            (paragraph-separate
             (if fill-prefix-regexp
                 (concat paragraph-separate
                         "\\|^"
                         fill-prefix-regexp
                         "[ \t]*$")
               paragraph-separate)))
     
       OMITTED-BACKWARD-MOVING-CODE ...
     
       (while (> arg 0)                ; forward-moving-code
         (beginning-of-line)
     
         (while (prog1 (and (not (eobp))
                            (looking-at paragraph-separate))
                  (forward-line 1)))
     
         (if fill-prefix-regexp
             (while (and (not (eobp))  ; then-part
                         (not (looking-at paragraph-separate))
                         (looking-at fill-prefix-regexp))
               (forward-line 1))
                                       ; else-part: the inner-if
           (if (re-search-forward paragraph-start nil t)
               (goto-char (match-beginning 0))
             (goto-char (point-max))))
     
         (setq arg (1- arg)))))        ; decrementer

The full definition for the `forward-paragraph' function not only
includes this code for going forwards, but also code for going
backwards.

If you are reading this inside of GNU Emacs and you want to see the
whole function, you can type `C-h f' (`describe-function') and the
name of the function.  This gives you the function documentation and
the name of the library containing the function's source.  Place
point over the name of the library and press the RET key; you will be
taken directly to the source.  (Be sure to install your sources!
Without them, you are like a person who tries to drive a car with his
eyes shut!)

Or - a good habit to get into - you can type `M-.' (`find-tag') and
the name of the function when prompted for it.  This will take you
directly to the source.  If the `find-tag' function first asks you
for the name of a `TAGS' table, give it the name of the `TAGS' file
such as `/usr/local/share/emacs/21.0.100/lisp/TAGS'.  (The exact path
to your `TAGS' file depends on how your copy of Emacs was installed.)

You can also create your own `TAGS' file for directories that lack
one.  *Note Create Your Own `TAGS' File: etags.

Create Your Own `TAGS' File
===========================

The `M-.' (`find-tag') command takes you directly to the source for a
function, variable, node, or other source.  The function depends on
tags tables to tell it where to go.

You often need to build and install tags tables yourself.  They are
not built automatically.  A tags table is called a `TAGS' file; the
name is in upper case letters.

You can create a `TAGS' file by calling the `etags' program that
comes as a part of the Emacs distribution.  Usually, `etags' is
compiled and installed when Emacs is built.  (`etags' is not an Emacs
Lisp function or a part of Emacs; it is a C program.)

To create a `TAGS' file, first switch to the directory in which you
want to create the file.  In Emacs you can do this with the `M-x cd'
command, or by visiting a file in the directory, or by listing the
directory with `C-x d' (`dired').  Then run the compile command, with
`etags *.el' as the command to execute

     M-x compile RET etags *.el RET

to create a `TAGS' file.

For example, if you have a large number of files in your `~/emacs'
directory, as I do--I have 137 `.el' files in it, of which I load
12--you can create a `TAGS' file for the Emacs Lisp files in that
directory.

The `etags' program takes all the usual shell `wildcards'.  For
example, if you have two directories for which you want a single
`TAGS file', type `etags *.el ../elisp/*.el', where `../elisp/' is
the second directory:

     M-x compile RET etags *.el ../elisp/*.el RET

Type

     M-x compile RET etags --help RET

to see a list of the options accepted by `etags' as well as a list of
supported languages.

The `etags' program handles more than 20 languages, including Emacs
Lisp, Common Lisp, Scheme, C, C++, Ada, Fortran, Java, LaTeX, Pascal,
Perl, Python, Texinfo, makefiles, and most assemblers.  The program
has no switches for specifying the language; it recognizes the
language in an input file according to its file name and contents.

`etags' is very helpful when you are writing code yourself and want
to refer back to functions you have already written.  Just run
`etags' again at intervals as you write new functions, so they become
part of the `TAGS' file.

If you think an appropriate `TAGS' file already exists for what you
want, but do not know where it is, you can use the `locate' program
to attempt to find it.

Type `M-x locate RET TAGS RET' and Emacs will list for you the full
path names of all your `TAGS' files.  On my system, this command
lists 34 `TAGS' files.  On the other hand, a `plain vanilla' system I
recently installed did not contain any `TAGS' files.

If the tags table you want has been created, you can use the `M-x
visit-tags-table' command to specify it.  Otherwise, you will need to
create the tag table yourself and then use `M-x visit-tags-table'.

Building Tags in the Emacs sources
..................................

The GNU Emacs sources come with a `Makefile' that contains a
sophisticated `etags' command that creates, collects, and merges tags
tables from all over the Emacs sources and puts the information into
one `TAGS' file in the `src/' directory below the top level of your
Emacs source directory.

To build this `TAGS' file, go to the top level of your Emacs source
directory and run the compile command `make tags':

     M-x compile RET make tags RET

(The `make tags' command works well with the GNU Emacs sources, as
well as with some other source packages.)

For more information, see *Note Tag Tables: (emacs)Tags.

Review
======

Here is a brief summary of some recently introduced functions.

`while'
     Repeatedly evaluate the body of the expression so long as the
     first element of the body tests true.  Then return `nil'.  (The
     expression is evaluated only for its side effects.)

     For example:

          (let ((foo 2))
            (while (> foo 0)
              (insert (format "foo is %d.\n" foo))
              (setq foo (1- foo))))
          
               =>      foo is 2.
                       foo is 1.
                       nil

     (The `insert' function inserts its arguments at point; the
     `format' function returns a string formatted from its arguments
     the way `message' formats its arguments; `\n' produces a new
     line.)

`re-search-forward'
     Search for a pattern, and if the pattern is found, move point to
     rest just after it.

     Takes four arguments, like `search-forward':

       1. A regular expression that specifies the pattern to search
          for.

       2. Optionally, the limit of the search.

       3. Optionally, what to do if the search fails, return `nil' or
          an error message.

       4. Optionally, how many times to repeat the search; if
          negative, the search goes backwards.

`let*'
     Bind some variables locally to particular values, and then
     evaluate the remaining arguments, returning the value of the
     last one.  While binding the local variables, use the local
     values of variables bound earlier, if any.

     For example:

          (let* ((foo 7)
                (bar (* 3 foo)))
            (message "`bar' is %d." bar))
               => `bar' is 21.

`match-beginning'
     Return the position of the start of the text found by the last
     regular expression search.

`looking-at'
     Return `t' for true if the text after point matches the argument,
     which should be a regular expression.

`eobp'
     Return `t' for true if point is at the end of the accessible part
     of a buffer.  The end of the accessible part is the end of the
     buffer if the buffer is not narrowed; it is the end of the
     narrowed part if the buffer is narrowed.

`prog1'
     Evaluate each argument in sequence and then return the value of
     the _first_.

     For example:

          (prog1 1 2 3 4)
               => 1

Exercises with `re-search-forward'
==================================

   * Write a function to search for a regular expression that matches
     two or more blank lines in sequence.

   * Write a function to search for duplicated words, such as `the
     the'.  *Note Syntax of Regular Expressions: (emacs)Regexps, for
     information on how to write a regexp (a regular expression) to
     match a string that is composed of two identical halves.  You
     can devise several regexps; some are better than others.  The
     function I use is described in an appendix, along with several
     regexps.  *Note `the-the' Duplicated Words Function: the-the.

Counting: Repetition and Regexps
********************************

Repetition and regular expression searches are powerful tools that you
often use when you write code in Emacs Lisp.  This chapter illustrates
the use of regular expression searches through the construction of
word count commands using `while' loops and recursion.

Counting words
==============

The standard Emacs distribution contains a function for counting the
number of lines within a region.  However, there is no corresponding
function for counting words.

Certain types of writing ask you to count words.  Thus, if you write
an essay, you may be limited to 800 words; if you write a novel, you
may discipline yourself to write 1000 words a day.  It seems odd to me
that Emacs lacks a word count command.  Perhaps people use Emacs
mostly for code or types of documentation that do not require word
counts; or perhaps they restrict themselves to the operating system
word count command, `wc'.  Alternatively, people may follow the
publishers' convention and compute a word count by dividing the
number of characters in a document by five.  In any event, here are
commands to count words.

The `count-words-region' Function
=================================

A word count command could count words in a line, paragraph, region,
or buffer.  What should the command cover?  You could design the
command to count the number of words in a complete buffer.  However,
the Emacs tradition encourages flexibility--you may want to count
words in just a section, rather than all of a buffer.  So it makes
more sense to design the command to count the number of words in a
region.  Once you have a `count-words-region' command, you can, if
you wish, count words in a whole buffer by marking it with `C-x h'
(`mark-whole-buffer').

Clearly, counting words is a repetitive act: starting from the
beginning of the region, you count the first word, then the second
word, then the third word, and so on, until you reach the end of the
region.  This means that word counting is ideally suited to recursion
or to a `while' loop.

Designing `count-words-region'
------------------------------

First, we will implement the word count command with a `while' loop,
then with recursion.  The command will, of course, be interactive.

The template for an interactive function definition is, as always:

     (defun NAME-OF-FUNCTION (ARGUMENT-LIST)
       "DOCUMENTATION..."
       (INTERACTIVE-EXPRESSION...)
       BODY...)

What we need to do is fill in the slots.

The name of the function should be self-explanatory and similar to the
existing `count-lines-region' name.  This makes the name easier to
remember.  `count-words-region' is a good choice.

The function counts words within a region.  This means that the
argument list must contain symbols that are bound to the two
positions, the beginning and end of the region.  These two positions
can be called `beginning' and `end' respectively.  The first line of
the documentation should be a single sentence, since that is all that
is printed as documentation by a command such as `apropos'.  The
interactive expression will be of the form `(interactive "r")', since
that will cause Emacs to pass the beginning and end of the region to
the function's argument list.  All this is routine.

The body of the function needs to be written to do three tasks:
first, to set up conditions under which the `while' loop can count
words, second, to run the `while' loop, and third, to send a message
to the user.

When a user calls `count-words-region', point may be at the beginning
or the end of the region.  However, the counting process must start
at the beginning of the region.  This means we will want to put point
there if it is not already there.  Executing `(goto-char beginning)'
ensures this.  Of course, we will want to return point to its
expected position when the function finishes its work.  For this
reason, the body must be enclosed in a `save-excursion' expression.

The central part of the body of the function consists of a `while'
loop in which one expression jumps point forward word by word, and
another expression counts those jumps.  The true-or-false-test of the
`while' loop should test true so long as point should jump forward,
and false when point is at the end of the region.

We could use `(forward-word 1)' as the expression for moving point
forward word by word, but it is easier to see what Emacs identifies
as a `word' if we use a regular expression search.

A regular expression search that finds the pattern for which it is
searching leaves point after the last character matched.  This means
that a succession of successful word searches will move point forward
word by word.

As a practical matter, we want the regular expression search to jump
over whitespace and punctuation between words as well as over the
words themselves.  A regexp that refuses to jump over interword
whitespace would never jump more than one word!  This means that the
regexp should include the whitespace and punctuation that follows a
word, if any, as well as the word itself.  (A word may end a buffer
and not have any following whitespace or punctuation, so that part of
the regexp must be optional.)

Thus, what we want for the regexp is a pattern defining one or more
word constituent characters followed, optionally, by one or more
characters that are not word constituents.  The regular expression for
this is:

     \w+\W*

The buffer's syntax table determines which characters are and are not
word constituents.  (*Note What Constitutes a Word or Symbol?:
Syntax, for more about syntax.  Also, see *Note Syntax:
(emacs)Syntax, and *Note Syntax Tables: (elisp)Syntax Tables.)

The search expression looks like this:

     (re-search-forward "\\w+\\W*")

(Note that paired backslashes precede the `w' and `W'.  A single
backslash has special meaning to the Emacs Lisp interpreter.  It
indicates that the following character is interpreted differently than
usual.  For example, the two characters, `\n', stand for `newline',
rather than for a backslash followed by `n'.  Two backslashes in a
row stand for an ordinary, `unspecial' backslash.)

We need a counter to count how many words there are; this variable
must first be set to 0 and then incremented each time Emacs goes
around the `while' loop.  The incrementing expression is simply:

     (setq count (1+ count))

Finally, we want to tell the user how many words there are in the
region.  The `message' function is intended for presenting this kind
of information to the user.  The message has to be phrased so that it
reads properly regardless of how many words there are in the region:
we don't want to say that "there are 1 words in the region".  The
conflict between singular and plural is ungrammatical.  We can solve
this problem by using a conditional expression that evaluates
different messages depending on the number of words in the region.
There are three possibilities: no words in the region, one word in the
region, and more than one word.  This means that the `cond' special
form is appropriate.

All this leads to the following function definition:

     ;;; First version; has bugs!
     (defun count-words-region (beginning end)
       "Print number of words in the region.
     Words are defined as at least one word-constituent
     character followed by at least one character that
     is not a word-constituent.  The buffer's syntax
     table determines which characters these are."
       (interactive "r")
       (message "Counting words in region ... ")
     
     ;;; 1. Set up appropriate conditions.
       (save-excursion
         (goto-char beginning)
         (let ((count 0))
     
     ;;; 2. Run the while loop.
           (while (< (point) end)
             (re-search-forward "\\w+\\W*")
             (setq count (1+ count)))
     
     ;;; 3. Send a message to the user.
           (cond ((zerop count)
                  (message
                   "The region does NOT have any words."))
                 ((= 1 count)
                  (message
                   "The region has 1 word."))
                 (t
                  (message
                   "The region has %d words." count))))))

As written, the function works, but not in all circumstances.

The Whitespace Bug in `count-words-region'
------------------------------------------

The `count-words-region' command described in the preceding section
has two bugs, or rather, one bug with two manifestations.  First, if
you mark a region containing only whitespace in the middle of some
text, the `count-words-region' command tells you that the region
contains one word!  Second, if you mark a region containing only
whitespace at the end of the buffer or the accessible portion of a
narrowed buffer, the command displays an error message that looks
like this:

     Search failed: "\\w+\\W*"

If you are reading this in Info in GNU Emacs, you can test for these
bugs yourself.

First, evaluate the function in the usual manner to install it.  Here
is a copy of the definition.  Place your cursor after the closing
parenthesis and type `C-x C-e' to install it.

     ;; First version; has bugs!
     (defun count-words-region (beginning end)
       "Print number of words in the region.
     Words are defined as at least one word-constituent character followed
     by at least one character that is not a word-constituent.  The buffer's
     syntax table determines which characters these are."
       (interactive "r")
       (message "Counting words in region ... ")
     
     ;;; 1. Set up appropriate conditions.
       (save-excursion
         (goto-char beginning)
         (let ((count 0))
     
     ;;; 2. Run the while loop.
           (while (< (point) end)
             (re-search-forward "\\w+\\W*")
             (setq count (1+ count)))
     
     ;;; 3. Send a message to the user.
           (cond ((zerop count)
                  (message "The region does NOT have any words."))
                 ((= 1 count) (message "The region has 1 word."))
                 (t (message "The region has %d words." count))))))

If you wish, you can also install this keybinding by evaluating it:

     (global-set-key "\C-c=" 'count-words-region)

To conduct the first test, set mark and point to the beginning and end
of the following line and then type `C-c =' (or `M-x
count-words-region' if you have not bound `C-c ='):

         one   two  three

Emacs will tell you, correctly, that the region has three words.

Repeat the test, but place mark at the beginning of the line and place
point just _before_ the word `one'.  Again type the command `C-c ='
(or `M-x count-words-region').  Emacs should tell you that the region
has no words, since it is composed only of the whitespace at the
beginning of the line.  But instead Emacs tells you that the region
has one word!

For the third test, copy the sample line to the end of the
`*scratch*' buffer and then type several spaces at the end of the
line.  Place mark right after the word `three' and point at the end
of line.  (The end of the line will be the end of the buffer.)  Type
`C-c =' (or `M-x count-words-region') as you did before.  Again,
Emacs should tell you that the region has no words, since it is
composed only of the whitespace at the end of the line.  Instead,
Emacs displays an error message saying `Search failed'.

The two bugs stem from the same problem.

Consider the first manifestation of the bug, in which the command
tells you that the whitespace at the beginning of the line contains
one word.  What happens is this: The `M-x count-words-region' command
moves point to the beginning of the region.  The `while' tests
whether the value of point is smaller than the value of `end', which
it is.  Consequently, the regular expression search looks for and
finds the first word.  It leaves point after the word.  `count' is
set to one.  The `while' loop repeats; but this time the value of
point is larger than the value of `end', the loop is exited; and the
function displays a message saying the number of words in the region
is one.  In brief, the regular expression search looks for and finds
the word even though it is outside the marked region.

In the second manifestation of the bug, the region is whitespace at
the end of the buffer.  Emacs says `Search failed'.  What happens is
that the true-or-false-test in the `while' loop tests true, so the
search expression is executed.  But since there are no more words in
the buffer, the search fails.

In both manifestations of the bug, the search extends or attempts to
extend outside of the region.

The solution is to limit the search to the region--this is a fairly
simple action, but as you may have come to expect, it is not quite as
simple as you might think.

As we have seen, the `re-search-forward' function takes a search
pattern as its first argument.  But in addition to this first,
mandatory argument, it accepts three optional arguments.  The optional
second argument bounds the search.  The optional third argument, if
`t', causes the function to return `nil' rather than signal an error
if the search fails.  The optional fourth argument is a repeat count.
(In Emacs, you can see a function's documentation by typing `C-h f',
the name of the function, and then <RET>.)

In the `count-words-region' definition, the value of the end of the
region is held by the variable `end' which is passed as an argument
to the function.  Thus, we can add `end' as an argument to the
regular expression search expression:

     (re-search-forward "\\w+\\W*" end)

However, if you make only this change to the `count-words-region'
definition and then test the new version of the definition on a
stretch of whitespace, you will receive an error message saying
`Search failed'.

What happens is this: the search is limited to the region, and fails
as you expect because there are no word-constituent characters in the
region.  Since it fails, we receive an error message.  But we do not
want to receive an error message in this case; we want to receive the
message that "The region does NOT have any words."

The solution to this problem is to provide `re-search-forward' with a
third argument of `t', which causes the function to return `nil'
rather than signal an error if the search fails.

However, if you make this change and try it, you will see the message
"Counting words in region ... " and ... you will keep on seeing that
message ..., until you type `C-g' (`keyboard-quit').

Here is what happens: the search is limited to the region, as before,
and it fails because there are no word-constituent characters in the
region, as expected.  Consequently, the `re-search-forward'
expression returns `nil'.  It does nothing else.  In particular, it
does not move point, which it does as a side effect if it finds the
search target.  After the `re-search-forward' expression returns
`nil', the next expression in the `while' loop is evaluated.  This
expression increments the count.  Then the loop repeats.  The
true-or-false-test tests true because the value of point is still less
than the value of end, since the `re-search-forward' expression did
not move point. ... and the cycle repeats ...

The `count-words-region' definition requires yet another
modification, to cause the true-or-false-test of the `while' loop to
test false if the search fails.  Put another way, there are two
conditions that must be satisfied in the true-or-false-test before the
word count variable is incremented: point must still be within the
region and the search expression must have found a word to count.

Since both the first condition and the second condition must be true
together, the two expressions, the region test and the search
expression, can be joined with an `and' special form and embedded in
the `while' loop as the true-or-false-test, like this:

     (and (< (point) end) (re-search-forward "\\w+\\W*" end t))

(*Note forward-paragraph::, for information about `and'.)

The `re-search-forward' expression returns `t' if the search succeeds
and as a side effect moves point.  Consequently, as words are found,
point is moved through the region.  When the search expression fails
to find another word, or when point reaches the end of the region,
the true-or-false-test tests false, the `while' loop exists, and the
`count-words-region' function displays one or other of its messages.

After incorporating these final changes, the `count-words-region'
works without bugs (or at least, without bugs that I have found!).
Here is what it looks like:

     ;;; Final version: `while'
     (defun count-words-region (beginning end)
       "Print number of words in the region."
       (interactive "r")
       (message "Counting words in region ... ")
     
     ;;; 1. Set up appropriate conditions.
       (save-excursion
         (let ((count 0))
           (goto-char beginning)
     
     ;;; 2. Run the while loop.
           (while (and (< (point) end)
                       (re-search-forward "\\w+\\W*" end t))
             (setq count (1+ count)))
     
     ;;; 3. Send a message to the user.
           (cond ((zerop count)
                  (message
                   "The region does NOT have any words."))
                 ((= 1 count)
                  (message
                   "The region has 1 word."))
                 (t
                  (message
                   "The region has %d words." count))))))

Count Words Recursively
=======================

You can write the function for counting words recursively as well as
with a `while' loop.  Let's see how this is done.

First, we need to recognize that the `count-words-region' function
has three jobs: it sets up the appropriate conditions for counting to
occur; it counts the words in the region; and it sends a message to
the user telling how many words there are.

If we write a single recursive function to do everything, we will
receive a message for every recursive call.  If the region contains 13
words, we will receive thirteen messages, one right after the other.
We don't want this!  Instead, we must write two functions to do the
job, one of which (the recursive function) will be used inside of the
other.  One function will set up the conditions and display the
message; the other will return the word count.

Let us start with the function that causes the message to be
displayed.  We can continue to call this `count-words-region'.

This is the function that the user will call.  It will be interactive.
Indeed, it will be similar to our previous versions of this function,
except that it will call `recursive-count-words' to determine how
many words are in the region.

We can readily construct a template for this function, based on our
previous versions:

     ;; Recursive version; uses regular expression search
     (defun count-words-region (beginning end)
       "DOCUMENTATION..."
       (INTERACTIVE-EXPRESSION...)
     
     ;;; 1. Set up appropriate conditions.
       (EXPLANATORY MESSAGE)
       (SET-UP FUNCTIONS...
     
     ;;; 2. Count the words.
         RECURSIVE CALL
     
     ;;; 3. Send a message to the user.
         MESSAGE PROVIDING WORD COUNT))

The definition looks straightforward, except that somehow the count
returned by the recursive call must be passed to the message
displaying the word count.  A little thought suggests that this can be
done by making use of a `let' expression: we can bind a variable in
the varlist of a `let' expression to the number of words in the
region, as returned by the recursive call; and then the `cond'
expression, using binding, can display the value to the user.

Often, one thinks of the binding within a `let' expression as somehow
secondary to the `primary' work of a function.  But in this case,
what you might consider the `primary' job of the function, counting
words, is done within the `let' expression.

Using `let', the function definition looks like this:

     (defun count-words-region (beginning end)
       "Print number of words in the region."
       (interactive "r")
     
     ;;; 1. Set up appropriate conditions.
       (message "Counting words in region ... ")
       (save-excursion
         (goto-char beginning)
     
     ;;; 2. Count the words.
         (let ((count (recursive-count-words end)))
     
     ;;; 3. Send a message to the user.
           (cond ((zerop count)
                  (message
                   "The region does NOT have any words."))
                 ((= 1 count)
                  (message
                   "The region has 1 word."))
                 (t
                  (message
                   "The region has %d words." count))))))

Next, we need to write the recursive counting function.

A recursive function has at least three parts: the `do-again-test',
the `next-step-expression', and the recursive call.

The do-again-test determines whether the function will or will not be
called again.  Since we are counting words in a region and can use a
function that moves point forward for every word, the do-again-test
can check whether point is still within the region.  The do-again-test
should find the value of point and determine whether point is before,
at, or after the value of the end of the region.  We can use the
`point' function to locate point.  Clearly, we must pass the value of
the end of the region to the recursive counting function as an
argument.

In addition, the do-again-test should also test whether the search
finds a word.  If it does not, the function should not call itself
again.

The next-step-expression changes a value so that when the recursive
function is supposed to stop calling itself, it stops.  More
precisely, the next-step-expression changes a value so that at the
right time, the do-again-test stops the recursive function from
calling itself again.  In this case, the next-step-expression can be
the expression that moves point forward, word by word.

The third part of a recursive function is the recursive call.

Somewhere, also, we also need a part that does the `work' of the
function, a part that does the counting.  A vital part!

But already, we have an outline of the recursive counting function:

     (defun recursive-count-words (region-end)
       "DOCUMENTATION..."
        DO-AGAIN-TEST
        NEXT-STEP-EXPRESSION
        RECURSIVE CALL)

Now we need to fill in the slots.  Let's start with the simplest cases
first:  if point is at or beyond the end of the region, there cannot
be any words in the region, so the function should return zero.
Likewise, if the search fails, there are no words to count, so the
function should return zero.

On the other hand, if point is within the region and the search
succeeds, the function should call itself again.

Thus, the do-again-test should look like this:

     (and (< (point) region-end)
          (re-search-forward "\\w+\\W*" region-end t))

Note that the search expression is part of the do-again-test--the
function returns `t' if its search succeeds and `nil' if it fails.
(*Note The Whitespace Bug in `count-words-region': Whitespace Bug,
for an explanation of how `re-search-forward' works.)

The do-again-test is the true-or-false test of an `if' clause.
Clearly, if the do-again-test succeeds, the then-part of the `if'
clause should call the function again; but if it fails, the else-part
should return zero since either point is outside the region or the
search failed because there were no words to find.

But before considering the recursive call, we need to consider the
next-step-expression.  What is it?  Interestingly, it is the search
part of the do-again-test.

In addition to returning `t' or `nil' for the do-again-test,
`re-search-forward' moves point forward as a side effect of a
successful search.  This is the action that changes the value of
point so that the recursive function stops calling itself when point
completes its movement through the region.  Consequently, the
`re-search-forward' expression is the next-step-expression.

In outline, then, the body of the `recursive-count-words' function
looks like this:

     (if DO-AGAIN-TEST-AND-NEXT-STEP-COMBINED
         ;; then
         RECURSIVE-CALL-RETURNING-COUNT
       ;; else
       RETURN-ZERO)

How to incorporate the mechanism that counts?

If you are not used to writing recursive functions, a question like
this can be troublesome.  But it can and should be approached
systematically.

We know that the counting mechanism should be associated in some way
with the recursive call.  Indeed, since the next-step-expression moves
point forward by one word, and since a recursive call is made for
each word, the counting mechanism must be an expression that adds one
to the value returned by a call to `recursive-count-words'.

Consider several cases:

   * If there are two words in the region, the function should return
     a value resulting from adding one to the value returned when it
     counts the first word, plus the number returned when it counts
     the remaining words in the region, which in this case is one.

   * If there is one word in the region, the function should return a
     value resulting from adding one to the value returned when it
     counts that word, plus the number returned when it counts the
     remaining words in the region, which in this case is zero.

   * If there are no words in the region, the function should return
     zero.

From the sketch we can see that the else-part of the `if' returns
zero for the case of no words.  This means that the then-part of the
`if' must return a value resulting from adding one to the value
returned from a count of the remaining words.

The expression will look like this, where `1+' is a function that
adds one to its argument.

     (1+ (recursive-count-words region-end))

The whole `recursive-count-words' function will then look like this:

     (defun recursive-count-words (region-end)
       "DOCUMENTATION..."
     
     ;;; 1. do-again-test
       (if (and (< (point) region-end)
                (re-search-forward "\\w+\\W*" region-end t))
     
     ;;; 2. then-part: the recursive call
           (1+ (recursive-count-words region-end))
     
     ;;; 3. else-part
         0))

Let's examine how this works:

If there are no words in the region, the else part of the `if'
expression is evaluated and consequently the function returns zero.

If there is one word in the region, the value of point is less than
the value of `region-end' and the search succeeds.  In this case, the
true-or-false-test of the `if' expression tests true, and the
then-part of the `if' expression is evaluated.  The counting
expression is evaluated.  This expression returns a value (which will
be the value returned by the whole function) that is the sum of one
added to the value returned by a recursive call.

Meanwhile, the next-step-expression has caused point to jump over the
first (and in this case only) word in the region.  This means that
when `(recursive-count-words region-end)' is evaluated a second time,
as a result of the recursive call, the value of point will be equal
to or greater than the value of region end.  So this time,
`recursive-count-words' will return zero.  The zero will be added to
one, and the original evaluation of `recursive-count-words' will
return one plus zero, which is one, which is the correct amount.

Clearly, if there are two words in the region, the first call to
`recursive-count-words' returns one added to the value returned by
calling `recursive-count-words' on a region containing the remaining
word--that is, it adds one to one, producing two, which is the
correct amount.

Similarly, if there are three words in the region, the first call to
`recursive-count-words' returns one added to the value returned by
calling `recursive-count-words' on a region containing the remaining
two words--and so on and so on.

With full documentation the two functions look like this:

The recursive function:

     (defun recursive-count-words (region-end)
       "Number of words between point and REGION-END."
     
     ;;; 1. do-again-test
       (if (and (< (point) region-end)
                (re-search-forward "\\w+\\W*" region-end t))
     
     ;;; 2. then-part: the recursive call
           (1+ (recursive-count-words region-end))
     
     ;;; 3. else-part
         0))

The wrapper:

     ;;; Recursive version
     (defun count-words-region (beginning end)
       "Print number of words in the region.
     
     Words are defined as at least one word-constituent
     character followed by at least one character that is
     not a word-constituent.  The buffer's syntax table
     determines which characters these are."
       (interactive "r")
       (message "Counting words in region ... ")
       (save-excursion
         (goto-char beginning)
         (let ((count (recursive-count-words end)))
           (cond ((zerop count)
                  (message
                   "The region does NOT have any words."))
                 ((= 1 count)
                  (message "The region has 1 word."))
                 (t
                  (message
                   "The region has %d words." count))))))

Exercise: Counting Punctuation
==============================

Using a `while' loop, write a function to count the number of
punctuation marks in a region--period, comma, semicolon, colon,
exclamation mark, and question mark.  Do the same using recursion.

Counting Words in a `defun'
***************************

Our next project is to count the number of words in a function
definition.  Clearly, this can be done using some variant of
`count-word-region'.  *Note Counting Words: Repetition and Regexps:
Counting Words.  If we are just going to count the words in one
definition, it is easy enough to mark the definition with the `C-M-h'
(`mark-defun') command, and then call `count-word-region'.

However, I am more ambitious: I want to count the words and symbols in
every definition in the Emacs sources and then print a graph that
shows how many functions there are of each length: how many contain 40
to 49 words or symbols, how many contain 50 to 59 words or symbols,
and so on.  I have often been curious how long a typical function is,
and this will tell.

Divide and Conquer
==================

Described in one phrase, the histogram project is daunting; but
divided into numerous small steps, each of which we can take one at a
time, the project becomes less fearsome.  Let us consider what the
steps must be:

   * First, write a function to count the words in one definition.
     This includes the problem of handling symbols as well as words.

   * Second, write a function to list the numbers of words in each
     function in a file.  This function can use the
     `count-words-in-defun' function.

   * Third, write a function to list the numbers of words in each
     function in each of several files.  This entails automatically
     finding the various files, switching to them, and counting the
     words in the definitions within them.

   * Fourth, write a function to convert the list of numbers that we
     created in step three to a form that will be suitable for
     printing as a graph.

   * Fifth, write a function to print the results as a graph.

This is quite a project!  But if we take each step slowly, it will not
be difficult.

What to Count?
==============

When we first start thinking about how to count the words in a
function definition, the first question is (or ought to be) what are
we going to count?  When we speak of `words' with respect to a Lisp
function definition, we are actually speaking, in large part, of
`symbols'.  For example, the following `multiply-by-seven' function
contains the five symbols `defun', `multiply-by-seven', `number',
`*', and `7'.  In addition, in the documentation string, it contains
the four words `Multiply', `NUMBER', `by', and `seven'.  The symbol
`number' is repeated, so the definition contains a total of ten words
and symbols.

     (defun multiply-by-seven (number)
       "Multiply NUMBER by seven."
       (* 7 number))

However, if we mark the `multiply-by-seven' definition with `C-M-h'
(`mark-defun'), and then call `count-words-region' on it, we will
find that `count-words-region' claims the definition has eleven
words, not ten!  Something is wrong!

The problem is twofold: `count-words-region' does not count the `*'
as a word, and it counts the single symbol, `multiply-by-seven', as
containing three words.  The hyphens are treated as if they were
interword spaces rather than intraword connectors:
`multiply-by-seven' is counted as if it were written `multiply by
seven'.

The cause of this confusion is the regular expression search within
the `count-words-region' definition that moves point forward word by
word.  In the canonical version of `count-words-region', the regexp
is:

     "\\w+\\W*"

This regular expression is a pattern defining one or more word
constituent characters possibly followed by one or more characters
that are not word constituents.  What is meant by `word constituent
characters' brings us to the issue of syntax, which is worth a section
of its own.

What Constitutes a Word or Symbol?
==================================

Emacs treats different characters as belonging to different "syntax
categories".  For example, the regular expression, `\\w+', is a
pattern specifying one or more _word constituent_ characters.  Word
constituent characters are members of one syntax category.  Other
syntax categories include the class of punctuation characters, such
as the period and the comma, and the class of whitespace characters,
such as the blank space and the tab character.  (For more
information, see *Note Syntax: (emacs)Syntax, and *Note Syntax
Tables: (elisp)Syntax Tables.)

Syntax tables specify which characters belong to which categories.
Usually, a hyphen is not specified as a `word constituent character'.
Instead, it is specified as being in the `class of characters that are
part of symbol names but not words.'  This means that the
`count-words-region' function treats it in the same way it treats an
interword white space, which is why `count-words-region' counts
`multiply-by-seven' as three words.

There are two ways to cause Emacs to count `multiply-by-seven' as one
symbol: modify the syntax table or modify the regular expression.

We could redefine a hyphen as a word constituent character by
modifying the syntax table that Emacs keeps for each mode.  This
action would serve our purpose, except that a hyphen is merely the
most common character within symbols that is not typically a word
constituent character; there are others, too.

Alternatively, we can redefine the regular expression used in the
`count-words' definition so as to include symbols.  This procedure
has the merit of clarity, but the task is a little tricky.

The first part is simple enough: the pattern must match "at least one
character that is a word or symbol constituent".  Thus:

     "\\(\\w\\|\\s_\\)+"

The `\\(' is the first part of the grouping construct that includes
the `\\w' and the `\\s_' as alternatives, separated by the `\\|'.
The `\\w' matches any word-constituent character and the `\\s_'
matches any character that is part of a symbol name but not a
word-constituent character.  The `+' following the group indicates
that the word or symbol constituent characters must be matched at
least once.

However, the second part of the regexp is more difficult to design.
What we want is to follow the first part with "optionally one or more
characters that are not constituents of a word or symbol".  At first,
I thought I could define this with the following:

     "\\(\\W\\|\\S_\\)*"

The upper case `W' and `S' match characters that are _not_ word or
symbol constituents.  Unfortunately, this expression matches any
character that is either not a word constituent or not a symbol
constituent.  This matches any character!

I then noticed that every word or symbol in my test region was
followed by white space (blank space, tab, or newline).  So I tried
placing a pattern to match one or more blank spaces after the pattern
for one or more word or symbol constituents.  This failed, too.  Words
and symbols are often separated by whitespace, but in actual code
parentheses may follow symbols and punctuation may follow words.  So
finally, I designed a pattern in which the word or symbol constituents
are followed optionally by characters that are not white space and
then followed optionally by white space.

Here is the full regular expression:

     "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"

The `count-words-in-defun' Function
===================================

We have seen that there are several ways to write a
`count-word-region' function.  To write a `count-words-in-defun', we
need merely adapt one of these versions.

The version that uses a `while' loop is easy to understand, so I am
going to adapt that.  Because `count-words-in-defun' will be part of
a more complex program, it need not be interactive and it need not
display a message but just return the count.  These considerations
simplify the definition a little.

On the other hand, `count-words-in-defun' will be used within a
buffer that contains function definitions.  Consequently, it is
reasonable to ask that the function determine whether it is called
when point is within a function definition, and if it is, to return
the count for that definition.  This adds complexity to the
definition, but saves us from needing to pass arguments to the
function.

These considerations lead us to prepare the following template:

     (defun count-words-in-defun ()
       "DOCUMENTATION..."
       (SET UP...
          (WHILE LOOP...)
        RETURN COUNT)

As usual, our job is to fill in the slots.

First, the set up.

We are presuming that this function will be called within a buffer
containing function definitions.  Point will either be within a
function definition or not.  For `count-words-in-defun' to work,
point must move to the beginning of the definition, a counter must
start at zero, and the counting loop must stop when point reaches the
end of the definition.

The `beginning-of-defun' function searches backwards for an opening
delimiter such as a `(' at the beginning of a line, and moves point
to that position, or else to the limit of the search.  In practice,
this means that `beginning-of-defun' moves point to the beginning of
an enclosing or preceding function definition, or else to the
beginning of the buffer.  We can use `beginning-of-defun' to place
point where we wish to start.

The `while' loop requires a counter to keep track of the words or
symbols being counted.  A `let' expression can be used to create a
local variable for this purpose, and bind it to an initial value of
zero.

The `end-of-defun' function works like `beginning-of-defun' except
that it moves point to the end of the definition.  `end-of-defun' can
be used as part of an expression that determines the position of the
end of the definition.

The set up for `count-words-in-defun' takes shape rapidly: first we
move point to the beginning of the definition, then we create a local
variable to hold the count, and finally, we record the position of
the end of the definition so the `while' loop will know when to stop
looping.

The code looks like this:

     (beginning-of-defun)
     (let ((count 0)
           (end (save-excursion (end-of-defun) (point))))

The code is simple.  The only slight complication is likely to concern
`end': it is bound to the position of the end of the definition by a
`save-excursion' expression that returns the value of point after
`end-of-defun' temporarily moves it to the end of the definition.

The second part of the `count-words-in-defun', after the set up, is
the `while' loop.

The loop must contain an expression that jumps point forward word by
word and symbol by symbol, and another expression that counts the
jumps.  The true-or-false-test for the `while' loop should test true
so long as point should jump forward, and false when point is at the
end of the definition.  We have already redefined the regular
expression for this (*note Syntax::), so the loop is straightforward:

     (while (and (< (point) end)
                 (re-search-forward
                  "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" end t)
       (setq count (1+ count)))

The third part of the function definition returns the count of words
and symbols.  This part is the last expression within the body of the
`let' expression, and can be, very simply, the local variable
`count', which when evaluated returns the count.

Put together, the `count-words-in-defun' definition looks like this:

     (defun count-words-in-defun ()
       "Return the number of words and symbols in a defun."
       (beginning-of-defun)
       (let ((count 0)
             (end (save-excursion (end-of-defun) (point))))
         (while
             (and (< (point) end)
                  (re-search-forward
                   "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"
                   end t))
           (setq count (1+ count)))
         count))

How to test this?  The function is not interactive, but it is easy to
put a wrapper around the function to make it interactive; we can use
almost the same code as for the recursive version of
`count-words-region':

     ;;; Interactive version.
     (defun count-words-defun ()
       "Number of words and symbols in a function definition."
       (interactive)
       (message
        "Counting words and symbols in function definition ... ")
       (let ((count (count-words-in-defun)))
         (cond
          ((zerop count)
           (message
            "The definition does NOT have any words or symbols."))
          ((= 1 count)
           (message
            "The definition has 1 word or symbol."))
          (t
           (message
            "The definition has %d words or symbols." count)))))

Let's re-use `C-c =' as a convenient keybinding:

     (global-set-key "\C-c=" 'count-words-defun)

Now we can try out `count-words-defun': install both
`count-words-in-defun' and `count-words-defun', and set the
keybinding, and then place the cursor within the following definition:

     (defun multiply-by-seven (number)
       "Multiply NUMBER by seven."
       (* 7 number))
          => 10

Success!  The definition has 10 words and symbols.

The next problem is to count the numbers of words and symbols in
several definitions within a single file.

Count Several `defuns' Within a File
====================================

A file such as `simple.el' may have 80 or more function definitions
within it.  Our long term goal is to collect statistics on many
files, but as a first step, our immediate goal is to collect
statistics on one file.

The information will be a series of numbers, each number being the
length of a function definition.  We can store the numbers in a list.

We know that we will want to incorporate the information regarding one
file with information about many other files; this means that the
function for counting definition lengths within one file need only
return the list of lengths.  It need not and should not display any
messages.

The word count commands contain one expression to jump point forward
word by word and another expression to count the jumps.  The function
to return the lengths of definitions can be designed to work the same
way, with one expression to jump point forward definition by
definition and another expression to construct the lengths' list.

This statement of the problem makes it elementary to write the
function definition.  Clearly, we will start the count at the
beginning of the file, so the first command will be `(goto-char
(point-min))'.  Next, we start the `while' loop; and the
true-or-false test of the loop can be a regular expression search for
the next function definition--so long as the search succeeds, point
is moved forward and then the body of the loop is evaluated.  The body
needs an expression that constructs the lengths' list.  `cons', the
list construction command, can be used to create the list.  That is
almost all there is to it.

Here is what this fragment of code looks like:

     (goto-char (point-min))
     (while (re-search-forward "^(defun" nil t)
       (setq lengths-list
             (cons (count-words-in-defun) lengths-list)))

What we have left out is the mechanism for finding the file that
contains the function definitions.

In previous examples, we either used this, the Info file, or we
switched back and forth to some other buffer, such as the `*scratch*'
buffer.

Finding a file is a new process that we have not yet discussed.

Find a File
===========

To find a file in Emacs, you use the `C-x C-f' (`find-file') command.
This command is almost, but not quite right for the lengths problem.

Let's look at the source for `find-file' (you can use the `find-tag'
command or `C-h f' (`describe-function') to find the source of a
function):

     (defun find-file (filename)
       "Edit file FILENAME.
     Switch to a buffer visiting file FILENAME,
     creating one if none already exists."
       (interactive "FFind file: ")
       (switch-to-buffer (find-file-noselect filename)))

The definition possesses short but complete documentation and an
interactive specification that prompts you for a file name when you
use the command interactively.  The body of the definition contains
two functions, `find-file-noselect' and `switch-to-buffer'.

According to its documentation as shown by `C-h f' (the
`describe-function' command), the `find-file-noselect' function reads
the named file into a buffer and returns the buffer.  However, the
buffer is not selected.  Emacs does not switch its attention (or
yours if you are using `find-file-noselect') to the named buffer.
That is what `switch-to-buffer' does: it switches the buffer to which
Emacs attention is directed; and it switches the buffer displayed in
the window to the new buffer.  We have discussed buffer switching
elsewhere.  (*Note Switching Buffers::.)

In this histogram project, we do not need to display each file on the
screen as the program determines the length of each definition within
it.  Instead of employing `switch-to-buffer', we can work with
`set-buffer', which redirects the attention of the computer program
to a different buffer but does not redisplay it on the screen.  So
instead of calling on `find-file' to do the job, we must write our
own expression.

The task is easy: use  `find-file-noselect' and `set-buffer'.

`lengths-list-file' in Detail
=============================

The core of the `lengths-list-file' function is a `while' loop
containing a function to move point forward `defun by defun' and a
function to count the number of words and symbols in each defun.
This core must be surrounded by functions that do various other tasks,
including finding the file, and ensuring that point starts out at the
beginning of the file.  The function definition looks like this:

     (defun lengths-list-file (filename)
       "Return list of definitions' lengths within FILE.
     The returned list is a list of numbers.
     Each number is the number of words or
     symbols in one function definition."
       (message "Working on `%s' ... " filename)
       (save-excursion
         (let ((buffer (find-file-noselect filename))
               (lengths-list))
           (set-buffer buffer)
           (setq buffer-read-only t)
           (widen)
           (goto-char (point-min))
           (while (re-search-forward "^(defun" nil t)
             (setq lengths-list
                   (cons (count-words-in-defun) lengths-list)))
           (kill-buffer buffer)
           lengths-list)))

The function is passed one argument, the name of the file on which it
will work.  It has four lines of documentation, but no interactive
specification.  Since people worry that a computer is broken if they
don't see anything going on, the first line of the body is a message.

The next line contains a `save-excursion' that returns Emacs'
attention to the current buffer when the function completes.  This is
useful in case you embed this function in another function that
presumes point is restored to the original buffer.

In the varlist of the `let' expression, Emacs finds the file and
binds the local variable `buffer' to the buffer containing the file.
At the same time, Emacs creates `lengths-list' as a local variable.

Next, Emacs switches its attention to the buffer.

In the following line, Emacs makes the buffer read-only.  Ideally,
this line is not necessary.  None of the functions for counting words
and symbols in a function definition should change the buffer.
Besides, the buffer is not going to be saved, even if it were changed.
This line is entirely the consequence of great, perhaps excessive,
caution.  The reason for the caution is that this function and those
it calls work on the sources for Emacs and it is very inconvenient if
they are inadvertently modified.  It goes without saying that I did
not realize a need for this line until an experiment went awry and
started to modify my Emacs source files ...

Next comes a call to widen the buffer if it is narrowed.  This
function is usually not needed--Emacs creates a fresh buffer if none
already exists; but if a buffer visiting the file already exists Emacs
returns that one.  In this case, the buffer may be narrowed and must
be widened.  If we wanted to be fully `user-friendly', we would
arrange to save the restriction and the location of point, but we
won't.

The `(goto-char (point-min))' expression moves point to the beginning
of the buffer.

Then comes a `while' loop in which the `work' of the function is
carried out.  In the loop, Emacs determines the length of each
definition and constructs a lengths' list containing the information.

Emacs kills the buffer after working through it.  This is to save
space inside of Emacs.  My version of Emacs 19 contained over 300
source files of interest; Emacs 21 contains over 800 source files.
Another function will apply `lengths-list-file' to each of the files.

Finally, the last expression within the `let' expression is the
`lengths-list' variable; its value is returned as the value of the
whole function.

You can try this function by installing it in the usual fashion.  Then
place your cursor after the following expression and type `C-x C-e'
(`eval-last-sexp').

     (lengths-list-file
      "/usr/local/share/emacs/21.0.100/lisp/emacs-lisp/debug.el")

(You may need to change the pathname of the file; the one here worked
with GNU Emacs version 21.0.100.  To change the expression, copy it to
the `*scratch*' buffer and edit it.

(Also, to see the full length of the list, rather than a truncated
version, you may have to evaluate the following:

     (custom-set-variables '(eval-expression-print-length nil))

(*Note Setting Variables with `defcustom': defcustom.  Then evaluate
the `lengths-list-file' expression.)

The lengths' list for `debug.el' takes less than a second to produce
and looks like this:

     (77 95 85 87 131 89 50 25 44 44 68 35 64 45 17 34 167 457)

(Using my old machine, the version 19 lengths' list for `debug.el'
took seven seconds to produce and looked like this:

     (75 41 80 62 20 45 44 68 45 12 34 235)

(The newer version of  `debug.el' contains more defuns than the
earlier one; and my new machine is much faster than the old one.)

Note that the length of the last definition in the file is first in
the list.

Count Words in `defuns' in Different Files
==========================================

In the previous section, we created a function that returns a list of
the lengths of each definition in a file.  Now, we want to define a
function to return a master list of the lengths of the definitions in
a list of files.

Working on each of a list of files is a repetitious act, so we can use
either a `while' loop or recursion.

Determine the lengths of `defuns'
---------------------------------

The design using a `while' loop is routine.  The argument passed the
function is a list of files.  As we saw earlier (*note Loop
Example::), you can write a `while' loop so that the body of the loop
is evaluated if such a list contains elements, but to exit the loop
if the list is empty.  For this design to work, the body of the loop
must contain an expression that shortens the list each time the body
is evaluated, so that eventually the list is empty.  The usual
technique is to set the value of the list to the value of the CDR of
the list each time the body is evaluated.

The template looks like this:

     (while TEST-WHETHER-LIST-IS-EMPTY
       BODY...
       SET-LIST-TO-CDR-OF-LIST)

Also, we remember that a `while' loop returns `nil' (the result of
evaluating the true-or-false-test), not the result of any evaluation
within its body.  (The evaluations within the body of the loop are
done for their side effects.)  However, the expression that sets the
lengths' list is part of the body--and that is the value that we want
returned by the function as a whole.  To do this, we enclose the
`while' loop within a `let' expression, and arrange that the last
element of the `let' expression contains the value of the lengths'
list.  (*Note Loop Example with an Incrementing Counter: Incrementing
Example.)

These considerations lead us directly to the function itself:

     ;;; Use `while' loop.
     (defun lengths-list-many-files (list-of-files)
       "Return list of lengths of defuns in LIST-OF-FILES."
       (let (lengths-list)
     
     ;;; true-or-false-test
         (while list-of-files
           (setq lengths-list
                 (append
                  lengths-list
     
     ;;; Generate a lengths' list.
                  (lengths-list-file
                   (expand-file-name (car list-of-files)))))
     
     ;;; Make files' list shorter.
           (setq list-of-files (cdr list-of-files)))
     
     ;;; Return final value of lengths' list.
         lengths-list))

`expand-file-name' is a built-in function that converts a file name
to the absolute, long, path name form of the directory in which the
function is called.

Thus, if `expand-file-name' is called on `debug.el' when Emacs is
visiting the `/usr/local/share/emacs/21.0.100/lisp/emacs-lisp/'
directory,

     debug.el

becomes

     /usr/local/share/emacs/21.0.100/lisp/emacs-lisp/debug.el

The only other new element of this function definition is the as yet
unstudied function `append', which merits a short section for itself.

The `append' Function
---------------------

The `append' function attaches one list to another.  Thus,

     (append '(1 2 3 4) '(5 6 7 8))

produces the list

     (1 2 3 4 5 6 7 8)

This is exactly how we want to attach two lengths' lists produced by
`lengths-list-file' to each other.  The results contrast with `cons',

     (cons '(1 2 3 4) '(5 6 7 8))

which constructs a new list in which the first argument to `cons'
becomes the first element of the new list:

     ((1 2 3 4) 5 6 7 8)

Recursively Count Words in Different Files
==========================================

Besides a `while' loop, you can work on each of a list of files with
recursion.  A recursive version of `lengths-list-many-files' is short
and simple.

The recursive function has the usual parts: the `do-again-test', the
`next-step-expression', and the recursive call.  The `do-again-test'
determines whether the function should call itself again, which it
will do if the `list-of-files' contains any remaining elements; the
`next-step-expression' resets the `list-of-files' to the CDR of
itself, so eventually the list will be empty; and the recursive call
calls itself on the shorter list.  The complete function is shorter
than this description!

     (defun recursive-lengths-list-many-files (list-of-files)
       "Return list of lengths of each defun in LIST-OF-FILES."
       (if list-of-files                     ; do-again-test
           (append
            (lengths-list-file
             (expand-file-name (car list-of-files)))
            (recursive-lengths-list-many-files
             (cdr list-of-files)))))

In a sentence, the function returns the lengths' list for the first of
the `list-of-files' appended to the result of calling itself on the
rest of the `list-of-files'.

Here is a test of `recursive-lengths-list-many-files', along with the
results of running `lengths-list-file' on each of the files
individually.

Install `recursive-lengths-list-many-files' and `lengths-list-file',
if necessary, and then evaluate the following expressions.  You may
need to change the files' pathnames; those here work when this Info
file and the Emacs sources are located in their customary places.  To
change the expressions, copy them to the `*scratch*' buffer, edit
them, and then evaluate them.

The results are shown after the `=>'.  (These results are for files
from Emacs Version 21.0.100; files from other versions of Emacs may
produce different results.)

     (cd "/usr/local/share/emacs/21.0.100/")
     
     (lengths-list-file "./lisp/macros.el")
          => (273 263 456 90)
     
     (lengths-list-file "./lisp/mail/mailalias.el")
          => (38 32 26 77 174 180 321 198 324)
     
     (lengths-list-file "./lisp/makesum.el")
          => (85 181)
     
     (recursive-lengths-list-many-files
      '("./lisp/macros.el"
        "./lisp/mail/mailalias.el"
        "./lisp/makesum.el"))
            => (273 263 456 90 38 32 26 77 174 180 321 198 324 85 181)

The `recursive-lengths-list-many-files' function produces the output
we want.

The next step is to prepare the data in the list for display in a
graph.

Prepare the Data for Display in a Graph
=======================================

The `recursive-lengths-list-many-files' function returns a list of
numbers.  Each number records the length of a function definition.
What we need to do now is transform this data into a list of numbers
suitable for generating a graph.  The new list will tell how many
functions definitions contain less than 10 words and symbols, how
many contain between 10 and 19 words and symbols, how many contain
between 20 and 29 words and symbols, and so on.

In brief, we need to go through the lengths' list produced by the
`recursive-lengths-list-many-files' function and count the number of
defuns within each range of lengths, and produce a list of those
numbers.

Based on what we have done before, we can readily foresee that it
should not be too hard to write a function that `CDRs' down the
lengths' list, looks at each element, determines which length range it
is in, and increments a counter for that range.

However, before beginning to write such a function, we should consider
the advantages of sorting the lengths' list first, so the numbers are
ordered from smallest to largest.  First, sorting will make it easier
to count the numbers in each range, since two adjacent numbers will
either be in the same length range or in adjacent ranges.  Second, by
inspecting a sorted list, we can discover the highest and lowest
number, and thereby determine the largest and smallest length range
that we will need.

Sorting Lists
-------------

Emacs contains a function to sort lists, called (as you might guess)
`sort'.  The `sort' function takes two arguments, the list to be
sorted, and a predicate that determines whether the first of two list
elements is "less" than the second.

As we saw earlier (*note Using the Wrong Type Object as an Argument:
Wrong Type of Argument.), a predicate is a function that determines
whether some property is true or false.  The `sort' function will
reorder a list according to whatever property the predicate uses;
this means that `sort' can be used to sort non-numeric lists by
non-numeric criteria--it can, for example, alphabetize a list.

The `<' function is used when sorting a numeric list.  For example,

     (sort '(4 8 21 17 33 7 21 7) '<)

produces this:

     (4 7 7 8 17 21 21 33)

(Note that in this example, both the arguments are quoted so that the
symbols are not evaluated before being passed to `sort' as arguments.)

Sorting the list returned by the `recursive-lengths-list-many-files'
function is straightforward; it uses the `<' function:

     (sort
      (recursive-lengths-list-many-files
       '("../lisp/macros.el"
         "../lisp/mailalias.el"
         "../lisp/makesum.el"))
      '<

which produces:

     (85 86 116 122 154 176 179 265)

(Note that in this example, the first argument to `sort' is not
quoted, since the expression must be evaluated so as to produce the
list that is passed to `sort'.)

Making a List of Files
----------------------

The `recursive-lengths-list-many-files' function requires a list of
files as its argument.  For our test examples, we constructed such a
list by hand; but the Emacs Lisp source directory is too large for us
to do for that.  Instead, we will write a function to do the job for
us.  In this function, we will use both a `while' loop and a
recursive call.

We did not have to write a function like this for older versions of
GNU Emacs, since they placed all the `.el' files in one directory.
Instead, we were able to use the `directory-files' function, which
lists the names of files that match a specified pattern within a
single directory.

However, recent versions of Emacs place Emacs Lisp files in
sub-directories of the top level `lisp' directory.  This
re-arrangement eases navigation.  For example, all the mail related
files are in a `lisp' sub-directory called `mail'.  But at the same
time, this arrangement forces us to create a file listing function
that descends into the sub-directories.

We can create this function, called `files-in-below-directory', using
familiar functions such as `car', `nthcdr', and `substring' in
conjunction with an existing function called
`directory-files-and-attributes'.  This latter function not only
lists all the filenames in a directory, including the names of
sub-directories, but also their attributes.

To restate our goal: to create a function that will enable us to feed
filenames to `recursive-lengths-list-many-files' as a list that looks
like this (but with more elements):

     ("../lisp/macros.el"
      "../lisp/mail/rmail.el"
      "../lisp/makesum.el")

The `directory-files-and-attributes' function returns a list of
lists.  Each of the lists within the main list consists of 13
elements.  The first element is a string that contains the name of the
file - which, in GNU/Linux, may be a `directory file', that is to
say, a file with the special attributes of a directory.  The second
element of the list is `t' for a directory, a string for symbolic
link (the string is the name linked to), or `nil'.

For example, the first `.el' file in the `lisp/' directory is
`abbrev.el'.  Its name is
`/usr/local/share/emacs/21.0.100/lisp/abbrev.el' and it is not a
directory or a symbolic link.

This is how `directory-files-and-attributes' lists that file and its
attributes:

     ("/usr/local/share/emacs/21.0.100/lisp/abbrev.el"
     nil
     1
     1000
     100
     (15019 32380)
     (14883 48041)
     (15214 49336)
     11583
     "-rw-rw-r--"
     t
     341385
     776)

On the other hand, `mail/' is a directory within the `lisp/'
directory.  The beginning of its listing looks like this:

     ("/usr/local/share/emacs/21.0.100/lisp/mail"
     t
     ...
     )

(Look at the documentation of `file-attributes' to learn about the
different attributes.  Bear in mind that the `file-attributes'
function does not list the filename, so its first element is
`directory-files-and-attributes''s second element.)

We will want our new function, `files-in-below-directory', to list
the `.el' files in the directory it is told to check, and in any
directories below that directory.

This gives us a hint on how to construct `files-in-below-directory':
within a directory, the function should add `.el' filenames to a
list; and if, within a directory, the function comes upon a
sub-directory, it should go into that sub-directory and repeat its
actions.

However, we should note that every directory contains a name that
refers to itself, called `.', ("dot") and a name that refers to its
parent directory, called `..' ("double dot").  (In `/', the root
directory, `..' refers to itself, since `/' has no parent.)  Clearly,
we do not want our `files-in-below-directory' function to enter those
directories, since they always lead us, directly or indirectly, to
the current directory.

Consequently, our `files-in-below-directory' function must do several
tasks:

   * Check to see whether it is looking at a filename that ends in
     `.el'; and if so, add its name to a list.

   * Check to see whether it is looking at a filename that is the
     name of a directory; and if so,

        - Check to see whether it is looking at `.'  or `..'; and if
          so skip it.

        - Or else, go into that directory and repeat the process.

Let's write a function definition to do these tasks.  We will use a
`while' loop to move from one filename to another within a directory,
checking what needs to be done; and we will use a recursive call to
repeat the actions on each sub-directory.  The recursive pattern is
`accumulate' (*note Recursive Pattern: _accumulate_: Accumulate.),
using `append' as the combiner.

Here is the function:

     (defun files-in-below-directory (directory)
       "List the .el files in DIRECTORY and in its sub-directories."
       ;; Although the function will be used non-interactively,
       ;; it will be easier to test if we make it interactive.
       ;; The directory will have a name such as
       ;;  "/usr/local/share/emacs/21.0.100/lisp/"
       (interactive "DDirectory name: ")
       (let (el-files-list
             (current-directory-list
              (directory-files-and-attributes directory t)))
         ;; while we are in the current directory
         (while current-directory-list
           (cond
            ;; check to see whether filename ends in `.el'
            ;; and if so, append its name to a list.
            ((equal ".el" (substring (car (car current-directory-list)) -3))
             (setq el-files-list
                   (cons (car (car current-directory-list)) el-files-list)))
            ;; check whether filename is that of a directory
            ((eq t (car (cdr (car current-directory-list))))
             ;; decide whether to skip or recurse
             (if
                 (equal (or "." "..")
                        (substring (car (car current-directory-list)) -1))
                 ;; then do nothing if filename is that of
                 ;;   current directory or parent
                 ()
               ;; else descend into the directory and repeat the process
               (setq el-files-list
                     (append
                      (files-in-below-directory
                       (car (car current-directory-list)))
                      el-files-list)))))
           ;; move to the next filename in the list; this also
           ;; shortens the list so the while loop eventually comes to an end
           (setq current-directory-list (cdr current-directory-list)))
         ;; return the filenames
         el-files-list))

The `files-in-below-directory' `directory-files' function takes one
argument, the name of a directory.

Thus, on my system,

     (length
      (files-in-below-directory "/usr/local/share/emacs/21.0.100/lisp/"))

tells me that my version 21.0.100 Lisp sources directory contains 754
`.el' files.

`files-in-below-directory' returns a list in reverse alphabetical
order.  An expression to sort the list in alphabetical order looks
like this:

     (sort
      (files-in-below-directory "/usr/local/share/emacs/21.0.100/lisp/")
      'string-lessp)

Counting function definitions
-----------------------------

Our immediate goal is to generate a list that tells us how many
function definitions contain fewer than 10 words and symbols, how many
contain between 10 and 19 words and symbols, how many contain between
20 and 29 words and symbols, and so on.

With a sorted list of numbers, this is easy: count how many elements
of the list are smaller than 10, then, after moving past the numbers
just counted, count how many are smaller than 20, then, after moving
past the numbers just counted, count how many are smaller than 30, and
so on.  Each of the numbers, 10, 20, 30, 40, and the like, is one
larger than the top of that range.  We can call the list of such
numbers the `top-of-ranges' list.

If we wished, we could generate this list automatically, but it is
simpler to write a list manually.  Here it is:

     (defvar top-of-ranges
      '(10  20  30  40  50
        60  70  80  90 100
       110 120 130 140 150
       160 170 180 190 200
       210 220 230 240 250
       260 270 280 290 300)
      "List specifying ranges for `defuns-per-range'.")

To change the ranges, we edit this list.

Next, we need to write the function that creates the list of the
number of definitions within each range.  Clearly, this function must
take the `sorted-lengths' and the `top-of-ranges' lists as arguments.

The `defuns-per-range' function must do two things again and again:
it must count the number of definitions within a range specified by
the current top-of-range value; and it must shift to the next higher
value in the `top-of-ranges' list after counting the number of
definitions in the current range.  Since each of these actions is
repetitive, we can use `while' loops for the job.  One loop counts
the number of definitions in the range defined by the current
top-of-range value, and the other loop selects each of the
top-of-range values in turn.

Several entries of the `sorted-lengths' list are counted for each
range; this means that the loop for the `sorted-lengths' list will be
inside the loop for the `top-of-ranges' list, like a small gear
inside a big gear.

The inner loop counts the number of definitions within the range.  It
is a simple counting loop of the type we have seen before.  (*Note A
loop with an incrementing counter: Incrementing Loop.)  The
true-or-false test of the loop tests whether the value from the
`sorted-lengths' list is smaller than the current value of the top of
the range.  If it is, the function increments the counter and tests
the next value from the `sorted-lengths' list.

The inner loop looks like this:

     (while LENGTH-ELEMENT-SMALLER-THAN-TOP-OF-RANGE
       (setq number-within-range (1+ number-within-range))
       (setq sorted-lengths (cdr sorted-lengths)))

The outer loop must start with the lowest value of the
`top-of-ranges' list, and then be set to each of the succeeding
higher values in turn.  This can be done with a loop like this:

     (while top-of-ranges
       BODY-OF-LOOP...
       (setq top-of-ranges (cdr top-of-ranges)))

Put together, the two loops look like this:

     (while top-of-ranges
     
       ;; Count the number of elements within the current range.
       (while LENGTH-ELEMENT-SMALLER-THAN-TOP-OF-RANGE
         (setq number-within-range (1+ number-within-range))
         (setq sorted-lengths (cdr sorted-lengths)))
     
       ;; Move to next range.
       (setq top-of-ranges (cdr top-of-ranges)))

In addition, in each circuit of the outer loop, Emacs should record
the number of definitions within that range (the value of
`number-within-range') in a list.  We can use `cons' for this
purpose.  (*Note `cons': cons.)

The `cons' function works fine, except that the list it constructs
will contain the number of definitions for the highest range at its
beginning and the number of definitions for the lowest range at its
end.  This is because `cons' attaches new elements of the list to the
beginning of the list, and since the two loops are working their way
through the lengths' list from the lower end first, the
`defuns-per-range-list' will end up largest number first.  But we
will want to print our graph with smallest values first and the
larger later.  The solution is to reverse the order of the
`defuns-per-range-list'.  We can do this using the `nreverse'
function, which reverses the order of a list.

For example,

     (nreverse '(1 2 3 4))

produces:

     (4 3 2 1)

Note that the `nreverse' function is "destructive"--that is, it
changes the list to which it is applied; this contrasts with the
`car' and `cdr' functions, which are non-destructive.  In this case,
we do not want the original `defuns-per-range-list', so it does not
matter that it is destroyed.  (The `reverse' function provides a
reversed copy of a list, leaving the original list as is.)

Put all together, the `defuns-per-range' looks like this:

     (defun defuns-per-range (sorted-lengths top-of-ranges)
       "SORTED-LENGTHS defuns in each TOP-OF-RANGES range."
       (let ((top-of-range (car top-of-ranges))
             (number-within-range 0)
             defuns-per-range-list)
     
         ;; Outer loop.
         (while top-of-ranges
     
           ;; Inner loop.
           (while (and
                   ;; Need number for numeric test.
                   (car sorted-lengths)
                   (< (car sorted-lengths) top-of-range))
     
             ;; Count number of definitions within current range.
             (setq number-within-range (1+ number-within-range))
             (setq sorted-lengths (cdr sorted-lengths)))
     
           ;; Exit inner loop but remain within outer loop.
     
           (setq defuns-per-range-list
                 (cons number-within-range defuns-per-range-list))
           (setq number-within-range 0)      ; Reset count to zero.
     
           ;; Move to next range.
           (setq top-of-ranges (cdr top-of-ranges))
           ;; Specify next top of range value.
           (setq top-of-range (car top-of-ranges)))
     
         ;; Exit outer loop and count the number of defuns larger than
         ;;   the largest top-of-range value.
         (setq defuns-per-range-list
               (cons
                (length sorted-lengths)
                defuns-per-range-list))
     
         ;; Return a list of the number of definitions within each range,
         ;;   smallest to largest.
         (nreverse defuns-per-range-list)))

The function is straightforward except for one subtle feature.  The
true-or-false test of the inner loop looks like this:

     (and (car sorted-lengths)
          (< (car sorted-lengths) top-of-range))

instead of like this:

     (< (car sorted-lengths) top-of-range)

The purpose of the test is to determine whether the first item in the
`sorted-lengths' list is less than the value of the top of the range.

The simple version of the test works fine unless the `sorted-lengths'
list has a `nil' value.  In that case, the `(car sorted-lengths)'
expression function returns `nil'.  The `<' function cannot compare a
number to `nil', which is an empty list, so Emacs signals an error and
stops the function from attempting to continue to execute.

The `sorted-lengths' list always becomes `nil' when the counter
reaches the end of the list.  This means that any attempt to use the
`defuns-per-range' function with the simple version of the test will
fail.

We solve the problem by using the `(car sorted-lengths)' expression
in conjunction with the `and' expression.  The `(car sorted-lengths)'
expression returns a non-`nil' value so long as the list has at least
one number within it, but returns `nil' if the list is empty.  The
`and' expression first evaluates the `(car sorted-lengths)'
expression, and if it is `nil', returns false _without_ evaluating the
`<' expression.  But if the `(car sorted-lengths)' expression returns
a non-`nil' value, the `and' expression evaluates the `<' expression,
and returns that value as the value of the `and' expression.

This way, we avoid an error.  *Note forward-paragraph::, for more
information about `and'.

Here is a short test of the `defuns-per-range' function.  First,
evaluate the expression that binds (a shortened) `top-of-ranges' list
to the list of values, then evaluate the expression for binding the
`sorted-lengths' list, and then evaluate the `defuns-per-range'
function.

     ;; (Shorter list than we will use later.)
     (setq top-of-ranges
      '(110 120 130 140 150
        160 170 180 190 200))
     
     (setq sorted-lengths
           '(85 86 110 116 122 129 154 176 179 200 265 300 300))
     
     (defuns-per-range sorted-lengths top-of-ranges)

The list returned looks like this:

     (2 2 2 0 0 1 0 2 0 0 4)

Indeed, there are two elements of the `sorted-lengths' list smaller
than 110, two elements between 110 and 119, two elements between 120
and 129, and so on.  There are four elements with a value of 200 or
larger.

Readying a Graph
****************

Our goal is to construct a graph showing the numbers of function
definitions of various lengths in the Emacs lisp sources.

As a practical matter, if you were creating a graph, you would
probably use a program such as `gnuplot' to do the job.  (`gnuplot'
is nicely integrated into GNU Emacs.)  In this case, however, we
create one from scratch, and in the process we will re-acquaint
ourselves with some of what we learned before and learn more.

In this chapter, we will first write a simple graph printing function.
This first definition will be a "prototype", a rapidly written
function that enables us to reconnoiter this unknown graph-making
territory.  We will discover dragons, or find that they are myth.
After scouting the terrain, we will feel more confident and enhance
the function to label the axes automatically.

Printing the Columns of a Graph
===============================

Since Emacs is designed to be flexible and work with all kinds of
terminals, including character-only terminals, the graph will need to
be made from one of the `typewriter' symbols.  An asterisk will do; as
we enhance the graph-printing function, we can make the choice of
symbol a user option.

We can call this function `graph-body-print'; it will take a
`numbers-list' as its only argument.  At this stage, we will not
label the graph, but only print its body.

The `graph-body-print' function inserts a vertical column of
asterisks for each element in the `numbers-list'.  The height of each
line is determined by the value of that element of the `numbers-list'.

Inserting columns is a repetitive act; that means that this function
can be written either with a `while' loop or recursively.

Our first challenge is to discover how to print a column of asterisks.
Usually, in Emacs, we print characters onto a screen horizontally,
line by line, by typing.  We have two routes we can follow: write our
own column-insertion function or discover whether one exists in Emacs.

To see whether there is one in Emacs, we can use the `M-x apropos'
command.  This command is like the `C-h a' (command-apropos) command,
except that the latter finds only those functions that are commands.
The `M-x apropos' command lists all symbols that match a regular
expression, including functions that are not interactive.

What we want to look for is some command that prints or inserts
columns.  Very likely, the name of the function will contain either
the word `print' or the word `insert' or the word `column'.
Therefore, we can simply type `M-x apropos RET print\|insert\|column
RET' and look at the result.  On my system, this command takes quite
some time, and then produces a list of 79 functions and variables.
Scanning down the list, the only function that looks as if it might
do the job is `insert-rectangle'.

Indeed, this is the function we want; its documentation says:

     insert-rectangle:
     Insert text of RECTANGLE with upper left corner at point.
     RECTANGLE's first line is inserted at point,
     its second line is inserted at a point vertically under point, etc.
     RECTANGLE should be a list of strings.

We can run a quick test, to make sure it does what we expect of it.

Here is the result of placing the cursor after the `insert-rectangle'
expression and typing `C-u C-x C-e' (`eval-last-sexp').  The function
inserts the strings `"first"', `"second"', and `"third"' at and below
point.  Also the function returns `nil'.

     (insert-rectangle '("first" "second" "third"))first
                                                   second
                                                   third
     nil

Of course, we won't be inserting the text of the `insert-rectangle'
expression itself into the buffer in which we are making the graph,
but will call the function from our program.  We shall, however, have
to make sure that point is in the buffer at the place where the
`insert-rectangle' function will insert its column of strings.

If you are reading this in Info, you can see how this works by
switching to another buffer, such as the `*scratch*' buffer, placing
point somewhere in the buffer, typing `M-:', typing the
`insert-rectangle' expression into the minibuffer at the prompt, and
then typing <RET>.  This causes Emacs to evaluate the expression in
the minibuffer, but to use as the value of point the position of
point in the `*scratch*' buffer.  (`M-:' is the keybinding for
`eval-expression'.)

We find when we do this that point ends up at the end of the last
inserted line--that is to say, this function moves point as a
side-effect.  If we were to repeat the command, with point at this
position, the next insertion would be below and to the right of the
previous insertion.  We don't want this!  If we are going to make a
bar graph, the columns need to be beside each other.

So we discover that each cycle of the column-inserting `while' loop
must reposition point to the place we want it, and that place will be
at the top, not the bottom, of the column.  Moreover, we remember
that when we print a graph, we do not expect all the columns to be
the same height.  This means that the top of each column may be at a
different height from the previous one.  We cannot simply reposition
point to the same line each time, but moved over to the right--or
perhaps we can...

We are planning to make the columns of the bar graph out of asterisks.
The number of asterisks in the column is the number specified by the
current element of the `numbers-list'.  We need to construct a list
of asterisks of the right length for each call to `insert-rectangle'.
If this list consists solely of the requisite number of asterisks,
then we will have position point the right number of lines above the
base for the graph to print correctly.  This could be difficult.

Alternatively, if we can figure out some way to pass
`insert-rectangle' a list of the same length each time, then we can
place point on the same line each time, but move it over one column
to the right for each new column.  If we do this, however, some of
the entries in the list passed to `insert-rectangle' must be blanks
rather than asterisks.  For example, if the maximum height of the
graph is 5, but the height of the column is 3, then
`insert-rectangle' requires an argument that looks like this:

     (" " " " "*" "*" "*")

This last proposal is not so difficult, so long as we can determine
the column height.  There are two ways for us to specify the column
height: we can arbitrarily state what it will be, which would work
fine for graphs of that height; or we can search through the list of
numbers and use the maximum height of the list as the maximum height
of the graph.  If the latter operation were difficult, then the former
procedure would be easiest, but there is a function built into Emacs
that determines the maximum of its arguments.  We can use that
function.  The function is called `max' and it returns the largest of
all its arguments, which must be numbers.  Thus, for example,

     (max  3 4 6 5 7 3)

returns 7.  (A corresponding function called `min' returns the
smallest of all its arguments.)

However, we cannot simply call `max' on the `numbers-list'; the `max'
function expects numbers as its argument, not a list of numbers.
Thus, the following expression,

     (max  '(3 4 6 5 7 3))

produces the following error message;

     Wrong type of argument:  number-or-marker-p, (3 4 6 5 7 3)

We need a function that passes a list of arguments to a function.
This function is `apply'.  This function `applies' its first argument
(a function) to its remaining arguments, the last of which may be a
list.

For example,

     (apply 'max 3 4 7 3 '(4 8 5))

returns 8.

(Incidentally, I don't know how you would learn of this function
without a book such as this.  It is possible to discover other
functions, like `search-forward' or `insert-rectangle', by guessing
at a part of their names and then using `apropos'.  Even though its
base in metaphor is clear--`apply' its first argument to the rest--I
doubt a novice would come up with that particular word when using
`apropos' or other aid.  Of course, I could be wrong; after all, the
function was first named by someone who had to invent it.)

The second and subsequent arguments to `apply' are optional, so we
can use `apply' to call a function and pass the elements of a list to
it, like this, which also returns 8:

     (apply 'max '(4 8 5))

This latter way is how we will use `apply'.  The
`recursive-lengths-list-many-files' function returns a numbers' list
to which we can apply `max' (we could also apply `max' to the sorted
numbers' list; it does not matter whether the list is sorted or not.)

Hence, the operation for finding the maximum height of the graph is
this:

     (setq max-graph-height (apply 'max numbers-list))

Now we can return to the question of how to create a list of strings
for a column of the graph.  Told the maximum height of the graph and
the number of asterisks that should appear in the column, the
function should return a list of strings for the `insert-rectangle'
command to insert.

Each column is made up of asterisks or blanks.  Since the function is
passed the value of the height of the column and the number of
asterisks in the column, the number of blanks can be found by
subtracting the number of asterisks from the height of the column.
Given the number of blanks and the number of asterisks, two `while'
loops can be used to construct the list:

     ;;; First version.
     (defun column-of-graph (max-graph-height actual-height)
       "Return list of strings that is one column of a graph."
       (let ((insert-list nil)
             (number-of-top-blanks
              (- max-graph-height actual-height)))
     
         ;; Fill in asterisks.
         (while (> actual-height 0)
           (setq insert-list (cons "*" insert-list))
           (setq actual-height (1- actual-height)))
     
         ;; Fill in blanks.
         (while (> number-of-top-blanks 0)
           (setq insert-list (cons " " insert-list))
           (setq number-of-top-blanks
                 (1- number-of-top-blanks)))
     
         ;; Return whole list.
         insert-list))

If you install this function and then evaluate the following
expression you will see that it returns the list as desired:

     (column-of-graph 5 3)

returns

     (" " " " "*" "*" "*")

As written, `column-of-graph' contains a major flaw: the symbols used
for the blank and for the marked entries in the column are
`hard-coded' as a space and asterisk.  This is fine for a prototype,
but you, or another user, may wish to use other symbols.  For example,
in testing the graph function, you many want to use a period in place
of the space, to make sure the point is being repositioned properly
each time the `insert-rectangle' function is called; or you might
want to substitute a `+' sign or other symbol for the asterisk.  You
might even want to make a graph-column that is more than one display
column wide.  The program should be more flexible.  The way to do
that is to replace the blank and the asterisk with two variables that
we can call `graph-blank' and `graph-symbol' and define those
variables separately.

Also, the documentation is not well written.  These considerations
lead us to the second version of the function:

     (defvar graph-symbol "*"
       "String used as symbol in graph, usually an asterisk.")
     
     (defvar graph-blank " "
       "String used as blank in graph, usually a blank space.
     graph-blank must be the same number of columns wide
     as graph-symbol.")

(For an explanation of `defvar', see *Note Initializing a Variable
with `defvar': defvar.)

     ;;; Second version.
     (defun column-of-graph (max-graph-height actual-height)
       "Return MAX-GRAPH-HEIGHT strings; ACTUAL-HEIGHT are graph-symbols.
     The graph-symbols are contiguous entries at the end
     of the list.
     The list will be inserted as one column of a graph.
     The strings are either graph-blank or graph-symbol."
     
       (let ((insert-list nil)
             (number-of-top-blanks
              (- max-graph-height actual-height)))
     
         ;; Fill in `graph-symbols'.
         (while (> actual-height 0)
           (setq insert-list (cons graph-symbol insert-list))
           (setq actual-height (1- actual-height)))
     
         ;; Fill in `graph-blanks'.
         (while (> number-of-top-blanks 0)
           (setq insert-list (cons graph-blank insert-list))
           (setq number-of-top-blanks
                 (1- number-of-top-blanks)))
     
         ;; Return whole list.
         insert-list))

If we wished, we could rewrite `column-of-graph' a third time to
provide optionally for a line graph as well as for a bar graph.  This
would not be hard to do.  One way to think of a line graph is that it
is no more than a bar graph in which the part of each bar that is
below the top is blank.  To construct a column for a line graph, the
function first constructs a list of blanks that is one shorter than
the value, then it uses `cons' to attach a graph symbol to the list;
then it uses `cons' again to attach the `top blanks' to the list.

It is easy to see how to write such a function, but since we don't
need it, we will not do it.  But the job could be done, and if it were
done, it would be done with `column-of-graph'.  Even more important,
it is worth noting that few changes would have to be made anywhere
else.  The enhancement, if we ever wish to make it, is simple.

Now, finally, we come to our first actual graph printing function.
This prints the body of a graph, not the labels for the vertical and
horizontal axes, so we can call this `graph-body-print'.

The `graph-body-print' Function
===============================

After our preparation in the preceding section, the
`graph-body-print' function is straightforward.  The function will
print column after column of asterisks and blanks, using the elements
of a numbers' list to specify the number of asterisks in each column.
This is a repetitive act, which means we can use a decrementing
`while' loop or recursive function for the job.  In this section, we
will write the definition using a `while' loop.

The `column-of-graph' function requires the height of the graph as an
argument, so we should determine and record that as a local variable.

This leads us to the following template for the `while' loop version
of this function:

     (defun graph-body-print (numbers-list)
       "DOCUMENTATION..."
       (let ((height  ...
              ...))
     
         (while numbers-list
           INSERT-COLUMNS-AND-REPOSITION-POINT
           (setq numbers-list (cdr numbers-list)))))

We need to fill in the slots of the template.

Clearly, we can use the `(apply 'max numbers-list)' expression to
determine the height of the graph.

The `while' loop will cycle through the `numbers-list' one element at
a time.  As it is shortened by the `(setq numbers-list (cdr
numbers-list))' expression, the CAR of each instance of the list is
the value of the argument for `column-of-graph'.

At each cycle of the `while' loop, the `insert-rectangle' function
inserts the list returned by `column-of-graph'.  Since the
`insert-rectangle' function moves point to the lower right of the
inserted rectangle, we need to save the location of point at the time
the rectangle is inserted, move back to that position after the
rectangle is inserted, and then move horizontally to the next place
from which `insert-rectangle' is called.

If the inserted columns are one character wide, as they will be if
single blanks and asterisks are used, the repositioning command is
simply `(forward-char 1)'; however, the width of a column may be
greater than one.  This means that the repositioning command should be
written `(forward-char symbol-width)'.  The `symbol-width' itself is
the length of a `graph-blank' and can be found using the expression
`(length graph-blank)'.  The best place to bind the `symbol-width'
variable to the value of the width of graph column is in the varlist
of the `let' expression.

These considerations lead to the following function definition:

     (defun graph-body-print (numbers-list)
       "Print a bar graph of the NUMBERS-LIST.
     The numbers-list consists of the Y-axis values."
     
       (let ((height (apply 'max numbers-list))
             (symbol-width (length graph-blank))
             from-position)
     
         (while numbers-list
           (setq from-position (point))
           (insert-rectangle
            (column-of-graph height (car numbers-list)))
           (goto-char from-position)
           (forward-char symbol-width)
           ;; Draw graph column by column.
           (sit-for 0)
           (setq numbers-list (cdr numbers-list)))
         ;; Place point for X axis labels.
         (forward-line height)
         (insert "\n")
     ))

The one unexpected expression in this function is the `(sit-for 0)'
expression in the `while' loop.  This expression makes the graph
printing operation more interesting to watch than it would be
otherwise.  The expression causes Emacs to `sit' or do nothing for a
zero length of time and then redraw the screen.  Placed here, it
causes Emacs to redraw the screen column by column.  Without it,
Emacs would not redraw the screen until the function exits.

We can test `graph-body-print' with a short list of numbers.

  1. Install `graph-symbol', `graph-blank', `column-of-graph', which
     are in *Note Columns of a graph::, and `graph-body-print'.

  2. Copy the following expression:

          (graph-body-print '(1 2 3 4 6 4 3 5 7 6 5 2 3))

  3. Switch to the `*scratch*' buffer and place the cursor where you
     want the graph to start.

  4. Type `M-:' (`eval-expression').

  5. Yank the `graph-body-print' expression into the minibuffer with
     `C-y' (`yank)'.

  6. Press <RET> to evaluate the `graph-body-print' expression.

Emacs will print a graph like this:

                         *
                     *   **
                     *  ****
                    *** ****
                   ********* *
                  ************
                 *************

The `recursive-graph-body-print' Function
=========================================

The `graph-body-print' function may also be written recursively.  The
recursive solution is divided into two parts: an outside `wrapper'
that uses a `let' expression to determine the values of several
variables that need only be found once, such as the maximum height of
the graph, and an inside function that is called recursively to print
the graph.

The `wrapper' is uncomplicated:

     (defun recursive-graph-body-print (numbers-list)
       "Print a bar graph of the NUMBERS-LIST.
     The numbers-list consists of the Y-axis values."
       (let ((height (apply 'max numbers-list))
             (symbol-width (length graph-blank))
             from-position)
         (recursive-graph-body-print-internal
          numbers-list
          height
          symbol-width)))

The recursive function is a little more difficult.  It has four parts:
the `do-again-test', the printing code, the recursive call, and the
`next-step-expression'.  The `do-again-test' is an `if' expression
that determines whether the `numbers-list' contains any remaining
elements; if it does, the function prints one column of the graph
using the printing code and calls itself again.  The function calls
itself again according to the value produced by the
`next-step-expression' which causes the call to act on a shorter
version of the `numbers-list'.

     (defun recursive-graph-body-print-internal
       (numbers-list height symbol-width)
       "Print a bar graph.
     Used within recursive-graph-body-print function."
     
       (if numbers-list
           (progn
             (setq from-position (point))
             (insert-rectangle
              (column-of-graph height (car numbers-list)))
             (goto-char from-position)
             (forward-char symbol-width)
             (sit-for 0)     ; Draw graph column by column.
             (recursive-graph-body-print-internal
              (cdr numbers-list) height symbol-width))))

After installation, this expression can be tested; here is a sample:

     (recursive-graph-body-print '(3 2 5 6 7 5 3 4 6 4 3 2 1))

Here is what `recursive-graph-body-print' produces:

                     *
                    **   *
                   ****  *
                   **** ***
                 * *********
                 ************
                 *************

Either of these two functions, `graph-body-print' or
`recursive-graph-body-print', create the body of a graph.

Need for Printed Axes
=====================

A graph needs printed axes, so you can orient yourself.  For a do-once
project, it may be reasonable to draw the axes by hand using Emacs'
Picture mode; but a graph drawing function may be used more than once.

For this reason, I have written enhancements to the basic
`print-graph-body' function that automatically print labels for the
horizontal and vertical axes.  Since the label printing functions do
not contain much new material, I have placed their description in an
appendix.  *Note A Graph with Labelled Axes: Full Graph.

Exercise
========

Write a line graph version of the graph printing functions.

Your `.emacs' File
******************

"You don't have to like Emacs to like it" - this seemingly
paradoxical statement is the secret of GNU Emacs.  The plain, `out of
the box' Emacs is a generic tool.  Most people who use it, customize
it to suit themselves.

GNU Emacs is mostly written in Emacs Lisp; this means that by writing
expressions in Emacs Lisp you can change or extend Emacs.

Emacs' Default Configuration
============================

There are those who appreciate Emacs' default configuration.  After
all, Emacs starts you in C mode when you edit a C file, starts you in
Fortran mode when you edit a Fortran file, and starts you in
Fundamental mode when you edit an unadorned file.  This all makes
sense, if you do not know who is going to use Emacs.  Who knows what a
person hopes to do with an unadorned file?  Fundamental mode is the
right default for such a file, just as C mode is the right default for
editing C code.  But when you do know who is going to use Emacs--you,
yourself--then it makes sense to customize Emacs.

For example, I seldom want Fundamental mode when I edit an otherwise
undistinguished file; I want Text mode.  This is why I customize
Emacs: so it suits me.

You can customize and extend Emacs by writing or adapting a
`~/.emacs' file.  This is your personal initialization file; its
contents, written in Emacs Lisp, tell Emacs what to do.(1)

A `~/.emacs' file contains Emacs Lisp code.  You can write this code
yourself; or you can use Emacs' `customize' feature to write the code
for you.  You can combine your own expressions and auto-written
Customize expressions in your `.emacs' file.

(I myself prefer to write my own expressions, except for those,
particularly fonts, that I find easier to manipulate using the
`customize' command.  I combine the two methods.)

Most of this chapter is about writing expressions yourself.  It
describes a simple `.emacs' file; for more information, see *Note The
Init File: (emacs)Init File, and *Note The Init File: (elisp)Init
File.

---------- Footnotes ----------

(1) You may also add `.el' to `~/.emacs' and call it a `~/.emacs.el'
file.  In the past, you were forbidden to type the extra keystrokes
that the name `~/.emacs.el' requires, but now you may.  The new
format is consistent with the Emacs Lisp file naming conventions; the
old format saves typing.

Site-wide Initialization Files
==============================

In addition to your personal initialization file, Emacs automatically
loads various site-wide initialization files, if they exist.  These
have the same form as your `.emacs' file, but are loaded by everyone.

Two site-wide initialization files, `site-load.el' and
`site-init.el', are loaded into Emacs and then `dumped' if a `dumped'
version of Emacs is created, as is most common.  (Dumped copies of
Emacs load more quickly.  However, once a file is loaded and dumped,
a change to it does not lead to a change in Emacs unless you load it
yourself or re-dump Emacs.  *Note Building Emacs: (elisp)Building
Emacs, and the `INSTALL' file.)

Three other site-wide initialization files are loaded automatically
each time you start Emacs, if they exist.  These are `site-start.el',
which is loaded _before_ your `.emacs' file, and `default.el', and
the terminal type file, which are both loaded _after_ your `.emacs'
file.

Settings and definitions in your `.emacs' file will overwrite
conflicting settings and definitions in a `site-start.el' file, if it
exists; but the settings and definitions in a `default.el' or
terminal type file will overwrite those in your `.emacs' file.  (You
can prevent interference from a terminal type file by setting
`term-file-prefix' to `nil'.  *Note A Simple Extension: Simple
Extension.)

The `INSTALL' file that comes in the distribution contains
descriptions of the `site-init.el' and `site-load.el' files.

The `loadup.el', `startup.el', and `loaddefs.el' files control
loading.  These files are in the `lisp' directory of the Emacs
distribution and are worth perusing.

The `loaddefs.el' file contains a good many suggestions as to what to
put into your own `.emacs' file, or into a site-wide initialization
file.

Specifying Variables using `defcustom'
======================================

You can specify variables using `defcustom' so that you and others
can then use Emacs' `customize' feature to set their values.  (You
cannot use `customize' to write function definitions; but you can
write `defuns' in your `.emacs' file.  Indeed, you can write any Lisp
expression in your `.emacs' file.)

The `customize' feature depends on the `defcustom' special form.
Although you can use `defvar' or `setq' for variables that users set,
the `defcustom' special form is designed for the job.

You can use your knowledge of `defvar' for writing the first three
arguments for `defcustom'.  The first argument to `defcustom' is the
name of the variable.  The second argument is the variable's initial
value, if any; and this value is set only if the value has not
already been set.  The third argument is the documentation.

The fourth and subsequent arguments to `defcustom' specify types and
options; these are not featured in `defvar'.  (These arguments are
optional.)

Each of these arguments consists of a keyword followed by a value.
Each keyword starts with the character `:'.

For example, the customizable user option variable `text-mode-hook'
looks like this:

     (defcustom text-mode-hook nil
       "Normal hook run when entering Text mode and many related modes."
       :type 'hook
       :options '(turn-on-auto-fill flyspell-mode)
       :group 'data)

The name of the variable is `text-mode-hook'; it has no default
value; and its documentation string tells you what it does.

The `:type' keyword tells Emacs what kind of data `text-mode-hook'
should be set to and how to display the value in a Customization
buffer.

The `:options' keyword specifies a suggested list of values for the
variable.  Currently, you can use `:options' only for a hook.  The
list is only a suggestion; it is not exclusive; a person who sets the
variable may set it to other values; the list shown following the
`:options' keyword is intended to offer convenient choices to a user.

Finally, the `:group' keyword tells the Emacs Customization command
in which group the variable is located.  This tells where to find it.

For more information, see *Note Writing Customization Definitions:
(elisp)Customization.

Consider `text-mode-hook' as an example.

There are two ways to customize this variable.  You can use the
customization command or write the appropriate expressions yourself.

Using the customization command,  you can type:

     M-x customize

and find that the group for editing files of data is called `data'.
Enter that group.  Text Mode Hook is the first member.  You can click
on its various options to set the values.  After you click on the
button to

     Save for Future Sessions

Emacs will write an expression into your `.emacs' file.  It will look
like this:

     (custom-set-variables
       ;; custom-set-variables was added by Custom --
       ;;                           don't edit or cut/paste it!
       ;; Your init file should contain only one such instance.
      '(text-mode-hook (quote (turn-on-auto-fill text-mode-hook-identify))))

(The `text-mode-hook-identify' function tells
`toggle-text-mode-auto-fill' which buffers are in Text mode.)

In spite of the warning, you certainly may edit, cut, and paste the
expression!  I do all time.  The purpose of the warning is to scare
those who do not know what they are doing, so they do not
inadvertently generate an error.

The `custom-set-variables' works somewhat differently than a `setq'.
While I have never learned the differences, I do modify the
`custom-set-variables' expressions in my `.emacs' file by hand:  I
make the changes in what appears to me to be a reasonable manner and
have not had any problems.  Others prefer to use the Customization
command and let Emacs do the work for them.

Another `custom-set-...' function is `custom-set-faces'.  This
function sets the various font faces.  Over time, I have set a
considerable number of faces.  Some of the time, I re-set them using
`customize'; other times, I simply edit the `custom-set-faces'
expression in my `.emacs' file itself.

The second way to customize your `text-mode-hook' is to set it
yourself in your `.emacs' file using code that has nothing to do with
the `custom-set-...' functions.

When you do this, and later use `customize', you will see a message
that says

     this option has been changed outside the customize buffer.

This message is only a warning.  If you click on the button to

     Save for Future Sessions

Emacs will write a `custom-set-...' expression near the end of your
`.emacs' file that will be evaluated after your hand-written
expression.  It will, therefore, overrule your hand-written
expression.  No harm will be done.  When you do this, however, be
careful to remember which expression is active; if you forget, you
may confuse yourself.

So long as you remember where the values are set, you will have no
trouble.  In any event, the values are always set in your
initialization file, which is usually called `.emacs'.

I myself use `customize' for hardly anything.  Mostly, I write
expressions myself.

Beginning a `.emacs' File
=========================

When you start Emacs, it loads your `.emacs' file unless you tell it
not to by specifying `-q' on the command line.  (The `emacs -q'
command gives you a plain, out-of-the-box Emacs.)

A `.emacs' file contains Lisp expressions.  Often, these are no more
than expressions to set values; sometimes they are function
definitions.

*Note The Init File `~/.emacs': (emacs)Init File, for a short
description of initialization files.

This chapter goes over some of the same ground, but is a walk among
extracts from a complete, long-used `.emacs' file--my own.

The first part of the file consists of comments: reminders to myself.
By now, of course, I remember these things, but when I started, I did
not.

     ;;;; Bob's .emacs file
     ; Robert J. Chassell
     ; 26 September 1985

Look at that date!  I started this file a long time ago.  I have been
adding to it ever since.

     ; Each section in this file is introduced by a
     ; line beginning with four semicolons; and each
     ; entry is introduced by a line beginning with
     ; three semicolons.

This describes the usual conventions for comments in Emacs Lisp.
Everything on a line that follows a semicolon is a comment.  Two,
three, and four semicolons are used as section and subsection
markers.  (*Note Comments: (elisp)Comments, for more about comments.)

     ;;;; The Help Key
     ; Control-h is the help key;
     ; after typing control-h, type a letter to
     ; indicate the subject about which you want help.
     ; For an explanation of the help facility,
     ; type control-h two times in a row.

Just remember: type `C-h' two times for help.

     ; To find out about any mode, type control-h m
     ; while in that mode.  For example, to find out
     ; about mail mode, enter mail mode and then type
     ; control-h m.

`Mode help', as I call this, is very helpful.  Usually, it tells you
all you need to know.

Of course, you don't need to include comments like these in your
`.emacs' file.  I included them in mine because I kept forgetting
about Mode help or the conventions for comments--but I was able to
remember to look here to remind myself.

Text and Auto Fill Mode
=======================

Now we come to the part that `turns on' Text mode and Auto Fill mode.

     ;;; Text mode and Auto Fill mode
     ; The next three lines put Emacs into Text mode
     ; and Auto Fill mode, and are for writers who
     ; want to start writing prose rather than code.
     
     (setq default-major-mode 'text-mode)
     (add-hook 'text-mode-hook 'text-mode-hook-identify)
     (add-hook 'text-mode-hook 'turn-on-auto-fill)

Here is the first part of this `.emacs' file that does something
besides remind a forgetful human!

The first of the two lines in parentheses tells Emacs to turn on Text
mode when you find a file, _unless_ that file should go into some
other mode, such as C mode.

When Emacs reads a file, it looks at the extension to the file name,
if any.  (The extension is the part that comes after a `.'.)  If the
file ends with a `.c' or `.h' extension then Emacs turns on C mode.
Also, Emacs looks at first nonblank line of the file; if the line
says `-*- C -*-', Emacs turns on C mode.  Emacs possesses a list of
extensions and specifications that it uses automatically.  In
addition, Emacs looks near the last page for a per-buffer, "local
variables list", if any.

*Note How Major Modes are Chosen: (emacs)Choosing Modes.

*Note Local Variables in Files: (emacs)File Variables.

Now, back to the `.emacs' file.

Here is the line again; how does it work?

     (setq default-major-mode 'text-mode)

This line is a short, but complete Emacs Lisp expression.

We are already familiar with `setq'.  It sets the following variable,
`default-major-mode', to the subsequent value, which is `text-mode'.
The single quote mark before `text-mode' tells Emacs to deal directly
with the `text-mode' variable, not with whatever it might stand for.
*Note Setting the Value of a Variable: set & setq, for a reminder of
how `setq' works.  The main point is that there is no difference
between the procedure you use to set a value in your `.emacs' file
and the procedure you use anywhere else in Emacs.

Here are the next two lines:

     (add-hook 'text-mode-hook 'text-mode-hook-identify)
     (add-hook 'text-mode-hook 'turn-on-auto-fill)

In these two lines, the `add-hook' command first adds
`text-mode-hook-identify' to the variable called `text-mode-hook' and
then adds `turn-on-auto-fill' to the variable.

`turn-on-auto-fill' is the name of a program, that, you guessed it!,
turns on Auto Fill mode.  `text-mode-hook-identify' is a function
that tells `toggle-text-mode-auto-fill' which buffers are in Text
mode.

Every time Emacs turns on Text mode, Emacs runs the commands `hooked'
onto Text mode.  So every time Emacs turns on Text mode, Emacs also
turns on Auto Fill mode.

In brief, the first line causes Emacs to enter Text mode when you edit
a file, unless the file name extension, first non-blank line, or local
variables tell Emacs otherwise.

Text mode among other actions, sets the syntax table to work
conveniently for writers.  In Text mode, Emacs considers an apostrophe
as part of a word like a letter; but Emacs does not consider a period
or a space as part of a word.  Thus, `M-f' moves you over `it's'.  On
the other hand, in C mode, `M-f' stops just after the `t' of `it's'.

The second and third lines causes Emacs to turn on Auto Fill mode when
it turns on Text mode.  In Auto Fill mode, Emacs automatically breaks
a line that is too wide and brings the excessively wide part of the
line down to the next line.  Emacs breaks lines between words, not
within them.

When Auto Fill mode is turned off, lines continue to the right as you
type them.  Depending on how you set the value of `truncate-lines',
the words you type either disappear off the right side of the screen,
or else are shown, in a rather ugly and unreadable manner, as a
continuation line on the screen.

In addition, in this part of my `.emacs' file, I tell the Emacs fill
commands to insert two spaces after a colon:

     (setq colon-double-space t)

Mail Aliases
============

Here is a `setq' that `turns on' mail aliases, along with more
reminders.

     ;;; Mail mode
     ; To enter mail mode, type `C-x m'
     ; To enter RMAIL (for reading mail),
     ; type `M-x rmail'
     
     (setq mail-aliases t)

This `setq' command sets the value of the variable `mail-aliases' to
`t'.  Since `t' means true, the line says, in effect, "Yes, use mail
aliases."

Mail aliases are convenient short names for long email addresses or
for lists of email addresses.  The file where you keep your `aliases'
is `~/.mailrc'.  You write an alias like this:

     alias geo george@foobar.wiz.edu

When you write a message to George, address it to `geo'; the mailer
will automatically expand `geo' to the full address.

Indent Tabs Mode
================

By default, Emacs inserts tabs in place of multiple spaces when it
formats a region.  (For example, you might indent many lines of text
all at once with the `indent-region' command.)  Tabs look fine on a
terminal or with ordinary printing, but they produce badly indented
output when you use TeX or Texinfo since TeX ignores tabs.

The following turns off Indent Tabs mode:

     ;;; Prevent Extraneous Tabs
     (setq-default indent-tabs-mode nil)

Note that this line uses `setq-default' rather than the `setq'
command that we have seen before.  The `setq-default' command sets
values only in buffers that do not have their own local values for
the variable.

*Note Tabs vs. Spaces: (emacs)Just Spaces.

*Note Local Variables in Files: (emacs)File Variables.

Some Keybindings
================

Now for some personal keybindings:

     ;;; Compare windows
     (global-set-key "\C-cw" 'compare-windows)

`compare-windows' is a nifty command that compares the text in your
current window with text in the next window.  It makes the comparison
by starting at point in each window, moving over text in each window
as far as they match.  I use this command all the time.

This also shows how to set a key globally, for all modes.

The command is `global-set-key'.  It is followed by the keybinding.
In a `.emacs' file, the keybinding is written as shown: `\C-c' stands
for `control-c', which means `press the control key and the `c' key
at the same time'.  The `w' means `press the `w' key'.  The
keybinding is surrounded by double quotation marks.  In
documentation, you would write this as `C-c w'.  (If you were binding
a <META> key, such as `M-c', rather than a <CTL> key, you would write
`\M-c'.  *Note Rebinding Keys in Your Init File: (emacs)Init
Rebinding, for details.)

The command invoked by the keys is `compare-windows'.  Note that
`compare-windows' is preceded by a single quote; otherwise, Emacs
would first try to evaluate the symbol to determine its value.

These three things, the double quotation marks, the backslash before
the `C', and the single quote mark are necessary parts of keybinding
that I tend to forget.  Fortunately, I have come to remember that I
should look at my existing `.emacs' file, and adapt what is there.

As for the keybinding itself: `C-c w'.  This combines the prefix key,
`C-c', with a single character, in this case, `w'.  This set of keys,
`C-c' followed by a single character, is strictly reserved for
individuals' own use.  (I call these `own' keys, since these are for
my own use.)  You should always be able to create such a keybinding
for your own use without stomping on someone else's keybinding.  If
you ever write an extension to Emacs, please avoid taking any of
these keys for public use.  Create a key like `C-c C-w' instead.
Otherwise, we will run out of `own' keys.

Here is another keybinding, with a comment:

     ;;; Keybinding for `occur'
     ; I use occur a lot, so let's bind it to a key:
     (global-set-key "\C-co" 'occur)

The `occur' command shows all the lines in the current buffer that
contain a match for a regular expression.  Matching lines are shown
in a buffer called `*Occur*'.  That buffer serves as a menu to jump
to occurrences.

Here is how to unbind a key, so it does not work:

     ;;; Unbind `C-x f'
     (global-unset-key "\C-xf")

There is a reason for this unbinding: I found I inadvertently typed
`C-x f' when I meant to type `C-x C-f'.  Rather than find a file, as
I intended, I accidentally set the width for filled text, almost
always to a width I did not want.  Since I hardly ever reset my
default width, I simply unbound the key.

The following rebinds an existing key:

     ;;; Rebind `C-x C-b' for `buffer-menu'
     (global-set-key "\C-x\C-b" 'buffer-menu)

By default, `C-x C-b' runs the `list-buffers' command.  This command
lists your buffers in _another_ window.  Since I almost always want
to do something in that window, I prefer the  `buffer-menu' command,
which not only lists the buffers, but moves point into that window.

Keymaps
=======

Emacs uses "keymaps" to record which keys call which commands.  When
you use `global-set-key' to set the keybinding for a single command
in all parts of Emacs, you are specifying the keybinding in
`current-global-map'.

Specific modes, such as C mode or Text mode, have their own keymaps;
the mode-specific keymaps override the global map that is shared by
all buffers.

The `global-set-key' function binds, or rebinds, the global keymap.
For example, the following binds the key `C-x C-b' to the function
`buffer-menu':

     (global-set-key "\C-x\C-b" 'buffer-menu)

Mode-specific keymaps are bound using the `define-key' function,
which takes a specific keymap as an argument, as well as the key and
the command.  For example, my `.emacs' file contains the following
expression to bind the `texinfo-insert-@group' command to `C-c C-c g':

     (define-key texinfo-mode-map "\C-c\C-cg" 'texinfo-insert-@group)

The `texinfo-insert-@group' function itself is a little extension to
Texinfo mode that inserts `@group' into a Texinfo file.  I use this
command all the time and prefer to type the three strokes `C-c C-c g'
rather than the six strokes `@ g r o u p'.  (`@group' and its
matching `@end group' are commands that keep all enclosed text
together on one page; many multi-line examples in this book are
surrounded by `@group ... @end group'.)

Here is the `texinfo-insert-@group' function definition:

     (defun texinfo-insert-@group ()
       "Insert the string @group in a Texinfo buffer."
       (interactive)
       (beginning-of-line)
       (insert "@group\n"))

(Of course, I could have used Abbrev mode to save typing, rather than
write a function to insert a word; but I prefer key strokes consistent
with other Texinfo mode key bindings.)

You will see numerous `define-key' expressions in `loaddefs.el' as
well as in the various mode libraries, such as `cc-mode.el' and
`lisp-mode.el'.

*Note Customizing Key Bindings: (emacs)Key Bindings, and *Note
Keymaps: (elisp)Keymaps, for more information about keymaps.

Loading Files
=============

Many people in the GNU Emacs community have written extensions to
Emacs.  As time goes by, these extensions are often included in new
releases.  For example, the Calendar and Diary packages are now part
of the standard GNU Emacs.

(Calc, which I consider a vital part of Emacs, would be part of the
standard distribution except that it was so large it was packaged
separately and no one has changed that.)

You can use a `load' command to evaluate a complete file and thereby
install all the functions and variables in the file into Emacs.  For
example:

     (load "~/emacs/slowsplit")

This evaluates, i.e. loads, the `slowsplit.el' file or if it exists,
the faster, byte compiled `slowsplit.elc' file from the `emacs'
sub-directory of your home directory.  The file contains the function
`split-window-quietly', which John Robinson wrote in 1989.

The `split-window-quietly' function splits a window with the minimum
of redisplay.  I installed it in 1989 because it worked well with the
slow 1200 baud terminals I was then using.  Nowadays, I only
occasionally come across such a slow connection, but I continue to use
the function because I like the way it leaves the bottom half of a
buffer in the lower of the new windows and the top half in the upper
window.

To replace the key binding for the default `split-window-vertically',
you must also unset that key and bind the keys to
`split-window-quietly', like this:

     (global-unset-key "\C-x2")
     (global-set-key "\C-x2" 'split-window-quietly)

If you load many extensions, as I do, then instead of specifying the
exact location of the extension file, as shown above, you can specify
that directory as part of Emacs' `load-path'.  Then, when Emacs loads
a file, it will search that directory as well as its default list of
directories.  (The default list is specified in `paths.h' when Emacs
is built.)

The following command adds your `~/emacs' directory to the existing
load path:

     ;;; Emacs Load Path
     (setq load-path (cons "~/emacs" load-path))

Incidentally, `load-library' is an interactive interface to the
`load' function.  The complete function looks like this:

     (defun load-library (library)
       "Load the library named LIBRARY.
     This is an interface to the function `load'."
       (interactive "sLoad library: ")
       (load library))

The name of the function, `load-library', comes from the use of
`library' as a conventional synonym for `file'.  The source for the
`load-library' command is in the `files.el' library.

Another interactive command that does a slightly different job is
`load-file'.  *Note Libraries of Lisp Code for Emacs: (emacs)Lisp
Libraries, for information on the distinction between `load-library'
and this command.

Autoloading
===========

Instead of installing a function by loading the file that contains it,
or by evaluating the function definition, you can make the function
available but not actually install it until it is first called.  This
is called "autoloading".

When you execute an autoloaded function, Emacs automatically evaluates
the file that contains the definition, and then calls the function.

Emacs starts quicker with autoloaded functions, since their libraries
are not loaded right away; but you need to wait a moment when you
first use such a function, while its containing file is evaluated.

Rarely used functions are frequently autoloaded.  The `loaddefs.el'
library contains hundreds of autoloaded functions, from
`bookmark-set' to `wordstar-mode'.  Of course, you may come to use a
`rare' function frequently.  When you do, you should load that
function's file with a `load' expression in your `.emacs' file.

In my `.emacs' file for Emacs version 21, I load 12 libraries that
contain functions that would otherwise be autoloaded.  (Actually, it
would have been better to include these files in my `dumped' Emacs
when I built it, but I forgot.  *Note Building Emacs: (elisp)Building
Emacs, and the `INSTALL' file for more about dumping.)

You may also want to include autoloaded expressions in your `.emacs'
file.  `autoload' is a built-in function that takes up to five
arguments, the final three of which are optional.  The first argument
is the name of the function to be autoloaded; the second is the name
of the file to be loaded.  The third argument is documentation for the
function, and the fourth tells whether the function can be called
interactively.  The fifth argument tells what type of
object--`autoload' can handle a keymap or macro as well as a function
(the default is a function).

Here is a typical example:

     (autoload 'html-helper-mode
       "html-helper-mode" "Edit HTML documents" t)

(`html-helper-mode' is an alternative to `html-mode', which is a
standard part of the distribution).

This expression autoloads the `html-helper-mode' function.  It takes
it from the `html-helper-mode.el' file (or from the byte compiled
file `html-helper-mode.elc', if it exists.)  The file must be located
in a directory specified by `load-path'.  The documentation says that
this is a mode to help you edit documents written in the HyperText
Markup Language.  You can call this mode interactively by typing `M-x
html-helper-mode'.  (You need to duplicate the function's regular
documentation in the autoload expression because the regular function
is not yet loaded, so its documentation is not available.)

*Note Autoload: (elisp)Autoload, for more information.

A Simple Extension: `line-to-top-of-window'
===========================================

Here is a simple extension to Emacs that moves the line point is on to
the top of the window.  I use this all the time, to make text easier
to read.

You can put the following code into a separate file and then load it
from your `.emacs' file, or you can include it within your `.emacs'
file.

Here is the definition:

     ;;; Line to top of window;
     ;;; replace three keystroke sequence  C-u 0 C-l
     (defun line-to-top-of-window ()
       "Move the line point is on to top of window."
       (interactive)
       (recenter 0))

Now for the keybinding.

Nowadays, function keys as well as mouse button events and non-ASCII
characters are written within square brackets, without quotation
marks.  (In Emacs version 18 and before, you had to write different
function key bindings for each different make of terminal.)

I bind `line-to-top-of-window' to my <F6> function key like this:

     (global-set-key [f6] 'line-to-top-of-window)

For more information, see *Note Rebinding Keys in Your Init File:
(emacs)Init Rebinding.

If you run two versions of GNU Emacs, such as versions 20 and 21, and
use one `.emacs' file, you can select which code to evaluate with the
following conditional:

     (cond
      ((string-equal (number-to-string 20) (substring (emacs-version) 10 12))
       ;; evaluate version 20 code
       ( ... ))
      ((string-equal (number-to-string 21) (substring (emacs-version) 10 12))
       ;; evaluate version 21 code
       ( ... )))

For example, in contrast to version 20, version 21 blinks its cursor
by default.  I hate such blinking, as well as some other features in
version 21, so I placed the following in my `.emacs' file(1):

     (if (string-equal "21" (substring (emacs-version) 10 12))
         (progn
           (blink-cursor-mode 0)
           ;; Insert newline when you press `C-n' (next-line)
           ;; at the end of the buffer
           (setq next-line-add-newlines t)
           ;; Turn on image viewing
           (auto-image-file-mode t)
           ;; Turn on menu bar (this bar has text)
           ;; (Use numeric argument to turn on)
           (menu-bar-mode 1)
           ;; Turn off tool bar (this bar has icons)
           ;; (Use numeric argument to turn on)
           (tool-bar-mode nil)
           ;; Turn off tooltip mode for tool bar
           ;; (This mode causes icon explanations to pop up)
           ;; (Use numeric argument to turn on)
           (tooltip-mode nil)
           ;; If tooltips turned on, make tips appear promptly
           (setq tooltip-delay 0.1)  ; default is one second
            ))

(You will note that instead of typing `(number-to-string 21)', I
decided to save typing and wrote `21' as a string, `"21"', rather
than convert it from an integer to a string.  In this instance, this
expression is better than the longer, but more general
`(number-to-string 21)'.  However, if you do not know ahead of time
what type of information will be returned, then the
`number-to-string' function will be needed.)

---------- Footnotes ----------

(1) When I start instances of Emacs that do not load my `.emacs' file
or any site file, I also turn off blinking:

     emacs -q --no-site-file -eval '(blink-cursor-mode nil)'

X11 Colors
==========

You can specify colors when you use Emacs with the MIT X Windowing
system.

I dislike the default colors and specify my own.

Here are the expressions in my `.emacs' file that set values:

     ;; Set cursor color
     (set-cursor-color "white")
     
     ;; Set mouse color
     (set-mouse-color "white")
     
     ;; Set foreground and background
     (set-foreground-color "white")
     (set-background-color "darkblue")
     
     ;;; Set highlighting colors for isearch and drag
     (set-face-foreground 'highlight "white")
     (set-face-background 'highlight "blue")
     
     (set-face-foreground 'region "cyan")
     (set-face-background 'region "blue")
     
     (set-face-foreground 'secondary-selection "skyblue")
     (set-face-background 'secondary-selection "darkblue")
     
     ;; Set calendar highlighting colors
     (setq calendar-load-hook
           '(lambda ()
              (set-face-foreground 'diary-face   "skyblue")
              (set-face-background 'holiday-face "slate blue")
              (set-face-foreground 'holiday-face "white")))

The various shades of blue soothe my eye and prevent me from seeing
the screen flicker.

Alternatively, I could have set my specifications in various X
initialization files.  For example, I could set the foreground,
background, cursor, and pointer (i.e., mouse) colors in my
`~/.Xresources' file like this:

     Emacs*foreground:   white
     Emacs*background:   darkblue
     Emacs*cursorColor:  white
     Emacs*pointerColor: white

In any event, since it is not part of Emacs, I set the root color of
my X window in my `~/.xinitrc' file, like this(1):

     # I use TWM for window manager.
     xsetroot -solid Navy -fg white &

---------- Footnotes ----------

(1) I occasionally run more modern window managers, such as Sawfish
with GNOME, Enlightenment, SCWM, or KDE; in those cases, I often
specify an image rather than a plain color.

Miscellaneous Settings for a `.emacs' File
==========================================

Here are a few miscellaneous settings:

   - Set the shape and color of the mouse cursor:
          ; Cursor shapes are defined in
          ; `/usr/include/X11/cursorfont.h';
          ; for example, the `target' cursor is number 128;
          ; the `top_left_arrow' cursor is number 132.
          
          (let ((mpointer (x-get-resource "*mpointer"
                                          "*emacs*mpointer")))
            ;; If you have not set your mouse pointer
            ;;     then set it, otherwise leave as is:
            (if (eq mpointer nil)
                (setq mpointer "132")) ; top_left_arrow
            (setq x-pointer-shape (string-to-int mpointer))
            (set-mouse-color "white"))

A Modified Mode Line
====================

Finally, a feature I really like: a modified mode line.

When I work over a network, I forget which machine I am using.  Also,
I tend to I lose track of where I am, and which line point is on.

So I reset my mode line to look like this:

     -:-- foo.texi   rattlesnake:/home/bob/  Line 1  (Texinfo Fill) Top

I am visiting a file called `foo.texi', on my machine `rattlesnake'
in my `/home/bob' buffer.  I am on line 1, in Texinfo mode, and am at
the top of the buffer.

My `.emacs' file has a section that looks like this:

     ;; Set a Mode Line that tells me which machine, which directory,
     ;; and which line I am on, plus the other customary information.
     (setq default-mode-line-format
      (quote
       (#("-" 0 1
          (help-echo
           "mouse-1: select window, mouse-2: delete others ..."))
        mode-line-mule-info
        mode-line-modified
        mode-line-frame-identification
        "    "
        mode-line-buffer-identification
        "    "
        (:eval (substring
                (system-name) 0 (string-match "\\..+" (system-name))))
        ":"
        default-directory
        #(" " 0 1
          (help-echo
           "mouse-1: select window, mouse-2: delete others ..."))
        (line-number-mode " Line %l ")
        global-mode-string
        #("   %[(" 0 6
          (help-echo
           "mouse-1: select window, mouse-2: delete others ..."))
        (:eval (mode-line-mode-name))
        mode-line-process
        minor-mode-alist
        #("%n" 0 2 (help-echo "mouse-2: widen" local-map (keymap ...)))
        ")%] "
        (-3 . "%P")
        ;;   "-%-"
        )))

Here, I redefine the default mode line.  Most of the parts are from
the original; but I make a few changes.  I set the _default_ mode
line format so as to permit various modes, such as Info, to override
it.

Many elements in the list are self-explanatory: `mode-line-modified'
is a variable that tells whether the buffer has been modified,
`mode-name' tells the name of the mode, and so on.  However, the
format looks complicated because of two features we have not
discussed.

The first string in the mode line is a dash or hyphen, `-'.  In the
old days, it would have been specified simply as `"-"'.  But
nowadays, Emacs can add properties to a string, such as highlighting
or, as in this case, a help feature.  If you place your mouse cursor
over the hyphen, some help information appears  (By default, you must
wait one second before the information appears.  You can change that
timing by changing the value of `tooltip-delay'.)

The new string format has a special syntax:

     #("-" 0 1 (help-echo "mouse-1: select window, ..."))

The `#(' begins a list.  The first element of the list is the string
itself, just one `-'.  The second and third elements specify the
range over which the fourth element applies.  A range starts _after_
a character, so a zero means the range starts just before the first
character; a 1 means that the range ends just after the first
character.  The third element is the property for the range.  It
consists of a property list,  a property name, in this case,
`help-echo', followed by a value, in this case, a string.  The
second, third, and fourth elements of this new string format can be
repeated.

*Note Text Properties in String: (elisp)Text Props and Strings, and
see *Note Mode Line Format: (elisp)Mode Line Format, for more
information.

`mode-line-buffer-identification' displays the current buffer name.
It is a list beginning `(#("%12b" 0 4 ...'.  The `#(' begins the list.

The `"%12b"' displays the current buffer name, using the
`buffer-name' function with which we are familiar; the `12' specifies
the maximum number of characters that will be displayed.  When a name
has fewer characters, whitespace is added to fill out to this number.
(Buffer names can and often should be longer than 12 characters;
this length works well in a typical 80 column wide window.)

`:eval' is a new feature in GNU Emacs version 21.  It says to
evaluate the following form and use the result as a string to display.
In this case, the expression displays the first component of the full
system name.  The end of the first component is a `.' (`period'), so
I use the `string-match' function to tell me the length of the first
component.  The substring from the zeroth character to that length is
the name of the machine.

This is the expression:

     (:eval (substring
             (system-name) 0 (string-match "\\..+" (system-name))))

`%[' and `%]' cause a pair of square brackets to appear for each
recursive editing level.  `%n' says `Narrow' when narrowing is in
effect.  `%P' tells you the percentage of the buffer that is above
the bottom of the window, or `Top', `Bottom', or `All'.  (A lower
case `p' tell you the percentage above the _top_ of the window.)
`%-' inserts enough dashes to fill out the line.

Remember, "You don't have to like Emacs to like it" -- your own Emacs
can have different colors, different commands, and different keys
than a default Emacs.

On the other hand, if you want to bring up a plain `out of the box'
Emacs, with no customization, type:

     emacs -q

This will start an Emacs that does _not_ load your `~/.emacs'
initialization file.  A plain, default Emacs.  Nothing more.

Debugging
*********

GNU Emacs has two debuggers, `debug' and `edebug'.  The first is
built into the internals of Emacs and is always with you; the second
requires that you instrument a function before you can use it.

Both debuggers are described extensively in *Note Debugging Lisp
Programs: (elisp)Debugging.  In this chapter, I will walk through a
short example of each.

`debug'
=======

Suppose you have written a function definition that is intended to
return the sum of the numbers 1 through a given number.  (This is the
`triangle' function discussed earlier.  *Note Example with
Decrementing Counter: Decrementing Example, for a discussion.)

However, your function definition has a bug.  You have mistyped `1='
for `1-'.  Here is the broken definition:

     (defun triangle-bugged (number)
       "Return sum of numbers 1 through NUMBER inclusive."
       (let ((total 0))
         (while (> number 0)
           (setq total (+ total number))
           (setq number (1= number)))      ; Error here.
         total))

If you are reading this in Info, you can evaluate this definition in
the normal fashion.  You will see `triangle-bugged' appear in the
echo area.

Now evaluate the `triangle-bugged' function with an argument of 4:

     (triangle-bugged 4)

In GNU Emacs version 21, you will create and enter a `*Backtrace*'
buffer that says:

     ---------- Buffer: *Backtrace* ----------
     Debugger entered--Lisp error: (void-function 1=)
       (1= number)
       (setq number (1= number))
       (while (> number 0) (setq total (+ total number))
             (setq number (1= number)))
       (let ((total 0)) (while (> number 0) (setq total ...)
         (setq number ...)) total)
       triangle-bugged(4)
       eval((triangle-bugged 4))
       eval-last-sexp-1(nil)
       eval-last-sexp(nil)
       call-interactively(eval-last-sexp)
     ---------- Buffer: *Backtrace* ----------

(I have reformatted this example slightly; the debugger does not fold
long lines.  As usual, you can quit the debugger by typing `q' in the
`*Backtrace*' buffer.)

In practice, for a bug as simple as this, the `Lisp error' line will
tell you what you need to know to correct the definition.  The
function `1=' is `void'.

In GNU Emacs 20 and before, you will see:

     Symbol's function definition is void: 1=

which has the same meaning as the `*Backtrace*' buffer line in
version 21.

However, suppose you are not quite certain what is going on?  You can
read the complete backtrace.

In this case, you need to run GNU Emacs 21, which automatically starts
the debugger that puts you in the `*Backtrace*' buffer; or else, you
need to start the debugger manually as described below.

Read the `*Backtrace*' buffer from the bottom up; it tells you what
Emacs did that led to the error.  Emacs made an interactive call to
`C-x C-e' (`eval-last-sexp'), which led to the evaluation of the
`triangle-bugged' expression.  Each line above tells you what the
Lisp interpreter evaluated next.

The third line from the top of the buffer is

     (setq number (1= number))

Emacs tried to evaluate this expression; in order to do so, it tried
to evaluate the inner expression shown on the second line from the
top:

     (1= number)

This is where the error occurred; as the top line says:

     Debugger entered--Lisp error: (void-function 1=)

You can correct the mistake, re-evaluate the function definition, and
then run your test again.

`debug-on-entry'
================

GNU Emacs 21 starts the debugger automatically when your function has
an error.  GNU Emacs version 20 and before did not; it simply
presented you with an error message.  You had to start the debugger
manually.

You can start the debugger manually for all versions of Emacs; the
advantage is that the debugger runs even if you do not have a bug in
your code.  Sometimes your code will be free of bugs!

You can enter the debugger when you call the function by calling
`debug-on-entry'.

Type:

     M-x debug-on-entry RET triangle-bugged RET

Now, evaluate the following:

     (triangle-bugged 5)

All versions of Emacs will create a `*Backtrace*' buffer and tell you
that it is beginning to evaluate the `triangle-bugged' function:

     ---------- Buffer: *Backtrace* ----------
     Debugger entered--entering a function:
     * triangle-bugged(5)
       eval((triangle-bugged 5))
       eval-last-sexp-1(nil)
       eval-last-sexp(nil)
       call-interactively(eval-last-sexp)
     ---------- Buffer: *Backtrace* ----------

In the `*Backtrace*' buffer, type `d'.  Emacs will evaluate the first
expression in `triangle-bugged'; the buffer will look like this:

     ---------- Buffer: *Backtrace* ----------
     Debugger entered--beginning evaluation of function call form:
     * (let ((total 0)) (while (> number 0) (setq total ...)
             (setq number ...)) total)
     * triangle-bugged(5)
       eval((triangle-bugged 5))
       eval-last-sexp-1(nil)
       eval-last-sexp(nil)
       call-interactively(eval-last-sexp)
     ---------- Buffer: *Backtrace* ----------

Now, type `d' again, eight times, slowly.  Each time you type `d',
Emacs will evaluate another expression in the function definition.

Eventually, the buffer will look like this:

     ---------- Buffer: *Backtrace* ----------
     Debugger entered--beginning evaluation of function call form:
     * (setq number (1= number))
     * (while (> number 0) (setq total (+ total number))
             (setq number (1= number)))
     * (let ((total 0)) (while (> number 0) (setq total ...)
             (setq number ...)) total)
     * triangle-bugged(5)
       eval((triangle-bugged 5))
       eval-last-sexp-1(nil)
       eval-last-sexp(nil)
       call-interactively(eval-last-sexp)
     ---------- Buffer: *Backtrace* ----------

Finally, after you type `d' two more times, Emacs will reach the
error, and the top two lines of the `*Backtrace*' buffer will look
like this:

     ---------- Buffer: *Backtrace* ----------
     Debugger entered--Lisp error: (void-function 1=)
     * (1= number)
     ...
     ---------- Buffer: *Backtrace* ----------

By typing `d', you were able to step through the function.

You can quit a `*Backtrace*' buffer by typing `q' in it; this quits
the trace, but does not cancel `debug-on-entry'.

To cancel the effect of `debug-on-entry', call
`cancel-debug-on-entry' and the name of the function, like this:

     M-x cancel-debug-on-entry RET triangle-bugged RET

(If you are reading this in Info, cancel `debug-on-entry' now.)

`debug-on-quit' and `(debug)'
=============================

In addition to setting `debug-on-error' or calling `debug-on-entry',
there are two other ways to start `debug'.

You can start `debug' whenever you type `C-g' (`keyboard-quit') by
setting the variable `debug-on-quit' to `t'.  This is useful for
debugging infinite loops.

Or, you can insert a line that says `(debug)' into your code where
you want the debugger to start, like this:

     (defun triangle-bugged (number)
       "Return sum of numbers 1 through NUMBER inclusive."
       (let ((total 0))
         (while (> number 0)
           (setq total (+ total number))
           (debug)                         ; Start debugger.
           (setq number (1= number)))      ; Error here.
         total))

The `debug' function is described in detail in *Note The Lisp
Debugger: (elisp)Debugger.

The `edebug' Source Level Debugger
==================================

Edebug is a source level debugger.  Edebug normally displays the
source of the code you are debugging, with an arrow at the left that
shows which line you are currently executing.

You can walk through the execution of a function, line by line, or run
quickly until reaching a "breakpoint" where execution stops.

Edebug is described in *Note Edebug: (elisp)edebug.

Here is a bugged function definition for `triangle-recursively'.
*Note Recursion in place of a counter: Recursive triangle function,
for a review of it.

     (defun triangle-recursively-bugged (number)
       "Return sum of numbers 1 through NUMBER inclusive.
     Uses recursion."
       (if (= number 1)
           1
         (+ number
            (triangle-recursively-bugged
             (1= number)))))               ; Error here.

Normally, you would install this definition by positioning your cursor
after the function's closing parenthesis and typing `C-x C-e'
(`eval-last-sexp') or else by positioning your cursor within the
definition and typing `C-M-x' (`eval-defun').  (By default, the
`eval-defun' command works only in Emacs Lisp mode or in Lisp
Interactive mode.)

However, to prepare this function definition for Edebug, you must
first "instrument" the code using a different command.  You can do
this by positioning your cursor within the definition and typing

     M-x edebug-defun RET

This will cause Emacs to load Edebug automatically if it is not
already loaded, and properly instrument the function.

After instrumenting the function, place your cursor after the
following expression and type `C-x C-e' (`eval-last-sexp'):

     (triangle-recursively-bugged 3)

You will be jumped back to the source for
`triangle-recursively-bugged' and the cursor positioned at the
beginning of the `if' line of the function.  Also, you will see an
arrowhead at the left hand side of that line.  The arrowhead marks
the line where the function is executing.  (In the following examples,
we show the arrowhead with `=>'; in a windowing system, you may see
the arrowhead as a solid triangle in the window `fringe'.)

     =>-!-(if (= number 1)

In the example, the location of point is displayed as `-!-' (in a
printed book, it is displayed with a five pointed star).

If you now press <SPC>, point will move to the next expression to be
executed; the line will look like this:

     =>(if -!-(= number 1)

As you continue to press <SPC>, point will move from expression to
expression.  At the same time, whenever an expression returns a value,
that value will be displayed in the echo area.  For example, after you
move point past `number', you will see the following:

     Result: 3 = C-c

This means the value of `number' is 3, which is ASCII `control-c'
(the third letter of the alphabet).

You can continue moving through the code until you reach the line with
the error.  Before evaluation, that line looks like this:

     =>        -!-(1= number)))))               ; Error here.

When you press <SPC> once again, you will produce an error message
that says:

     Symbol's function definition is void: 1=

This is the bug.

Press `q' to quit Edebug.

To remove instrumentation from a function definition, simply
re-evaluate it with a command that does not instrument it.  For
example, you could place your cursor after the definition's closing
parenthesis and type `C-x C-e'.

Edebug does a great deal more than walk with you through a function.
You can set it so it races through on its own, stopping only at an
error or at specified stopping points; you can cause it to display the
changing values of various expressions; you can find out how many
times a function is called, and more.

Edebug is described in *Note Edebug: (elisp)edebug.

Debugging Exercises
===================

   * Install the `count-words-region' function and then cause it to
     enter the built-in debugger when you call it.  Run the command
     on a region containing two words.  You will need to press `d' a
     remarkable number of times.  On your system, is a `hook' called
     after the command finishes?  (For information on hooks, see
     *Note Command Loop Overview: (elisp)Command Overview.)

   * Copy `count-words-region' into the `*scratch*' buffer,
     instrument the function for Edebug, and walk through its
     execution.  The function does not need to have a bug, although
     you can introduce one if you wish.  If the function lacks a bug,
     the walk-through completes without problems.

   * While running Edebug, type `?' to see a list of all the Edebug
     commands.  (The `global-edebug-prefix' is usually `C-x X', i.e.
     `<CTL>-x' followed by an upper case `X'; use this prefix for
     commands made outside of the Edebug debugging buffer.)

   * In the Edebug debugging buffer, use the `p'
     (`edebug-bounce-point') command to see where in the region the
     `count-words-region' is working.

   * Move point to some spot further down function and then type the
     `h' (`edebug-goto-here') command to jump to that location.

   * Use the `t' (`edebug-trace-mode') command to cause Edebug to
     walk through the function on its own; use an upper case `T' for
     `edebug-Trace-fast-mode'.

   * Set a breakpoint, then run Edebug in Trace mode until it reaches
     the stopping point.

Conclusion
**********

We have now reached the end of this Introduction.  You have now
learned enough about programming in Emacs Lisp to set values, to write
simple `.emacs' files for yourself and your friends, and write simple
customizations and extensions to Emacs.

This is a place to stop.  Or, if you wish, you can now go onward, and
teach yourself.

You have learned some of the basic nuts and bolts of programming.  But
only some.  There are a great many more brackets and hinges that are
easy to use that we have not touched.

A path you can follow right now lies among the sources to GNU Emacs
and in *Note The GNU Emacs Lisp Reference Manual: (elisp)Top.

The Emacs Lisp sources are an adventure.  When you read the sources
and come across a function or expression that is unfamiliar, you need
to figure out or find out what it does.

Go to the Reference Manual.  It is a thorough, complete, and fairly
easy-to-read description of Emacs Lisp.  It is written not only for
experts, but for people who know what you know.  (The `Reference
Manual' comes with the standard GNU Emacs distribution.  Like this
introduction, it comes as a Texinfo source file, so you can read it
on-line and as a typeset, printed book.)

Go to the other on-line help that is part of GNU Emacs: the on-line
documentation for all functions, and `find-tags', the program that
takes you to sources.

Here is an example of how I explore the sources.  Because of its name,
`simple.el' is the file I looked at first, a long time ago.  As it
happens some of the functions in `simple.el' are complicated, or at
least look complicated at first sight.  The `open-line' function, for
example, looks complicated.

You may want to walk through this function slowly, as we did with the
`forward-sentence' function.  (*Note forward-sentence::.)  Or you may
want to skip that function and look at another, such as `split-line'.
You don't need to read all the functions.  According to
`count-words-in-defun', the `split-line' function contains 27 words
and symbols.

Even though it is short, `split-line' contains four expressions we
have not studied: `skip-chars-forward', `indent-to', `current-column'
and `?\n'.

Consider the `skip-chars-forward' function.  (It is part of the
function definition for `back-to-indentation', which is shown in
*Note Review: Review.)

In GNU Emacs, you can find out more about `skip-chars-forward' by
typing `C-h f' (`describe-function') and the name of the function.
This gives you the function documentation.

You may be able to guess what is done by a well named function such as
`indent-to'; or you can look it up, too.  Incidentally, the
`describe-function' function itself is in `help.el'; it is one of
those long, but decipherable functions.  You can look up
`describe-function' using the `C-h f' command!

In this instance, since the code is Lisp, the `*Help*' buffer
contains the name of the library containing the function's source.
You can put point over the name of the library and press the RET key,
which in this situation is bound to `help-follow', and be taken
directly to the source, in the same way as `M-.' (`find-tag').

The definition for `describe-function' illustrates how to customize
the `interactive' expression without using the standard character
codes; and it shows how to create a temporary buffer.

(The `indent-to' function is written in C rather than Emacs Lisp; it
is a `built-in' function.  `help-follow' only provides you with the
documentation of a built-in function; it does not take you to the
source.  But `find-tag' will take you to the source, if properly set
up.)

You can look at a function's source using `find-tag', which is bound
to `M-.'  Finally, you can find out what the Reference Manual has to
say by visiting the manual in Info, and typing `i' (`Info-index') and
the name of the function, or by looking up `skip-chars-forward' in
the index to a printed copy of the manual.

Similarly, you can find out what is meant by `?\n'.  You can try
using `Info-index' with `?\n'.  It turns out that this action won't
help; but don't give up.  If you search the index for `\n' without
the `?', you will be taken directly to the relevant section of the
manual.  (*Note Character Type: (elisp)Character Type.  `?\n' stands
for the newline character.)

Other interesting source files include `paragraphs.el',
`loaddefs.el', and `loadup.el'.  The `paragraphs.el' file includes
short, easily understood functions as well as longer ones.  The
`loaddefs.el' file contains the many standard autoloads and many
keymaps.  I have never looked at it all; only at parts.  `loadup.el'
is the file that loads the standard parts of Emacs; it tells you a
great deal about how Emacs is built.  (*Note Building Emacs:
(elisp)Building Emacs, for more about building.)

As I said, you have learned some nuts and bolts; however, and very
importantly, we have hardly touched major aspects of programming; I
have said nothing about how to sort information, except to use the
predefined `sort' function; I have said nothing about how to store
information, except to use variables and lists; I have said nothing
about how to write programs that write programs.  These are topics for
another, and different kind of book, a different kind of learning.

What you have done is learn enough for much practical work with GNU
Emacs.  What you have done is get started.  This is the end of a
beginning.

The `the-the' Function
**********************

Sometimes when you you write text, you duplicate words--as with "you
you" near the beginning of this sentence.  I find that most
frequently, I duplicate "the'; hence, I call the function for
detecting duplicated words, `the-the'.

As a first step, you could use the following regular expression to
search for duplicates:

     \\(\\w+[ \t\n]+\\)\\1

This regexp matches one or more word-constituent characters followed
by one or more spaces, tabs, or newlines.  However, it does not detect
duplicated words on different lines, since the ending of the first
word, the end of the line, is different from the ending of the second
word, a space.  (For more information about regular expressions, see
*Note Regular Expression Searches: Regexp Search, as well as *Note
Syntax of Regular Expressions: (emacs)Regexps, and *Note Regular
Expressions: (elisp)Regular Expressions.)

You might try searching just for duplicated word-constituent
characters but that does not work since the pattern detects doubles
such as the two occurrences of `th' in `with the'.

Another possible regexp searches for word-constituent characters
followed by non-word-constituent characters, reduplicated.  Here,
`\\w+' matches one or more word-constituent characters and `\\W*'
matches zero or more non-word-constituent characters.

     \\(\\(\\w+\\)\\W*\\)\\1

Again, not useful.

Here is the pattern that I use.  It is not perfect, but good enough.
`\\b' matches the empty string, provided it is at the beginning or
end of a word; `[^@ \n\t]+' matches one or more occurrences of any
characters that are _not_ an @-sign, space, newline, or tab.

     \\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b

One can write more complicated expressions, but I found that this
expression is good enough, so I use it.

Here is the `the-the' function, as I include it in my `.emacs' file,
along with a handy global key binding:

     (defun the-the ()
       "Search forward for for a duplicated word."
       (interactive)
       (message "Searching for for duplicated words ...")
       (push-mark)
       ;; This regexp is not perfect
       ;; but is fairly good over all:
       (if (re-search-forward
            "\\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b" nil 'move)
           (message "Found duplicated word.")
         (message "End of buffer")))
     
     ;; Bind `the-the' to  C-c \
     (global-set-key "\C-c\\" 'the-the)


Here is test text:

     one two two three four five
     five six seven

You can substitute the other regular expressions shown above in the
function definition and try each of them on this list.

Handling the Kill Ring
**********************

The kill ring is a list that is transformed into a ring by the
workings of the `rotate-yank-pointer' function.  The `yank' and
`yank-pop' commands use the `rotate-yank-pointer' function.  This
appendix describes the `rotate-yank-pointer' function as well as both
the `yank' and the `yank-pop' commands.

The `rotate-yank-pointer' Function
==================================

The `rotate-yank-pointer' function changes the element in the kill
ring to which `kill-ring-yank-pointer' points.  For example, it can
change  `kill-ring-yank-pointer' from pointing to the second element
to point to the third element.

Here is the code for `rotate-yank-pointer':

     (defun rotate-yank-pointer (arg)
       "Rotate the yanking point in the kill ring."
       (interactive "p")
       (let ((length (length kill-ring)))
         (if (zerop length)
             ;; then-part
             (error "Kill ring is empty")
           ;; else-part
           (setq kill-ring-yank-pointer
                 (nthcdr (% (+ arg
                               (- length
                                  (length
                                   kill-ring-yank-pointer)))
                            length)
                         kill-ring)))))

`rotate-yank-pointer' in Outline
--------------------------------

The `rotate-yank-pointer' function looks complex, but as usual, it
can be understood by taking it apart piece by piece.  First look at
it in skeletal form:

     (defun rotate-yank-pointer (arg)
       "Rotate the yanking point in the kill ring."
       (interactive "p")
       (let VARLIST
         BODY...)

This function takes one argument, called `arg'.  It has a brief
documentation string; and it is interactive with a small `p', which
means that the argument must be a processed prefix passed to the
function as a number.

The body of the function definition is a `let' expression, which
itself has a body as well as a VARLIST.

The `let' expression declares a variable that will be only usable
within the bounds of this function.  This variable is called `length'
and is bound to a value that is equal to the number of items in the
kill ring.  This is done by using the function called `length'.
(Note that this function has the same name as the variable called
`length'; but one use of the word is to name the function and the
other is to name the variable.  The two are quite distinct.
Similarly, an English speaker will distinguish between the meanings
of the word `ship' when he says: "I must ship this package
immediately." and "I must get aboard the ship immediately.")

The function `length' tells the number of items there are in a list,
so `(length kill-ring)' returns the number of items there are in the
kill ring.

The Body of `rotate-yank-pointer'
---------------------------------

The body of `rotate-yank-pointer' is a `let' expression and the body
of the `let' expression is an `if' expression.

The purpose of the `if' expression is to find out whether there is
anything in the kill ring.  If the kill ring is empty, the `error'
function stops evaluation of the function and prints a message in the
echo area.  On the other hand, if the kill ring has something in it,
the work of the function is done.

Here is the if-part and then-part of the `if' expression:

     (if (zerop length)                      ; if-part
         (error "Kill ring is empty")        ; then-part
       ...

If there is not anything in the kill ring, its length must be zero and
an error message sent to the user: `Kill ring is empty'.  The `if'
expression uses the function `zerop' which returns true if the value
it is testing is zero.  When `zerop' tests true, the then-part of the
`if' is evaluated.  The then-part is a list starting with the
function `error', which is a function that is similar to the
`message' function (*note message::), in that it prints a one-line
message in the echo area.  However, in addition to printing a
message, `error' also stops evaluation of the function within which
it is embedded.  This means that the rest of the function will not be
evaluated if the length of the kill ring is zero.

Digression about the word `error'
.................................

(In my opinion, it is slightly misleading, at least to humans, to use
the term `error' as the name of the `error' function.  A better term
would be `cancel'.  Strictly speaking, of course, you cannot point
to, much less rotate a pointer to a list that has no length, so from
the point of view of the computer, the word `error' is correct.  But
a human expects to attempt this sort of thing, if only to find out
whether the kill ring is full or empty.  This is an act of
exploration.

(From the human point of view, the act of exploration and discovery is
not necessarily an error, and therefore should not be labelled as one,
even in the bowels of a computer.  As it is, the code in Emacs implies
that a human who is acting virtuously, by exploring his or her
environment, is making an error.  This is bad.  Even though the
computer takes the same steps as it does when there is an `error', a
term such as `cancel' would have a clearer connotation.)

The else-part of the `if' expression
....................................

The else-part of the `if' expression is dedicated to setting the
value of `kill-ring-yank-pointer' when the kill ring has something in
it.  The code looks like this:

     (setq kill-ring-yank-pointer
           (nthcdr (% (+ arg
                         (- length
                            (length kill-ring-yank-pointer)))
                      length)
                   kill-ring)))))

This needs some examination.  Clearly, `kill-ring-yank-pointer' is
being set to be equal to some CDR of the kill ring, using the
`nthcdr' function that is described in an earlier section.  (*Note
copy-region-as-kill::.)  But exactly how does it do this?

Before looking at the details of the code let's first consider the
purpose of the `rotate-yank-pointer' function.

The `rotate-yank-pointer' function changes what
`kill-ring-yank-pointer' points to.  If `kill-ring-yank-pointer'
starts by pointing to the first element of a list, a call to
`rotate-yank-pointer' causes it to point to the second element; and
if `kill-ring-yank-pointer' points to the second element, a call to
`rotate-yank-pointer' causes it to point to the third element.  (And
if `rotate-yank-pointer' is given an argument greater than 1, it
jumps the pointer that many elements.)

The `rotate-yank-pointer' function uses `setq' to reset what the
`kill-ring-yank-pointer' points to.  If `kill-ring-yank-pointer'
points to the first element of the kill ring, then, in the simplest
case, the `rotate-yank-pointer' function must cause it to point to
the second element.  Put another way, `kill-ring-yank-pointer' must
be reset to have a value equal to the CDR of the kill ring.

That is, under these circumstances,

     (setq kill-ring-yank-pointer
        ("some text" "a different piece of text" "yet more text"))
     
     (setq kill-ring
        ("some text" "a different piece of text" "yet more text"))

the code should do this:

     (setq kill-ring-yank-pointer (cdr kill-ring))

As a result, the `kill-ring-yank-pointer' will look like this:

     kill-ring-yank-pointer
          => ("a different piece of text" "yet more text"))

The actual `setq' expression uses the `nthcdr' function to do the job.

As we have seen before (*note nthcdr::), the `nthcdr' function works
by repeatedly taking the CDR of a list--it takes the CDR of the CDR
of the CDR ...

The two following expressions produce the same result:

     (setq kill-ring-yank-pointer (cdr kill-ring))
     
     (setq kill-ring-yank-pointer (nthcdr 1 kill-ring))

In the `rotate-yank-pointer' function, however, the first argument to
`nthcdr' is a rather complex looking expression with lots of
arithmetic inside of it:

     (% (+ arg
           (- length
              (length kill-ring-yank-pointer)))
        length)

As usual, we need to look at the most deeply embedded expression first
and then work our way towards the light.

The most deeply embedded expression is `(length
kill-ring-yank-pointer)'.  This finds the length of the current value
of the `kill-ring-yank-pointer'.  (Remember that the
`kill-ring-yank-pointer' is the name of a variable whose value is a
list.)

The measurement of the length is inside the expression:

     (- length (length kill-ring-yank-pointer))

In this expression, the first `length' is the variable that was
assigned the length of the kill ring in the `let' statement at the
beginning of the function.  (One might think this function would be
clearer if the variable `length' were named `length-of-kill-ring'
instead; but if you look at the text of the whole function, you will
see that it is so short that naming this variable `length' is not a
bother, unless you are pulling the function apart into very tiny
pieces as we are doing here.)

So the line `(- length (length kill-ring-yank-pointer))' tells the
difference between the length of the kill ring and the length of the
list whose name is `kill-ring-yank-pointer'.

To see how all this fits into the `rotate-yank-pointer' function,
let's begin by analyzing the case where `kill-ring-yank-pointer'
points to the first element of the kill ring, just as `kill-ring'
does, and see what happens when `rotate-yank-pointer' is called with
an argument of 1.

The variable `length' and the value of the expression `(length
kill-ring-yank-pointer)' will be the same since the variable `length'
is the length of the kill ring and the `kill-ring-yank-pointer' is
pointing to the whole kill ring.  Consequently, the value of

     (- length (length kill-ring-yank-pointer))

will be zero.  Since the value of `arg' will be 1, this will mean
that the value of the whole expression

     (+ arg (- length (length kill-ring-yank-pointer)))

will be 1.

Consequently, the argument to `nthcdr' will be found as the result of
the expression

     (% 1 length)

The `%' remainder function
..........................

To understand `(% 1 length)', we need to understand `%'.  According
to its documentation (which I just found by typing `C-h f % <RET>'),
the `%' function returns the remainder of its first argument divided
by its second argument.  For example, the remainder of 5 divided by 2
is 1.  (2 goes into 5 twice with a remainder of 1.)

What surprises people who don't often do arithmetic is that a smaller
number can be divided by a larger number and have a remainder.  In the
example we just used, 5 was divided by 2.  We can reverse that and
ask, what is the result of dividing 2 by 5?  If you can use
fractions, the answer is obviously 2/5 or .4; but if, as here, you
can only use whole numbers, the result has to be something different.
Clearly, 5 can go into 2 zero times, but what of the remainder?  To
see what the answer is, consider a case that has to be familiar from
childhood:

   * 5 divided by 5 is 1 with a remainder of 0;

   * 6 divided by 5 is 1 with a remainder of 1;

   * 7 divided by 5 is 1 with a remainder of 2.

   * Similarly, 10 divided by 5 is 2 with a remainder of 0;

   * 11 divided by 5 is 2 with a remainder of 1;

   * 12 divided by 5 is 1 with a remainder of 2.

By considering the cases as parallel, we can see that

   * zero divided by 5 must be zero with a remainder of zero;

   * 1 divided by 5 must be zero with a remainder of 1;

   * 2 divided by 5 must be zero with a remainder of 2;

and so on.

So, in this code, if the value of `length' is 5, then the result of
evaluating

     (% 1 5)

is 1.  (I just checked this by placing the cursor after the expression
and typing `C-x C-e'.  Indeed, 1 is printed in the echo area.)

Using `%' in `rotate-yank-pointer'
..................................

When the `kill-ring-yank-pointer' points to the beginning of the kill
ring, and the argument passed to `rotate-yank-pointer' is 1, the `%'
expression returns 1:

     (- length (length kill-ring-yank-pointer))
          => 0

therefore,

     (+ arg (- length (length kill-ring-yank-pointer)))
          => 1

and consequently:

     (% (+ arg (- length (length kill-ring-yank-pointer)))
        length)
          => 1

regardless of the value of `length'.

As a result of this, the `setq kill-ring-yank-pointer' expression
simplifies to:

     (setq kill-ring-yank-pointer (nthcdr 1 kill-ring))

What it does is now easy to understand.  Instead of pointing as it did
to the first element of the kill ring, the `kill-ring-yank-pointer'
is set to point to the second element.

Clearly, if the argument passed to `rotate-yank-pointer' is two, then
the `kill-ring-yank-pointer' is set to `(nthcdr 2 kill-ring)'; and so
on for different values of the argument.

Similarly, if the `kill-ring-yank-pointer' starts out pointing to the
second element of the kill ring, its length is shorter than the
length of the kill ring by 1, so the computation of the remainder is
based on the expression `(% (+ arg 1) length)'.  This means that the
`kill-ring-yank-pointer' is moved from the second element of the kill
ring to the third element if the argument passed to
`rotate-yank-pointer' is 1.

Pointing to the last element
............................

The final question is, what happens if the `kill-ring-yank-pointer'
is set to the _last_ element of the kill ring?  Will a call to
`rotate-yank-pointer' mean that nothing more can be taken from the
kill ring?  The answer is no.  What happens is different and useful.
The `kill-ring-yank-pointer' is set to point to the beginning of the
kill ring instead.

Let's see how this works by looking at the code, assuming the length
of the kill ring is 5 and the argument passed to
`rotate-yank-pointer' is 1.  When the `kill-ring-yank-pointer' points
to the last element of the kill ring, its length is 1.  The code
looks like this:

     (% (+ arg (- length (length kill-ring-yank-pointer))) length)

When the variables are replaced by their numeric values, the
expression looks like this:

     (% (+ 1 (- 5 1)) 5)

This expression can be evaluated by looking at the most embedded inner
expression first and working outwards:  The value of `(- 5 1)' is 4;
the sum of `(+ 1 4)' is 5; and the remainder of dividing 5 by 5 is
zero.  So what `rotate-yank-pointer' will do is

     (setq kill-ring-yank-pointer (nthcdr 0 kill-ring))

which will set the `kill-ring-yank-pointer' to point to the beginning
of the kill ring.

So what happens with successive calls to `rotate-yank-pointer' is that
it moves the `kill-ring-yank-pointer' from element to element in the
kill ring until it reaches the end; then it jumps back to the
beginning.  And this is why the kill ring is called a ring, since by
jumping back to the beginning, it is as if the list has no end!  (And
what is a ring, but an entity with no end?)

`yank'
======

After learning about `rotate-yank-pointer', the code for the `yank'
function is almost easy.  It has only one tricky part, which is the
computation of the argument to be passed to `rotate-yank-pointer'.

The code looks like this:

     (defun yank (&optional arg)
       "Reinsert the last stretch of killed text.
     More precisely, reinsert the stretch of killed text most
     recently killed OR yanked.
     With just C-U as argument, same but put point in front
     (and mark at end).  With argument n, reinsert the nth
     most recently killed stretch of killed text.
     See also the command \\[yank-pop]."
     
       (interactive "*P")
       (rotate-yank-pointer (if (listp arg) 0
                              (if (eq arg '-) -1
                                (1- arg))))
       (push-mark (point))
       (insert (car kill-ring-yank-pointer))
       (if (consp arg)
           (exchange-point-and-mark)))

Glancing over this code, we can understand the last few lines readily
enough.  The mark is pushed, that is, remembered; then the first
element (the CAR) of what the `kill-ring-yank-pointer' points to is
inserted; and then, if the argument passed the function is a `cons',
point and mark are exchanged so the point is put in the front of the
inserted text rather than at the end.  This option is explained in
the documentation.  The function itself is interactive with `"*P"'.
This means it will not work on a read-only buffer, and that the
unprocessed prefix argument is passed to the function.

Passing the argument
....................

The hard part of `yank' is understanding the computation that
determines the value of the argument passed to `rotate-yank-pointer'.
Fortunately, it is not so difficult as it looks at first sight.

What happens is that the result of evaluating one or both of the `if'
expressions will be a number and that number will be the argument
passed to `rotate-yank-pointer'.

Laid out with comments, the code looks like this:

     (if (listp arg)                         ; if-part
         0                                   ; then-part
       (if (eq arg '-)                       ; else-part, inner if
           -1                                ; inner if's then-part
         (1- arg))))                         ; inner if's else-part

This code consists of two `if' expression, one the else-part of the
other.

The first or outer `if' expression tests whether the argument passed
to `yank' is a list.  Oddly enough, this will be true if `yank' is
called without an argument--because then it will be passed the value
of `nil' for the optional argument and an evaluation of `(listp nil)'
returns true!  So, if no argument is passed to `yank', the argument
passed to `rotate-yank-pointer' inside of `yank' is zero.  This means
the pointer is not moved and the first element to which
`kill-ring-yank-pointer' points is inserted, as we expect.
Similarly, if the argument for `yank' is `C-u', this will be read as
a list, so again, a zero will be passed to `rotate-yank-pointer'.
(`C-u' produces an unprocessed prefix argument of `(4)', which is a
list of one element.)  At the same time, later in the function, this
argument will be read as a `cons' so point will be put in the front
and mark at the end of the insertion.  (The `P' argument to
`interactive' is designed to provide these values for the case when
an optional argument is not provided or when it is `C-u'.)

The then-part of the outer `if' expression handles the case when
there is no argument or when it is `C-u'.  The else-part handles the
other situations.  The else-part is itself another `if' expression.

The inner `if' expression tests whether the argument is a minus sign.
(This is done by pressing the <META> and `-' keys at the same time,
or the <ESC> key and then the `-' key).  In this case, the
`rotate-yank-pointer' function is passed `-1' as an argument.  This
moves the `kill-ring-yank-pointer' backwards, which is what is
desired.

If the true-or-false-test of the inner `if' expression is false (that
is, if the argument is not a minus sign), the else-part of the
expression is evaluated.  This is the expression `(1- arg)'.  Because
of the two `if' expressions, it will only occur when the argument is
a positive number or when it is a negative number (not just a minus
sign on its own).  What `(1- arg)' does is decrement the number and
return it.  (The `1-' function subtracts one from its argument.)
This means that if the argument to `rotate-yank-pointer' is 1, it is
reduced to zero, which means the first element to which
`kill-ring-yank-pointer' points is yanked back, as you would expect.

Passing a negative argument
...........................

Finally, the question arises, what happens if either the remainder
function, `%', or the `nthcdr' function is passed a negative
argument, as they quite well may?

The answers can be found by a quick test.  When `(% -1 5)' is
evaluated, a negative number is returned; and if `nthcdr' is called
with a negative number, it returns the same value as if it were
called with a first argument of zero.  This can be seen by evaluating
the following code.

Here the `=>' points to the result of evaluating the code preceding
it.  This was done by positioning the cursor after the code and
typing `C-x C-e' (`eval-last-sexp') in the usual fashion.  You can do
this if you are reading this in Info inside of GNU Emacs.

     (% -1 5)
          => -1
     
     (setq animals '(cats dogs elephants))
          => (cats dogs elephants)
     
     (nthcdr 1 animals)
          => (dogs elephants)
     
     (nthcdr 0 animals)
          => (cats dogs elephants)
     
     (nthcdr -1 animals)
          => (cats dogs elephants)

So, if a minus sign or a negative number is passed to `yank', the
`kill-ring-yank-point' is rotated backwards until it reaches the
beginning of the list.  Then it stays there.  Unlike the other case,
when it jumps from the end of the list to the beginning of the list,
making a ring, it stops.  This makes sense.  You often want to get
back to the most recently clipped out piece of text, but you don't
usually want to insert text from as many as thirty kill commands ago.
So you need to work through the ring to get to the end, but won't
cycle around it inadvertently if you are trying to come back to the
beginning.

Incidentally, any number passed to `yank' with a minus sign preceding
it will be treated as -1.  This is evidently a simplification for
writing the program.  You don't need to jump back towards the
beginning of the kill ring more than one place at a time and doing
this is easier than writing a function to determine the magnitude of
the number that follows the minus sign.

`yank-pop'
==========

After understanding `yank', the `yank-pop' function is easy.  Leaving
out the documentation to save space, it looks like this:

     (defun yank-pop (arg)
       (interactive "*p")
       (if (not (eq last-command 'yank))
           (error "Previous command was not a yank"))
       (setq this-command 'yank)
       (let ((before (< (point) (mark))))
         (delete-region (point) (mark))
         (rotate-yank-pointer arg)
         (set-mark (point))
         (insert (car kill-ring-yank-pointer))
         (if before (exchange-point-and-mark))))

The function is interactive with a small `p' so the prefix argument
is processed and passed to the function.  The command can only be
used after a previous yank; otherwise an error message is sent.  This
check uses the variable `last-command' which is discussed elsewhere.
(*Note copy-region-as-kill::.)

The `let' clause sets the variable `before' to true or false
depending whether point is before or after mark and then the region
between point and mark is deleted.  This is the region that was just
inserted by the previous yank and it is this text that will be
replaced.  Next the `kill-ring-yank-pointer' is rotated so that the
previously inserted text is not reinserted yet again.  Mark is set at
the beginning of the place the new text will be inserted and then the
first element to which `kill-ring-yank-pointer' points is inserted.
This leaves point after the new text.  If in the previous yank, point
was left before the inserted text, point and mark are now exchanged
so point is again left in front of the newly inserted text.  That is
all there is to it!

A Graph with Labelled Axes
**************************

Printed axes help you understand a graph.  They convey scale.  In an
earlier chapter (*note Readying a Graph: Readying a Graph.), we wrote
the code to print the body of a graph.  Here we write the code for
printing and labelling vertical and horizontal axes, along with the
body itself.

Labelled Example Graph
======================

Since insertions fill a buffer to the right and below point, the new
graph printing function should first print the Y or vertical axis,
then the body of the graph, and finally the X or horizontal axis.
This sequence lays out for us the contents of the function:

  1. Set up code.

  2. Print Y axis.

  3. Print body of graph.

  4. Print X axis.

Here is an example of how a finished graph should look:

         10 -
                       *
                       *  *
                       *  **
                       *  ***
          5 -      *   *******
                 * *** *******
                 *************
               ***************
          1 - ****************
              |   |    |    |
              1   5   10   15

In this graph, both the vertical and the horizontal axes are labelled
with numbers.  However, in some graphs, the horizontal axis is time
and would be better labelled with months, like this:

          5 -      *
                 * ** *
                 *******
               ********** **
          1 - **************
              |    ^      |
              Jan  June   Jan

Indeed, with a little thought, we can easily come up with a variety of
vertical and horizontal labelling schemes.  Our task could become
complicated.  But complications breed confusion.  Rather than permit
this, it is better choose a simple labelling scheme for our first
effort, and to modify or replace it later.

These considerations suggest the following outline for the
`print-graph' function:

     (defun print-graph (numbers-list)
       "DOCUMENTATION..."
       (let ((height  ...
             ...))
         (print-Y-axis height ... )
         (graph-body-print numbers-list)
         (print-X-axis ... )))

We can work on each part of the `print-graph' function definition in
turn.

The `print-graph' Varlist
=========================

In writing the `print-graph' function, the first task is to write the
varlist in the `let' expression.  (We will leave aside for the moment
any thoughts about making the function interactive or about the
contents of its documentation string.)

The varlist should set several values.  Clearly, the top of the label
for the vertical axis must be at least the height of the graph, which
means that we must obtain this information here.  Note that the
`print-graph-body' function also requires this information.  There is
no reason to calculate the height of the graph in two different
places, so we should change `print-graph-body' from the way we
defined it earlier to take advantage of the calculation.

Similarly, both the function for printing the X axis labels and the
`print-graph-body' function need to learn the value of the width of
each symbol.  We can perform the calculation here and change the
definition for `print-graph-body' from the way we defined it in the
previous chapter.

The length of the label for the horizontal axis must be at least as
long as the graph.  However, this information is used only in the
function that prints the horizontal axis, so it does not need to be
calculated here.

These thoughts lead us directly to the following form for the varlist
in the `let' for `print-graph':

     (let ((height (apply 'max numbers-list)) ; First version.
           (symbol-width (length graph-blank)))

As we shall see, this expression is not quite right.

The `print-Y-axis' Function
===========================

The job of the `print-Y-axis' function is to print a label for the
vertical axis that looks like this:

         10 -
     
     
     
     
          5 -
     
     
     
          1 -

The function should be passed the height of the graph, and then should
construct and insert the appropriate numbers and marks.

It is easy enough to see in the figure what the Y axis label should
look like; but to say in words, and then to write a function
definition to do the job is another matter.  It is not quite true to
say that we want a number and a tic every five lines: there are only
three lines between the `1' and the `5' (lines 2, 3, and 4), but four
lines between the `5' and the `10' (lines 6, 7, 8, and 9).  It is
better to say that we want a number and a tic mark on the base line
(number 1) and then that we want a number and a tic on the fifth line
from the bottom and on every line that is a multiple of five.

What height should the label be?
--------------------------------

The next issue is what height the label should be?  Suppose the
maximum height of tallest column of the graph is seven.  Should the
highest label on the Y axis be `5 -', and should the graph stick up
above the label?  Or should the highest label be `7 -', and mark the
peak of the graph?  Or should the highest label be `10 -', which is a
multiple of five, and be higher than the topmost value of the graph?

The latter form is preferred.  Most graphs are drawn within rectangles
whose sides are an integral number of steps long--5, 10, 15, and so
on for a step distance of five.  But as soon as we decide to use a
step height for the vertical axis, we discover that the simple
expression in the varlist for computing the height is wrong.  The
expression is `(apply 'max numbers-list)'.  This returns the precise
height, not the maximum height plus whatever is necessary to round up
to the nearest multiple of five.  A more complex expression is
required.

As usual in cases like this, a complex problem becomes simpler if it
is divided into several smaller problems.

First, consider the case when the highest value of the graph is an
integral multiple of five--when it is 5, 10, 15 ,or some higher
multiple of five.  We can use this value as the Y axis height.

A fairly simply way to determine whether a number is a multiple of
five is to divide it by five and see if the division results in a
remainder.  If there is no remainder, the number is a multiple of
five.  Thus, seven divided by five has a remainder of two, and seven
is not an integral multiple of five.  Put in slightly different
language, more reminiscent of the classroom, five goes into seven
once, with a remainder of two.  However, five goes into ten twice,
with no remainder: ten is an integral multiple of five.

Side Trip: Compute a Remainder
------------------------------

In Lisp, the function for computing a remainder is `%'.  The function
returns the remainder of its first argument divided by its second
argument.  As it happens, `%' is a function in Emacs Lisp that you
cannot discover using `apropos': you find nothing if you type `M-x
apropos <RET> remainder <RET>'.  The only way to learn of the
existence of `%' is to read about it in a book such as this or in the
Emacs Lisp sources.  The `%' function is used in the code for
`rotate-yank-pointer', which is described in an appendix.  (*Note The
Body of `rotate-yank-pointer': rotate-yk-ptr body.)

You can try the `%' function by evaluating the following two
expressions:

     (% 7 5)
     
     (% 10 5)

The first expression returns 2 and the second expression returns 0.

To test whether the returned value is zero or some other number, we
can use the `zerop' function.  This function returns `t' if its
argument, which must be a number, is zero.

     (zerop (% 7 5))
          => nil
     
     (zerop (% 10 5))
          => t

Thus, the following expression will return `t' if the height of the
graph is evenly divisible by five:

     (zerop (% height 5))

(The value of `height', of course, can be found from `(apply 'max
numbers-list)'.)

On the other hand, if the value of `height' is not a multiple of
five, we want to reset the value to the next higher multiple of five.
This is straightforward arithmetic using functions with which we are
already familiar.  First, we divide the value of `height' by five to
determine how many times five goes into the number.  Thus, five goes
into twelve twice.  If we add one to this quotient and multiply by
five, we will obtain the value of the next multiple of five that is
larger than the height.  Five goes into twelve twice.  Add one to two,
and multiply by five; the result is fifteen, which is the next
multiple of five that is higher than twelve.  The Lisp expression for
this is:

     (* (1+ (/ height 5)) 5)

For example, if you evaluate the following, the result is 15:

     (* (1+ (/ 12 5)) 5)

All through this discussion, we have been using `five' as the value
for spacing labels on the Y axis; but we may want to use some other
value.  For generality, we should replace `five' with a variable to
which we can assign a value.  The best name I can think of for this
variable is `Y-axis-label-spacing'.

Using this term, and an `if' expression, we produce the following:

     (if (zerop (% height Y-axis-label-spacing))
         height
       ;; else
       (* (1+ (/ height Y-axis-label-spacing))
          Y-axis-label-spacing))

This expression returns the value of `height' itself if the height is
an even multiple of the value of the `Y-axis-label-spacing' or else
it computes and returns a value of `height' that is equal to the next
higher multiple of the value of the `Y-axis-label-spacing'.

We can now include this expression in the `let' expression of the
`print-graph' function (after first setting the value of
`Y-axis-label-spacing'):

     (defvar Y-axis-label-spacing 5
       "Number of lines from one Y axis label to next.")
     
     ...
     (let* ((height (apply 'max numbers-list))
            (height-of-top-line
             (if (zerop (% height Y-axis-label-spacing))
                 height
               ;; else
               (* (1+ (/ height Y-axis-label-spacing))
                  Y-axis-label-spacing)))
            (symbol-width (length graph-blank))))
     ...

(Note use of the  `let*' function: the initial value of height is
computed once by the `(apply 'max numbers-list)' expression and then
the resulting value of  `height' is used to compute its final value.
*Note The `let*' expression: fwd-para let, for more about `let*'.)

Construct a Y Axis Element
--------------------------

When we print the vertical axis, we want to insert strings such as
`5 -' and `10 - ' every five lines.  Moreover, we want the numbers
and dashes to line up, so shorter numbers must be padded with leading
spaces.  If some of the strings use two digit numbers, the strings
with single digit numbers must include a leading blank space before
the number.

To figure out the length of the number, the `length' function is
used.  But the `length' function works only with a string, not with a
number.  So the number has to be converted from being a number to
being a string.  This is done with the `number-to-string' function.
For example,

     (length (number-to-string 35))
          => 2
     
     (length (number-to-string 100))
          => 3

(`number-to-string' is also called `int-to-string'; you will see this
alternative name in various sources.)

In addition, in each label, each number is followed by a string such
as ` - ', which we will call the `Y-axis-tic' marker.  This variable
is defined with `defvar':

     (defvar Y-axis-tic " - "
        "String that follows number in a Y axis label.")

The length of the Y label is the sum of the length of the Y axis tic
mark and the length of the number of the top of the graph.

     (length (concat (number-to-string height) Y-axis-tic)))

This value will be calculated by the `print-graph' function in its
varlist as `full-Y-label-width' and passed on.  (Note that we did not
think to include this in the varlist when we first proposed it.)

To make a complete vertical axis label, a tic mark is concatenated
with a number; and the two together may be preceded by one or more
spaces depending on how long the number is.  The label consists of
three parts: the (optional) leading spaces, the number, and the tic
mark.  The function is passed the value of the number for the specific
row, and the value of the width of the top line, which is calculated
(just once) by `print-graph'.

     (defun Y-axis-element (number full-Y-label-width)
       "Construct a NUMBERed label element.
     A numbered element looks like this `  5 - ',
     and is padded as needed so all line up with
     the element for the largest number."
       (let* ((leading-spaces
              (- full-Y-label-width
                 (length
                  (concat (number-to-string number)
                          Y-axis-tic)))))
         (concat
          (make-string leading-spaces ? )
          (number-to-string number)
          Y-axis-tic)))

The `Y-axis-element' function concatenates together the leading
spaces, if any; the number, as a string; and the tic mark.

To figure out how many leading spaces the label will need, the
function subtracts the actual length of the label--the length of the
number plus the length of the tic mark--from the desired label width.

Blank spaces are inserted using the `make-string' function.  This
function takes two arguments: the first tells it how long the string
will be and the second is a symbol for the character to insert, in a
special format.  The format is a question mark followed by a blank
space, like this, `? '.  *Note Character Type: (elisp)Character Type,
for a description of the syntax for characters.

The `number-to-string' function is used in the concatenation
expression, to convert the number to a string that is concatenated
with the leading spaces and the tic mark.

Create a Y Axis Column
----------------------

The preceding functions provide all the tools needed to construct a
function that generates a list of numbered and blank strings to insert
as the label for the vertical axis:

     (defun Y-axis-column (height width-of-label)
       "Construct list of Y axis labels and blank strings.
     For HEIGHT of line above base and WIDTH-OF-LABEL."
       (let (Y-axis)
         (while (> height 1)
           (if (zerop (% height Y-axis-label-spacing))
               ;; Insert label.
               (setq Y-axis
                     (cons
                      (Y-axis-element height width-of-label)
                      Y-axis))
             ;; Else, insert blanks.
             (setq Y-axis
                   (cons
                    (make-string width-of-label ? )
                    Y-axis)))
           (setq height (1- height)))
         ;; Insert base line.
         (setq Y-axis
               (cons (Y-axis-element 1 width-of-label) Y-axis))
         (nreverse Y-axis)))

In this function, we start with the value of `height' and
repetitively subtract one from its value.  After each subtraction, we
test to see whether the value is an integral multiple of the
`Y-axis-label-spacing'.  If it is, we construct a numbered label
using the `Y-axis-element' function; if not, we construct a blank
label using the `make-string' function.  The base line consists of
the number one followed by a tic mark.

The Not Quite Final Version of `print-Y-axis'
---------------------------------------------

The list constructed by the `Y-axis-column' function is passed to the
`print-Y-axis' function, which inserts the list as a column.

     (defun print-Y-axis (height full-Y-label-width)
       "Insert Y axis using HEIGHT and FULL-Y-LABEL-WIDTH.
     Height must be the maximum height of the graph.
     Full width is the width of the highest label element."
     ;; Value of height and full-Y-label-width
     ;; are passed by `print-graph'.
       (let ((start (point)))
         (insert-rectangle
          (Y-axis-column height full-Y-label-width))
         ;; Place point ready for inserting graph.
         (goto-char start)
         ;; Move point forward by value of full-Y-label-width
         (forward-char full-Y-label-width)))

The `print-Y-axis' uses the `insert-rectangle' function to insert the
Y axis labels created by the `Y-axis-column' function.  In addition,
it places point at the correct position for printing the body of the
graph.

You can test `print-Y-axis':

  1. Install

          Y-axis-label-spacing
          Y-axis-tic
          Y-axis-element
          Y-axis-column
          print-Y-axis

  2. Copy the following expression:

          (print-Y-axis 12 5)

  3. Switch to the `*scratch*' buffer and place the cursor where you
     want the axis labels to start.

  4. Type `M-:' (`eval-expression').

  5. Yank the `graph-body-print' expression into the minibuffer with
     `C-y' (`yank)'.

  6. Press <RET> to evaluate the expression.

Emacs will print labels vertically, the top one being `10 - '.  (The
`print-graph' function will pass the value of `height-of-top-line',
which in this case would end up as 15.)

The `print-X-axis' Function
===========================

X axis labels are much like Y axis labels, except that the tics are
on a line above the numbers.  Labels should look like this:

         |   |    |    |
         1   5   10   15

The first tic is under the first column of the graph and is preceded
by several blank spaces.  These spaces provide room in rows above for
the Y axis labels.  The second, third, fourth, and subsequent tics
are all spaced equally, according to the value of
`X-axis-label-spacing'.

The second row of the X axis consists of numbers, preceded by several
blank spaces and also separated according to the value of the variable
`X-axis-label-spacing'.

The value of the variable `X-axis-label-spacing' should itself be
measured in units of `symbol-width', since you may want to change the
width of the symbols that you are using to print the body of the
graph without changing the ways the graph is labelled.

Similarities and differences
----------------------------

The `print-X-axis' function is constructed in more or less the same
fashion as the `print-Y-axis' function except that it has two lines:
the line of tic marks and the numbers.  We will write a separate
function to print each line and then combine them within the
`print-X-axis' function.

This is a three step process:

  1. Write a function to print the X axis tic marks,
     `print-X-axis-tic-line'.

  2. Write a function to print the X numbers,
     `print-X-axis-numbered-line'.

  3. Write a function to print both lines, the `print-X-axis'
     function, using `print-X-axis-tic-line' and
     `print-X-axis-numbered-line'.

X Axis Tic Marks
----------------

The first function should print the X axis tic marks.  We must specify
the tic marks themselves and their spacing:

     (defvar X-axis-label-spacing
       (if (boundp 'graph-blank)
           (* 5 (length graph-blank)) 5)
       "Number of units from one X axis label to next.")

(Note that the value of `graph-blank' is set by another `defvar'.
The `boundp' predicate checks whether it has already been set;
`boundp' returns `nil' if it has not.  If `graph-blank' were unbound
and we did not use this conditional construction, in GNU Emacs 21, we
would enter the debugger and see an error message saying
`Debugger entered--Lisp error: (void-variable graph-blank)'.)

Here is the `defvar' for `X-axis-tic-symbol':

     (defvar X-axis-tic-symbol "|"
       "String to insert to point to a column in X axis.")

The goal is to make a line that looks like this:

            |   |    |    |

The first tic is indented so that it is under the first column, which
is indented to provide space for the Y axis labels.

A tic element consists of the blank spaces that stretch from one tic
to the next plus a tic symbol.  The number of blanks is determined by
the width of the tic symbol and the `X-axis-label-spacing'.

The code looks like this:

     ;;; X-axis-tic-element
     ...
     (concat
      (make-string
       ;; Make a string of blanks.
       (-  (* symbol-width X-axis-label-spacing)
           (length X-axis-tic-symbol))
       ? )
      ;; Concatenate blanks with tic symbol.
      X-axis-tic-symbol)
     ...

Next, we determine how many blanks are needed to indent the first tic
mark to the first column of the graph.  This uses the value of
`full-Y-label-width' passed it by the `print-graph' function.

The code to make `X-axis-leading-spaces' looks like this:

     ;; X-axis-leading-spaces
     ...
     (make-string full-Y-label-width ? )
     ...

We also need to determine the length of the horizontal axis, which is
the length of the numbers list, and the number of tics in the
horizontal axis:

     ;; X-length
     ...
     (length numbers-list)
     
     ;; tic-width
     ...
     (* symbol-width X-axis-label-spacing)
     
     ;; number-of-X-tics
     (if (zerop (% (X-length tic-width)))
         (/ (X-length tic-width))
       (1+ (/ (X-length tic-width))))

All this leads us directly to the function for printing the X axis
tic line:

     (defun print-X-axis-tic-line
       (number-of-X-tics X-axis-leading-spaces X-axis-tic-element)
       "Print tics for X axis."
         (insert X-axis-leading-spaces)
         (insert X-axis-tic-symbol)  ; Under first column.
         ;; Insert second tic in the right spot.
         (insert (concat
                  (make-string
                   (-  (* symbol-width X-axis-label-spacing)
                       ;; Insert white space up to second tic symbol.
                       (* 2 (length X-axis-tic-symbol)))
                   ? )
                  X-axis-tic-symbol))
         ;; Insert remaining tics.
         (while (> number-of-X-tics 1)
           (insert X-axis-tic-element)
           (setq number-of-X-tics (1- number-of-X-tics))))

The line of numbers is equally straightforward:

First, we create a numbered element with blank spaces before each
number:

     (defun X-axis-element (number)
       "Construct a numbered X axis element."
       (let ((leading-spaces
              (-  (* symbol-width X-axis-label-spacing)
                  (length (number-to-string number)))))
         (concat (make-string leading-spaces ? )
                 (number-to-string number))))

Next, we create the function to print the numbered line, starting with
the number "1" under the first column:

     (defun print-X-axis-numbered-line
       (number-of-X-tics X-axis-leading-spaces)
       "Print line of X-axis numbers"
       (let ((number X-axis-label-spacing))
         (insert X-axis-leading-spaces)
         (insert "1")
         (insert (concat
                  (make-string
                   ;; Insert white space up to next number.
                   (-  (* symbol-width X-axis-label-spacing) 2)
                   ? )
                  (number-to-string number)))
         ;; Insert remaining numbers.
         (setq number (+ number X-axis-label-spacing))
         (while (> number-of-X-tics 1)
           (insert (X-axis-element number))
           (setq number (+ number X-axis-label-spacing))
           (setq number-of-X-tics (1- number-of-X-tics)))))

Finally, we need to write the `print-X-axis' that uses
`print-X-axis-tic-line' and `print-X-axis-numbered-line'.

The function must determine the local values of the variables used by
both `print-X-axis-tic-line' and `print-X-axis-numbered-line', and
then it must call them.  Also, it must print the carriage return that
separates the two lines.

The function consists of a varlist that specifies five local
variables, and calls to each of the two line printing functions:

     (defun print-X-axis (numbers-list)
       "Print X axis labels to length of NUMBERS-LIST."
       (let* ((leading-spaces
               (make-string full-Y-label-width ? ))
            ;; symbol-width is provided by graph-body-print
            (tic-width (* symbol-width X-axis-label-spacing))
            (X-length (length numbers-list))
            (X-tic
             (concat
              (make-string
               ;; Make a string of blanks.
               (-  (* symbol-width X-axis-label-spacing)
                   (length X-axis-tic-symbol))
               ? )
              ;; Concatenate blanks with tic symbol.
              X-axis-tic-symbol))
            (tic-number
             (if (zerop (% X-length tic-width))
                 (/ X-length tic-width)
               (1+ (/ X-length tic-width)))))
         (print-X-axis-tic-line tic-number leading-spaces X-tic)
         (insert "\n")
         (print-X-axis-numbered-line tic-number leading-spaces)))

You can test `print-X-axis':

  1. Install `X-axis-tic-symbol', `X-axis-label-spacing',
     `print-X-axis-tic-line', as well as `X-axis-element',
     `print-X-axis-numbered-line', and `print-X-axis'.

  2. Copy the following expression:

          (progn
           (let ((full-Y-label-width 5)
                 (symbol-width 1))
             (print-X-axis
              '(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16))))

  3. Switch to the `*scratch*' buffer and place the cursor where you
     want the axis labels to start.

  4. Type `M-:' (`eval-expression').

  5. Yank the test expression into the minibuffer with `C-y' (`yank)'.

  6. Press <RET> to evaluate the expression.

Emacs will print the horizontal axis like this:

          |   |    |    |    |
          1   5   10   15   20

Printing the Whole Graph
========================

Now we are nearly ready to print the whole graph.

The function to print the graph with the proper labels follows the
outline we created earlier (*note A Graph with Labelled Axes: Full
Graph.), but with additions.

Here is the outline:

     (defun print-graph (numbers-list)
       "DOCUMENTATION..."
       (let ((height  ...
             ...))
         (print-Y-axis height ... )
         (graph-body-print numbers-list)
         (print-X-axis ... )))

Changes for the Final Version
-----------------------------

The final version is different from what we planned in two ways:
first, it contains additional values calculated once in the varlist;
second, it carries an option to specify the labels' increment per row.
This latter feature turns out to be essential; otherwise, a graph may
have more rows than fit on a display or on a sheet of paper.

This new feature requires a change to the `Y-axis-column' function,
to add `vertical-step' to it.  The function looks like this:

     ;;; Final version.
     (defun Y-axis-column
       (height width-of-label &optional vertical-step)
       "Construct list of labels for Y axis.
     HEIGHT is maximum height of graph.
     WIDTH-OF-LABEL is maximum width of label.
     VERTICAL-STEP, an option, is a positive integer
     that specifies how much a Y axis label increments
     for each line.  For example, a step of 5 means
     that each line is five units of the graph."
       (let (Y-axis
             (number-per-line (or vertical-step 1)))
         (while (> height 1)
           (if (zerop (% height Y-axis-label-spacing))
               ;; Insert label.
               (setq Y-axis
                     (cons
                      (Y-axis-element
                       (* height number-per-line)
                       width-of-label)
                      Y-axis))
             ;; Else, insert blanks.
             (setq Y-axis
                   (cons
                    (make-string width-of-label ? )
                    Y-axis)))
           (setq height (1- height)))
         ;; Insert base line.
         (setq Y-axis (cons (Y-axis-element
                             (or vertical-step 1)
                             width-of-label)
                            Y-axis))
         (nreverse Y-axis)))

The values for the maximum height of graph and the width of a symbol
are computed by `print-graph' in its `let' expression; so
`graph-body-print' must be changed to accept them.

     ;;; Final version.
     (defun graph-body-print (numbers-list height symbol-width)
       "Print a bar graph of the NUMBERS-LIST.
     The numbers-list consists of the Y-axis values.
     HEIGHT is maximum height of graph.
     SYMBOL-WIDTH is number of each column."
       (let (from-position)
         (while numbers-list
           (setq from-position (point))
           (insert-rectangle
            (column-of-graph height (car numbers-list)))
           (goto-char from-position)
           (forward-char symbol-width)
           ;; Draw graph column by column.
           (sit-for 0)
           (setq numbers-list (cdr numbers-list)))
         ;; Place point for X axis labels.
         (forward-line height)
         (insert "\n")))

Finally, the code for the `print-graph' function:

     ;;; Final version.
     (defun print-graph
       (numbers-list &optional vertical-step)
       "Print labelled bar graph of the NUMBERS-LIST.
     The numbers-list consists of the Y-axis values.
     
     Optionally, VERTICAL-STEP, a positive integer,
     specifies how much a Y axis label increments for
     each line.  For example, a step of 5 means that
     each row is five units."
       (let* ((symbol-width (length graph-blank))
              ;; `height' is both the largest number
              ;; and the number with the most digits.
              (height (apply 'max numbers-list))
              (height-of-top-line
               (if (zerop (% height Y-axis-label-spacing))
                   height
                 ;; else
                 (* (1+ (/ height Y-axis-label-spacing))
                    Y-axis-label-spacing)))
              (vertical-step (or vertical-step 1))
              (full-Y-label-width
               (length
                (concat
                 (number-to-string
                  (* height-of-top-line vertical-step))
                 Y-axis-tic))))
     
         (print-Y-axis
          height-of-top-line full-Y-label-width vertical-step)
         (graph-body-print
          numbers-list height-of-top-line symbol-width)
         (print-X-axis numbers-list)))

Testing `print-graph'
---------------------

We can test the `print-graph' function with a short list of numbers:

  1. Install the final versions of `Y-axis-column',
     `graph-body-print', and `print-graph' (in addition to the rest
     of the code.)

  2. Copy the following expression:

          (print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1))

  3. Switch to the `*scratch*' buffer and place the cursor where you
     want the axis labels to start.

  4. Type `M-:' (`eval-expression').

  5. Yank the test expression into the minibuffer with `C-y' (`yank)'.

  6. Press <RET> to evaluate the expression.

Emacs will print a graph that looks like this:

     10 -
     
     
              *
             **   *
      5 -   ****  *
            **** ***
          * *********
          ************
      1 - *************
     
          |   |    |    |
          1   5   10   15

On the other hand, if you pass `print-graph' a `vertical-step' value
of 2, by evaluating this expression:

     (print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1) 2)

The graph looks like this:

     20 -
     
     
              *
             **   *
     10 -   ****  *
            **** ***
          * *********
          ************
      2 - *************
     
          |   |    |    |
          1   5   10   15

(A question: is the `2' on the bottom of the vertical axis a bug or a
feature?  If you think it is a bug, and should be a `1' instead, (or
even a `0'), you can modify the sources.)

Graphing Numbers of Words and Symbols
-------------------------------------

Now for the graph for which all this code was written: a graph that
shows how many function definitions contain fewer than 10 words and
symbols, how many contain between 10 and 19 words and symbols, how
many contain between 20 and 29 words and symbols, and so on.

This is a multi-step process.  First make sure you have loaded all the
requisite code.

It is a good idea to reset the value of `top-of-ranges' in case you
have set it to some different value.  You can evaluate the following:

     (setq top-of-ranges
      '(10  20  30  40  50
        60  70  80  90 100
       110 120 130 140 150
       160 170 180 190 200
       210 220 230 240 250
       260 270 280 290 300)

Next create a list of the number of words and symbols in each range.

Evaluate the following:

     (setq list-for-graph
            (defuns-per-range
              (sort
               (recursive-lengths-list-many-files
                (directory-files "/usr/local/emacs/lisp"
                                 t ".+el$"))
               '<)
              top-of-ranges))

On my machine, this takes about an hour.  It looks though 303 Lisp
files in my copy of Emacs version 19.23.  After all that computing,
the `list-for-graph' has this value:

     (537 1027 955 785 594 483 349 292 224 199 166 120 116 99
     90 80 67 48 52 45 41 33 28 26 25 20 12 28 11 13 220)

This means that my copy of Emacs has 537 function definitions with
fewer than 10 words or symbols in them, 1,027 function definitions
with 10 to 19 words or symbols in them, 955 function definitions with
20 to 29 words or symbols in them, and so on.

Clearly, just by looking at this list we can see that most function
definitions contain ten to thirty words and symbols.

Now for printing.  We do _not_ want to print a graph that is 1,030
lines high ...  Instead, we should print a graph that is fewer than
twenty-five lines high.  A graph that height can be displayed on
almost any monitor, and easily printed on a sheet of paper.

This means that each value in `list-for-graph' must be reduced to
one-fiftieth its present value.

Here is a short function to do just that, using two functions we have
not yet seen, `mapcar' and `lambda'.

     (defun one-fiftieth (full-range)
       "Return list, each number one-fiftieth of previous."
      (mapcar '(lambda (arg) (/ arg 50)) full-range))

A `lambda' Expression: Useful Anonymity
---------------------------------------

`lambda' is the symbol for an anonymous function, a function without
a name.  Every time you use an anonymous function, you need to
include its whole body.

Thus,

     (lambda (arg) (/ arg 50))

is a function definition that says `return the value resulting from
dividing whatever is passed to me as `arg' by 50'.

Earlier, for example, we had a function `multiply-by-seven'; it
multiplied its argument by 7.  This function is similar, except it
divides its argument by 50; and, it has no name.  The anonymous
equivalent of `multiply-by-seven' is:

     (lambda (number) (* 7 number))

(*Note The `defun' Special Form: defun.)

If we want to multiply 3 by 7, we can write:

     (multiply-by-seven 3)
      \_______________/ ^
              |         |
           function  argument



This expression returns 21.

Similarly, we can write:

     ((lambda (number) (* 7 number)) 3)
      \____________________________/ ^
                    |                |
           anonymous function     argument



If we want to divide 100 by 50, we can write:

     ((lambda (arg) (/ arg 50)) 100)
      \______________________/  \_/
                  |              |
         anonymous function   argument



This expression returns 2.  The 100 is passed to the function, which
divides that number by 50.

*Note Lambda Expressions: (elisp)Lambda Expressions, for more about
`lambda'.  Lisp and lambda expressions derive from the Lambda
Calculus.

The `mapcar' Function
---------------------

`mapcar' is a function that calls its first argument with each
element of its second argument, in turn.  The second argument must be
a sequence.

The `map' part of the name comes from the mathematical phrase,
`mapping over a domain', meaning to apply a function to each of the
elements in a domain.  The mathematical phrase is based on the
metaphor of a surveyor walking, one step at a time, over an area he is
mapping.  And `car', of course, comes from the Lisp notion of the
first of a list.

For example,

     (mapcar '1+ '(2 4 6))
          => (3 5 7)

The function `1+' which adds one to its argument, is executed on
_each_ element of the list, and a new list is returned.

Contrast this with `apply', which applies its first argument to all
the remaining.  (*Note Readying a Graph: Readying a Graph, for a
explanation of `apply'.)

In the definition of `one-fiftieth', the first argument is the
anonymous function:

     (lambda (arg) (/ arg 50))

and the second argument is `full-range', which will be bound to
`list-for-graph'.

The whole expression looks like this:

     (mapcar '(lambda (arg) (/ arg 50)) full-range))

*Note Mapping Functions: (elisp)Mapping Functions, for more about
`mapcar'.

Using the `one-fiftieth' function, we can generate a list in which
each element is one-fiftieth the size of the corresponding element in
`list-for-graph'.

     (setq fiftieth-list-for-graph
           (one-fiftieth list-for-graph))

The resulting list looks like this:

     (10 20 19 15 11 9 6 5 4 3 3 2 2
     1 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 4)

This, we are almost ready to print!  (We also notice the loss of
information: many of the higher ranges are 0, meaning that fewer than
50 defuns had that many words or symbols--but not necessarily meaning
that none had that many words or symbols.)

Another Bug ... Most Insidious
------------------------------

I said `almost ready to print'!  Of course, there is a bug in the
`print-graph' function ...  It has a `vertical-step' option, but not
a `horizontal-step' option.  The `top-of-range' scale goes from 10 to
300 by tens.  But the `print-graph' function will print only by ones.

This is a classic example of what some consider the most insidious
type of bug, the bug of omission.  This is not the kind of bug you can
find by studying the code, for it is not in the code; it is an omitted
feature.  Your best actions are to try your program early and often;
and try to arrange, as much as you can, to write code that is easy to
understand and easy to change.  Try to be aware, whenever you can,
that whatever you have written, _will_ be rewritten, if not soon,
eventually.  A hard maxim to follow.

It is the `print-X-axis-numbered-line' function that needs the work;
and then the `print-X-axis' and the `print-graph' functions need to
be adapted.  Not much needs to be done; there is one nicety: the
numbers ought to line up under the tic marks.  This takes a little
thought.

Here is the corrected `print-X-axis-numbered-line':

     (defun print-X-axis-numbered-line
       (number-of-X-tics X-axis-leading-spaces
        &optional horizontal-step)
       "Print line of X-axis numbers"
       (let ((number X-axis-label-spacing)
             (horizontal-step (or horizontal-step 1)))
         (insert X-axis-leading-spaces)
         ;; Delete extra leading spaces.
         (delete-char
          (- (1-
              (length (number-to-string horizontal-step)))))
         (insert (concat
                  (make-string
                   ;; Insert white space.
                   (-  (* symbol-width
                          X-axis-label-spacing)
                       (1-
                        (length
                         (number-to-string horizontal-step)))
                       2)
                   ? )
                  (number-to-string
                   (* number horizontal-step))))
         ;; Insert remaining numbers.
         (setq number (+ number X-axis-label-spacing))
         (while (> number-of-X-tics 1)
           (insert (X-axis-element
                    (* number horizontal-step)))
           (setq number (+ number X-axis-label-spacing))
           (setq number-of-X-tics (1- number-of-X-tics)))))

If you are reading this in Info, you can see the new versions of
`print-X-axis' `print-graph' and evaluate them.  If you are reading
this in a printed book, you can see the changed lines here (the full
text is too much to print).

     (defun print-X-axis (numbers-list horizontal-step)
       "Print X axis labels to length of NUMBERS-LIST.
     Optionally, HORIZONTAL-STEP, a positive integer,
     specifies how much an X  axis label increments for
     each column."
     ;; Value of symbol-width and full-Y-label-width
     ;; are passed by `print-graph'.
       (let* ((leading-spaces
               (make-string full-Y-label-width ? ))
            ;; symbol-width is provided by graph-body-print
            (tic-width (* symbol-width X-axis-label-spacing))
            (X-length (length numbers-list))
            (X-tic
             (concat
              (make-string
               ;; Make a string of blanks.
               (-  (* symbol-width X-axis-label-spacing)
                   (length X-axis-tic-symbol))
               ? )
              ;; Concatenate blanks with tic symbol.
              X-axis-tic-symbol))
            (tic-number
             (if (zerop (% X-length tic-width))
                 (/ X-length tic-width)
               (1+ (/ X-length tic-width)))))
     
         (print-X-axis-tic-line
          tic-number leading-spaces X-tic)
         (insert "\n")
         (print-X-axis-numbered-line
          tic-number leading-spaces horizontal-step)))

     (defun print-graph
       (numbers-list &optional vertical-step horizontal-step)
       "Print labelled bar graph of the NUMBERS-LIST.
     The numbers-list consists of the Y-axis values.
     
     Optionally, VERTICAL-STEP, a positive integer,
     specifies how much a Y axis label increments for
     each line.  For example, a step of 5 means that
     each row is five units.
     
     Optionally, HORIZONTAL-STEP, a positive integer,
     specifies how much an X  axis label increments for
     each column."
       (let* ((symbol-width (length graph-blank))
              ;; `height' is both the largest number
              ;; and the number with the most digits.
              (height (apply 'max numbers-list))
              (height-of-top-line
               (if (zerop (% height Y-axis-label-spacing))
                   height
                 ;; else
                 (* (1+ (/ height Y-axis-label-spacing))
                    Y-axis-label-spacing)))
              (vertical-step (or vertical-step 1))
              (full-Y-label-width
               (length
                (concat
                 (number-to-string
                  (* height-of-top-line vertical-step))
                 Y-axis-tic))))
         (print-Y-axis
          height-of-top-line full-Y-label-width vertical-step)
         (graph-body-print
             numbers-list height-of-top-line symbol-width)
         (print-X-axis numbers-list horizontal-step)))

The Printed Graph
-----------------

When made and installed, you can call the `print-graph' command like
this:

     (print-graph fiftieth-list-for-graph 50 10)

Here is the graph:



     1000 -  *
             **
             **
             **
             **
      750 -  ***
             ***
             ***
             ***
             ****
      500 - *****
            ******
            ******
            ******
            *******
      250 - ********
            *********                     *
            ***********                   *
            *************                 *
       50 - ***************** *           *
            |   |    |    |    |    |    |    |
           10  50  100  150  200  250  300  350



The largest group of functions contain 10 - 19 words and symbols each.

GNU Free Documentation License
******************************

                       Version 1.1, March 2000
     Copyright (C) 2000 Free Software Foundation, Inc.
     59 Temple Place, Suite 330, Boston, MA  02111-1307, USA
     
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  4. MODIFICATIONS

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Index
*****

% (remainder function):
          See ``Side Trip: Compute a Remainder''.
(debug) in code:
          See ```debug-on-quit' and `(debug)'''.
* (multiplication):
          See ``The `defun' Special Form''.
* for read-only buffer:
          See ``A Read-only Buffer''.
*scratch* buffer:
          See ``An Example: `print-elements-of-list'''.
.emacs file:
          See ``Your `.emacs' File''.
.emacs file, beginning of:
          See ``Beginning a `.emacs' File''.
/ (division):
          See ``What happens in a large buffer''.
<= (less than or equal):
          See ``The parts of the function definition''.
> (greater than):
          See ```if' in more detail''.
Accumulate, type of recursive pattern:
          See ``Recursive Pattern: _accumulate_''.
add-hook:
          See ``Text and Auto Fill Mode''.
and <1>:
          See ``The `let*' expression''.
and:
          See ``The `kill-new' function''.
and, introduced:
          See ``The `kill-new' function''.
Anonymous function:
          See ``A `lambda' Expression: Useful Anonymity''.
append-to-buffer:
          See ``The Definition of `append-to-buffer'''.
apply:
          See ``Printing the Columns of a Graph''.
apropos:
          See ``Printing the Columns of a Graph''.
Argument as local variable:
          See ``Putting the function definition together''.
argument defined:
          See ``Arguments''.
argument list defined:
          See ``The `defun' Special Form''.
Argument, wrong type of:
          See ``Using the Wrong Type Object as an Argument''.
Arguments:
          See ``Arguments''.
Arguments' data types:
          See ``Arguments' Data Types''.
Arguments, variable number of:
          See ``Variable Number of Arguments''.
Asterisk for read-only buffer:
          See ``A Read-only Buffer''.
Auto Fill mode turned on:
          See ``Text and Auto Fill Mode''.
autoload:
          See ``Autoloading''.
Automatic mode selection:
          See ``Text and Auto Fill Mode''.
Axis, print horizontal:
          See ``The `print-X-axis' Function''.
Axis, print vertical:
          See ``The `print-Y-axis' Function''.
beginning-of-buffer:
          See ``Complete Definition of `beginning-of-buffer'''.
bind defined:
          See ``Setting the Value of a Variable''.
body defined:
          See ``The `defun' Special Form''.
Body of graph:
          See ``Readying a Graph''.
Buffer size:
          See ``Buffer Size and the Location of Point''.
Buffer, history of word:
          See ``Buffer Names''.
buffer-file-name:
          See ``Buffer Names''.
buffer-menu, bound to key:
          See ``Some Keybindings''.
buffer-name:
          See ``Buffer Names''.
Bug, most insidious type:
          See ``Another Bug ... Most Insidious''.
Building robots:
          See ``Building Robots: Extending the Metaphor''.
Building Tags in the Emacs sources:
          See ``Create Your Own `TAGS' File''.
Byte compiling:
          See ``Byte Compiling''.
C language primitives:
          See ``An Aside about Primitive Functions''.
C, a digression into:
          See ``Digression into C''.
call defined:
          See ``Switching Buffers''.
cancel-debug-on-entry:
          See ```debug-on-entry'''.
car, introduced:
          See ```car', `cdr', `cons': Fundamental Functions''.
cdr, introduced:
          See ```car', `cdr', `cons': Fundamental Functions''.
Changing a function definition:
          See ``Change a Function Definition''.
Chest of Drawers, metaphor for a symbol:
          See ``Symbols as a Chest of Drawers''.
Clipping text:
          See ``Cutting and Storing Text''.
Code installation:
          See ``Install Code Permanently''.
command defined:
          See ``How to Evaluate''.
Comments in Lisp code:
          See ``Change a Function Definition''.
Common Lisp:
          See ``Lisp History''.
compare-windows:
          See ``Some Keybindings''.
concat:
          See ``Arguments' Data Types''.
cond:
          See ``Recursion Example Using `cond'''.
condition-case:
          See ```condition-case'''.
Conditional 'twixt two versions of Emacs:
          See ``A Simple Extension: `line-to-top-of-window'''.
Conditional with if:
          See ``The `if' Special Form''.
cons, example:
          See ``The `kill-new' function''.
cons, introduced:
          See ```cons'''.
copy-region-as-kill:
          See ```copy-region-as-kill'''.
copy-to-buffer:
          See ``The Definition of `copy-to-buffer'''.
Count words recursively:
          See ``Count Words Recursively''.
count-words-in-defun:
          See ``The `count-words-in-defun' Function''.
count-words-region:
          See ``The `count-words-region' Function''.
Counting:
          See ``Counting''.
Counting words in a defun <1>:
          See ``The `count-words-in-defun' Function''.
Counting words in a defun:
          See ``Counting Words in a `defun'''.
current-buffer:
          See ``Getting Buffers''.
Customizing your .emacs file:
          See ``Your `.emacs' File''.
Cutting and storing text:
          See ``Cutting and Storing Text''.
Data types:
          See ``Arguments' Data Types''.
debug:
          See ```debug'''.
debug-on-entry:
          See ```debug-on-entry'''.
debug-on-quit:
          See ```debug-on-quit' and `(debug)'''.
debugging:
          See ``Debugging''.
default-mode-line-format:
          See ``A Modified Mode Line''.
default.el init file:
          See ``Site-wide Initialization Files''.
defcustom:
          See ``Specifying Variables using `defcustom'''.
Deferment in recursion:
          See ``Recursion without Deferments''.
Defermentless solution:
          See ``No Deferment Solution''.
Definition installation:
          See ``Install a Function Definition''.
Definition writing:
          See ``How To Write Function Definitions''.
Definition, how to change:
          See ``Change a Function Definition''.
defun:
          See ``The `defun' Special Form''.
defvar:
          See ``Initializing a Variable with `defvar'''.
defvar for a user customizable variable:
          See ```defvar' and an asterisk''.
defvar with an asterisk:
          See ```defvar' and an asterisk''.
delete-and-extract-region <1>:
          See ``Digression into C''.
delete-and-extract-region:
          See ```delete-and-extract-region'''.
Deleting text:
          See ``Cutting and Storing Text''.
describe-function:
          See ``A Simplified `beginning-of-buffer' Definition''.
describe-function, introduced:
          See ``Finding More Information''.
Digression into C:
          See ``Digression into C''.
directory-files:
          See ``Making a List of Files''.
Division:
          See ``What happens in a large buffer''.
dolist:
          See ``The `dolist' Macro''.
dotimes:
          See ``The `dotimes' Macro''.
Drawers, Chest of, metaphor for a symbol:
          See ``Symbols as a Chest of Drawers''.
Duplicated words function:
          See ``The `the-the' Function''.
edebug:
          See ``The `edebug' Source Level Debugger''.
edit-options:
          See ```defvar' and an asterisk''.
Else:
          See ``If-then-else Expressions''.
Emacs version, choosing:
          See ``A Simple Extension: `line-to-top-of-window'''.
empty list defined:
          See ``Lisp Atoms''.
empty string defined:
          See ``Review''.
eobp:
          See ``Between paragraphs''.
eq:
          See ``Review''.
eq (example of use):
          See ```last-command' and `this-command'''.
equal:
          See ``Review''.
Erasing text:
          See ``Cutting and Storing Text''.
error:
          See ``The Body of `rotate-yank-pointer'''.
Error for symbol without function:
          See ``Error Message for a Symbol Without a Function''.
Error for symbol without value:
          See ``Error Message for a Symbol Without a Value''.
Error message generation:
          See ``Generate an Error Message''.
etags:
          See ``Create Your Own `TAGS' File''.
evaluate defined:
          See ``Run a Program''.
Evaluating inner lists:
          See ``Evaluating Inner Lists''.
Evaluation:
          See ``Evaluation''.
Evaluation practice:
          See ``Practicing Evaluation''.
Every, type of recursive pattern:
          See ``Recursive Pattern: _every_''.
Example variable, fill-column:
          See ```fill-column', an Example Variable''.
expression defined:
          See ``Lisp Atoms''.
Falsehood and truth in Emacs Lisp:
          See ``Truth and Falsehood in Emacs Lisp''.
FDL, GNU Free Documentation License:
          See ``GNU Free Documentation License''.
files-in-below-directory:
          See ``Making a List of Files''.
fill-column, an example variable:
          See ```fill-column', an Example Variable''.
Find a File:
          See ``Find a File''.
Find function documentation:
          See ``Finding More Information''.
Find source of function:
          See ``Finding More Information''.
find-tags:
          See ``Finding More Information''.
Flowers in a field:
          See ``Lisp Lists''.
Focusing attention (narrowing):
          See ``Narrowing and Widening''.
form defined:
          See ``Lisp Atoms''.
Formatting convention:
          See ```save-excursion' in `append-to-buffer'''.
Formatting help:
          See ``GNU Emacs Helps You Type Lists''.
forward-paragraph:
          See ```forward-paragraph': a Goldmine of Functions''.
forward-sentence:
          See ```forward-sentence'''.
function defined:
          See ``Generate an Error Message''.
function definition defined:
          See ``The `defun' Special Form''.
Function definition installation:
          See ``Install a Function Definition''.
Function definition writing:
          See ``How To Write Function Definitions''.
Function definition, how to change:
          See ``Change a Function Definition''.
Functions, primitive:
          See ``An Aside about Primitive Functions''.
Generate an error message:
          See ``Generate an Error Message''.
Getting a buffer:
          See ``Getting Buffers''.
Global set key:
          See ``Some Keybindings''.
global-set-key:
          See ``Some Keybindings''.
global-unset-key:
          See ``Some Keybindings''.
Graph prototype:
          See ``Readying a Graph''.
Graph, printing all:
          See ``Printing the Whole Graph''.
graph-body-print:
          See ``The `graph-body-print' Function''.
graph-body-print Final version.:
          See ``Changes for the Final Version''.
Handling the kill ring:
          See ``Handling the Kill Ring''.
Help typing lists:
          See ``GNU Emacs Helps You Type Lists''.
Horizontal axis printing:
          See ``The `print-X-axis' Function''.
if:
          See ``The `if' Special Form''.
if-part defined:
          See ```if' in more detail''.
indent-tabs-mode:
          See ``Indent Tabs Mode''.
Indentation for formatting:
          See ```save-excursion' in `append-to-buffer'''.
Initialization file:
          See ``Your `.emacs' File''.
Initializing a variable:
          See ``Initializing a Variable with `defvar'''.
Inner list evaluation:
          See ``Evaluating Inner Lists''.
insert-buffer:
          See ``The Definition of `insert-buffer'''.
insert-buffer-substring:
          See ``An Overview of `append-to-buffer'''.
Insidious type of bug:
          See ``Another Bug ... Most Insidious''.
Install a Function Definition:
          See ``Install a Function Definition''.
Install code permanently:
          See ``Install Code Permanently''.
interactive:
          See ``Make a Function Interactive''.
interactive function defined:
          See ``How to Evaluate''.
Interactive functions:
          See ``Make a Function Interactive''.
Interactive options:
          See ``Different Options for `interactive'''.
interactive, example use of:
          See ``The Interactive Expression in `insert-buffer'''.
Interpreter, Lisp, explained:
          See ``Run a Program''.
Interpreter, what it does:
          See ``The Lisp Interpreter''.
Keep, type of recursive pattern:
          See ``Recursive Pattern: _keep_''.
Key setting globally:
          See ``Some Keybindings''.
Key unbinding:
          See ``Some Keybindings''.
Keymaps:
          See ``Keymaps''.
Keyword:
          See ``Optional Arguments''.
Kill ring handling:
          See ``Handling the Kill Ring''.
Kill ring overview:
          See ``Kill Ring Overview''.
kill-append:
          See ``The `kill-append' function''.
kill-new:
          See ``The `kill-new' function''.
kill-region:
          See ```kill-region'''.
Killing text:
          See ``Cutting and Storing Text''.
lambda:
          See ``A `lambda' Expression: Useful Anonymity''.
length:
          See ``Find the Length of a List: `length'''.
lengths-list-file:
          See ```lengths-list-file' in Detail''.
lengths-list-many-files:
          See ``Determine the lengths of `defuns'''.
let:
          See ```let'''.
let expression sample:
          See ``Sample `let' Expression''.
let expression, parts of:
          See ``The Parts of a `let' Expression''.
let variables uninitialized:
          See ``Uninitialized Variables in a `let' Statement''.
Library, as term for `file':
          See ``Finding More Information''.
line-to-top-of-window:
          See ``A Simple Extension: `line-to-top-of-window'''.
Lisp Atoms:
          See ``Lisp Atoms''.
Lisp history:
          See ``Lisp History''.
Lisp interpreter, explained:
          See ``Run a Program''.
Lisp interpreter, what it does:
          See ``The Lisp Interpreter''.
Lisp Lists:
          See ``Lisp Lists''.
Lisp macro:
          See ```delete-and-extract-region'''.
list-buffers, rebound:
          See ``Some Keybindings''.
Lists in a computer:
          See ``How Lists are Implemented''.
load-library:
          See ``Loading Files''.
load-path:
          See ``Loading Files''.
Loading files:
          See ``Loading Files''.
local variable defined:
          See ```let' Prevents Confusion''.
Local variables list, per-buffer,:
          See ``Text and Auto Fill Mode''.
Location of point:
          See ``Buffer Size and the Location of Point''.
looking-at:
          See ``Between paragraphs''.
Loops:
          See ```while'''.
Loops and recursion:
          See ``Loops and Recursion''.
Maclisp:
          See ``Lisp History''.
Macro, lisp:
          See ```delete-and-extract-region'''.
Mail aliases:
          See ``Mail Aliases''.
make tags:
          See ``Create Your Own `TAGS' File''.
make-string:
          See ``Construct a Y Axis Element''.
mapcar:
          See ``The `mapcar' Function''.
mark:
          See ```save-excursion'''.
mark-whole-buffer:
          See ``The Definition of `mark-whole-buffer'''.
match-beginning:
          See ``No fill prefix''.
max:
          See ``Printing the Columns of a Graph''.
message:
          See ``The `message' Function''.
min:
          See ``Printing the Columns of a Graph''.
Mode line format:
          See ``A Modified Mode Line''.
Mode selection, automatic:
          See ``Text and Auto Fill Mode''.
Motion by sentence and paragraph:
          See ``Regular Expression Searches''.
Narrowing:
          See ``Narrowing and Widening''.
narrowing defined:
          See ``Buffer Size and the Location of Point''.
nil:
          See ``Truth and Falsehood in Emacs Lisp''.
nil, history of word:
          See ``Buffer Names''.
No deferment solution:
          See ``No Deferment Solution''.
nreverse:
          See ``Counting function definitions''.
nth:
          See ```nth'''.
nthcdr <1>:
          See ```copy-region-as-kill'''.
nthcdr:
          See ```nthcdr'''.
nthcdr, example:
          See ``The `kill-new' function''.
number-to-string:
          See ``Construct a Y Axis Element''.
occur:
          See ``Some Keybindings''.
optional:
          See ``Optional Arguments''.
Optional arguments:
          See ``Optional Arguments''.
Options for interactive:
          See ``Different Options for `interactive'''.
or:
          See ``The `or' in the Body''.
other-buffer:
          See ``Getting Buffers''.
Paragraphs, movement by:
          See ``Regular Expression Searches''.
Parts of a Recursive Definition:
          See ``The Parts of a Recursive Definition''.
Parts of let expression:
          See ``The Parts of a `let' Expression''.
Passing information to functions:
          See ``Arguments''.
Pasting text:
          See ``Yanking Text Back''.
Patterns, searching for:
          See ``Regular Expression Searches''.
Per-buffer, local variables list:
          See ``Text and Auto Fill Mode''.
Permanent code installation:
          See ``Install Code Permanently''.
point:
          See ```save-excursion'''.
point defined:
          See ``Buffer Size and the Location of Point''.
Point location:
          See ``Buffer Size and the Location of Point''.
Point, mark, buffer preservation:
          See ```save-excursion'''.
Practicing evaluation:
          See ``Practicing Evaluation''.
Preserving point, mark, and buffer:
          See ```save-excursion'''.
Primitive functions:
          See ``An Aside about Primitive Functions''.
Primitives written in C:
          See ``An Aside about Primitive Functions''.
Print horizontal axis:
          See ``The `print-X-axis' Function''.
Print vertical axis:
          See ``The `print-Y-axis' Function''.
print-elements-of-list:
          See ``An Example: `print-elements-of-list'''.
print-elements-recursively:
          See ``Recursion with a List''.
print-graph Final version.:
          See ``Changes for the Final Version''.
print-graph varlist:
          See ``The `print-graph' Varlist''.
print-X-axis:
          See ``X Axis Tic Marks''.
print-X-axis-numbered-line:
          See ``X Axis Tic Marks''.
print-X-axis-tic-line:
          See ``X Axis Tic Marks''.
print-Y-axis:
          See ``The Not Quite Final Version of `print-Y-axis'''.
Printing the whole graph:
          See ``Printing the Whole Graph''.
prog1:
          See ``Between paragraphs''.
progn:
          See ``The `progn' Special Form''.
Program, running one:
          See ``Run a Program''.
Prototype graph:
          See ``Readying a Graph''.
re-search-forward:
          See ``The `re-search-forward' Function''.
Read-only buffer:
          See ``A Read-only Buffer''.
Readying a graph:
          See ``Readying a Graph''.
Rebinding keys:
          See ``Keymaps''.
Recursion:
          See ``Recursion''.
Recursion and loops:
          See ``Loops and Recursion''.
Recursion without Deferments:
          See ``Recursion without Deferments''.
Recursive Definition Parts:
          See ``The Parts of a Recursive Definition''.
Recursive pattern: accumulate:
          See ``Recursive Pattern: _accumulate_''.
Recursive pattern: every:
          See ``Recursive Pattern: _every_''.
Recursive pattern: keep:
          See ``Recursive Pattern: _keep_''.
Recursive Patterns:
          See ``Recursive Patterns''.
recursive-count-words:
          See ``Count Words Recursively''.
recursive-graph-body-print:
          See ``The `recursive-graph-body-print' Function''.
recursive-lengths-list-many-files:
          See ``Recursively Count Words in Different Files''.
Recursively counting words:
          See ``Count Words Recursively''.
regexp-quote:
          See ``The `let*' expression''.
Region, what it is:
          See ```save-excursion'''.
Regular expression searches:
          See ``Regular Expression Searches''.
Regular expressions for word counting:
          See ``Counting: Repetition and Regexps''.
Remainder function, %:
          See ``Side Trip: Compute a Remainder''.
Repetition (loops):
          See ``Loops and Recursion''.
Repetition for word counting:
          See ``Counting: Repetition and Regexps''.
Retrieving text:
          See ``Yanking Text Back''.
reverse:
          See ``Counting function definitions''.
Ring, making a list like a:
          See ``Handling the Kill Ring''.
Robots, building:
          See ``Building Robots: Extending the Metaphor''.
rotate-yank-pointer <1>:
          See ``The `rotate-yank-pointer' Function''.
rotate-yank-pointer:
          See ``Yanking Text Back''.
Run a program:
          See ``Run a Program''.
Sample let expression:
          See ``Sample `let' Expression''.
save-excursion:
          See ```save-excursion'''.
save-restriction:
          See ``The `save-restriction' Special Form''.
search-forward:
          See ``The `search-forward' Function''.
Searches, illustrating:
          See ``Regular Expression Searches''.
sentence-end:
          See ``The Regular Expression for `sentence-end'''.
Sentences, movement by:
          See ``Regular Expression Searches''.
set:
          See ``Using `set'''.
set-buffer:
          See ``Switching Buffers''.
setcar:
          See ```setcar'''.
setcdr:
          See ```setcdr'''.
setcdr, example:
          See ``The `kill-new' function''.
setq:
          See ``Using `setq'''.
Setting a key globally:
          See ``Some Keybindings''.
Setting value of variable:
          See ``Setting the Value of a Variable''.
side effect defined:
          See ``Evaluation''.
Simple extension in .emacs file:
          See ``A Simple Extension: `line-to-top-of-window'''.
simplified-beginning-of-buffer:
          See ``A Simplified `beginning-of-buffer' Definition''.
site-init.el init file:
          See ``Site-wide Initialization Files''.
site-load.el init file:
          See ``Site-wide Initialization Files''.
Size of buffer:
          See ``Buffer Size and the Location of Point''.
Solution without deferment:
          See ``No Deferment Solution''.
sort:
          See ``Sorting Lists''.
Source level debugger:
          See ``The `edebug' Source Level Debugger''.
Special form:
          See ``Complications''.
Special form of defun:
          See ``The `defun' Special Form''.
Storing and cutting text:
          See ``Cutting and Storing Text''.
string defined:
          See ``Lisp Atoms''.
switch-to-buffer:
          See ``Switching Buffers''.
Switching to a buffer:
          See ``Switching Buffers''.
Symbol names:
          See ``Symbol Names and Function Definitions''.
Symbol without function error:
          See ``Error Message for a Symbol Without a Function''.
Symbol without value error:
          See ``Error Message for a Symbol Without a Value''.
Symbolic expressions, introduced:
          See ``Lisp Atoms''.
Symbols as a Chest of Drawers:
          See ``Symbols as a Chest of Drawers''.
Syntax categories and tables:
          See ``What Constitutes a Word or Symbol?''.
Tabs, preventing:
          See ``Indent Tabs Mode''.
TAGS file, create own:
          See ``Create Your Own `TAGS' File''.
Tags in the Emacs sources:
          See ``Create Your Own `TAGS' File''.
TAGS table, specifying:
          See ``Finding More Information''.
Text between double quotation marks:
          See ``Lisp Atoms''.
Text Mode turned on:
          See ``Text and Auto Fill Mode''.
Text retrieval:
          See ``Yanking Text Back''.
the-the:
          See ``The `the-the' Function''.
then-part defined:
          See ```if' in more detail''.
top-of-ranges:
          See ``Counting function definitions''.
triangle-bugged:
          See ```debug'''.
triangle-recursively:
          See ``Recursion in Place of a Counter''.
Truth and falsehood in Emacs Lisp:
          See ``Truth and Falsehood in Emacs Lisp''.
Types of data:
          See ``Arguments' Data Types''.
Unbinding key:
          See ``Some Keybindings''.
Uninitialized let variables:
          See ``Uninitialized Variables in a `let' Statement''.
Variable initialization:
          See ``Initializing a Variable with `defvar'''.
Variable number of arguments:
          See ``Variable Number of Arguments''.
Variable, example of, fill-column:
          See ```fill-column', an Example Variable''.
Variable, setting value:
          See ``Setting the Value of a Variable''.
Variables:
          See ``Variables''.
varlist defined:
          See ``The Parts of a `let' Expression''.
Version of Emacs, choosing:
          See ``A Simple Extension: `line-to-top-of-window'''.
Vertical axis printing:
          See ``The `print-Y-axis' Function''.
what-line:
          See ```what-line'''.
while:
          See ```while'''.
Whitespace in lists:
          See ``Whitespace in Lists''.
Whole graph printing:
          See ``Printing the Whole Graph''.
Widening:
          See ``Narrowing and Widening''.
Widening, example of:
          See ```what-line'''.
Word counting in a defun:
          See ``Counting Words in a `defun'''.
Words and symbols in defun:
          See ``What to Count?''.
Words, counted recursively:
          See ``Count Words Recursively''.
Words, duplicated:
          See ``The `the-the' Function''.
Writing a function definition:
          See ``How To Write Function Definitions''.
Wrong type of argument:
          See ``Using the Wrong Type Object as an Argument''.
X axis printing:
          See ``The `print-X-axis' Function''.
X-axis-element:
          See ``X Axis Tic Marks''.
Y axis printing:
          See ``The `print-Y-axis' Function''.
Y-axis-column:
          See ``Create a Y Axis Column''.
Y-axis-column Final version.:
          See ``Changes for the Final Version''.
Y-axis-label-spacing:
          See ``Side Trip: Compute a Remainder''.
Y-axis-tic:
          See ``Construct a Y Axis Element''.
yank <1>:
          See ```yank'''.
yank:
          See ``Yanking Text Back''.
yank-pop:
          See ```yank-pop'''.
zap-to-char:
          See ```zap-to-char'''.
zerop:
          See ``The Body of `rotate-yank-pointer'''.
About the Author
****************

     Robert J. Chassell has worked with GNU Emacs since 1985.  He
     writes and edits, teaches Emacs and Emacs Lisp, and speaks
     throughout the world on software freedom.  Chassell was a
     founding Director and Treasurer of the Free Software Foundation,
     Inc.  He is co-author of the `Texinfo' manual, and has edited
     more than a dozen other books.  He graduated from Cambridge
     University, in England.  He has an abiding interest in social
     and economic history and flies his own airplane.