(command-line): If no error data, print "", not nil.

[gnu-emacs] / lispref / help.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/help
@node Documentation, Files, Modes, Top
@chapter Documentation
@cindex documentation strings

  GNU Emacs Lisp has convenient on-line help facilities, most of which
derive their information from the documentation strings associated with
functions and variables.  This chapter describes how to write good
documentation strings for your Lisp programs, as well as how to write
programs to access documentation.

  Note that the documentation strings for Emacs are not the same thing
as the Emacs manual.  Manuals have their own source files, written in
the Texinfo language; documentation strings are specified in the
definitions of the functions and variables they apply to.  A collection
of documentation strings is not sufficient as a manual because a good
manual is not organized in that fashion; it is organized in terms of
topics of discussion.

@menu
* Documentation Basics::      Good style for doc strings.
                                Where to put them.  How Emacs stores them.
* Accessing Documentation::   How Lisp programs can access doc strings.
* Keys in Documentation::     Substituting current key bindings.
* Describing Characters::     Making printable descriptions of
                                non-printing characters and key sequences.
* Help Functions::            Subroutines used by Emacs help facilities.
@end menu

@node Documentation Basics
@comment  node-name,  next,  previous,  up
@section Documentation Basics
@cindex documentation conventions
@cindex writing a documentation string
@cindex string, writing a doc string

  A documentation string is written using the Lisp syntax for strings,
with double-quote characters surrounding the text of the string.  This
is because it really is a Lisp string object.  The string serves as
documentation when it is written in the proper place in the definition
of a function or variable.  In a function definition, the documentation
string follows the argument list.  In a variable definition, the
documentation string follows the initial value of the variable.

  When you write a documentation string, make the first line a complete
sentence (or two complete sentences) since some commands, such as
@code{apropos}, show 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 @kbd{C-h f}
(@code{describe-function}) or @kbd{C-h v} (@code{describe-variable}).
@xref{Documentation Tips}.

  Documentation strings may contain several special substrings, which
stand for key bindings to be looked up in the current keymaps when the
documentation is displayed.  This allows documentation strings to refer
to the keys for related commands and be accurate even when a user
rearranges the key bindings.  (@xref{Accessing Documentation}.)

  Within the Lisp world, a documentation string is kept with the
function or variable that it describes:

@itemize @bullet
@item
The documentation for a function is stored in the function definition
itself (@pxref{Lambda Expressions}).  The function
@code{documentation} knows how to extract it.

@item
@kindex variable-documentation
The documentation for a variable is stored in the variable's property
list under the property name @code{variable-documentation}.  The
function @code{documentation-property} knows how to extract it.
@end itemize

@cindex @file{DOC} (documentation) file
@cindex @file{emacs/etc/DOC-@var{version}}
@cindex @file{etc/DOC-@var{version}}
To save space, the documentation for preloaded functions and variables
(including primitive functions and autoloaded functions) are stored in
the file @file{emacs/etc/DOC-@var{version}}.  The data structure inside
Emacs has an integer offset into the file, where the documentation
string ought to be.  The functions @code{documentation} the
@code{documentation-property} read the documentation from the file
@file{emacs/etc/DOC-@var{version}} when they notice the integer there;
this is transparent to the user.  Keeping the documentation strings out
of the Emacs core image saves a significant amount of space.
@xref{Building Emacs}.

  For information on the uses of documentation strings, see @ref{Help, ,
Help, emacs, The GNU Emacs Manual}.

@c Wordy to prevent overfull hbox.  --rjc 15mar92
  The @file{emacs/lib-src} directory contains two utilities that you can
use to print nice-looking hardcopy for the file
@file{emacs/etc/DOC-@var{version}}.  These are @file{sorted-doc.c} and
@file{digest-doc.c}.

@node Accessing Documentation
@section Access to Documentation Strings

@defun documentation-property symbol property &optional verbatim
This function returns the documentation string that is recorded
@var{symbol}'s property list under property @var{property}.  It
retrieves the text from the file @file{emacs/etc/DOC-@var{version}} if
necessary, and runs @code{substitute-command-keys} to substitute actual
key bindings.  (This substitution is not done if @var{verbatim} is
non-@code{nil}; the @var{verbatim} argument exists only as of Emacs 19.)

@smallexample
@group
(documentation-property 'command-line-processed
   'variable-documentation)
     @result{} "t once command line has been processed"
@end group
@group
(symbol-plist 'command-line-processed)
     @result{} (variable-documentation 188902)
@end group
@end smallexample
@end defun

@defun documentation function &optional verbatim
This function returns the documentation string of @var{function}.
This function will access the documentation string if it is stored in
the @file{emacs/etc/DOC-@var{version}} file.

In addition, @code{documentation} runs @code{substitute-command-keys}
on the resulting string, so the value contains the actual (current) key
bindings.  (This is not done if @var{verbatim} is non-@code{nil}; the
@var{verbatim} argument exists only as of Emacs 19.)

The function @code{documentation} signals a @code{void-function} error
unless @var{function} has a function definition.  However, it is ok if
the function definition has no documentation string.  In that case,
@code{documentation} returns @code{nil}.
@end defun

@c Wordy to prevent overfull hboxes.  --rjc 15mar92
  Here is an example of using the two functions, @code{documentation} and
@code{documentation-property}, to display the documentation strings for
several symbols in a @samp{*Help*} buffer.

@smallexample
@group
(defun describe-symbols (pattern)
  "Describe the Emacs Lisp symbols matching PATTERN.
All symbols that have PATTERN in their name are described
in the `*Help*' buffer."
  (interactive "sDescribe symbols matching: ")
  (let ((describe-func
         (function 
          (lambda (s)
@end group
@group
            ;; @r{Print description of symbol.}
            (if (fboundp s)             ; @r{It is a function.}
                (princ
                 (format "%s\t%s\n%s\n\n" s
                   (if (commandp s) 
                       (let ((keys (where-is-internal s)))
                         (if keys
                             (concat
                              "Keys: "
                              (mapconcat 'key-description 
                                         keys " "))
                           "Keys: none"))
                     "Function")
@end group
@group
                   (or (documentation s) 
                       "not documented"))))
            
            (if (boundp s)              ; @r{It is a variable.}
@end group
@group
                (princ
                 (format "%s\t%s\n%s\n\n" s
                   (if (user-variable-p s) 
                       "Option " "Variable")
@end group
@group
                   (or (documentation-property 
                         s 'variable-documentation)
                       "not documented")))))))
        sym-list)
@end group

@group
    ;; @r{Build a list of symbols that match pattern.}
    (mapatoms (function 
               (lambda (sym)
                 (if (string-match pattern (symbol-name sym))
                     (setq sym-list (cons sym sym-list))))))
@end group

@group
    ;; @r{Display the data.}
    (with-output-to-temp-buffer "*Help*"
      (mapcar describe-func (sort sym-list 'string<))
      (print-help-return-message))))
@end group
@end smallexample

  The @code{describe-symbols} function works like @code{apropos},
but provides more information.

@smallexample
@group
(describe-symbols "goal")

---------- Buffer: *Help* ----------
goal-column     Option 
*Semipermanent goal column for vertical motion, as set by C-x C-n, or nil.
@end group
@c Do not blithely break or fill these lines.
@c That makes them incorrect.

@group
set-goal-column Command: C-x C-n
Set the current horizontal position as a goal for C-n and C-p.
@end group
@c DO NOT put a blank line here!  That is factually inaccurate!
@group
Those commands will move to this position in the line moved to
rather than trying to keep the same horizontal position.
With a non-nil argument, clears out the goal column
so that C-n and C-p resume vertical motion.
The goal column is stored in the variable `goal-column'.
@end group

@group
temporary-goal-column   Variable
Current goal column for vertical motion.
It is the column where point was
at the start of current run of vertical motion commands.
When the `track-eol' feature is doing its job, the value is 9999.
---------- Buffer: *Help* ----------
@end group
@end smallexample

@defun Snarf-documentation filename
  This function is used only during Emacs initialization, just before
the runnable Emacs is dumped.  It finds the file offsets of the
documentation strings stored in the file @var{filename}, and records
them in the in-core function definitions and variable property lists in
place of the actual strings.  @xref{Building Emacs}.

  Emacs finds the file @var{filename} in the @file{emacs/etc} directory.
When the dumped Emacs is later executed, the same file is found in the
directory @code{doc-directory}.  Usually @var{filename} is
@code{"DOC-@var{version}"}.
@end defun

@c Emacs 19 feature
@defvar doc-directory
This variable holds the name of the directory which should contion the
file @code{"DOC-@var{version}"} that contains documentation strings for
built-in and preloaded functions and variables.

In most cases, this is the same as @code{data-directory}.  They may be
different when you run Emacs from the directory where you built it,
without actually installing it.  See @code{data-directory} in @ref{Help
Functions}.

In older Emacs versions, @code{exec-directory} was used for this.
@end defvar

@node Keys in Documentation
@section Substituting Key Bindings in Documentation
@cindex documentation, keys in
@cindex keys in documentation strings
@cindex substituting keys in documentation

  When documentation strings refer to key sequences, they should do so
based on the current, actual key bindings.  They can do so using certain
special text sequences described below.  Accessing documentation strings
in the usual way substitutes current key binding information for these
special sequences.  This works by calling @code{substitute-command-keys}.
You can also call that function yourself.

  Here is a list of the special sequences and what they mean:

@table @code
@item \[@var{command}]
stands for a key sequence that will invoke @var{command}, or @samp{M-x
@var{command}} if @var{command} has no key bindings.

@item \@{@var{mapvar}@} 
stands for a summary of the value of @var{mapvar}, which should be a
keymap.  The summary is made by @code{describe-bindings}.

@item \<@var{mapvar}> 
stands for no text itself.  It is used for a side effect: it specifies
@var{mapvar} as the keymap for any following @samp{\[@var{command}]}
sequences in this documentation string.
@end table

@strong{Please note:} each @samp{\} must be doubled when written in a
string in Emacs Lisp.

@defun substitute-command-keys string
This function scans @var{string} for the above special sequences and
replaces them by what they stand for, returning the result as a string.
This permits display of documentation that refers accurately to the
users's own customized key bindings.
@end defun

  Here are examples of the special sequences:

@smallexample
@group
(substitute-command-keys 
   "To abort recursive edit, type: \\[abort-recursive-edit]")
@result{} "To abort recursive edit, type: C-]"
@end group

@group
(substitute-command-keys 
   "The keys that are defined for the minibuffer here are:
  \\@{minibuffer-local-must-match-map@}")
@result{} "The keys that are defined for the minibuffer here are:
@end group

?               minibuffer-completion-help
SPC             minibuffer-complete-word
TAB             minibuffer-complete
LFD             minibuffer-complete-and-exit
RET             minibuffer-complete-and-exit
C-g             abort-recursive-edit
"

@group
(substitute-command-keys
   "To abort a recursive edit from the minibuffer, type\
\\<minibuffer-local-must-match-map>\\[abort-recursive-edit].")
@result{} "To abort a recursive edit from the minibuffer, type C-g."
@end group
@end smallexample

@node Describing Characters
@section Describing Characters for Help Messages

  These functions convert events, key sequences or characters to textual
descriptions.  These descriptions are useful for including arbitrary
text characters or key sequences in messages, because they convert
non-printing and whitespace characters to sequences of printing
characters.  The description of a non-whitespace printing character is
the character itself.

@defun key-description sequence
@cindex Emacs event standard notation
This function returns a string containing the Emacs standard notation
for the input events in @var{sequence}.  The argument @var{sequence} may
be a string, vector or list.  @xref{Input Events}, for more information
about valid events.  See also the examples for
@code{single-key-description}, below.
@end defun

@defun single-key-description event
@cindex event printing
@cindex character printing
@cindex control character printing
@cindex meta character printing
This function returns a string describing @var{event} in the standard
Emacs notation for keyboard input.  A normal printing character appears
as itself, but a control character turns into a string starting with
@samp{C-}, a meta character turns into a string starting with @samp{M-},
and space, linefeed, etc.@: appear as @samp{SPC}, @samp{LFD}, etc.  A
function key symbol appears as itself.  An event which is a list appears
as the name of the symbol in the @sc{car} of the list.

@smallexample
@group
(single-key-description ?\C-x)
     @result{} "C-x"
@end group
@group
(key-description "\C-x \M-y \n \t \r \f123")
     @result{} "C-x SPC M-y SPC LFD SPC TAB SPC RET SPC C-l 1 2 3"
@end group
@group
(single-key-description 'C-mouse-1)
     @result{} "C-mouse-1"
@end group
@end smallexample
@end defun

@defun text-char-description character
This function returns a string describing @var{character} in the
standard Emacs notation for characters that appear in text---like
@code{single-key-description}, except that control characters are
represented with a leading caret (which is how control characters in
Emacs buffers are usually displayed).

@smallexample
@group
(text-char-description ?\C-c)
     @result{} "^C"
@end group
@group
(text-char-description ?\M-m)
     @result{} "M-m"
@end group
@group
(text-char-description ?\C-\M-m)
     @result{} "M-^M"
@end group
@end smallexample
@end defun

@node Help Functions
@section Help Functions

  Emacs provides a variety of on-line help functions, all accessible to
the user as subcommands of the prefix @kbd{C-h}.  For more information
about them, see @ref{Help, , Help, emacs, The GNU Emacs Manual}.  Here
we describe some program-level interfaces to the same information.

@deffn Command apropos regexp &optional do-all predicate
  This function finds all symbols whose names contain a match for the
regular expression @var{regexp}, and returns a list of them.  
It also displays the symbols in a buffer named @samp{*Help*}, each with a
one-line description.

@c Emacs 19 feature
  If @var{do-all} is non-@code{nil}, then @code{apropos} also shows
key bindings for the functions that are found.

  If @var{predicate} is non-@code{nil}, it should be a function to be
called on each symbol that has matched @var{regexp}.  Only symbols for
which @var{predicate} returns a non-@code{nil} value are listed or
displayed.

  In the first of the following examples, @code{apropos} finds all the
symbols with names containing @samp{exec}.  In the second example, it
finds and returns only those symbols that are also commands.
(We don't show the output that results in the @samp{*Help*} buffer.)

@smallexample
@group
(apropos "exec")
     @result{} (Buffer-menu-execute command-execute exec-directory
    exec-path execute-extended-command execute-kbd-macro
    executing-kbd-macro executing-macro)
@end group

@group
(apropos "exec" nil 'commandp)
     @result{} (Buffer-menu-execute execute-extended-command)
@end group
@ignore
@group
---------- Buffer: *Help* ----------
Buffer-menu-execute
  Function: Save and/or delete buffers marked with
  M-x Buffer-menu-save or M-x Buffer-menu-delete commands.
execute-extended-command      ESC x
  Function: Read function name, then read its 
  arguments and call it.
---------- Buffer: *Help* ----------
@end group
@end ignore
@end smallexample

  The command @kbd{C-h a} (@code{command-apropos}) calls @code{apropos},
but specifies a @var{predicate} to restrict the output to symbols that
are commands.  The call to @code{apropos} looks like this:

@smallexample
(apropos string t 'commandp)
@end smallexample
@end deffn

@c Emacs 19 feature
@deffn Command super-apropos regexp &optional do-all
This function differs from @code{apropos} in that it searches
documentation strings as well as symbol names for matches for
@var{regexp}.  By default, it searches only the documentation strings,
and only those of functions and variables that are included in Emacs
when it is dumped.  If @var{do-all} is non-@code{nil}, it scans the
names and documentation strings of all functions and variables.
@end deffn

@defvar help-map
The value of this variable is a local keymap for characters following the
Help key, @kbd{C-h}.
@end defvar

@deffn {Prefix Command} help-command
This symbol is not a function; its function definition is actually the
keymap known as @code{help-map}.  It is defined in @file{help.el} as
follows:

@smallexample
@group
(define-key global-map "\C-h" 'help-command)
(fset 'help-command help-map)
@end group
@end smallexample
@end deffn

@defun print-help-return-message &optional function
This function builds a string which is a message explaining how to
restore the previous state of the windows after a help command.  After
building the message, it applies @var{function} to it if @var{function}
is non-@code{nil}.  Otherwise it calls @code{message} to display it in
the echo area.

This function expects to be called inside a
@code{with-output-to-temp-buffer} special form, and expects
@code{standard-output} to have the value bound by that special form.
For an example of its use, see the long example in @ref{Accessing
Documentation}.
@end defun

@defvar help-char
The value of this variable is the help character---the character that
Emacs recognizes as meaning Help.  By default, it is 8, which is
@kbd{C-h}.  When Emacs reads this character, if @code{help-form} is
non-@code{nil} Lisp expression, it evaluates that expression, and
displays the result in a window if it is a string.

Usually the value of @code{help-form}'s value is @code{nil}.  Then the
help character has no special meaning at the level of command input, and
it becomes part of a key sequence in the normal way.  The standard key
binding of @kbd{C-h} is a prefix key for several general-purpose help
features.

The help character is special after prefix keys, too.  If it has no
binding as a subcommand of the prefix key, it runs
@code{describe-prefix-bindings}, which displays a list of all the
subcommands of the prefix key.
@end defvar

@defvar help-form
If this variable is non-@code{nil}, its value is a form to evaluate
whenever the character @code{help-char} is read.  If evaluating the form
produces a string, that string is displayed.

A command that calls @code{read-event} or @code{read-char} probably
should bind @code{help-form} to a non-@code{nil} expression while it
does input.  (The exception is when @kbd{C-h} is meaningful input.)
Evaluating this expression should result in a string that explains what
the input is for and how to enter it properly.

Entry to the minibuffer binds this variable to the value of
@code{minibuffer-help-form} (@pxref{Minibuffer Misc}).
@end defvar

@defvar prefix-help-command
This variable holds a function to print help for a prefix character.
The function is called when the user types a prefix key followed by the
help character, and the help character has no binding after that prefix.
The variable's default value is @code{describe-prefix-bindings}.
@end defvar

@defun describe-prefix-bindings
This function calls @code{describe-bindings} to display a list of all
the subcommands of the prefix key of the most recent key sequence.  The
prefix described consists of all but the last event of that key
sequence.
@end defun

  The following two functions are found in the library @file{helper}.
They are for modes that want to provide help without relinquishing
control, such as the ``electric'' modes.  You must load that library
with @code{(require 'helper)} in order to use them.  Their names begin
with @samp{Helper} to distinguish them from the ordinary help functions.

@deffn Command Helper-describe-bindings
This command pops up a window displaying a help buffer containing a
listing of all of the key bindings from both the local and global keymaps.
It works by calling @code{describe-bindings}.
@end deffn

@deffn Command Helper-help
This command provides help for the current mode.  It prompts the user
in the minibuffer with the message @samp{Help (Type ? for further
options)}, and then provides assistance in finding out what the key
bindings are, and what the mode is intended for.  It returns @code{nil}.

This can be customized by changing the map @code{Helper-help-map}.
@end deffn

@c Emacs 19 feature
@defvar data-directory
This variable holds the name of the directory in which Emacs finds
certain documentation and text files that come with Emacs.  In older
Emacs versions, @code{exec-directory} was used for this.
@end defvar

@c Emacs 19 feature
@defmac make-help-screen fname help-line help-text help-map
This macro defines a help command named @var{fname} which acts like a 
prefix key which shows a list of the subcommands it offers.

When invoked, @var{fname} displays @var{help-text} in a window, then
reads and executes a key sequence according to @var{help-map}.  The
string @var{help-text} should describe of the bindings available in
@var{help-map}.

The command @var{fname} is defined to handle a few events itself, by
scrolling the display of @var{help-text}.  When @var{fname} reads one of
those special events, it does the scrolling and then reads another
event.  When it reads an event which is not one of those few, and which
has a binding in @var{help-map}, it executes that key's binding and
then returns.

The argument @var{help-line} should be a single-line summary of the
alternatives in @var{help-map}.  In the current version of Emacs, this
argument is used only if you set the option @code{three-step-help} to
@code{t}.
@end defmac

@defopt three-step-help
If this variable is non-@code{nil}, commands defined with
@code{make-help-screen} display their @var{help-line} strings in the
echo area at first, and display the longer @var{help-text} strings only
if the user types the help character again.
@end defopt