entered into RCS

[gnu-emacs] / lispref / objects.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/objects
@node Lisp Data Types, Numbers, Introduction, Top
@chapter Lisp Data Types
@cindex object
@cindex Lisp object
@cindex type
@cindex data type

  A Lisp @dfn{object} is a piece of data used and manipulated by Lisp
programs.  For our purposes, a @dfn{type} or @dfn{data type} is a set of
possible objects.

  Every object belongs to at least one type.  Objects of the same type
have similar structures and may usually be used in the same contexts.
Types can overlap, and objects can belong to two or more types.
Consequently, we can ask whether an object belongs to a particular type,
but not for ``the'' type of an object.

@cindex primitive type
  A few fundamental object types are built into Emacs.  These, from
which all other types are constructed, are called @dfn{primitive
types}.  Each object belongs to one and only one primitive type.  These
types include @dfn{integer}, @dfn{float}, @dfn{cons}, @dfn{symbol},
@dfn{string}, @dfn{vector}, @dfn{subr}, @dfn{byte-code function}, and
several special types, such as @dfn{buffer}, that are related to
editing.  (@xref{Editing Types}.)

  Each primitive type has a corresponding Lisp function that checks
whether an object is a member of that type.

  Note that Lisp is unlike many other languages in that Lisp objects are
@dfn{self-typing}: the primitive type of the object is implicit in the
object itself.  For example, if an object is a vector, nothing can treat
it as a number; Lisp knows it is a vector, not a number.

  In most languages, the programmer must declare the data type of each
variable, and the type is known by the compiler but not represented in
the data.  Such type declarations do not exist in Emacs Lisp.  A Lisp
variable can have any type of value, and it remembers whatever value
you store in it, type and all.

  This chapter describes the purpose, printed representation, and read
syntax of each of the standard types in GNU Emacs Lisp.  Details on how
to use these types can be found in later chapters.

@menu
* Printed Representation::      How Lisp objects are represented as text.
* Comments::                    Comments and their formatting conventions.
* Programming Types::           Types found in all Lisp systems.
* Editing Types::               Types specific to Emacs.
* Type Predicates::             Tests related to types.
* Equality Predicates::         Tests of equality between any two objects.
@end menu

@node Printed Representation
@comment  node-name,  next,  previous,  up
@section Printed Representation and Read Syntax
@cindex printed representation
@cindex read syntax

  The @dfn{printed representation} of an object is the format of the
output generated by the Lisp printer (the function @code{prin1}) for
that object.  The @dfn{read syntax} of an object is the format of the
input accepted by the Lisp reader (the function @code{read}) for that
object.  Most objects have more than one possible read syntax.  Some
types of object have no read syntax; except for these cases, the printed
representation of an object is also a read syntax for it.

  In other languages, an expression is text; it has no other form.  In
Lisp, an expression is primarily a Lisp object and only secondarily the
text that is the object's read syntax.  Often there is no need to
emphasize this distinction, but you must keep it in the back of your
mind, or you will occasionally be very confused.

@cindex hash notation
  Every type has a printed representation.  Some types have no read
syntax, since it may not make sense to enter objects of these types
directly in a Lisp program.  For example, the buffer type does not have
a read syntax.  Objects of these types are printed in @dfn{hash
notation}: the characters @samp{#<} followed by a descriptive string
(typically the type name followed by the name of the object), and closed
with a matching @samp{>}.  Hash notation cannot be read at all, so the
Lisp reader signals the error @code{invalid-read-syntax} whenever it
encounters @samp{#<}.
@kindex invalid-read-syntax

@example
(current-buffer)
     @result{} #<buffer objects.texi>
@end example

  When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (@pxref{Evaluation}).  However,
evaluation and reading are separate activities.  Reading returns the
Lisp object represented by the text that is read; the object may or may
not be evaluated later.  @xref{Input Functions}, for a description of
@code{read}, the basic function for reading objects.

@node Comments
@comment  node-name,  next,  previous,  up
@section Comments
@cindex comments
@cindex @samp{;} in comment

  A @dfn{comment} is text that is written in a program only for the sake
of humans that read the program, and that has no effect on the meaning
of the program.  In Lisp, a semicolon (@samp{;}) starts a comment if it
is not within a string or character constant.  The comment continues to
the end of line.  The Lisp reader discards comments; they do not become
part of the Lisp objects which represent the program within the Lisp
system.

  @xref{Comment Tips}, for conventions for formatting comments.

@node Programming Types
@section Programming Types
@cindex programming types

  There are two general categories of types in Emacs Lisp: those having
to do with Lisp programming, and those having to do with editing.  The
former exist in many Lisp implementations, in one form or another.  The
latter are unique to Emacs Lisp.

@menu
* Integer Type::        Numbers without fractional parts.
* Floating Point Type:: Numbers with fractional parts and with a large range.
* Character Type::      The representation of letters, numbers and
                        control characters.
* Symbol Type::         A multi-use object that refers to a function,
                        variable, or property list, and has a unique identity.
* Sequence Type::       Both lists and arrays are classified as sequences.
* Cons Cell Type::      Cons cells, and lists (which are made from cons cells).
* Array Type::          Arrays include strings and vectors.
* String Type::         An (efficient) array of characters.
* Vector Type::         One-dimensional arrays.
* Function Type::       A piece of executable code you can call from elsewhere.
* Macro Type::          A method of expanding an expression into another
                          expression, more fundamental but less pretty.
* Primitive Function Type::     A function written in C, callable from Lisp.
* Byte-Code Type::      A function written in Lisp, then compiled.
* Autoload Type::       A type used for automatically loading seldom-used
                        functions.
@end menu

@node Integer Type
@subsection Integer Type

  Integers were the only kind of number in Emacs version 18.  The range
of values for integers is @minus{}8388608 to 8388607 (24 bits; i.e.,
@ifinfo
-2**23
@end ifinfo
@tex
$-2^{23}$
@end tex
to
@ifinfo
2**23 - 1)
@end ifinfo
@tex
$2^{23}-1$)
@end tex
on most machines, but is 25 or 26 bits on some systems.  It is important
to note that the Emacs Lisp arithmetic functions do not check for
overflow.  Thus @code{(1+ 8388607)} is @minus{}8388608 on 24-bit
implementations.@refill

  The read syntax for integers is a sequence of (base ten) digits with an
optional sign at the beginning and an optional period at the end.  The
printed representation produced by the Lisp interpreter never has a
leading @samp{+} or a final @samp{.}.

@example
@group
-1               ; @r{The integer -1.}
1                ; @r{The integer 1.}
1.               ; @r{Also The integer 1.}
+1               ; @r{Also the integer 1.}
16777217         ; @r{Also the integer 1!} 
                 ; @r{  (on a 24-bit or 25-bit implementation)}
@end group
@end example

  @xref{Numbers}, for more information.

@node Floating Point Type
@subsection Floating Point Type

  Emacs version 19 supports floating point numbers (though there is a
compilation option to disable them).  The precise range of floating
point numbers is machine-specific.

  The printed representation for floating point numbers requires either
a decimal point (with at least one digit following), an exponent, or
both.  For example, @samp{1500.0}, @samp{15e2}, @samp{15.0e2},
@samp{1.5e3}, and @samp{.15e4} are five ways of writing a floating point
number whose value is 1500.  They are all equivalent.

  @xref{Numbers}, for more information.

@node Character Type
@subsection Character Type
@cindex @sc{ASCII} character codes

  A @dfn{character} in Emacs Lisp is nothing more than an integer.  In
other words, characters are represented by their character codes.  For
example, the character @kbd{A} is represented as the @w{integer 65}.

  Individual characters are not often used in programs.  It is far more
common to work with @emph{strings}, which are sequences composed of
characters.  @xref{String Type}.

  Characters in strings, buffers, and files are currently limited to the
range of 0 to 255---eight bits.  If you store a larger integer into a
string, buffer or file, it is truncated to that range.  Characters that
represent keyboard input have a much wider range.

@cindex read syntax for characters
@cindex printed representation for characters
@cindex syntax for characters
  Since characters are really integers, the printed representation of a
character is a decimal number.  This is also a possible read syntax for
a character, but writing characters that way in Lisp programs is a very
bad idea.  You should @emph{always} use the special read syntax formats
that Emacs Lisp provides for characters.  These syntax formats start
with a question mark.

  The usual read syntax for alphanumeric characters is a question mark
followed by the character; thus, @samp{?A} for the character
@kbd{A}, @samp{?B} for the character @kbd{B}, and @samp{?a} for the
character @kbd{a}.  

  For example:

@example
?Q @result{} 81     ?q @result{} 113
@end example

  You can use the same syntax for punctuation characters, but it is
often a good idea to add a @samp{\} so that the Emacs commands for
editing Lisp code don't get confused.  For example, @samp{?\ } is the
way to write the space character.  If the character is @samp{\}, you
@emph{must} use a second @samp{\} to quote it: @samp{?\\}.

@cindex whitespace
@cindex bell character
@cindex @samp{\a}
@cindex backspace
@cindex @samp{\b}
@cindex tab
@cindex @samp{\t}
@cindex vertical tab
@cindex @samp{\v}
@cindex formfeed
@cindex @samp{\f}
@cindex newline
@cindex @samp{\n}
@cindex return
@cindex @samp{\r}
@cindex escape
@cindex @samp{\e}
  You can express the characters Control-g, backspace, tab, newline,
vertical tab, formfeed, return, and escape as @samp{?\a}, @samp{?\b},
@samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, @samp{?\r}, @samp{?\e},
respectively.  Those values are 7, 8, 9, 10, 11, 12, 13, and 27 in
decimal.  Thus,

@example
?\a @result{} 7                 ; @r{@kbd{C-g}}
?\b @result{} 8                 ; @r{backspace, @key{BS}, @kbd{C-h}}
?\t @result{} 9                 ; @r{tab, @key{TAB}, @kbd{C-i}}
?\n @result{} 10                ; @r{newline, @key{LFD}, @kbd{C-j}}
?\v @result{} 11                ; @r{vertical tab, @kbd{C-k}}
?\f @result{} 12                ; @r{formfeed character, @kbd{C-l}}
?\r @result{} 13                ; @r{carriage return, @key{RET}, @kbd{C-m}}
?\e @result{} 27                ; @r{escape character, @key{ESC}, @kbd{C-[}}
?\\ @result{} 92                ; @r{backslash character, @kbd{\}}
@end example

@cindex escape sequence
  These sequences which start with backslash are also known as
@dfn{escape sequences}, because backslash plays the role of an escape
character; this usage has nothing to do with the character @key{ESC}.

@cindex control characters
  Control characters may be represented using yet another read syntax.
This consists of a question mark followed by a backslash, caret, and the
corresponding non-control character, in either upper or lower case.  For
example, both @samp{?\^I} and @samp{?\^i} are valid read syntax for the
character @kbd{C-i}, the character whose value is 9.

  Instead of the @samp{^}, you can use @samp{C-}; thus, @samp{?\C-i} is
equivalent to @samp{?\^I} and to @samp{?\^i}:

@example
?\^I @result{} 9     ?\C-I @result{} 9
@end example

  For use in strings and buffers, you are limited to the control
characters that exist in @sc{ASCII}, but for keyboard input purposes,
you can turn any character into a control character with @samp{C-}.  The
character codes for these non-@sc{ASCII} control characters include the
2**22 bit as well as the code for the corresponding non-control
character.  Ordinary terminals have no way of generating non-@sc{ASCII}
control characters, but you can generate them straightforwardly using an
X terminal.

  You can think of the @key{DEL} character as @kbd{Control-?}:

@example
?\^? @result{} 127     ?\C-? @result{} 127
@end example

  For representing control characters to be found in files or strings,
we recommend the @samp{^} syntax; for control characters in keyboard
input, we prefer the @samp{C-} syntax.  This does not affect the meaning
of the program, but may guide the understanding of people who read it.

@cindex meta characters
  A @dfn{meta character} is a character typed with the @key{META}
modifier key.  The integer that represents such a character has the
2**23 bit set (which on most machines makes it a negative number).  We
use high bits for this and other modifiers to make possible a wide range
of basic character codes.

  In a string, the 2**7 bit indicates a meta character, so the meta
characters that can fit in a string have codes in the range from 128 to
255, and are the meta versions of the ordinary @sc{ASCII} characters.
(In Emacs versions 18 and older, this convention was used for characters
outside of strings as well.)

  The read syntax for meta characters uses @samp{\M-}.  For example,
@samp{?\M-A} stands for @kbd{M-A}.  You can use @samp{\M-} together with
octal character codes (see below), with @samp{\C-}, or with any other
syntax for a character.  Thus, you can write @kbd{M-A} as @samp{?\M-A},
or as @samp{?\M-\101}.  Likewise, you can write @kbd{C-M-b} as
@samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}.

  The case of an ordinary letter is indicated by its character code as
part of @sc{ASCII}, but @sc{ASCII} has no way to represent whether a
control character is upper case or lower case.  Emacs uses the 2**21 bit
to indicate that the shift key was used for typing a control character.
This distinction is possible only when you use X terminals or other
special terminals; ordinary terminals do not indicate the distinction to
the computer in any way.

@cindex hyper characters
@cindex super characters
@cindex alt characters
  The X Window System defines three other modifier bits that can be set
in a character: @dfn{hyper}, @dfn{super} and @dfn{alt}.  The syntaxes
for these bits are @samp{\H-}, @samp{\s-} and @samp{\A-}.  Thus,
@samp{?\H-\M-\A-x} represents @kbd{Alt-Hyper-Meta-x}.  Numerically, the
bit values are 2**18 for alt, 2**19 for super and 2**20 for hyper.

@cindex @samp{?} in character constant
@cindex question mark in character constant
@cindex @samp{\} in character constant
@cindex backslash in character constant
@cindex octal character code
  Finally, the most general read syntax consists of a question mark
followed by a backslash and the character code in octal (up to three
octal digits); thus, @samp{?\101} for the character @kbd{A},
@samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the
character @kbd{C-b}.  Although this syntax can represent any @sc{ASCII}
character, it is preferred only when the precise octal value is more
important than the @sc{ASCII} representation.

@example
@group
?\012 @result{} 10         ?\n @result{} 10         ?\C-j @result{} 10
?\101 @result{} 65         ?A @result{} 65
@end group
@end example

  A backslash is allowed, and harmless, preceding any character without
a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}.
There is no reason to add a backslash before most characters.  However,
you should add a backslash before any of the characters
@samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing
Lisp code.  Also add a backslash before whitespace characters such as
space, tab, newline and formfeed.  However, it is cleaner to use one of
the easily readable escape sequences, such as @samp{\t}, instead of an
actual whitespace character such as a tab.

@node Symbol Type
@subsection Symbol Type

  A @dfn{symbol} in GNU Emacs Lisp is an object with a name.  The symbol
name serves as the printed representation of the symbol.  In ordinary
use, the name is unique---no two symbols have the same name.

  A symbol can serve as a variable, as a function name, or to hold a
property list.  Or it may serve only to be distinct from all other Lisp
objects, so that its presence in a data structure may be recognized
reliably.  In a given context, usually only one of these uses is
intended.  But you can use one symbol in all of these ways,
independently.

@cindex @samp{\} in symbols
@cindex backslash in symbols
  A symbol name can contain any characters whatever.  Most symbol names
are written with letters, digits, and the punctuation characters
@samp{-+=*/}.  Such names require no special punctuation; the characters
of the name suffice as long as the name does not look like a number.
(If it does, write a @samp{\} at the beginning of the name to force
interpretation as a symbol.)  The characters @samp{_~!@@$%^&:<>@{@}} are
less often used but also require no special punctuation.  Any other
characters may be included in a symbol's name by escaping them with a
backslash.  In contrast to its use in strings, however, a backslash in
the name of a symbol simply quotes the single character that follows the
backslash.  For example, in a string, @samp{\t} represents a tab
character; in the name of a symbol, however, @samp{\t} merely quotes the
letter @kbd{t}.  To have a symbol with a tab character in its name, you
must actually use a tab (preceded with a backslash).  But it's rare to
do such a thing.

@cindex CL note---case of letters
@quotation
@b{Common Lisp note:} In Common Lisp, lower case letters are always
``folded'' to upper case, unless they are explicitly escaped.  This is
in contrast to Emacs Lisp, in which upper case and lower case letters
are distinct.
@end quotation

  Here are several examples of symbol names.  Note that the @samp{+} in
the fifth example is escaped to prevent it from being read as a number.
This is not necessary in the last example because the rest of the name
makes it invalid as a number.

@example
@group
foo                 ; @r{A symbol named @samp{foo}.}
FOO                 ; @r{A symbol named @samp{FOO}, different from @samp{foo}.}
char-to-string      ; @r{A symbol named @samp{char-to-string}.}
@end group
@group
1+                  ; @r{A symbol named @samp{1+}}
                    ;   @r{(not @samp{+1}, which is an integer).}
@end group
@group
\+1                 ; @r{A symbol named @samp{+1}}
                    ;   @r{(not a very readable name).}
@end group
@group
\(*\ 1\ 2\)         ; @r{A symbol named @samp{(* 1 2)} (a worse name).}
@c the @'s in this next line use up three characters, hence the
@c apparent misalignment of the comment.
+-*/_~!@@$%^&=:<>@{@}  ; @r{A symbol named @samp{+-*/_~!@@$%^&=:<>@{@}}.}
                    ;   @r{These characters need not be escaped.}
@end group
@end example

@node Sequence Type
@subsection Sequence Types

  A @dfn{sequence} is a Lisp object that represents an ordered set of
elements.  There are two kinds of sequence in Emacs Lisp, lists and
arrays.  Thus, an object of type list or of type array is also
considered a sequence.

  Arrays are further subdivided into strings and vectors.  Vectors can
hold elements of any type, but string elements must be characters in the
range from 0 to 255.  However, the characters in a string can have text
properties like characters in a buffer (@pxref{Text Properties});
vectors do not support text properties even when their elements happen
to be characters.

  Lists, strings and vectors are different, but they have important
similarities.  For example, all have a length @var{l}, and all have
elements which can be indexed from zero to @var{l} minus one.  Also,
several functions, called sequence functions, accept any kind of
sequence.  For example, the function @code{elt} can be used to extract
an element of a sequence, given its index.  @xref{Sequences Arrays
Vectors}.

  It is impossible to read the same sequence twice, since sequences are
always created anew upon reading.  If you read the read syntax for a
sequence twice, you get two sequences with equal contents.  There is one
exception: the empty list @code{()} always stands for the same object,
@code{nil}.

@node Cons Cell Type
@subsection Cons Cell and List Types
@cindex address field of register
@cindex decrement field of register

  A @dfn{cons cell} is an object comprising two pointers named the
@sc{car} and the @sc{cdr}.  Each of them can point to any Lisp object.

  A @dfn{list} is a series of cons cells, linked together so that the
@sc{cdr} of each cons cell points either to another cons cell or to the
empty list.  @xref{Lists}, for functions that work on lists.  Because
most cons cells are used as part of lists, the phrase @dfn{list
structure} has come to refer to any structure made out of cons cells.

  The names @sc{car} and @sc{cdr} have only historical meaning now.  The
original Lisp implementation ran on an @w{IBM 704} computer which
divided words into two parts, called the ``address'' part and the
``decrement''; @sc{car} was an instruction to extract the contents of
the address part of a register, and @sc{cdr} an instruction to extract
the contents of the decrement.  By contrast, ``cons cells'' are named
for the function @code{cons} that creates them, which in turn is named
for its purpose, the construction of cells.

@cindex atom
  Because cons cells are so central to Lisp, we also have a word for
``an object which is not a cons cell''.  These objects are called
@dfn{atoms}.

@cindex parenthesis
  The read syntax and printed representation for lists are identical, and
consist of a left parenthesis, an arbitrary number of elements, and a
right parenthesis.

   Upon reading, each object inside the parentheses becomes an element
of the list.  That is, a cons cell is made for each element.  The
@sc{car} of the cons cell points to the element, and its @sc{cdr} points
to the next cons cell of the list, which holds the next element in the
list.  The @sc{cdr} of the last cons cell is set to point to @code{nil}.

@cindex box diagrams, for lists
@cindex diagrams, boxed, for lists
  A list can be illustrated by a diagram in which the cons cells are
shown as pairs of boxes.  (The Lisp reader cannot read such an
illustration; unlike the textual notation, which can be understood by
both humans and computers, the box illustrations can be understood only
by humans.)  The following represents the three-element list @code{(rose
violet buttercup)}:

@example
@group
    ___ ___      ___ ___      ___ ___
   |___|___|--> |___|___|--> |___|___|--> nil
     |            |            |
     |            |            |
      --> rose     --> violet   --> buttercup
@end group
@end example

  In this diagram, each box represents a slot that can refer to any Lisp
object.  Each pair of boxes represents a cons cell.  Each arrow is a
reference to a Lisp object, either an atom or another cons cell.

  In this example, the first box, the @sc{car} of the first cons cell,
refers to or ``contains'' @code{rose} (a symbol).  The second box, the
@sc{cdr} of the first cons cell, refers to the next pair of boxes, the
second cons cell.  The @sc{car} of the second cons cell refers to
@code{violet} and the @sc{cdr} refers to the third cons cell.  The
@sc{cdr} of the third (and last) cons cell refers to @code{nil}.

Here is another diagram of the same list, @code{(rose violet
buttercup)}, sketched in a different manner:

@smallexample
@group
 ---------------       ----------------       -------------------
| car   | cdr   |     | car    | cdr   |     | car       | cdr   |
| rose  |   o-------->| violet |   o-------->| buttercup |  nil  |
|       |       |     |        |       |     |           |       |
 ---------------       ----------------       -------------------
@end group
@end smallexample

@cindex @samp{(@dots{})} in lists
@cindex @code{nil} in lists
@cindex empty list
  A list with no elements in it is the @dfn{empty list}; it is identical
to the symbol @code{nil}.  In other words, @code{nil} is both a symbol
and a list.

  Here are examples of lists written in Lisp syntax:

@example
(A 2 "A")            ; @r{A list of three elements.}
()                   ; @r{A list of no elements (the empty list).}
nil                  ; @r{A list of no elements (the empty list).}
("A ()")             ; @r{A list of one element: the string @code{"A ()"}.}
(A ())               ; @r{A list of two elements: @code{A} and the empty list.}
(A nil)              ; @r{Equivalent to the previous.}
((A B C))            ; @r{A list of one element}
                     ;   @r{(which is a list of three elements).}
@end example

  Here is the list @code{(A ())}, or equivalently @code{(A nil)},
depicted with boxes and arrows:

@example
@group
    ___ ___      ___ ___
   |___|___|--> |___|___|--> nil
     |            |
     |            |
      --> A        --> nil
@end group
@end example

@menu
* Dotted Pair Notation::        An alternative syntax for lists.
* Association List Type::       A specially constructed list.
@end menu

@node Dotted Pair Notation
@comment  node-name,  next,  previous,  up
@subsubsection Dotted Pair Notation
@cindex dotted pair notation
@cindex @samp{.} in lists

  @dfn{Dotted pair notation} is an alternative syntax for cons cells
that represents the @sc{car} and @sc{cdr} explicitly.  In this syntax,
@code{(@var{a} .@: @var{b})} stands for a cons cell whose @sc{car} is
the object @var{a}, and whose @sc{cdr} is the object @var{b}.  Dotted
pair notation is therefore more general than list syntax.  In the dotted
pair notation, the list @samp{(1 2 3)} is written as @samp{(1 .  (2 . (3
. nil)))}.  For @code{nil}-terminated lists, the two notations produce
the same result, but list notation is usually clearer and more
convenient when it is applicable.  When printing a list, the dotted pair
notation is only used if the @sc{cdr} of a cell is not a list.

  Here's how box notation can illustrate dotted pairs.  This example
shows the pair @code{(rose . violet)}:

@example
@group
    ___ ___
   |___|___|--> violet
     |
     |
      --> rose
@end group
@end example

  Dotted pair notation can be combined with list notation to represent a
chain of cons cells with a non-@code{nil} final @sc{cdr}.  For example,
@code{(rose violet . buttercup)} is equivalent to @code{(rose . (violet
. buttercup))}.  The object looks like this:

@example
@group
    ___ ___      ___ ___
   |___|___|--> |___|___|--> buttercup
     |            |
     |            |
      --> rose     --> violet
@end group
@end example

  These diagrams make it evident why @w{@code{(rose .@: violet .@:
buttercup)}} is invalid syntax; it would require a cons cell that has
three parts rather than two.

  The list @code{(rose violet)} is equivalent to @code{(rose . (violet))}
and looks like this:

@example
@group
    ___ ___      ___ ___
   |___|___|--> |___|___|--> nil
     |            |
     |            |
      --> rose     --> violet
@end group
@end example

  Similarly, the three-element list @code{(rose violet buttercup)}
is equivalent to @code{(rose . (violet . (buttercup)))}.
@ifinfo
It looks like this:

@example
@group
    ___ ___      ___ ___      ___ ___
   |___|___|--> |___|___|--> |___|___|--> nil
     |            |            |
     |            |            |
      --> rose     --> violet   --> buttercup
@end group
@end example
@end ifinfo

@node Association List Type
@comment  node-name,  next,  previous,  up
@subsubsection Association List Type

  An @dfn{association list} or @dfn{alist} is a specially-constructed
list whose elements are cons cells.  In each element, the @sc{car} is
considered a @dfn{key}, and the @sc{cdr} is considered an
@dfn{associated value}.  (In some cases, the associated value is stored
in the @sc{car} of the @sc{cdr}.)  Association lists are often used as
stacks, since it is easy to add or remove associations at the front of
the list.

  For example,

@example
(setq alist-of-colors
      '((rose . red) (lily . white)  (buttercup . yellow)))
@end example

@noindent
sets the variable @code{alist-of-colors} to an alist of three elements.  In the
first element, @code{rose} is the key and @code{red} is the value.

  @xref{Association Lists}, for a further explanation of alists and for
functions that work on alists.

@node Array Type
@subsection Array Type

  An @dfn{array} is composed of an arbitrary number of slots for
referring to other Lisp objects, arranged in a contiguous block of
memory.  Accessing any element of an array takes the same amount of
time.  In contrast, accessing an element of a list requires time
proportional to the position of the element in the list.  (Elements at
the end of a list take longer to access than elements at the beginning
of a list.)

  Emacs defines two types of array, strings and vectors.  A string is an
array of characters and a vector is an array of arbitrary objects.  Both
are one-dimensional.  (Most other programming languages support
multidimensional arrays, but they are not essential; you can get the
same effect with an array of arrays.)  Each type of array has its own
read syntax; see @ref{String Type}, and @ref{Vector Type}.

  An array may have any length up to the largest integer; but once
created, it has a fixed size.  The first element of an array has index
zero, the second element has index 1, and so on.  This is called
@dfn{zero-origin} indexing.  For example, an array of four elements has
indices 0, 1, 2, @w{and 3}.

  The array type is contained in the sequence type and contains both the
string type and the vector type.

@node String Type
@subsection String Type

  A @dfn{string} is an array of characters.  Strings are used for many
purposes in Emacs, as can be expected in a text editor; for example, as
the names of Lisp symbols, as messages for the user, and to represent
text extracted from buffers.  Strings in Lisp are constants: evaluation
of a string returns the same string.

@cindex @samp{"} in strings
@cindex double-quote in strings
@cindex @samp{\} in strings
@cindex backslash in strings
  The read syntax for strings is a double-quote, an arbitrary number of
characters, and another double-quote, @code{"like this"}.  The Lisp
reader accepts the same formats for reading the characters of a string
as it does for reading single characters (without the question mark that
begins a character literal).  You can enter a nonprinting character such
as tab, @kbd{C-a} or @kbd{M-C-A} using the convenient escape sequences,
like this: @code{"\t, \C-a, \M-\C-a"}.  You can include a double-quote
in a string by preceding it with a backslash; thus, @code{"\""} is a
string containing just a single double-quote character.
(@xref{Character Type}, for a description of the read syntax for
characters.)

  If you use the @samp{\M-} syntax to indicate a meta character in a
string constant, this sets the 2**7 bit of the character in the string.
This is not the same representation that the meta modifier has in a
character on its own (not inside a string).  @xref{Character Type}.

  Strings cannot hold characters that have the hyper, super, or alt
modifiers; they can hold @sc{ASCII} control characters, but no others.
They do not distinguish case in @sc{ASCII} control characters.

  The printed representation of a string consists of a double-quote, the
characters it contains, and another double-quote.  However, you must
escape any backslash or double-quote characters in the string with a
backslash, like this: @code{"this \" is an embedded quote"}.

  The newline character is not special in the read syntax for strings;
if you write a new line between the double-quotes, it becomes a
character in the string.  But an escaped newline---one that is preceded
by @samp{\}---does not become part of the string; i.e., the Lisp reader
ignores an escaped newline while reading a string.
@cindex newline in strings

@example
"It is useful to include newlines
in documentation strings,
but the newline is \
ignored if escaped."
     @result{} "It is useful to include newlines 
in documentation strings, 
but the newline is ignored if escaped."
@end example

  A string can hold properties of the text it contains, in addition to
the characters themselves.  This enables programs that copy text between
strings and buffers to preserve the properties with no special effort.
@xref{Text Properties}.  Strings with text properties have a special
read and print syntax:

@example
#("@var{characters}" @var{property-data}...)
@end example

@noindent
where @var{property-data} consists of zero or more elements, in groups
of three as follows:

@example
@var{beg} @var{end} @var{plist}
@end example

@noindent
The elements @var{beg} and @var{end} are integers, and together specify
a range of indices in the string; @var{plist} is the property list for
that range.

  @xref{Strings and Characters}, for functions that work on strings.

@node Vector Type
@subsection Vector Type

  A @dfn{vector} is a one-dimensional array of elements of any type.  It
takes a constant amount of time to access any element of a vector.  (In
a list, the access time of an element is proportional to the distance of
the element from the beginning of the list.)

  The printed representation of a vector consists of a left square
bracket, the elements, and a right square bracket.  This is also the
read syntax.  Like numbers and strings, vectors are considered constants
for evaluation.

@example
[1 "two" (three)]      ; @r{A vector of three elements.}
     @result{} [1 "two" (three)]
@end example

  @xref{Vectors}, for functions that work with vectors.

@node Function Type
@subsection Function Type

  Just as functions in other programming languages are executable,
@dfn{Lisp function} objects are pieces of executable code.  However,
functions in Lisp are primarily Lisp objects, and only secondarily the
text which represents them.  These Lisp objects are lambda expressions:
lists whose first element is the symbol @code{lambda} (@pxref{Lambda
Expressions}).

  In most programming languages, it is impossible to have a function
without a name.  In Lisp, a function has no intrinsic name.  A lambda
expression is also called an @dfn{anonymous function} (@pxref{Anonymous
Functions}).  A named function in Lisp is actually a symbol with a valid
function in its function cell (@pxref{Defining Functions}).

  Most of the time, functions are called when their names are written in
Lisp expressions in Lisp programs.  However, you can construct or obtain
a function object at run time and then call it with the primitive
functions @code{funcall} and @code{apply}.  @xref{Calling Functions}.

@node Macro Type
@subsection Macro Type

  A @dfn{Lisp macro} is a user-defined construct that extends the Lisp
language.  It is represented as an object much like a function, but with
different parameter-passing semantics.  A Lisp macro has the form of a
list whose first element is the symbol @code{macro} and whose @sc{cdr}
is a Lisp function object, including the @code{lambda} symbol.

  Lisp macro objects are usually defined with the built-in
@code{defmacro} function, but any list that begins with @code{macro} is
a macro as far as Emacs is concerned.  @xref{Macros}, for an explanation
of how to write a macro.

@node Primitive Function Type
@subsection Primitive Function Type
@cindex special forms

  A @dfn{primitive function} is a function callable from Lisp but
written in the C programming language.  Primitive functions are also
called @dfn{subrs} or @dfn{built-in functions}.  (The word ``subr'' is
derived from ``subroutine''.)  Most primitive functions evaluate all
their arguments when they are called.  A primitive function that does
not evaluate all its arguments is called a @dfn{special form}
(@pxref{Special Forms}).@refill

  It does not matter to the caller of a function whether the function is
primitive.  However, this does matter if you try to substitute a
function written in Lisp for a primitive of the same name.  The reason
is that the primitive function may be called directly from C code.
Calls to the redefined function from Lisp will use the new definition,
but calls from C code may still use the built-in definition.

  The term @dfn{function} refers to all Emacs functions, whether written
in Lisp or C.  @xref{Function Type}, for information about the
functions written in Lisp.

  Primitive functions have no read syntax and print in hash notation
with the name of the subroutine.

@example
@group
(symbol-function 'car)          ; @r{Access the function cell}
                                ;   @r{of the symbol.}
     @result{} #<subr car>
(subrp (symbol-function 'car))  ; @r{Is this a primitive function?}
     @result{} t                       ; @r{Yes.}
@end group
@end example

@node Byte-Code Type
@subsection Byte-Code Function Type

The byte compiler produces @dfn{byte-code function objects}.
Internally, a byte-code function object is much like a vector; however,
the evaluator handles this data type specially when it appears as a
function to be called.  @xref{Byte Compilation}, for information about
the byte compiler.

The printed representation for a byte-code function object is like that
for a vector, with an additional @samp{#} before the opening @samp{[}.

@node Autoload Type
@subsection Autoload Type

  An @dfn{autoload object} is a list whose first element is the symbol
@code{autoload}.  It is stored as the function definition of a symbol as
a placeholder for the real definition; it says that the real definition
is found in a file of Lisp code that should be loaded when necessary.
The autoload object contains the name of the file, plus some other
information about the real definition.

  After the file has been loaded, the symbol should have a new function
definition that is not an autoload object.  The new definition is then
called as if it had been there to begin with.  From the user's point of
view, the function call works as expected, using the function definition
in the loaded file.

  An autoload object is usually created with the function
@code{autoload}, which stores the object in the function cell of a
symbol.  @xref{Autoload}, for more details.

@node Editing Types
@section Editing Types
@cindex editing types

  The types in the previous section are common to many Lisp dialects.
Emacs Lisp provides several additional data types for purposes connected
with editing.

@menu
* Buffer Type::         The basic object of editing.
* Marker Type::         A position in a buffer.
* Window Type::         Buffers are displayed in windows.
* Frame Type::          Windows subdivide frames.
* Window Configuration Type::   Recording the way a frame is subdivided.
* Process Type::        A process running on the underlying OS.
* Stream Type::         Receive or send characters.
* Keymap Type::         What function a keystroke invokes.
* Syntax Table Type::   What a character means.
* Display Table Type::  How display tables are represented.
* Overlay Type::        How an overlay is represented.
@end menu

@node Buffer Type
@subsection Buffer Type

  A @dfn{buffer} is an object that holds text that can be edited
(@pxref{Buffers}).  Most buffers hold the contents of a disk file
(@pxref{Files}) so they can be edited, but some are used for other
purposes.  Most buffers are also meant to be seen by the user, and
therefore displayed, at some time, in a window (@pxref{Windows}).  But a
buffer need not be displayed in any window.

  The contents of a buffer are much like a string, but buffers are not
used like strings in Emacs Lisp, and the available operations are
different.  For example, insertion of text into a buffer is very
efficient, whereas ``inserting'' text into a string requires
concatenating substrings, and the result is an entirely new string
object.

  Each buffer has a designated position called @dfn{point}
(@pxref{Positions}).  At any time, one buffer is the @dfn{current
buffer}.  Most editing commands act on the contents of the current
buffer in the neighborhood of point.  Many of the standard Emacs
functions manipulate or test the characters in the current buffer; a
whole chapter in this manual is devoted to describing these functions
(@pxref{Text}).

  Several other data structures are associated with each buffer:

@itemize @bullet
@item
a local syntax table (@pxref{Syntax Tables});

@item
a local keymap (@pxref{Keymaps}); and,

@item
a local variable binding list (@pxref{Buffer-Local Variables}).
@end itemize

@noindent
The local keymap and variable list contain entries that individually
override global bindings or values.  These are used to customize the
behavior of programs in different buffers, without actually changing the
programs.

  Buffers have no read syntax.  They print in hash notation with the
buffer name.

@example
@group
(current-buffer)
     @result{} #<buffer objects.texi>
@end group
@end example

@node Marker Type
@subsection Marker Type

  A @dfn{marker} denotes a position in a specific buffer.  Markers
therefore have two components: one for the buffer, and one for the
position.  Changes in the buffer's text automatically relocate the
position value as necessary to ensure that the marker always points
between the same two characters in the buffer.

  Markers have no read syntax.  They print in hash notation, giving the
current character position and the name of the buffer.

@example
@group
(point-marker)
     @result{} #<marker at 10779 in objects.texi>
@end group
@end example

@xref{Markers}, for information on how to test, create, copy, and move
markers.

@node Window Type
@subsection Window Type

  A @dfn{window} describes the portion of the terminal screen that Emacs
uses to display a buffer.  Every window has one associated buffer, whose
contents appear in the window.  By contrast, a given buffer may appear
in one window, no window, or several windows.

  Though many windows may exist simultaneously, at any time one window
is designated the @dfn{selected window}.  This is the window where the
cursor is (usually) displayed when Emacs is ready for a command.  The
selected window usually displays the current buffer, but this is not
necessarily the case.

  Windows are grouped on the screen into frames; each window belongs to
one and only one frame.  @xref{Frame Type}.

  Windows have no read syntax.  They print in hash notation, giving the
window number and the name of the buffer being displayed.  The window
numbers exist to identify windows uniquely, since the buffer displayed
in any given window can change frequently.

@example
@group
(selected-window)
     @result{} #<window 1 on objects.texi>
@end group
@end example

  @xref{Windows}, for a description of the functions that work on windows.

@node Frame Type
@subsection Frame Type

  A @var{frame} is a rectangle on the screen that contains one or more
Emacs windows.  A frame initially contains a single main window (plus
perhaps a minibuffer window) which you can subdivide vertically or
horizontally into smaller windows.

  Frames have no read syntax.  They print in hash notation, giving the
frame's title, plus its address in core (useful to identify the frame
uniquely).

@example
@group
(selected-frame)
     @result{} #<frame xemacs@@mole.gnu.ai.mit.edu 0xdac80>
@end group
@end example

  @xref{Frames}, for a description of the functions that work on frames.

@node Window Configuration Type
@subsection Window Configuration Type
@cindex screen layout

  A @dfn{window configuration} stores information about the positions,
sizes, and contents of the windows in a frame, so you can recreate the
same arrangement of windows later.

  Window configurations do not have a read syntax.  They print as
@samp{#<window-configuration>}.  @xref{Window Configurations}, for a
description of several functions related to window configurations.

@node Process Type
@subsection Process Type

  The word @dfn{process} usually means a running program.  Emacs itself
runs in a process of this sort.  However, in Emacs Lisp, a process is a
Lisp object that designates a subprocess created by the Emacs process.
Programs such as shells, GDB, ftp, and compilers, running in
subprocesses of Emacs, extend the capabilities of Emacs.

  An Emacs subprocess takes textual input from Emacs and returns textual
output to Emacs for further manipulation.  Emacs can also send signals
to the subprocess.

  Process objects have no read syntax.  They print in hash notation,
giving the name of the process:

@example
@group
(process-list)
     @result{} (#<process shell>)
@end group
@end example

@xref{Processes}, for information about functions that create, delete,
return information about, send input or signals to, and receive output
from processes.

@node Stream Type
@subsection Stream Type

  A @dfn{stream} is an object that can be used as a source or sink for
characters---either to supply characters for input or to accept them as
output.  Many different types can be used this way: markers, buffers,
strings, and functions.  Most often, input streams (character sources)
obtain characters from the keyboard, a buffer, or a file, and output
streams (character sinks) send characters to a buffer, such as a
@file{*Help*} buffer, or to the echo area.

  The object @code{nil}, in addition to its other meanings, may be used
as a stream.  It stands for the value of the variable
@code{standard-input} or @code{standard-output}.  Also, the object
@code{t} as a stream specifies input using the minibuffer
(@pxref{Minibuffers}) or output in the echo area (@pxref{The Echo
Area}).

  Streams have no special printed representation or read syntax, and
print as whatever primitive type they are.

  @xref{Read and Print}, for a description of functions
related to streams, including parsing and printing functions.

@node Keymap Type
@subsection Keymap Type

  A @dfn{keymap} maps keys typed by the user to commands.  This mapping
controls how the user's command input is executed.  A keymap is actually
a list whose @sc{car} is the symbol @code{keymap}.

  @xref{Keymaps}, for information about creating keymaps, handling prefix
keys, local as well as global keymaps, and changing key bindings.

@node Syntax Table Type
@subsection Syntax Table Type

  A @dfn{syntax table} is a vector of 256 integers.  Each element of the
vector defines how one character is interpreted when it appears in a
buffer.  For example, in C mode (@pxref{Major Modes}), the @samp{+}
character is punctuation, but in Lisp mode it is a valid character in a
symbol.  These modes specify different interpretations by changing the
syntax table entry for @samp{+}, at index 43 in the syntax table.

  Syntax tables are used only for scanning text in buffers, not for
reading Lisp expressions.  The table the Lisp interpreter uses to read
expressions is built into the Emacs source code and cannot be changed;
thus, to change the list delimiters to be @samp{@{} and @samp{@}}
instead of @samp{(} and @samp{)} would be impossible.

  @xref{Syntax Tables}, for details about syntax classes and how to make
and modify syntax tables.

@node Display Table Type
@subsection Display Table Type

  A @dfn{display table} specifies how to display each character code.
Each buffer and each window can have its own display table.  A display
table is actually a vector of length 261.  @xref{Display Tables}.

@node Overlay Type
@subsection Overlay Type

  An @dfn{overlay} specifies temporary alteration of the display
appearance of a part of a buffer.  It contains markers delimiting a
range of the buffer, plus a property list (a list whose elements are
alternating property names and values).  Overlays are used to present
parts of the buffer temporarily in a different display style.

  @xref{Overlays}, for how to create and use overlays.  They have no
read syntax, and print in hash notation, giving the buffer name and
range of positions.

@node Type Predicates
@section Type Predicates
@cindex predicates
@cindex type checking
@kindex wrong-type-argument

  The Emacs Lisp interpreter itself does not perform type checking on
the actual arguments passed to functions when they are called.  It could
not do so, since function arguments in Lisp do not have declared data
types, as they do in other programming languages.  It is therefore up to
the individual function to test whether each actual argument belongs to
a type that the function can use.

  All built-in functions do check the types of their actual arguments
when appropriate, and signal a @code{wrong-type-argument} error if an
argument is of the wrong type.  For example, here is what happens if you
pass an argument to @code{+} that it cannot handle:

@example
@group
(+ 2 'a)
     @error{} Wrong type argument: integer-or-marker-p, a
@end group
@end example

@cindex type predicates
@cindex testing types
  Lisp provides functions, called @dfn{type predicates}, to test whether
an object is a member of a given type.  (Following a convention of long
standing, the names of most Emacs Lisp predicates end in @samp{p}.)

Here is a table of predefined type predicates, in alphabetical order,
with references to further information.

@table @code
@item atom
@xref{List-related Predicates, atom}.

@item arrayp
@xref{Array Functions, arrayp}.

@item bufferp
@xref{Buffer Basics, bufferp}.

@item byte-code-function-p
@xref{Byte-Code Type, byte-code-function-p}.

@item case-table-p
@xref{Case Table, case-table-p}.

@item char-or-string-p
@xref{Predicates for Strings, char-or-string-p}.

@item commandp
@xref{Interactive Call, commandp}.

@item consp
@xref{List-related Predicates, consp}.

@item floatp
@xref{Predicates on Numbers, floatp}.

@item frame-live-p
@xref{Deleting Frames, frame-live-p}.

@item framep
@xref{Frames, framep}.

@item integer-or-marker-p
@xref{Predicates on Markers, integer-or-marker-p}.

@item integerp
@xref{Predicates on Numbers, integerp}.

@item keymapp
@xref{Creating Keymaps, keymapp}.

@item listp
@xref{List-related Predicates, listp}.

@item markerp
@xref{Predicates on Markers, markerp}.

@item wholenump
@xref{Predicates on Numbers, wholenump}.

@item nlistp
@xref{List-related Predicates, nlistp}.

@item numberp
@xref{Predicates on Numbers, numberp}.

@item number-or-marker-p
@xref{Predicates on Markers, number-or-marker-p}.

@item overlayp
@xref{Overlays, overlayp}.

@item processp
@xref{Processes, processp}.

@item sequencep
@xref{Sequence Functions, sequencep}.

@item stringp
@xref{Predicates for Strings, stringp}.

@item subrp
@xref{Function Cells, subrp}.

@item symbolp
@xref{Symbols, symbolp}.

@item syntax-table-p
@xref{Syntax Tables, syntax-table-p}.

@item user-variable-p
@xref{Defining Variables, user-variable-p}.

@item vectorp
@xref{Vectors, vectorp}.

@item window-configuration-p
@xref{Window Configurations, window-configuration-p}.

@item window-live-p
@xref{Deleting Windows, window-live-p}.

@item windowp
@xref{Basic Windows, windowp}.
@end table

@node Equality Predicates
@section Equality Predicates
@cindex equality

  Here we describe two functions that test for equality between any two
objects.  Other functions test equality between objects of specific
types, e.g., strings.  For these predicates, see the appropriate chapter
describing the data type.

@defun eq object1 object2
This function returns @code{t} if @var{object1} and @var{object2} are
the same object, @code{nil} otherwise.  The ``same object'' means that a
change in one will be reflected by the same change in the other.

@code{eq} returns @code{t} if @var{object1} and @var{object2} are
integers with the same value.  Also, since symbol names are normally
unique, if the arguments are symbols with the same name, they are
@code{eq}.  For other types (e.g., lists, vectors, strings), two
arguments with the same contents or elements are not necessarily
@code{eq} to each other: they are @code{eq} only if they are the same
object.

(The @code{make-symbol} function returns an uninterned symbol that is
not interned in the standard @code{obarray}.  When uninterned symbols
are in use, symbol names are no longer unique.  Distinct symbols with
the same name are not @code{eq}.  @xref{Creating Symbols}.)

@example
@group
(eq 'foo 'foo)
     @result{} t
@end group

@group
(eq 456 456)
     @result{} t
@end group

@group
(eq "asdf" "asdf")
     @result{} nil
@end group

@group
(eq '(1 (2 (3))) '(1 (2 (3))))
     @result{} nil
@end group

@group
(setq foo '(1 (2 (3))))
     @result{} (1 (2 (3)))
(eq foo foo)
     @result{} t
(eq foo '(1 (2 (3))))
     @result{} nil
@end group

@group
(eq [(1 2) 3] [(1 2) 3])
     @result{} nil
@end group

@group
(eq (point-marker) (point-marker))
     @result{} nil
@end group
@end example

@end defun

@defun equal object1 object2
This function returns @code{t} if @var{object1} and @var{object2} have
equal components, @code{nil} otherwise.  Whereas @code{eq} tests if its
arguments are the same object, @code{equal} looks inside nonidentical
arguments to see if their elements are the same.  So, if two objects are
@code{eq}, they are @code{equal}, but the converse is not always true.

@example
@group
(equal 'foo 'foo)
     @result{} t
@end group

@group
(equal 456 456)
     @result{} t
@end group

@group
(equal "asdf" "asdf")
     @result{} t
@end group
@group
(eq "asdf" "asdf")
     @result{} nil
@end group

@group
(equal '(1 (2 (3))) '(1 (2 (3))))
     @result{} t
@end group
@group
(eq '(1 (2 (3))) '(1 (2 (3))))
     @result{} nil
@end group

@group
(equal [(1 2) 3] [(1 2) 3])
     @result{} t
@end group
@group
(eq [(1 2) 3] [(1 2) 3])
     @result{} nil
@end group

@group
(equal (point-marker) (point-marker))
     @result{} t
@end group

@group
(eq (point-marker) (point-marker))
     @result{} nil
@end group
@end example

Comparison of strings uses @code{string=}, and is case-sensitive.

@example
@group
(equal "asdf" "ASDF")
     @result{} nil
@end group
@end example
@end defun

  The test for equality is implemented recursively, and circular lists may
therefore cause infinite recursion (leading to an error).