Zap before-change-function, ftp-setup-write-file-hooks,

[bpt/emacs.git] / lispref / objects.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999
@c   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{hash-table}, @dfn{subr}, and
@dfn{byte-code function}, plus 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.
* Circular Objects::            Read syntax for circular structure.
* 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.  @xref{Read and Print}.

  Most objects have more than one possible read syntax.  Some types of
object have no read syntax, since it may not make sense to enter objects
of these types directly in a Lisp program.  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---for example, the buffer type has none.  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.

  The @samp{#@@@var{count}} construct, which skips the next @var{count}
characters, is useful for program-generated comments containing binary
data.  The Emacs Lisp byte compiler uses this in its output files
(@pxref{Byte Compilation}).  It isn't meant for source files, however.

  @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.
* Char-Table Type::     One-dimensional sparse arrays indexed by characters.
* Bool-Vector Type::    One-dimensional arrays of @code{t} or @code{nil}.
* Hash Table Type::     Super-fast lookup tables.
* 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

  The range of values for integers in Emacs Lisp is @minus{}134217728 to
134217727 (28 bits; i.e.,
@ifnottex
-2**27
@end ifnottex
@tex
@math{-2^{27}}
@end tex
to
@ifnottex
2**27 - 1)
@end ifnottex
@tex
@math{2^{28}-1})
@end tex
on most machines.  (Some machines may provide a wider range.)  It is
important to note that the Emacs Lisp arithmetic functions do not check
for overflow.  Thus @code{(1+ 134217727)} is @minus{}134217728 on most
machines.

  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.}
268435457        ; @r{Also the integer 1 on a 28-bit implementation.}
@end group
@end example

  @xref{Numbers}, for more information.

@node Floating Point Type
@subsection Floating Point Type

  Emacs 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 524287---nineteen bits.  But not all values in that range
are valid character codes.  Codes 0 through 127 are @sc{ascii} codes; the
rest are non-@sc{ascii} (@pxref{Non-ASCII Characters}).  Characters that represent
keyboard input have a much wider range, to encode modifier keys such as
Control, Meta and Shift.

@cindex read syntax for characters
@cindex printed representation for characters
@cindex syntax for characters
@cindex @samp{?} in character constant
@cindex question mark in character constant
  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.  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, @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{\}}
?\d @result{} 127               ; @r{delete character, @key{DEL}}
@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

  In strings and buffers, the only control characters allowed are those
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
@tex
@math{2^{26}}
@end tex
@ifnottex
2**26
@end ifnottex
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 X
and other window systems.

  For historical reasons, Emacs treats the @key{DEL} character as
the control equivalent of @kbd{?}:

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

@noindent
As a result, it is currently not possible to represent the character
@kbd{Control-?}, which is a meaningful input character under X, using
@samp{\C-}.  It is not easy to change this, as various Lisp files refer
to @key{DEL} in this way.

  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.  Which one you use 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
@tex
@math{2^{27}}
@end tex
@ifnottex
2**27
@end ifnottex
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
@tex
@math{2^{7}}
@end tex
@ifnottex
2**7
@end ifnottex
bit attached to an @sc{ascii} character indicates a meta character; thus, 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 a graphic character is indicated by its character code;
for example, @sc{ascii} distinguishes between the characters @samp{a}
and @samp{A}.  But @sc{ascii} has no way to represent whether a control
character is upper case or lower case.  Emacs uses the
@tex
@math{2^{25}}
@end tex
@ifnottex
2**25
@end ifnottex
bit to indicate that the shift key was used in typing a control
character.  This distinction is possible only when you use X terminals
or other special terminals; ordinary terminals do not report the
distinction to the computer in any way.  The Lisp syntax for
the shift bit is @samp{\S-}; thus, @samp{?\C-\S-o} or @samp{?\C-\S-O} 
represents the shifted-control-o character.

@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-}.  (Case is
significant in these prefixes.)  Thus, @samp{?\H-\M-\A-x} represents
@kbd{Alt-Hyper-Meta-x}.
@tex
Numerically, the
bit values are @math{2^{22}} for alt, @math{2^{23}} for super and @math{2^{24}} for hyper.
@end tex
@ifnottex
Numerically, the
bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.
@end ifnottex

@cindex @samp{\} in character constant
@cindex backslash in character constant
@cindex octal character code
  Finally, the most general read syntax for a character represents the
character code in either octal or hex.  To use octal, write a question
mark followed by a backslash and the octal character code (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

  To use hex, write a question mark followed by a backslash, @samp{x},
and the hexadecimal character code.  You can use any number of hex
digits, so you can represent any character code in this way.
Thus, @samp{?\x41} for the character @kbd{A}, @samp{?\x1} for the
character @kbd{C-a}, and @code{?\x8e0} for the character
@iftex
@samp{@`a}.
@end iftex
@ifnottex
@samp{a} with grave accent.
@end ifnottex

  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.

  A symbol whose name starts with a colon (@samp{:}) is called a
@dfn{keyword symbol}.  These symbols automatically act as constants, and
are normally used only by comparing an unknown symbol with a few
specific alternatives.

@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 @samp{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.  In Emacs
Lisp, 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 sixth 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

@cindex @samp{#:} read syntax
  Normally the Lisp reader interns all symbols (@pxref{Creating
Symbols}).  To prevent interning, you can write @samp{#:} before the
name of the symbol.

@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, vectors, char-tables and
bool-vectors.  Vectors can hold elements of any type, but string
elements must be characters, and bool-vector elements must be @code{t}
or @code{nil}.  The characters in a string can have text properties like
characters in a buffer (@pxref{Text Properties}); vectors and
bool-vectors do not support text properties even when their elements
happen to be characters.  Char-tables are like vectors except that they
are indexed by any valid character code.

  Lists, strings and the other array types 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.
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 generally 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
@cindex pointers

  A @dfn{cons cell} is an object that consists of two slots, called the
@sc{car} slot and the @sc{cdr} slot.  Each slot can @dfn{hold} or
@dfn{refer to} any Lisp object.  We also say that ``the @sc{car} of
this cons cell is'' whatever object its @sc{car} slot currently holds,
and likewise for the @sc{cdr}.

@quotation
A note to C programmers: in Lisp, we do not distinguish between
``holding'' a value and ``pointing to'' the value, because pointers in
Lisp are implicit.
@end quotation

  A @dfn{list} is a series of cons cells, linked together so that the
@sc{cdr} slot of each cons cell holds either the next cons cell or 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} derive from the history of Lisp.  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 was 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} slot of the cons cell holds the element, and its @sc{cdr}
slot refers to the next cons cell of the list, which holds the next
element in the list.  The @sc{cdr} slot of the last cons cell is set to
hold @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, like dominoes.  (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.)  This picture 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 hold or refer to
any Lisp object.  Each pair of boxes represents a cons cell.  Each arrow
represents a reference to a Lisp object, either an atom or another cons
cell.

  In this example, the first box, which holds the @sc{car} of the first
cons cell, refers to or ``holds'' @code{rose} (a symbol).  The second
box, holding 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 is @code{violet}, and its @sc{cdr} is the third cons cell.  The
@sc{cdr} of the third (and last) cons cell is @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, you can use either
notation, but list notation is usually clearer and more convenient.
When printing a list, the dotted pair notation is only used if the
@sc{cdr} of a cons cell is not a list.

  Here's an example using boxes to illustrate dotted pair notation.
This example shows the pair @code{(rose . violet)}:

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

  You can combine dotted pair notation with list notation to represent
conveniently a chain of cons cells with a non-@code{nil} final @sc{cdr}.
You write a dot after the last element of the list, followed by the
@sc{cdr} of the final cons cell.  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

  The syntax @code{(rose .@: violet .@: buttercup)} is invalid because
there is nothing that it could mean.  If anything, it would say to put
@code{buttercup} in the @sc{cdr} of a cons cell whose @sc{cdr} is already
used for @code{violet}.

  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)))}.
@ifnottex
It looks like this:

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

@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.  @xref{Hash Tables}, for another kind of
lookup table, which is much faster for handling a large number of keys.

@node Array Type
@subsection Array Type

  An @dfn{array} is composed of an arbitrary number of slots for
holding or referring to other Lisp objects, arranged in a contiguous block of
memory.  Accessing any element of an array takes approximately 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 four types of array: strings, vectors, bool-vectors, and
char-tables.

  A string is an array of characters and a vector is an array of
arbitrary objects.  A bool-vector can hold only @code{t} or @code{nil}.
These kinds of array may have any length up to the largest integer.
Char-tables are sparse arrays indexed by any valid character code; they
can hold arbitrary objects.

  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
largest possible index value is one less than the length of the array.
Once an array is created, its length is fixed.

  All Emacs Lisp arrays 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 the following sections for details.

  The array type is contained in the sequence type and
contains the string type, the vector type, the bool-vector type, and the
char-table 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.

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

@menu
* Syntax for Strings::
* Non-ASCII in Strings::
* Nonprinting Characters::
* Text Props and Strings::
@end menu

@node Syntax for Strings
@subsubsection Syntax for Strings

@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"}.  To include a
double-quote in a string, precede it with a backslash; thus, @code{"\""}
is a string containing just a single double-quote character.  Likewise,
you can include a backslash by preceding it with another backslash, like
this: @code{"this \\ is a single embedded backslash"}.

@cindex newline in strings
  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.  An escaped space
@w{@samp{\ }} is likewise ignored.

@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

@node Non-ASCII in Strings
@subsubsection Non-@sc{ascii} Characters in Strings

  You can include a non-@sc{ascii} international character in a string
constant by writing it literally.  There are two text representations
for non-@sc{ascii} characters in Emacs strings (and in buffers): unibyte
and multibyte.  If the string constant is read from a multibyte source,
such as a multibyte buffer or string, or a file that would be visited as
multibyte, then the character is read as a multibyte character, and that
makes the string multibyte.  If the string constant is read from a
unibyte source, then the character is read as unibyte and that makes the
string unibyte.

  You can also represent a multibyte non-@sc{ascii} character with its
character code: use a hex escape, @samp{\x@var{nnnnnnn}}, with as many
digits as necessary.  (Multibyte non-@sc{ascii} character codes are all
greater than 256.)  Any character which is not a valid hex digit
terminates this construct.  If the next character in the string could be
interpreted as a hex digit, write @w{@samp{\ }} (backslash and space) to
terminate the hex escape---for example, @w{@samp{\x8e0\ }} represents
one character, @samp{a} with grave accent.  @w{@samp{\ }} in a string
constant is just like backslash-newline; it does not contribute any
character to the string, but it does terminate the preceding hex escape.

  Using a multibyte hex escape forces the string to multibyte.  You can
represent a unibyte non-@sc{ascii} character with its character code,
which must be in the range from 128 (0200 octal) to 255 (0377 octal).
This forces a unibyte string.
  
  @xref{Text Representations}, for more information about the two
text representations.

@node Nonprinting Characters
@subsubsection Nonprinting Characters in Strings

  You can use the same backslash escape-sequences in a string constant
as in character literals (but do not use the question mark that begins a
character constant).  For example, you can write a string containing the
nonprinting characters tab and @kbd{C-a}, with commas and spaces between
them, like this: @code{"\t, \C-a"}.  @xref{Character Type}, for a
description of the read syntax for characters.

  However, not all of the characters you can write with backslash
escape-sequences are valid in strings.  The only control characters that
a string can hold are the @sc{ascii} control characters.  Strings do not
distinguish case in @sc{ascii} control characters.

  Properly speaking, strings cannot hold meta characters; but when a
string is to be used as a key sequence, there is a special convention
that provides a way to represent meta versions of @sc{ascii} characters in a
string.  If you use the @samp{\M-} syntax to indicate a meta character
in a string constant, this sets the
@tex
@math{2^{7}}
@end tex
@ifnottex
2**7
@end ifnottex
bit of the character in the string.  If the string is used in
@code{define-key} or @code{lookup-key}, this numeric code is translated
into the equivalent meta character.  @xref{Character Type}.

  Strings cannot hold characters that have the hyper, super, or alt
modifiers.

@node Text Props and Strings
@subsubsection Text Properties in Strings

  A string can hold properties for the characters it contains, in
addition to the characters themselves.  This enables programs that copy
text between strings and buffers to copy the text's properties with no
special effort.  @xref{Text Properties}, for an explanation of what text
properties mean.  Strings with text properties use 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.  For example,

@example
#("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))
@end example

@noindent
represents a string whose textual contents are @samp{foo bar}, in which
the first three characters have a @code{face} property with value
@code{bold}, and the last three have a @code{face} property with value
@code{italic}.  (The fourth character has no text properties, so its
property list is @code{nil}.  It is not actually necessary to mention
ranges with @code{nil} as the property list, since any characters not
mentioned in any range will default to having no properties.)

@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 Char-Table Type
@subsection Char-Table Type

  A @dfn{char-table} is a one-dimensional array of elements of any type,
indexed by character codes.  Char-tables have certain extra features to
make them more useful for many jobs that involve assigning information
to character codes---for example, a char-table can have a parent to
inherit from, a default value, and a small number of extra slots to use for
special purposes.  A char-table can also specify a single value for
a whole character set.

  The printed representation of a char-table is like a vector
except that there is an extra @samp{#^} at the beginning.

  @xref{Char-Tables}, for special functions to operate on char-tables.
Uses of char-tables include:

@itemize @bullet
@item
Case tables (@pxref{Case Tables}).

@item
Character category tables (@pxref{Categories}).

@item
Display Tables (@pxref{Display Tables}).

@item
Syntax tables (@pxref{Syntax Tables}).
@end itemize

@node Bool-Vector Type
@subsection Bool-Vector Type

  A @dfn{bool-vector} is a one-dimensional array of elements that
must be @code{t} or @code{nil}.

  The printed representation of a bool-vector is like a string, except
that it begins with @samp{#&} followed by the length.  The string
constant that follows actually specifies the contents of the bool-vector
as a bitmap---each ``character'' in the string contains 8 bits, which
specify the next 8 elements of the bool-vector (1 stands for @code{t},
and 0 for @code{nil}).  The least significant bits of the character 
correspond to the lowest indices in the bool-vector.  If the length is not a
multiple of 8, the printed representation shows extra elements, but
these extras really make no difference.

@example
(make-bool-vector 3 t)
     @result{} #&3"\007"
(make-bool-vector 3 nil)
     @result{} #&3"\0"
;; @r{These are equal since only the first 3 bits are used.}
(equal #&3"\377" #&3"\007")
     @result{} t
@end example

@node Hash Table Type
@subsection Hash Table Type

    A hash table is a very fast kind of lookup table, somewhat like an
alist in that it maps keys to corresponding values, but much faster.
Hash tables are a new feature in Emacs 21; they have no read syntax, and
print using hash notation.  @xref{Hash Tables}.

@example
(make-hash-table)
     @result{} #<hash-table 'eql nil 0/65 0x83af980>
@end example

@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 argument-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.

  @strong{Warning}: Lisp macros and keyboard macros (@pxref{Keyboard
Macros}) are entirely different things.  When we use the word ``macro''
without qualification, we mean a Lisp macro, not a keyboard 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 redefine a primitive
with a function written in Lisp.  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.  Therefore, @strong{we discourage
redefinition of primitive functions}.

  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 and read syntax 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,
where it serves as a placeholder for the real definition.  The autoload
object says that the real definition is found in a file of Lisp code
that should be loaded when necessary.  It 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 used for general programming
purposes, and most of them are common to most 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.
* Frame Configuration Type::    Recording the status of all frames.
* Process Type::        A process running on the underlying OS.
* Stream Type::         Receive or send characters.
* Keymap Type::         What function a keystroke invokes.
* 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, you can insert text efficiently into an
existing buffer, altering the buffer's contents, 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 list of buffer-local variable bindings (@pxref{Buffer-Local Variables}).

@item
overlays (@pxref{Overlays}).

@item
text properties for the text in the buffer (@pxref{Text Properties}).
@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.

  A buffer may be @dfn{indirect}, which means it shares the text
of another buffer, but presents it differently.  @xref{Indirect Buffers}.

  Buffers have no read syntax.  They print in hash notation, showing 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 @dfn{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 emacs@@psilocin.gnu.org 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; their print syntax
looks like @samp{#<window-configuration>}.  @xref{Window
Configurations}, for a description of several functions related to
window configurations.

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

  A @dfn{frame configuration} stores information about the positions,
sizes, and contents of the windows in all frames.  It is actually
a list whose @sc{car} is @code{frame-configuration} and whose
@sc{cdr} is an alist.  Each alist element describes one frame,
which appears as the @sc{car} of that element.

  @xref{Frame Configurations}, for a description of several functions
related to frame 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 Overlay Type
@subsection Overlay Type

  An @dfn{overlay} specifies properties that apply to a part of a
buffer.  Each overlay applies to a specified range of the buffer, and
contains a property list (a list whose elements are alternating property
names and values).  Overlay properties are used to present parts of the
buffer temporarily in a different display style.  Overlays have no read
syntax, and print in hash notation, giving the buffer name and range of
positions.

  @xref{Overlays}, for how to create and use overlays.

@node Circular Objects
@section Read Syntax for Circular Objects
@cindex circular structure, read syntax
@cindex shared structure, read syntax
@cindex @samp{#@var{n}=} read syntax
@cindex @samp{#@var{n}#} read syntax

  In Emacs 21, to represent shared or circular structure within a
complex of Lisp objects, you can use the reader constructs
@samp{#@var{n}=} and @samp{#@var{n}#}.

  Use @code{#@var{n}=} before an object to label it for later reference;
subsequently, you can use @code{#@var{n}#} to refer the same object in
another place.  Here, @var{n} is some integer.  For example, here is how
to make a list in which the first element recurs as the third element:

@example
(#1=(a) b #1#)
@end example

@noindent
This differs from ordinary syntax such as this

@example
((a) b (a))
@end example

@noindent
which would result in a list whose first and third elements
look alike but are not the same Lisp object.  This shows the difference:

@example
(prog1 nil
  (setq x '(#1=(a) b #1#)))
(eq (nth 0 x) (nth 2 x))
     @result{} t
(setq x '((a) b (a)))
(eq (nth 0 x) (nth 2 x))
     @result{} nil
@end example

  You can also use the same syntax to make a circular structure, which
appears as an ``element'' within itself.  Here is an example:

@example
#1=(a #1#)
@end example

@noindent
This makes a list whose second element is the list itself.
Here's how you can see that it really works:

@example
(prog1 nil
  (setq x '#1=(a #1#)))
(eq x (cadr x))
     @result{} t
@end example

  The Lisp printer can produce this syntax to record circular and shared
structure in a Lisp object, if you bind the variable @code{print-circle}
to a non-@code{nil} value.  @xref{Output Variables}.

@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: number-or-marker-p, a
@end group
@end example

@cindex type predicates
@cindex testing types
  If you want your program to handle different types differently, you
must do explicit type checking.  The most common way to check the type
of an object is to call a @dfn{type predicate} function.  Emacs has a
type predicate for each type, as well as some predicates for
combinations of types.

  A type predicate function takes one argument; it returns @code{t} if
the argument belongs to the appropriate type, and @code{nil} otherwise.
Following a general Lisp convention for predicate functions, most type
predicates' names end with @samp{p}.

  Here is an example which uses the predicates @code{listp} to check for
a list and @code{symbolp} to check for a symbol.

@example
(defun add-on (x)
  (cond ((symbolp x)
         ;; If X is a symbol, put it on LIST.
         (setq list (cons x list)))
        ((listp x)
         ;; If X is a list, add its elements to LIST.
         (setq list (append x list)))
        (t
         ;; We handle only symbols and lists.
         (error "Invalid argument %s in add-on" x))))
@end example

  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 bool-vector-p
@xref{Bool-Vectors, bool-vector-p}.

@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 Tables, case-table-p}.

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

@item char-table-p
@xref{Char-Tables, char-table-p}.

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

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

@item display-table-p
@xref{Display Tables, display-table-p}.

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

@item frame-configuration-p
@xref{Frame Configurations, frame-configuration-p}.

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

@item framep
@xref{Frames, framep}.

@item functionp
@xref{Functions, functionp}.

@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 keywordp
@xref{Constant Variables}.

@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

  The most general way to check the type of an object is to call the
function @code{type-of}.  Recall that each object belongs to one and
only one primitive type; @code{type-of} tells you which one (@pxref{Lisp
Data Types}).  But @code{type-of} knows nothing about non-primitive
types.  In most cases, it is more convenient to use type predicates than
@code{type-of}.

@defun type-of object
This function returns a symbol naming the primitive type of
@var{object}.  The value is one of the symbols @code{symbol},
@code{integer}, @code{float}, @code{string}, @code{cons}, @code{vector},
@code{char-table}, @code{bool-vector}, @code{hash-table}, @code{subr},
@code{compiled-function}, @code{marker}, @code{overlay}, @code{window},
@code{buffer}, @code{frame}, @code{process}, or
@code{window-configuration}.

@example
(type-of 1)
     @result{} integer
(type-of 'nil)
     @result{} symbol
(type-of '())    ; @r{@code{()} is @code{nil}.}
     @result{} symbol
(type-of '(x))
     @result{} cons
@end example
@end defun

@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.

@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

The @code{make-symbol} function returns an uninterned symbol, distinct
from the symbol that is used if you write the name in a Lisp expression.
Distinct symbols with the same name are not @code{eq}.  @xref{Creating
Symbols}.

@example
@group
(eq (make-symbol "foo") 'foo)
     @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 or contents 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 is case-sensitive, but does not take account of
text properties---it compares only the characters in the strings.
A unibyte string never equals a multibyte string unless the
contents are entirely @sc{ascii} (@pxref{Text Representations}).

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

However, two distinct buffers are never considered @code{equal}, even if
their textual contents are the same.
@end defun

  The test for equality is implemented recursively; for example, given
two cons cells @var{x} and @var{y}, @code{(equal @var{x} @var{y})}
returns @code{t} if and only if both the expressions below return
@code{t}:

@example
(equal (car @var{x}) (car @var{y}))
(equal (cdr @var{x}) (cdr @var{y}))
@end example

Because of this recursive method, circular lists may therefore cause
infinite recursion (leading to an error).