[sxemacs] / info / lispref / objects.texi

@c -*-texinfo-*-
@c This is part of the SXEmacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
@c Copyright (C) 2005 Sebastian Freundt <hroptatyr@sxemacs.org>
@c See the file lispref.texi for copying conditions.
@setfilename ../../info/objects.info

@node Lisp Data Types, Numbers, Packaging, 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 SXEmacs.  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{character} (starting with XEmacs 20.0),
@dfn{float}, @dfn{cons}, @dfn{symbol}, @dfn{string}, @dfn{vector},
@dfn{bit-vector}, @dfn{subr}, @dfn{compiled-function}, @dfn{hash-table},
@dfn{range-table}, @dfn{char-table}, @dfn{weak-list}, 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 SXEmacs 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 SXEmacs 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.
* Primitive Types::             List of all primitive types in SXEmacs.
* Programming Types::           Types found in all Lisp systems.
* Editing Types::               Types specific to SXEmacs.
* Window-System Types::         Types specific to windowing systems.
* Type Predicates::             Tests related to types.
* Equality Predicates::         Tests of equality between any two objects.
@end menu


@node Printed Representation
@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
@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 SXEmacs 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 Primitive Types
@section Primitive Types
@cindex primitive types

  For reference, here is a list of all the primitive types that may
exist in SXEmacs.  Note that some of these types may not exist
in some SXEmacs executables; that depends on the options that
SXEmacs was configured with.

@itemize @bullet
@item
bit-vector
@item
buffer
@item
char-table
@item
character
@item
charset
@item
coding-system
@item
cons
@item
color-instance
@item
compiled-function
@item
console
@item
database
@item
device
@item
event
@item
extent
@item
face
@item
float
@item
font-instance
@item
frame
@item
glyph
@item
hash-table
@item
image-instance
@item
integer
@item
keymap
@item
marker
@item
process
@item
range-table
@item
specifier
@item
string
@item
subr
@item
subwindow
@item
symbol
@item
toolbar-button
@item
tooltalk-message
@item
tooltalk-pattern
@item
vector
@item
weak-list
@item
window
@item
window-configuration
@item
x-resource
@end itemize

In addition, the following special types are created internally
but will never be seen by Lisp code.  You may encounter them,
however, if you are debugging SXEmacs.  The printed representation
of these objects begins @samp{#<INTERNAL EMACS BUG}, which indicates
to the Lisp programmer that he has found an internal bug in SXEmacs
if he ever encounters any of these objects.

@itemize @bullet
@item
char-table-entry
@item
command-builder
@item
extent-auxiliary
@item
extent-info
@item
lcrecord-list
@item
lstream
@item
opaque
@item
opaque-list
@item
popup-data
@item
symbol-value-buffer-local
@item
symbol-value-forward
@item
symbol-value-lisp-magic
@item
symbol-value-varalias
@item
toolbar-data
@end itemize


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

  There are two general categories of types in SXEmacs 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 SXEmacs 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.
* Bit Vector Type::     An (efficient) array of bits.
* 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.
* Compiled-Function Type::      A function written in Lisp, then compiled.
* Autoload Type::       A type used for automatically loading seldom-used
                        functions.
* Char Table Type::     A mapping from characters to Lisp objects.
* Hash Table Type::     A fast mapping between Lisp objects.
* Range Table Type::    A mapping from ranges of integers to Lisp objects.
* Weak List Type::      A list with special garbage-collection properties.
@end menu


@node Integer Type
@subsection Integer Type

  The range of values for integers in SXEmacs Lisp is @minus{}134217728 to
134217727 (28 bits; i.e.,
@ifinfo
-2**27
@end ifinfo
@tex
$-2^{27}$
@end tex
to
@ifinfo
2**27 - 1)
@end ifinfo
@tex
$2^{28}-1$)
@end tex
on most machines.  (Some machines, in particular 64-bit machines such as
the DEC Alpha, may provide a wider range.)  It is important to note that
the XEmacs Lisp arithmetic functions do not check for overflow.  Thus
@code{(1+ 134217727)} is @minus{}134217728 on most machines. (However,
you @emph{will} get an error if you attempt to read an out-of-range
number using the Lisp reader.)

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

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

  @xref{Numbers}, for more information.


@node Floating Point Type
@subsection Floating Point Type

  SXEmacs supports floating point numbers.  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
@cindex char-int confoundance disease

  In XEmacs version 19, and in all versions of FSF GNU Emacs, a
@dfn{character} in XEmacs Lisp is nothing more than an integer.
This is yet another holdover from XEmacs Lisp's derivation from
vintage-1980 Lisps; modern versions of Lisp consider this equivalence
a bad idea, and have separate character types.  In XEmacs version 20,
and of course all SXEmacs versions, the modern convention is followed,
and characters are their own primitive types.  This change was necessary
in order for @sc{mule}, i.e. Asian-language, support to be correctly
implemented.

  Even in XEmacs version 20, remnants of the equivalence between
characters and integers still exist; this is termed the @dfn{char-int
confoundance disease}.  In particular, many functions such as @code{eq},
@code{equal}, and @code{memq} have equivalent functions (@code{old-eq},
@code{old-equal}, @code{old-memq}, etc.) that pretend like characters
are integers are the same.  Byte code compiled under any version 19
Emacs will have all such functions mapped to their @code{old-} equivalents
when the byte code is read into XEmacs 20.  This is to preserve
compatibility---Emacs 19 converts all constant characters to the equivalent
integer during byte-compilation, and thus there is no other way to preserve
byte-code compatibility even if the code has specifically been written
with the distinction between characters and integers in mind.

  Every character has an equivalent integer, called the @dfn{character
code}.  For example, the character @kbd{A} is represented as the
@w{integer 65}, following the standard @sc{ascii} representation of
characters.  If SXEmacs was not compiled with @sc{mule} support, the
range of this integer will always be 0 to 255---eight bits, or one
byte. (Integers outside this range are accepted but silently truncated;
however, you should most decidedly @emph{not} rely on this, because it
will not work under SXEmacs with @sc{mule} support.)  When @sc{mule}
support is present, the range of character codes is much
larger. (Currently, 19 bits are used.)

  FSF GNU Emacs uses kludgy character codes above 255 to represent
keyboard input of @sc{ascii} characters in combination with certain
modifiers.  SXEmacs does not use this (a more general mechanism is
used that does not distinguish between @sc{ascii} keys and other
keys), so you will never find character codes above 255 in a
non-@sc{mule} SXEmacs.

  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}.

@cindex read syntax for characters
@cindex printed representation for characters
@cindex syntax for characters

  The read syntax for characters begins with a question mark, followed
by the character (if it's printable) or some symbolic representation of
it.  In SXEmacs and XEmacs 20+, where characters are their own type,
this is also the print representation.  In XEmacs 19, however, where
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 SXEmacs Lisp
provides for characters.

  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
;; @r{Under SXEmacs:}
?Q @result{} ?Q    ?q @result{} ?q
(char-int ?Q) @result{} 81
;; @r{Under XEmacs 19:}
?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 SXEmacs 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{?\\}.

SXEmacs always prints punctuation characters with a @samp{\} in front of
them, to avoid confusion.

@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.  Their character codes are 7, 8, 9, 10, 11, 12, 13, and 27
in decimal.  Thus,

@example
;; @r{Under SXEmacs:}
?\a @result{} ?\^G              ; @r{@kbd{C-g}}
(char-int ?\a) @result{} 7
?\b @result{} ?\^H              ; @r{backspace, @key{BS}, @kbd{C-h}}
(char-int ?\b) @result{} 8
?\t @result{} ?\t               ; @r{tab, @key{TAB}, @kbd{C-i}}
(char-int ?\t) @result{} 9
?\n @result{} ?\n               ; @r{newline, @key{LFD}, @kbd{C-j}}
?\v @result{} ?\^K              ; @r{vertical tab, @kbd{C-k}}
?\f @result{} ?\^L              ; @r{formfeed character, @kbd{C-l}}
?\r @result{} ?\r               ; @r{carriage return, @key{RET}, @kbd{C-m}}
?\e @result{} ?\^[              ; @r{escape character, @key{ESC}, @kbd{C-[}}
?\\ @result{} ?\\               ; @r{backslash character, @kbd{\}}
;; @r{Under XEmacs 19:}
?\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
;; @r{Under SXEmacs:}
?\^I @result{} ?\t   ?\C-I @result{} ?\t
(char-int ?\^I) @result{} 9
;; @r{Under XEmacs 19:}
?\^I @result{} 9     ?\C-I @result{} 9
@end example

  There is also a character read syntax beginning with @samp{\M-}.  This
sets the high bit of the character code (same as adding 128 to the
character code).  For example, @samp{?\M-A} stands for the character
with character code 193, or 128 plus 65.  You should @emph{not} use this
syntax in your programs.  It is a holdover of yet another confoundance
disease from earlier Emacsen. (This was used to represent keyboard input
with the @key{META} key set, thus the @samp{M}; however, it conflicts
with the legitimate @sc{iso}-8859-1 interpretation of the character code.
For example, character code 193 is a lowercase @samp{a} with an acute
accent, in @sc{iso}-8859-1.)

@ignore @c None of this crap applies to SXEmacs nor XEmacs.
  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
@iftex
$2^{26}$
@end iftex
@ifinfo
2**26
@end ifinfo
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.

  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.  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.  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
@iftex
$2^{27}$
@end iftex
@ifinfo
2**27
@end ifinfo
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
@iftex
$2^{7}$
@end iftex
@ifinfo
2**7
@end ifinfo
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
@iftex
$2^{25}$
@end iftex
@ifinfo
2**25
@end ifinfo
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}.
@iftex
Numerically, the
bit values are $2^{22}$ for alt, $2^{23}$ for super and $2^{24}$ for hyper.
@end iftex
@ifinfo
Numerically, the
bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.
@end ifinfo
@end ignore

@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
;; @r{Under SXEmacs:}
?\012 @result{} ?\n        ?\n @result{} ?\n        ?\C-j @result{} ?\n
?\101 @result{} ?A         ?A @result{} ?A
@end group
@group
;; @r{Under XEmacs 19:}
?\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 SXEmacs 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 SXEmacs 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.  In SXEmacs
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


@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 SXEmacs 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, and bit vectors.
Vectors can hold elements of any type, but string elements must be
characters, and bit vector elements must be either 0 or 1.  However, the
characters in a string can have extents (@pxref{Extents}) and text
properties (@pxref{Text Properties}) like characters in a buffer;
vectors do not support extents or text properties even when their
elements happen to be characters.

  Lists, strings, vectors, and bit 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
@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
@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.)

  SXEmacs defines three types of array, strings, vectors, and bit
vectors.  A string is an array of characters, a vector is an array of
arbitrary objects, and a bit vector is an array of 1's and 0's.  All 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}, @ref{Vector Type}, and @ref{Bit
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 the
string type, the vector type, and the bit vector type.


@node String Type
@subsection String Type

  A @dfn{string} is an array of characters.  Strings are used for many
purposes in SXEmacs, 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 or @kbd{C-a} using the convenient escape sequences, like this:
@code{"\t, \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.)

@ignore @c More ill-conceived FSF Emacs crap.
  If you use the @samp{\M-} syntax to indicate a meta character in a
string constant, this sets the
@iftex
$2^{7}$
@end iftex
@ifinfo
2**7
@end ifinfo
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.
@end ignore

  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"}.

  An alternative syntax allows insertion of raw backslashes into a
string, like this: @code{#r"this \ is an embedded backslash"}.  In  such
a string, each character following a backslash is included literally in
the string, and all backslashes are left in the string.  This means that
@code{#r"\""} is a valid string literal with two characters, a backslash and a
double-quote.  It also means that a string  with this syntax @emph{cannot end
in a single backslash}.

  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 extents and 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 extents and properties
with no special effort.  @xref{Extents}, @xref{Text Properties}.

  Note that FSF GNU Emacs has a special read and print syntax for
strings with text properties, but SXEmacs does not currently implement
this.  It was judged better not to include this in SXEmacs because it
entails that @code{equal} return @code{nil} when passed a string with
text properties and the equivalent string without text properties, which
is often counter-intuitive.

@ignore @c Not in SXEmacs nor XEmacs
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{start} @var{end} @var{plist}
@end example

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

  @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 Bit Vector Type
@subsection Bit Vector Type

  A @dfn{bit vector} is a one-dimensional array of 1's and 0's.  It
takes a constant amount of time to access any element of a bit vector,
as for vectors.  Bit vectors have an extremely compact internal
representation (one machine bit per element), which makes them ideal
for keeping track of unordered sets, large collections of boolean values,
etc.

  The printed representation of a bit vector consists of @samp{#*}
followed by the bits in the vector.  This is also the read syntax.  Like
numbers, strings, and vectors, bit vectors are considered constants for
evaluation.

@example
#*00101000      ; @r{A bit vector of eight elements.}
     @result{} #*00101000
@end example

  @xref{Bit Vectors}, for functions that work with bit 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 SXEmacs 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 SXEmacs 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 Compiled-Function Type
@subsection Compiled-Function Type

  The byte compiler produces @dfn{compiled-function objects}.  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 compiled-function object is normally
@samp{#<compiled-function...>}.  If @code{print-readably} is true,
however, it is @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 Char Table Type
@subsection Char Table Type
@cindex char table type

(not yet documented)


@node Hash Table Type
@subsection Hash Table Type
@cindex hash table type

  A @dfn{hash table} is a table providing an arbitrary mapping from
one Lisp object to another, using an internal indexing method
called @dfn{hashing}.  Hash tables are very fast (much more efficient
that using an association list, when there are a large number of
elements in the table).

Hash tables have a special read syntax beginning with
@samp{#s(hash-table} (this is an example of @dfn{structure} read
syntax.  This notation is also used for printing when
@code{print-readably} is @code{t}.

Otherwise they print in hash notation (The ``hash'' in ``hash notation''
has nothing to do with the ``hash'' in ``hash table''), giving the
number of elements, total space allocated for elements, and a unique
number assigned at the time the hash table was created. (Hash tables
automatically resize as necessary so there is no danger of running out
of space for elements.)

@example
@group
(make-hash-table :size 50)
     @result{} #<hash-table 0/107 0x313a>
@end group
@end example

@xref{Hash Tables}, for information on how to create and work with hash
tables.


@node Range Table Type
@subsection Range Table Type
@cindex range table type

  A @dfn{range table} is a table that maps from ranges of integers to
arbitrary Lisp objects.  Range tables automatically combine overlapping
ranges that map to the same Lisp object, and operations are provided
for mapping over all of the ranges in a range table.

  Range tables have a special read syntax beginning with
@samp{#s(range-table} (this is an example of @dfn{structure} read syntax,
which is also used for char tables and faces).

@example
@group
(setq x (make-range-table))
(put-range-table 20 50 'foo x)
(put-range-table 100 200 "bar" x)
x
     @result{} #s(range-table data ((20 50) foo (100 200) "bar"))
@end group
@end example

@xref{Range Tables}, for information on how to create and work with range
tables.


@node Weak List Type
@subsection Weak List Type
@cindex weak list type

(not yet documented)


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

  The types in the previous section are common to many Lisp dialects.
SXEmacs 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.
* Extent Type::         A range in a buffer or string, maybe with properties.
* Window Type::         Buffers are displayed in windows.
* Frame Type::          Windows subdivide frames.
* Device Type::         Devices group all frames on a display.
* Console Type::        Consoles group all devices with the same keyboard.
* Window Configuration Type::   Recording the way a frame is subdivided.
* Event Type::          An interesting occurrence in the system.
* 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.
* Database Type::       A connection to an external DBM or DB database.
* Charset Type::        A character set (e.g. all Kanji characters),
                          under SXEmacs/MULE.
* Coding System Type::  An object encapsulating a way of converting between
                          different textual encodings, under SXEmacs/MULE.
* ToolTalk Message Type:: A message, in the ToolTalk IPC protocol.
* ToolTalk Pattern Type:: A pattern, in the ToolTalk IPC protocol.
@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 SXEmacs 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 SXEmacs
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});

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

@item
a list of extents (@pxref{Extents});

@item
and various other related 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.  @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 50661 in objects.texi>
@end group
@end example

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


@node Extent Type
@subsection Extent Type

  An @dfn{extent} specifies temporary alteration of the display
appearance of a part of a buffer (or string).  It contains markers
delimiting a range of the buffer, plus a property list (a list whose
elements are alternating property names and values).  Extents are used
to present parts of the buffer temporarily in a different display style.
They have no read syntax, and print in hash notation, giving the buffer
name and range of positions.

  Extents can exist over strings as well as buffers; the primary use
of this is to preserve extent and text property information as text
is copied from one buffer to another or between different parts of
a buffer.

  Extents have no read syntax.  They print in hash notation, giving the
range of text they cover, the name of the buffer or string they are in,
the address in core, and a summary of some of the properties attached to
the extent.

@example
@group
(extent-at (point))
     @result{} #<extent [51742, 51748) font-lock text-prop 0x90121e0 in buffer objects.texi>
@end group
@end example

  @xref{Extents}, for how to create and use extents.

  Extents are used to implement text properties.  @xref{Text Properties}.


@node Window Type
@subsection Window Type

  A @dfn{window} describes the portion of the frame that SXEmacs uses to
display a buffer. (In standard window-system usage, a @dfn{window} is
what SXEmacs calls a @dfn{frame}; SXEmacs confusingly uses the term
``window'' to refer to what is called a @dfn{pane} in standard
window-system usage.) 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 SXEmacs 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
name of the buffer being displayed and a unique number assigned at the
time the window was created. (This number can be useful because the
buffer displayed in any given window can change frequently.)

@example
@group
(selected-window)
     @result{} #<window on "objects.texi" 0x266c>
@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 (a @dfn{window} in standard
window-system terminology) that contains one or more non-overlapping
SXEmacs windows (@dfn{panes} in standard window-system terminology).  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 type, name as used for resourcing, and a unique number assigned
at the time the frame was created.

@example
@group
(selected-frame)
     @result{} #<x-frame "emacs" 0x9db>
@end group
@end example

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


@node Device Type
@subsection Device Type

  A @dfn{device} represents a single display on which frames exist.
Normally, there is only one device object, but there may be more
than one if SXEmacs is being run on a multi-headed display (e.g. an
X server with attached color and mono screens) or if SXEmacs is
simultaneously driving frames attached to different consoles, e.g.
an X display and a @sc{tty} connection.

  Devices do not have a read syntax.  They print in hash notation,
giving the device's type, connection name, and a unique number assigned
at the time the device was created.

@example
@group
(selected-device)
     @result{} #<x-device on ":0.0" 0x5b9>
@end group
@end example

  @xref{Consoles and Devices}, for a description of several functions
related to devices.


@node Console Type
@subsection Console Type

  A @dfn{console} represents a single keyboard to which devices
(i.e. displays on which frames exist) are connected.  Normally, there is
only one console object, but there may be more than one if SXEmacs is
simultaneously driving frames attached to different X servers and/or
@sc{tty} connections.

SXEmacs is capable of driving multiple X and @sc{tty} connections at the
same time, and provides a robust mechanism for handling the differing
display capabilities of such heterogeneous environments.  A buffer with
embedded glyphs and multiple fonts and colors, for example, will display
reasonably if it simultaneously appears on a frame on a color X display,
a frame on a mono X display, and a frame on a @sc{tty} connection.

  Consoles do not have a read syntax.  They print in hash notation,
giving the console's type, connection name, and a unique number assigned
at the time the console was created.

@example
@group
(selected-console)
     @result{} #<x-console on "localhost:0" 0x5b7>
@end group
@end example

  @xref{Consoles and Devices}, for a description of several functions
related to consoles.


@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 in hash
notation, giving a unique number assigned at the time the window
configuration was created.

@example
@group
(current-window-configuration)
     @result{} #<window-configuration 0x2db4>
@end group
@end example

  @xref{Window Configurations}, for a description of several functions
related to window configurations.


@node Event Type
@subsection Event Type

(not yet documented)


@node Process Type
@subsection Process Type

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

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

  Process objects have no read syntax.  They print in hash notation,
giving the name of the process, its associated process ID, and the
current state of the process:

@example
@group
(process-list)
     @result{} (#<process "shell" pid 2909 state:run>)
@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.

  NOTE: In SXEmacs, a keymap is a separate primitive type.  In FSF GNU
Emacs, 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

  Under SXEmacs and XEmacs 20+, a @dfn{syntax table} is a particular
type of char table.  Under XEmacs 19, a syntax table a vector of 256
integers.  In both cases, each element 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{+}.

  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 SXEmacs 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 256, although in SXEmacs and XEmacs
20+ this may change to be a particular type of char table.
@xref{Display Tables}.


@node Database Type
@subsection Database Type
@cindex database type

(not yet documented)


@node Charset Type
@subsection Charset Type
@cindex charset type

(not yet documented)


@node Coding System Type
@subsection Coding System Type
@cindex coding system type

(not yet documented)


@node ToolTalk Message Type
@subsection ToolTalk Message Type

(not yet documented)


@node ToolTalk Pattern Type
@subsection ToolTalk Pattern Type

(not yet documented)


@node Window-System Types
@section Window-System Types
@cindex window system types

  SXEmacs also has some types that represent objects such as faces
(collections of display characters), fonts, and pixmaps that are
commonly found in windowing systems.

@menu
* Face Type::           A collection of display characteristics.
* Glyph Type::          An image appearing in a buffer or elsewhere.
* Specifier Type::      A way of controlling display characteristics on
                          a per-buffer, -frame, -window, or -device level.
* Font Instance Type::  The way a font appears on a particular device.
* Color Instance Type:: The way a color appears on a particular device.
* Image Instance Type:: The way an image appears on a particular device.
* Toolbar Button Type:: An object representing a button in a toolbar.
* Subwindow Type::      An externally-controlled window-system window
                          appearing in a buffer.
* X Resource Type::     A miscellaneous X resource, if Epoch support was
                          compiled into SXEmacs.
@end menu


@node Face Type
@subsection Face Type
@cindex face type

(not yet documented)


@node Glyph Type
@subsection Glyph Type
@cindex glyph type

(not yet documented)


@node Specifier Type
@subsection Specifier Type
@cindex specifier type

(not yet documented)


@node Font Instance Type
@subsection Font Instance Type
@cindex font instance type

(not yet documented)


@node Color Instance Type
@subsection Color Instance Type
@cindex color instance type

(not yet documented)


@node Image Instance Type
@subsection Image Instance Type
@cindex image instance type

(not yet documented)


@node Toolbar Button Type
@subsection Toolbar Button Type
@cindex toolbar button type

(not yet documented)


@node Subwindow Type
@subsection Subwindow Type
@cindex subwindow type

(not yet documented)


@node X Resource Type
@subsection X Resource Type
@cindex X resource type

(not yet documented)


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

  The SXEmacs 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
  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.  SXEmacs 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)))
@need 3000
        (t
         ;; We only handle 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 annotationp
@xref{Annotation Primitives, annotationp}.

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

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

@item bit-vector-p
@xref{Bit Vector Functions, bit-vector-p}.

@item bitp
@xref{Bit Vector Functions, bitp}.

@item boolean-specifier-p
@xref{Specifier Types, boolean-specifier-p}.

@item buffer-glyph-p
@xref{Glyph Types, buffer-glyph-p}.

@item buffer-live-p
@xref{Killing Buffers, buffer-live-p}.

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

@item button-event-p
@xref{Event Predicates, button-event-p}.

@item button-press-event-p
@xref{Event Predicates, button-press-event-p}.

@item button-release-event-p
@xref{Event Predicates, button-release-event-p}.

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

@item char-int-p
@xref{Character Codes, char-int-p}.

@item char-or-char-int-p
@xref{Character Codes, char-or-char-int-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 characterp
@xref{Predicates for Characters, characterp}.

@item color-instance-p
@xref{Colors, color-instance-p}.

@item color-pixmap-image-instance-p
@xref{Image Instance Types, color-pixmap-image-instance-p}.

@item color-specifier-p
@xref{Specifier Types, color-specifier-p}.

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

@item compiled-function-p
@xref{Compiled-Function Type, compiled-function-p}.

@item console-live-p
@xref{Connecting to a Console or Device, console-live-p}.

@item consolep
@xref{Consoles and Devices, consolep}.

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

@item database-live-p
@xref{Connecting to a Database, database-live-p}.

@item databasep
@xref{Databases, databasep}.

@item device-live-p
@xref{Connecting to a Console or Device, device-live-p}.

@item device-or-frame-p
@xref{Basic Device Functions, device-or-frame-p}.

@item devicep
@xref{Consoles and Devices, devicep}.

@item eval-event-p
@xref{Event Predicates, eval-event-p}.

@item event-live-p
@xref{Event Predicates, event-live-p}.

@item eventp
@xref{Events, eventp}.

@item extent-live-p
@xref{Creating and Modifying Extents, extent-live-p}.

@item extentp
@xref{Extents, extentp}.

@item face-boolean-specifier-p
@xref{Specifier Types, face-boolean-specifier-p}.

@item facep
@xref{Basic Face Functions, facep}.

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

@item font-instance-p
@xref{Fonts, font-instance-p}.

@item font-specifier-p
@xref{Specifier Types, font-specifier-p}.

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

@item framep
@xref{Frames, framep}.

@item functionp
(not yet documented)

@item generic-specifier-p
@xref{Specifier Types, generic-specifier-p}.

@item glyphp
@xref{Glyphs, glyphp}.

@item hash-table-p
@xref{Hash Tables, hash-table-p}.

@item icon-glyph-p
@xref{Glyph Types, icon-glyph-p}.

@item image-instance-p
@xref{Images, image-instance-p}.

@item image-specifier-p
@xref{Specifier Types, image-specifier-p}.

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

@item integer-or-char-p
@xref{Predicates for Characters, integer-or-char-p}.

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

@item integer-specifier-p
@xref{Specifier Types, integer-specifier-p}.

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

@item itimerp
(not yet documented)

@item key-press-event-p
@xref{Event Predicates, key-press-event-p}.

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

@item keywordp
(not yet documented)

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

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

@item misc-user-event-p
@xref{Event Predicates, misc-user-event-p}.

@item mono-pixmap-image-instance-p
@xref{Image Instance Types, mono-pixmap-image-instance-p}.

@item motion-event-p
@xref{Event Predicates, motion-event-p}.

@item mouse-event-p
@xref{Event Predicates, mouse-event-p}.

@item natnum-specifier-p
@xref{Specifier Types, natnum-specifier-p}.

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

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

@item nothing-image-instance-p
@xref{Image Instance Types, nothing-image-instance-p}.

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

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

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

@item pointer-glyph-p
@xref{Glyph Types, pointer-glyph-p}.

@item pointer-image-instance-p
@xref{Image Instance Types, pointer-image-instance-p}.

@item process-event-p
@xref{Event Predicates, process-event-p}.

@item processp
@xref{Processes, processp}.

@item range-table-p
@xref{Range Tables, range-table-p}.

@item ringp
(not yet documented)

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

@item specifierp
@xref{Specifiers, specifierp}.

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

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

@item subwindow-image-instance-p
@xref{Image Instance Types, subwindow-image-instance-p}.

@item subwindowp
@xref{Subwindows, subwindowp}.

@item symbolp
@xref{Symbols, symbolp}.

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

@item text-image-instance-p
@xref{Image Instance Types, text-image-instance-p}.

@item timeout-event-p
@xref{Event Predicates, timeout-event-p}.

@item toolbar-button-p
@xref{Toolbar, toolbar-button-p}.

@item toolbar-specifier-p
@xref{Toolbar, toolbar-specifier-p}.

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

@item vectorp
@xref{Vectors, vectorp}.

@item weak-list-p
@xref{Weak Lists, weak-list-p}.

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

@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 @code{bit-vector}, @code{buffer},
@code{char-table}, @code{character}, @code{charset},
@code{coding-system}, @code{cons}, @code{color-instance},
@code{compiled-function}, @code{console}, @code{database},
@code{device}, @code{event}, @code{extent}, @code{face}, @code{float},
@code{font-instance}, @code{frame}, @code{glyph}, @code{hash-table},
@code{image-instance}, @code{integer}, @code{keymap}, @code{marker},
@code{process}, @code{range-table}, @code{specifier}, @code{string},
@code{subr}, @code{subwindow}, @code{symbol}, @code{toolbar-button},
@code{tooltalk-message}, @code{tooltalk-pattern}, @code{vector},
@code{weak-list}, @code{window}, @code{window-configuration}, or
@code{x-resource}.

@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.

(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}.)

NOTE: Under XEmacs 19, characters are really just integers, and thus
characters and integers are @code{eq}.  Under SXEmacs and XEmacs 20+, it
was necessary to preserve remnants of this in function such as
@code{old-eq} in order to maintain byte-code compatibility.  Byte code
compiled under any Emacs 19 will automatically have calls to @code{eq}
mapped to @code{old-eq} when executed under SXEmacs or XEmacs 20+.

@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 old-eq object1 object2
This function exists under SXEmacs and XEmacs 20+ and is exactly like
@code{eq} except that it suffers from the char-int confoundance disease.
In other words, it returns @code{t} if given a character and the
equivalent integer, even though the objects are of different types!
You should @emph{not} ever call this function explicitly in your
code.  However, be aware that all calls to @code{eq} in byte code
compiled under version 19 map to @code{old-eq} in SXEmacs or XEmacs 20+.

Likewise for @code{old-equal}, @code{old-memq}, @code{old-member},
@code{old-assq} and  @code{old-assoc}.

@example
@group
;; @r{Remember, this does not apply under XEmacs 19.}
?A
     @result{} ?A
(char-int ?A)
     @result{} 65
(old-eq ?A 65)
     @result{} t               ; @r{Eek, we've been infected.}
(eq ?A 65)
     @result{} nil             ; @r{We are still healthy.}
@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 is case-sensitive.

Note that in FSF GNU Emacs, comparison of strings takes into account
their text properties, and you have to use @code{string-equal} if you
want only the strings themselves compared.  This difference does not
exist in SXEmacs; @code{equal} and @code{string-equal} always return
the same value on the same strings.

@ignore @c Not true in SXEmacs nor XEmacs
Comparison of strings is case-sensitive and takes account of text
properties as well as the characters in the strings.  To compare
two strings' characters without comparing their text properties,
use @code{string=} (@pxref{Text Comparison}).
@end ignore

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

Two distinct buffers are never @code{equal}, even if their contents
are the same.
@end defun

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