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[Emacs-diffs] Changes to emacs/lispref/objects.texi [gnus-5_10-branch]
From: |
Miles Bader |
Subject: |
[Emacs-diffs] Changes to emacs/lispref/objects.texi [gnus-5_10-branch] |
Date: |
Sat, 04 Sep 2004 08:31:08 -0400 |
Index: emacs/lispref/objects.texi
diff -c /dev/null emacs/lispref/objects.texi:1.42.2.1
*** /dev/null Sat Sep 4 12:02:45 2004
--- emacs/lispref/objects.texi Sat Sep 4 12:01:14 2004
***************
*** 0 ****
--- 1,1926 ----
+ @c -*-texinfo-*-
+ @c This is part of the GNU Emacs Lisp Reference Manual.
+ @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2003
+ @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. (Actually, a small number of Emacs
+ Lisp variables can only take on values of a certain type.
+ @xref{Variables with Restricted Values}.)
+
+ 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{}268435456 to
+ 268435455 (29 bits; i.e.,
+ @ifnottex
+ -2**28
+ @end ifnottex
+ @tex
+ @math{-2^{28}}
+ @end tex
+ to
+ @ifnottex
+ 2**28 - 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+ 268435455)} is @minus{}268435456 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.}
+ 536870913 ; @r{Also the integer 1 on a 29-bit implementation.}
+ @end group
+ @end example
+
+ @xref{Numbers}, for more information.
+
+ @node Floating Point Type
+ @subsection Floating Point Type
+
+ Floating point numbers are the computer equivalent of scientific
+ notation. The precise number of significant figures and the range of
+ possible exponents is machine-specific; Emacs always uses the C data
+ type @code{double} to store the value.
+
+ 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 @acronym{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
+ @acronym{ASCII} codes; the rest are address@hidden
+ (@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 open-paren 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}
+ @cindex space
+ @cindex @samp{\s}
+ You can express the characters control-g, backspace, tab, newline,
+ vertical tab, formfeed, space, return, del, and escape as @samp{?\a},
+ @samp{?\b}, @samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f},
+ @samp{?\s}, @samp{?\r}, @samp{?\d}, and @samp{?\e}, respectively.
+ Thus,
+
+ @example
+ ?\a @result{} 7 ; @r{control-g, @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-[}}
+ ?\s @result{} 32 ; @r{space character, @key{SPC}}
+ ?\\ @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 terminology has nothing to do with the
+ character @key{ESC}. @samp{\s} is meant for use only in character
+ constants; in string constants, just write the space.
+
+ @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 @acronym{ASCII}; but for keyboard input purposes, you can turn
+ any character into a control character with @samp{C-}. The character
+ codes for these address@hidden 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 address@hidden
+ 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. 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 @acronym{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 @acronym{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, @acronym{ASCII} distinguishes between the characters @samp{a}
+ and @samp{A}. But @acronym{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
+ @anchor{modifier bits}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}. (Note that @samp{\s} with no following @samp{-}
+ represents the space character.)
+ @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 @acronym{ASCII}
+ character, it is preferred only when the precise octal value is more
+ important than the @acronym{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 Latin-1 character
+ @iftex
+ @address@hidden
+ @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. You can 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} or @samp{\s},
+ instead of an actual whitespace character such as a tab or a space.
+ (If you do write backslash followed by a space, you should write
+ an extra space after the character constant to separate it from the
+ following text.)
+
+ @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{_~!@@$%^&:<>@address@hidden 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 seventh 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.
+ +-*/_~!@@$%^&=:<>@address@hidden ; @r{A symbol named
@samp{+-*/_~!@@$%^&=:<>@address@hidden
+ ; @r{These characters need not be escaped.}
+ @end group
+ @end example
+
+ @ifinfo
+ @c This uses ``colon'' instead of a literal `:' because Info cannot
+ @c cope with a `:' in a menu
+ @cindex @address@hidden read syntax
+ @end ifinfo
+ @ifnotinfo
+ @cindex @samp{#:} read syntax
+ @end ifnotinfo
+ 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}. Char-tables are like vectors except that they are
+ indexed by any valid character code. The characters in a string can
+ have text properties like characters in a buffer (@pxref{Text
+ Properties}), but vectors do not support text properties, even when
+ their elements happen to be characters.
+
+ 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 address@hidden 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
+ @address@hidden }} 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 address@hidden Characters in Strings
+
+ You can include a address@hidden international character in a string
+ constant by writing it literally. There are two text representations
+ for address@hidden 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 address@hidden character with its
+ character code: use a hex escape, @address@hidden, with as many
+ digits as necessary. (Multibyte address@hidden 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 @address@hidden }} (backslash and space) to
+ terminate the hex escape---for example, @address@hidden }} represents
+ one character, @samp{a} with grave accent. @address@hidden }} 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.
+
+ You can represent a unibyte address@hidden character with its
+ character code, which must be in the range from 128 (0200 octal) to
+ 255 (0377 octal). If you write all such character codes in octal and
+ the string contains no other characters forcing it to be multibyte,
+ this produces a unibyte string. However, using any hex escape in a
+ string (even for an @acronym{ASCII} character) forces the string to be
+ multibyte.
+
+ @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 @acronym{ASCII} control characters. Strings do not
+ distinguish case in @acronym{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 @acronym{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.
+
+ @example
+ (make-bool-vector 3 t)
+ @result{} #&3"^G"
+ (make-bool-vector 3 nil)
+ @result{} #&3"^@@"
+ @end example
+
+ @noindent
+ These results make sense, because the binary code for @samp{C-g} is
+ 111 and @samp{C-@@} is the character with code 0.
+
+ If the length is not a multiple of 8, the printed representation
+ shows extra elements, but these extras really make no difference. For
+ instance, in the next example, the two bool-vectors are equal, because
+ only the first 3 bits are used:
+
+ @example
+ (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})address@hidden
+
+ 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 @address@hidden read syntax
+ @cindex @address@hidden read syntax
+
+ In Emacs 21, to represent shared or circular structures within a
+ complex of Lisp objects, you can use the reader constructs
+ @address@hidden and @address@hidden
+
+ Use @address@hidden before an object to label it for later reference;
+ subsequently, you can use @address@hidden 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 address@hidden 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 '()) ; @address@hidden()} 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. For
+ technical reasons, a unibyte string and a multibyte string are
+ @code{equal} if and only if they contain the same sequence of
+ character codes and all these codes are either in the range 0 through
+ 127 (@acronym{ASCII}) or 160 through 255 (@code{eight-bit-graphic}).
+ (@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).
+
+ @ignore
+ arch-tag: 9711a66e-4749-4265-9e8c-972d55b67096
+ @end ignore
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