@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
+@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2002, 2003,
+@c 2004, 2005, 2006 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/objects
@node Lisp Data Types, Numbers, Introduction, Top
@cindex primitive type
A few fundamental object types are built into Emacs. These, from
-which all other types are constructed, are called @dfn{primitive
-types}. Each object belongs to one and only one primitive type. These
-types include @dfn{integer}, @dfn{float}, @dfn{cons}, @dfn{symbol},
-@dfn{string}, @dfn{vector}, @dfn{subr}, @dfn{byte-code function}, and
-several special types, such as @dfn{buffer}, that are related to
-editing. (@xref{Editing Types}.)
+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.
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.
+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
* 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
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.
+that object. Every data type has a unique printed representation.
+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. This
+is not necessarily unique; many kinds of object have more than one
+syntax. @xref{Read and Print}.
@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
+ In most cases, an object's printed representation is also a read
+syntax for the object. However, some types have no read syntax, since
+it does not make sense to enter objects of these types as constants in
+a Lisp program. These objects are printed in @dfn{hash notation},
+which consists of the characters @samp{#<}, a descriptive string
+(typically the type name followed by the name of the object), and a
+closing @samp{>}. For example:
@example
(current-buffer)
@result{} #<buffer objects.texi>
@end example
+@noindent
+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
+
+ 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.
+
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,
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
* 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.
-* Symbol Type:: A multi-use object that refers to a function,
- variable, property list, or itself.
-* Lisp Function Type:: A piece of executable code you can call from elsewhere.
-* Lisp Macro Type:: A method of expanding an expression into another
+* 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.
@node Integer Type
@subsection Integer Type
- Integers were the only kind of number in Emacs version 18. The range
-of values for integers is @minus{}8388608 to 8388607 (24 bits; i.e.,
-@ifinfo
--2**23
-@end ifinfo
+ The range of values for integers in Emacs Lisp is @minus{}268435456 to
+268435455 (29 bits; i.e.,
+@ifnottex
+-2**28
+@end ifnottex
@tex
-$-2^{23}$
+@math{-2^{28}}
@end tex
to
-@ifinfo
-2**23 - 1)
-@end ifinfo
+@ifnottex
+2**28 - 1)
+@end ifnottex
@tex
-$2^{23}-1$)
+@math{2^{28}-1})
@end tex
-on most machines, but is 25 or 26 bits on some systems. It is important
-to note that the Emacs Lisp arithmetic functions do not check for
-overflow. Thus @code{(1+ 8388607)} is @minus{}8388608 on 24-bit
-implementations.@refill
+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
@group
-1 ; @r{The integer -1.}
1 ; @r{The integer 1.}
-1. ; @r{Also The integer 1.}
+1. ; @r{Also the integer 1.}
+1 ; @r{Also the integer 1.}
-16777217 ; @r{Also the integer 1!}
- ; @r{ (on a 24-bit or 25-bit implementation)}
+536870913 ; @r{Also the integer 1 on a 29-bit implementation.}
@end group
@end example
@node Floating Point Type
@subsection Floating Point Type
- Emacs version 19 supports floating point numbers (though there is a
-compilation option to disable them). The precise range of floating
-point numbers is machine-specific.
+ Floating point numbers are the computer equivalent of scientific
+notation; you can think of a floating point number as a fraction
+together with a power of ten. The precise number of significant
+figures and the range of possible exponents is machine-specific; Emacs
+uses the C data type @code{double} to store the value, and internally
+this records a power of 2 rather than a power of 10.
The printed representation for floating point numbers requires either
a decimal point (with at least one digit following), an exponent, or
@node Character Type
@subsection Character Type
-@cindex @sc{ASCII} character codes
+@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
common to work with @emph{strings}, which are sequences composed of
characters. @xref{String Type}.
- Characters in strings, buffers, and files are currently limited to the
-range of 0 to 255---eight bits. If you store a larger integer into a
-string, buffer or file, it is truncated to that range. Characters that
-represent keyboard input have a much wider range.
+ 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 non-@acronym{ASCII}
+(@pxref{Non-ASCII Characters}). Characters that represent keyboard
+input have a much wider range, to encode modifier keys such as
+Control, Meta and Shift.
@cindex read syntax for characters
@cindex printed representation for characters
@cindex syntax for characters
+@cindex @samp{?} in character constant
+@cindex question mark in character constant
Since characters are really integers, the printed representation of a
character is a decimal number. This is also a possible read syntax for
a character, but writing characters that way in Lisp programs is a very
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}.
+character @kbd{a}.
For example:
You can use the same syntax for punctuation characters, but it is
often a good idea to add a @samp{\} so that the Emacs commands for
-editing Lisp code don't get confused. For example, @samp{?\ } is the
-way to write the space character. If the character is @samp{\}, you
-@emph{must} use a second @samp{\} to quote it: @samp{?\\}.
+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{\r}
@cindex escape
@cindex @samp{\e}
- You can express the characters Control-g, backspace, tab, newline,
-vertical tab, formfeed, return, and escape as @samp{?\a}, @samp{?\b},
-@samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, @samp{?\r}, @samp{?\e},
-respectively. Those values are 7, 8, 9, 10, 11, 12, 13, and 27 in
-decimal. Thus,
+@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.
+(@samp{?\s} followed by a dash has a different meaning---it applies
+the ``super'' modifier to the following character.) Thus,
@example
-?\a @result{} 7 ; @r{@kbd{C-g}}
+?\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, @key{LFD}, @kbd{C-j}}
+?\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 usage has nothing to do with the character @key{ESC}.
+@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 in character
+constants; in string constants, just write the space.
@cindex control characters
Control characters may be represented using yet another read syntax.
?\^I @result{} 9 ?\C-I @result{} 9
@end example
- For use in strings and buffers, you are limited to the control
-characters that exist in @sc{ASCII}, but for keyboard input purposes,
-you can turn any character into a control character with @samp{C-}. The
-character codes for these non-@sc{ASCII} control characters include the
-2**22 bit as well as the code for the corresponding non-control
-character. Ordinary terminals have no way of generating non-@sc{ASCII}
-control characters, but you can generate them straightforwardly using an
-X terminal.
+ 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 non-@acronym{ASCII} control characters include the
+@tex
+@math{2^{26}}
+@end tex
+@ifnottex
+2**26
+@end ifnottex
+bit as well as the code for the corresponding non-control
+character. Ordinary terminals have no way of generating non-@acronym{ASCII}
+control characters, but you can generate them straightforwardly using X
+and other window systems.
- You can think of the @key{DEL} character as @kbd{Control-?}:
+ 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. This does not affect the meaning
-of the program, but may guide the understanding of people who read it.
+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
-2**23 bit set (which on most machines makes it a negative number). We
-use high bits for this and other modifiers to make possible a wide range
-of basic character codes.
+@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 2**7 bit indicates a meta character, so the meta
-characters that can fit in a string have codes in the range from 128 to
-255, and are the meta versions of the ordinary @sc{ASCII} characters.
-(In Emacs versions 18 and older, this convention was used for characters
-outside of strings as well.)
+ 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
or as @samp{?\M-\101}. Likewise, you can write @kbd{C-M-b} as
@samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}.
- The case of an ordinary letter is indicated by its character code as
-part of @sc{ASCII}, but @sc{ASCII} has no way to represent whether a
-control character is upper case or lower case. Emacs uses the 2**21 bit
-to indicate that the shift key was used for typing a control character.
-This distinction is possible only when you use X terminals or other
-special terminals; ordinary terminals do not indicate the distinction to
-the computer in any way.
+ 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 modifier bits that can be set
+ 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-}. Thus,
-@samp{?\H-\M-\A-x} represents @kbd{Alt-Hyper-Meta-x}. Numerically, the
-bit values are 2**18 for alt, 2**19 for super and 2**20 for hyper.
+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 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
+ Finally, the most general read syntax for a character represents the
+character code in either octal or hex. To use octal, write a question
+mark followed by a backslash and the octal character code (up to three
octal digits); thus, @samp{?\101} for the character @kbd{A},
@samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the
-character @kbd{C-b}. Although this syntax can represent any @sc{ASCII}
+character @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 @sc{ASCII} representation.
+important than the @acronym{ASCII} representation.
@example
@group
@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
+@samp{@`a}.
+@end iftex
+@ifnottex
+@samp{a} with grave accent.
+@end ifnottex
+
A backslash is allowed, and harmless, preceding any character without
a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}.
There is no reason to add a backslash before most characters. However,
you should add a backslash before any of the characters
@samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing
-Lisp code. Also add a backslash before whitespace characters such as
+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}, instead of an
-actual whitespace character such as a tab.
+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 @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 Lisp use, with one single obarray (@pxref{Creating Symbols},
+a symbol's 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
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
@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{_~!@@$%^&:<>@{@}} are
+interpretation as a symbol.) The characters @samp{_~!@@$%^&:<>@{@}?} 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
+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. This is
-in contrast to Emacs Lisp, in which upper case and lower case letters
-are distinct.
+@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 last example because the rest of the name
+This is not necessary in the fourth example because the rest of the name
makes it invalid as a number.
@example
@end group
@end example
+@ifinfo
+@c This uses ``colon'' instead of a literal `:' because Info cannot
+@c cope with a `:' in a menu
+@cindex @samp{#@var{colon}} 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
arrays. Thus, an object of type list or of type array is also
considered a sequence.
- Arrays are further subdivided into strings and vectors. Vectors can
-hold elements of any type, but string elements must be characters in the
-range from 0 to 255. However, the characters in a string can have text
-properties like characters in a buffer (@pxref{Text Properties});
-vectors do not support text properties even when their elements happen
-to be characters.
-
- Lists, strings and vectors are different, but they have important
-similarities. For example, all have a length @var{l}, and all have
-elements which can be indexed from zero to @var{l} minus one. Also,
-several functions, called sequence functions, accept any kind of
+ 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 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}.
+ 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 comprising two pointers named the
-@sc{car} and the @sc{cdr}. Each of them can point to any Lisp object.
+ 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}.
- 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.
+@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
- 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.
+ 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. The empty list is actually the symbol @code{nil}.
+@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.
@cindex atom
Because cons cells are so central to Lisp, we also have a word for
@dfn{atoms}.
@cindex parenthesis
+@cindex @samp{(@dots{})} in lists
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.
+right parenthesis. Here are examples of lists:
+
+@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
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}.
+@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}.
+ 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.
+
+@menu
+* Box Diagrams:: Drawing pictures of lists.
+* Dotted Pair Notation:: A general syntax for cons cells.
+* Association List Type:: A specially constructed list.
+@end menu
+
+@node Box Diagrams
+@subsubsection Drawing Lists as Box Diagrams
@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)}:
+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
+ --- --- --- --- --- ---
+ | | |--> | | |--> | | |--> 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 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, 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}.
+ 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
+ Here is another diagram of the same list, @code{(rose violet
buttercup)}, sketched in a different manner:
@smallexample
@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
+ --- --- --- ---
+ | | |--> | | |--> 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
+ Here is a more complex illustration, showing the three-element list,
+@code{((pine needles) oak maple)}, the first element of which is a
+two-element list:
+
+@example
+@group
+ --- --- --- --- --- ---
+ | | |--> | | |--> | | |--> nil
+ --- --- --- --- --- ---
+ | | |
+ | | |
+ | --> oak --> maple
+ |
+ | --- --- --- ---
+ --> | | |--> | | |--> nil
+ --- --- --- ---
+ | |
+ | |
+ --> pine --> needles
+@end group
+@end example
+
+ The same list represented in the second box notation looks like this:
+
+@example
+@group
+ -------------- -------------- --------------
+| car | cdr | | car | cdr | | car | cdr |
+| o | o------->| oak | o------->| maple | nil |
+| | | | | | | | | |
+ -- | --------- -------------- --------------
+ |
+ |
+ | -------------- ----------------
+ | | car | cdr | | car | cdr |
+ ------>| pine | o------->| needles | nil |
+ | | | | | |
+ -------------- ----------------
+@end group
+@end example
@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,
+ @dfn{Dotted pair notation} is a general 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)}:
+the object @var{a} and whose @sc{cdr} is the object @var{b}. Dotted
+pair notation is more general than list syntax because the @sc{cdr}
+does not have to be a list. However, it is more cumbersome in cases
+where list syntax would work. In 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
+ --- ---
+ | | |--> 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:
+ You can combine dotted pair notation with list notation to represent
+conveniently a chain of cons cells with a non-@code{nil} final @sc{cdr}.
+You write a dot after the last element of the list, followed by the
+@sc{cdr} of the final cons cell. For example, @code{(rose violet
+. buttercup)} is equivalent to @code{(rose . (violet . buttercup))}.
+The object looks like this:
@example
@group
- ___ ___ ___ ___
- |___|___|--> |___|___|--> buttercup
+ --- --- --- ---
+ | | |--> | | |--> 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 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))}
+ The list @code{(rose violet)} is equivalent to @code{(rose . (violet))},
and looks like this:
@example
@group
- ___ ___ ___ ___
- |___|___|--> |___|___|--> nil
+ --- --- --- ---
+ | | |--> | | |--> nil
+ --- --- --- ---
| |
| |
--> rose --> violet
Similarly, the three-element list @code{(rose violet buttercup)}
is equivalent to @code{(rose . (violet . (buttercup)))}.
-@ifinfo
+@ifnottex
It looks like this:
@example
@group
- ___ ___ ___ ___ ___ ___
- |___|___|--> |___|___|--> |___|___|--> nil
+ --- --- --- --- --- ---
+ | | |--> | | |--> | | |--> nil
+ --- --- --- --- --- ---
| | |
| | |
--> rose --> violet --> buttercup
@end group
@end example
-@end ifinfo
+@end ifnottex
@node Association List Type
@comment node-name, next, previous, up
@example
(setq alist-of-colors
- '((rose . red) (lily . white) (buttercup . yellow)))
+ '((rose . red) (lily . white) (buttercup . yellow)))
@end example
@noindent
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.
+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
-referring to other Lisp objects, arranged in a contiguous block of
-memory. Accessing any element of an array takes the same amount of
-time. In contrast, accessing an element of a list requires time
-proportional to the position of the element in the list. (Elements at
-the end of a list take longer to access than elements at the beginning
-of a list.)
-
- Emacs defines two types of array, strings and vectors. A string is an
-array of characters and a vector is an array of arbitrary objects. Both
-are one-dimensional. (Most other programming languages support
-multidimensional arrays, but they are not essential; you can get the
-same effect with an array of arrays.) Each type of array has its own
-read syntax; see @ref{String Type}, and @ref{Vector Type}.
-
- An array may have any length up to the largest integer; but once
-created, it has a fixed size. The first element of an array has index
-zero, the second element has index 1, and so on. This is called
-@dfn{zero-origin} indexing. For example, an array of four elements has
-indices 0, 1, 2, @w{and 3}.
-
- The array type is contained in the sequence type and contains both the
-string type and the vector type.
+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 nested one-dimensional arrays.) Each
+type of array has its own read syntax; see the following sections for
+details.
+
+ The array type is a subset of 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
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"}. The Lisp
-reader accepts the same formats for reading the characters of a string
-as it does for reading single characters (without the question mark that
-begins a character literal). You can enter a nonprinting character such
-as tab, @kbd{C-a} or @kbd{M-C-A} using the convenient escape sequences,
-like this: @code{"\t, \C-a, \M-\C-a"}. You can include a double-quote
-in a string by preceding it with a backslash; thus, @code{"\""} is a
-string containing just a single double-quote character.
-(@xref{Character Type}, for a description of the read syntax for
-characters.)
-
- If you use the @samp{\M-} syntax to indicate a meta character in a
-string constant, this sets the 2**7 bit of the character in the string.
-This is not the same representation that the meta modifier has in a
-character on its own (not inside a string). @xref{Character Type}.
-
- Strings cannot hold characters that have the hyper, super, or alt
-modifiers; they can hold @sc{ASCII} control characters, but no others.
-They do not distinguish case in @sc{ASCII} control characters.
-
- The printed representation of a string consists of a double-quote, the
-characters it contains, and another double-quote. However, you must
-escape any backslash or double-quote characters in the string with a
-backslash, like this: @code{"this \" is an embedded quote"}.
+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.
-@cindex newline in strings
+ignores an escaped newline while reading a string. An escaped space
+@w{@samp{\ }} is likewise ignored.
@example
"It is useful to include newlines
in documentation strings,
but the newline is \
ignored if escaped."
- @result{} "It is useful to include newlines
-in documentation strings,
+ @result{} "It is useful to include newlines
+in documentation strings,
but the newline is ignored if escaped."
@end example
- A string can hold properties of the text it contains, in addition to
-the characters themselves. This enables programs that copy text between
-strings and buffers to preserve the properties with no special effort.
-@xref{Text Properties}. Strings with text properties have a special
-read and print syntax:
+@node Non-ASCII in Strings
+@subsubsection Non-@acronym{ASCII} Characters in Strings
+
+ You can include a non-@acronym{ASCII} international character in a string
+constant by writing it literally. There are two text representations
+for non-@acronym{ASCII} characters in Emacs strings (and in buffers): unibyte
+and multibyte. If the string constant is read from a multibyte source,
+such as a multibyte buffer or string, or a file that would be visited as
+multibyte, then the character is read as a multibyte character, and that
+makes the string multibyte. If the string constant is read from a
+unibyte source, then the character is read as unibyte and that makes the
+string unibyte.
+
+ You can also represent a multibyte non-@acronym{ASCII} character with its
+character code: use a hex escape, @samp{\x@var{nnnnnnn}}, with as many
+digits as necessary. (Multibyte non-@acronym{ASCII} character codes are all
+greater than 256.) Any character which is not a valid hex digit
+terminates this construct. If the next character in the string could be
+interpreted as a hex digit, write @w{@samp{\ }} (backslash and space) to
+terminate the hex escape---for example, @w{@samp{\x8e0\ }} represents
+one character, @samp{a} with grave accent. @w{@samp{\ }} in a string
+constant is just like backslash-newline; it does not contribute any
+character to the string, but it does terminate the preceding hex escape.
+
+ You can represent a unibyte non-@acronym{ASCII} 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}...)
@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.
+that range. For example,
+
+@example
+#("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))
+@end example
- @xref{Strings and Characters}, for functions that work on strings.
+@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
@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 have no read syntax, and print using hash notation.
+@xref{Hash Tables}, for functions that operate on hash tables.
+
+@example
+(make-hash-table)
+ @result{} #<hash-table 'eql nil 0/65 0x83af980>
+@end example
+
@node Function Type
@subsection Function 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
+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.
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
(@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.
+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
function to be called. @xref{Byte Compilation}, for information about
the byte compiler.
-The printed representation for a byte-code function object is like that
-for a vector, with an additional @samp{#} before the opening @samp{[}.
+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 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.
+@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
@section Editing Types
@cindex editing types
- The types in the previous section are common to many Lisp dialects.
-Emacs Lisp provides several additional data types for purposes connected
-with editing.
+ 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.
* 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.
-* Syntax Table Type:: What a character means.
-* Display Table Type:: How display tables are represented.
* Overlay Type:: How an overlay is represented.
@end menu
The contents of a buffer are much like a string, but buffers are not
used like strings in Emacs Lisp, and the available operations are
-different. For example, insertion of text into a buffer is very
-efficient, whereas ``inserting'' text into a string requires
-concatenating substrings, and the result is an entirely new string
-object.
+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
a local keymap (@pxref{Keymaps}); and,
@item
-a local variable binding list (@pxref{Buffer-Local Variables}).
+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
behavior of programs in different buffers, without actually changing the
programs.
- Buffers have no read syntax. They print in hash notation with the
+ 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
@node Frame Type
@subsection Frame Type
- A @var{frame} is a rectangle on the screen that contains one or more
+ 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.
@example
@group
(selected-frame)
- @result{} #<frame xemacs@@mole.gnu.ai.mit.edu 0xdac80>
+ @result{} #<frame emacs@@psilocin.gnu.org 0xdac80>
@end group
@end example
sizes, and contents of the windows in a frame, so you can recreate the
same arrangement of windows later.
- Window configurations do not have a read syntax. They print as
-@samp{#<window-configuration>}. @xref{Window Configurations}, for a
-description of several functions related to window configurations.
+ 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
Streams have no special printed representation or read syntax, and
print as whatever primitive type they are.
- @xref{Streams, Reading and Printing}, for a description of functions
+ @xref{Read and Print}, for a description of functions
related to streams, including parsing and printing functions.
@node Keymap Type
@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
+@node Overlay Type
+@subsection Overlay Type
- A @dfn{syntax table} is a vector of 256 integers. Each element of the
-vector defines how one character is interpreted when it appears in a
-buffer. For example, in C mode (@pxref{Major Modes}), the @samp{+}
-character is punctuation, but in Lisp mode it is a valid character in a
-symbol. These modes specify different interpretations by changing the
-syntax table entry for @samp{+}, at index 43 in the syntax table.
+ 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.
- Syntax tables are used only for scanning text in buffers, not for
-reading Lisp expressions. The table the Lisp interpreter uses to read
-expressions is built into the Emacs source code and cannot be changed;
-thus, to change the list delimiters to be @samp{@{} and @samp{@}}
-instead of @samp{(} and @samp{)} would be impossible.
+ @xref{Overlays}, for how to create and use overlays.
- @xref{Syntax Tables}, for details about syntax classes and how to make
-and modify syntax tables.
+@node Circular Objects
+@section Read Syntax for Circular Objects
+@cindex circular structure, read syntax
+@cindex shared structure, read syntax
+@cindex @samp{#@var{n}=} read syntax
+@cindex @samp{#@var{n}#} read syntax
-@node Display Table Type
-@subsection Display Table Type
+ To represent shared or circular structures within a complex of Lisp
+objects, you can use the reader constructs @samp{#@var{n}=} and
+@samp{#@var{n}#}.
- A @dfn{display table} specifies how to display each character code.
-Each buffer and each window can have its own display table. A display
-table is actually a vector of length 261. @xref{Display Tables}.
+ Use @code{#@var{n}=} before an object to label it for later reference;
+subsequently, you can use @code{#@var{n}#} to refer the same object in
+another place. Here, @var{n} is some integer. For example, here is how
+to make a list in which the first element recurs as the third element:
-@node Overlay Type
-@subsection Overlay Type
+@example
+(#1=(a) b #1#)
+@end example
- An @dfn{overlay} specifies temporary alteration of the display
-appearance of a part of a buffer. It contains markers delimiting a
-range of the buffer, plus a property list (a list whose elements are
-alternating property names and values). Overlays are used to present
-parts of the buffer temporarily in a different display style.
+@noindent
+This differs from ordinary syntax such as this
- @xref{Overlays}, for how to create and use overlays. They have no
-read syntax, and print in hash notation, giving the buffer name and
-range of positions.
+@example
+((a) b (a))
+@end example
+
+@noindent
+which would result in a list whose first and third elements
+look alike but are not the same Lisp object. This shows the difference:
+
+@example
+(prog1 nil
+ (setq x '(#1=(a) b #1#)))
+(eq (nth 0 x) (nth 2 x))
+ @result{} t
+(setq x '((a) b (a)))
+(eq (nth 0 x) (nth 2 x))
+ @result{} nil
+@end example
+
+ You can also use the same syntax to make a circular structure, which
+appears as an ``element'' within itself. Here is an example:
+
+@example
+#1=(a #1#)
+@end example
+
+@noindent
+This makes a list whose second element is the list itself.
+Here's how you can see that it really works:
+
+@example
+(prog1 nil
+ (setq x '#1=(a #1#)))
+(eq x (cadr x))
+ @result{} t
+@end example
+
+ The Lisp printer can produce this syntax to record circular and shared
+structure in a Lisp object, if you bind the variable @code{print-circle}
+to a non-@code{nil} value. @xref{Output Variables}.
@node Type Predicates
@section Type Predicates
-@cindex predicates
@cindex type checking
@kindex wrong-type-argument
@example
@group
(+ 2 'a)
- @error{} Wrong type argument: integer-or-marker-p, a
+ @error{} Wrong type argument: number-or-marker-p, a
@end group
@end example
@cindex type predicates
@cindex testing types
- Lisp provides functions, called @dfn{type predicates}, to test whether
-an object is a member of a given type. (Following a convention of long
-standing, the names of most Emacs Lisp predicates end in @samp{p}.)
+ 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,
+ Here is a table of predefined type predicates, in alphabetical order,
with references to further information.
@table @code
@item arrayp
@xref{Array Functions, arrayp}.
+@item bool-vector-p
+@xref{Bool-Vectors, bool-vector-p}.
+
@item bufferp
@xref{Buffer Basics, bufferp}.
@xref{Byte-Code Type, byte-code-function-p}.
@item case-table-p
-@xref{Case Table, 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 hash-table-p
+@xref{Other Hash, hash-table-p}.
+
@item integer-or-marker-p
@xref{Predicates on Markers, integer-or-marker-p}.
@item keymapp
@xref{Creating Keymaps, keymapp}.
+@item keywordp
+@xref{Constant Variables}.
+
@item listp
@xref{List-related Predicates, listp}.
@item windowp
@xref{Basic Windows, windowp}.
+
+@item booleanp
+@xref{nil and t, booleanp}.
+
+@item string-or-null-p
+@xref{Predicates for Strings, string-or-null-p}.
@end table
+ The most general way to check the type of an object is to call the
+function @code{type-of}. Recall that each object belongs to one and
+only one primitive type; @code{type-of} tells you which one (@pxref{Lisp
+Data Types}). But @code{type-of} knows nothing about non-primitive
+types. In most cases, it is more convenient to use type predicates than
+@code{type-of}.
+
+@defun type-of object
+This function returns a symbol naming the primitive type of
+@var{object}. The value is one of the symbols @code{symbol},
+@code{integer}, @code{float}, @code{string}, @code{cons}, @code{vector},
+@code{char-table}, @code{bool-vector}, @code{hash-table}, @code{subr},
+@code{compiled-function}, @code{marker}, @code{overlay}, @code{window},
+@code{buffer}, @code{frame}, @code{process}, or
+@code{window-configuration}.
+
+@example
+(type-of 1)
+ @result{} integer
+(type-of 'nil)
+ @result{} symbol
+(type-of '()) ; @r{@code{()} is @code{nil}.}
+ @result{} symbol
+(type-of '(x))
+ @result{} cons
+@end example
+@end defun
+
@node Equality Predicates
@section Equality Predicates
@cindex equality
@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.
+the same object, @code{nil} otherwise.
@code{eq} returns @code{t} if @var{object1} and @var{object2} are
integers with the same value. Also, since symbol names are normally
@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}.)
+object, meaning that a change in the contents of one will be reflected
+by the same change in the contents of the other.
@example
@group
@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 are the same. So, if two objects are
-@code{eq}, they are @code{equal}, but the converse is not always true.
+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
@end group
@end example
-Comparison of strings uses @code{string=}, and is case-sensitive.
+@cindex equality of strings
+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
@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, and circular lists may
-therefore cause infinite recursion (leading to an error).
+ 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