X-Git-Url: https://code.delx.au/gnu-emacs/blobdiff_plain/a9f0a989a17f47f9d25b7a426b4e82a8ff684ee4..7d90e71da10a760e269c24c1e789fe50dc60e279:/lispref/objects.texi diff --git a/lispref/objects.texi b/lispref/objects.texi index f2c082b56b..4a693f186d 100644 --- a/lispref/objects.texi +++ b/lispref/objects.texi @@ -1,6 +1,7 @@ @c -*-texinfo-*- @c This is part of the GNU Emacs Lisp Reference Manual. -@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998 Free Software Foundation, Inc. +@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 @@ -22,12 +23,12 @@ but not for ``the'' type of an object. @cindex primitive type A few fundamental object types are built into Emacs. These, from -which all other types are constructed, are called @dfn{primitive -types}. Each object belongs to one and only one primitive type. These -types include @dfn{integer}, @dfn{float}, @dfn{cons}, @dfn{symbol}, -@dfn{string}, @dfn{vector}, @dfn{subr}, @dfn{byte-code function}, plus -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. @@ -41,7 +42,9 @@ it as a number; Lisp knows it is a vector, not a number. 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 @@ -52,6 +55,7 @@ to use these types can be found in later chapters. * 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 @@ -64,36 +68,37 @@ to use these types can be found in later chapters. 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. +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---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 + 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}: 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{>}. For example: @example (current-buffer) @result{} # @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, @@ -146,6 +151,7 @@ latter are unique to Emacs Lisp. * 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. @@ -158,24 +164,24 @@ latter are unique to Emacs Lisp. @node Integer Type @subsection Integer Type - The range of values for integers in Emacs Lisp is @minus{}134217728 to -134217727 (28 bits; i.e., -@ifinfo --2**27 -@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 -$-2^{27}$ +@math{-2^{28}} @end tex to -@ifinfo -2**27 - 1) -@end ifinfo +@ifnottex +2**28 - 1) +@end ifnottex @tex -$2^{28}-1$) +@math{2^{28}-1}) @end tex on most machines. (Some machines may provide a wider range.) It is important to note that the Emacs Lisp arithmetic functions do not check -for overflow. Thus @code{(1+ 134217727)} is @minus{}134217728 on most +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 @@ -187,9 +193,9 @@ leading @samp{+} or a final @samp{.}. @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.} -268435457 ; @r{Also the integer 1 on a 28-bit implementation.} +536870913 ; @r{Also the integer 1 on a 29-bit implementation.} @end group @end example @@ -198,9 +204,12 @@ leading @samp{+} or a final @samp{.}. @node Floating Point Type @subsection Floating Point Type - Emacs supports floating point numbers (though there is a compilation -option to disable them). The precise range of floating point numbers is -machine-specific. + 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 @@ -212,7 +221,7 @@ number whose value is 1500. They are all equivalent. @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 @@ -222,16 +231,19 @@ example, the character @kbd{A} is represented as the @w{integer 65}. 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 ASCII codes; the -rest are non-ASCII (@pxref{Non-ASCII Characters}). Characters that represent -keyboard input have a much wider range, to encode modifier keys such as + 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 @@ -242,7 +254,7 @@ 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}. +character @kbd{a}. For example: @@ -252,9 +264,9 @@ character @kbd{a}. 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 @@ -273,13 +285,16 @@ way to write the space character. If the character is @samp{\}, you @cindex @samp{\r} @cindex escape @cindex @samp{\e} - You can express the characters Control-g, backspace, tab, newline, -vertical tab, formfeed, return, and escape as @samp{?\a}, @samp{?\b}, -@samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, @samp{?\r}, @samp{?\e}, -respectively. Thus, +@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{@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, @kbd{C-j}} @@ -287,13 +302,17 @@ respectively. Thus, ?\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 only in character +constants; in string constants, just write the space. @cindex control characters Control characters may be represented using yet another read syntax. @@ -310,19 +329,19 @@ equivalent to @samp{?\^I} and to @samp{?\^i}: @end example In strings and buffers, the only control characters allowed are those -that exist in @sc{ASCII}; but for keyboard input purposes, you can turn +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-@sc{ASCII} control characters include the +codes for these non-@acronym{ASCII} control characters include the @tex -$2^{26}$ +@math{2^{26}} @end tex -@ifinfo +@ifnottex 2**26 -@end ifinfo +@end ifnottex bit as well as the code for the corresponding non-control -character. Ordinary terminals have no way of generating non-@sc{ASCII} -control characters, but you can generate them straightforwardly using an -X terminal. +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. For historical reasons, Emacs treats the @key{DEL} character as the control equivalent of @kbd{?}: @@ -347,27 +366,26 @@ people who read it. A @dfn{meta character} is a character typed with the @key{META} modifier key. The integer that represents such a character has the @tex -$2^{27}$ +@math{2^{27}} @end tex -@ifinfo +@ifnottex 2**27 -@end ifinfo -bit set (which on most machines makes it a negative number). We -use high bits for this and other modifiers to make possible a wide range -of basic character codes. +@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 -$2^{7}$ +@math{2^{7}} @end tex -@ifinfo +@ifnottex 2**7 -@end ifinfo -bit attached to an ASCII character indicates a meta character; thus, the -meta characters that can fit in a string have codes in the range from -128 to 255, and are the meta versions of the ordinary @sc{ASCII} -characters. (In Emacs versions 18 and older, this convention was used -for characters outside of strings as well.) +@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 @@ -377,39 +395,41 @@ or as @samp{?\M-\101}. Likewise, you can write @kbd{C-M-b} as @samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}. The case of a graphic character is indicated by its character code; -for example, @sc{ASCII} distinguishes between the characters @samp{a} -and @samp{A}. But @sc{ASCII} has no way to represent whether a control +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 -$2^{25}$ +@math{2^{25}} @end tex -@ifinfo +@ifnottex 2**25 -@end ifinfo +@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. +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-}. (Case is significant in these prefixes.) Thus, @samp{?\H-\M-\A-x} represents -@kbd{Alt-Hyper-Meta-x}. +@kbd{Alt-Hyper-Meta-x}. (Note that @samp{\s} with no following @samp{-} +represents the space character.) @tex -Numerically, the -bit values are $2^{22}$ for alt, $2^{23}$ for super and $2^{24}$ for hyper. +Numerically, the bit values are @math{2^{22}} for alt, @math{2^{23}} +for super and @math{2^{24}} for hyper. @end tex -@ifinfo +@ifnottex Numerically, the bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper. -@end ifinfo +@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 @@ -418,9 +438,9 @@ 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 @@ -433,30 +453,34 @@ important than the @sc{ASCII} representation. 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{?\x8c0} for the character +character @kbd{C-a}, and @code{?\x8e0} for the Latin-1 character @iftex @samp{@`a}. @end iftex -@ifinfo +@ifnottex @samp{a} with grave accent. -@end ifinfo +@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 @@ -465,6 +489,11 @@ 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 @@ -472,7 +501,7 @@ are written with letters, digits, and the punctuation characters @samp{-+=*/}. Such names require no special punctuation; the characters of the name suffice as long as the name does not look like a number. (If it does, write a @samp{\} at the beginning of the name to force -interpretation as a symbol.) The characters @samp{_~!@@$%^&:<>@{@}} are +interpretation as a symbol.) The characters @samp{_~!@@$%^&:<>@{@}?} 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 @@ -492,7 +521,7 @@ Lisp, upper case and lower case letters are distinct. Here are several examples of symbol names. Note that the @samp{+} in the fifth example is escaped to prevent it from being read as a number. -This is not necessary in the sixth example because the rest of the name +This is not necessary in the fourth example because the rest of the name makes it invalid as a number. @example @@ -518,6 +547,18 @@ char-to-string ; @r{A symbol named @samp{char-to-string}.} @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 @@ -529,11 +570,11 @@ considered a sequence. Arrays are further subdivided into strings, vectors, char-tables and bool-vectors. Vectors can hold elements of any type, but string elements must be characters, and bool-vector elements must be @code{t} -or @code{nil}. The characters in a string can have text properties like -characters in a buffer (@pxref{Text Properties}); vectors and -bool-vectors do not support text properties even when their elements -happen to be characters. Char-tables are like vectors except that they -are indexed by any valid character code. +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 @@ -553,24 +594,26 @@ same object, @code{nil}. @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 @@ -580,22 +623,52 @@ for its purpose, the construction of cells. @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. +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 @@ -608,18 +681,19 @@ 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 @@ -639,19 +713,6 @@ buttercup)}, sketched in a different manner: 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: @@ -666,30 +727,67 @@ depicted with boxes and arrows: @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 first 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)}: +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 @@ -702,10 +800,12 @@ shows the pair @code{(rose . violet)}: @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 @@ -718,11 +818,12 @@ chain of cons cells with a non-@code{nil} final @sc{cdr}. For example, @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 @@ -738,7 +839,7 @@ and looks like this: Similarly, the three-element list @code{(rose violet buttercup)} is equivalent to @code{(rose . (violet . (buttercup)))}. -@ifinfo +@ifnottex It looks like this: @example @@ -751,7 +852,7 @@ It looks like this: --> rose --> violet --> buttercup @end group @end example -@end ifinfo +@end ifnottex @node Association List Type @comment node-name, next, previous, up @@ -769,7 +870,7 @@ the list. @example (setq alist-of-colors - '((rose . red) (lily . white) (buttercup . yellow))) + '((rose . red) (lily . white) (buttercup . yellow))) @end example @noindent @@ -777,13 +878,14 @@ 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. +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 +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 @@ -807,8 +909,9 @@ 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. +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 contained in the sequence type and contains the string type, the vector type, the bool-vector type, and the @@ -859,17 +962,17 @@ ignores an escaped newline while reading a string. An escaped space 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 @node Non-ASCII in Strings -@subsubsection Non-ASCII Characters in Strings +@subsubsection Non-@acronym{ASCII} Characters in Strings - You can include a non-@sc{ASCII} international character in a string + You can include a non-@acronym{ASCII} international character in a string constant by writing it literally. There are two text representations -for non-@sc{ASCII} characters in Emacs strings (and in buffers): unibyte +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 @@ -877,23 +980,25 @@ 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. -@c ??? Change this? - You can also represent a multibyte non-@sc{ASCII} character with its -character code, using a hex escape, @samp{\x@var{nnnnnnn}}, with as many -digits as necessary. (Multibyte non-@sc{ASCII} character codes are all + 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 character that would follow is a hex -digit, write @w{@samp{\ }} to terminate the hex escape---for example, -@w{@samp{\x8c0\ }} represents one character, @samp{a} with grave accent. -@w{@samp{\ }} in a string constant is just like backslash-newline; it does -not contribute any character to the string, but it does terminate the -preceding hex escape. - - Using a multibyte hex escape forces the string to multibyte. You can -represent a unibyte non-@sc{ASCII} character with its character code, -which must be in the range from 128 (0200 octal) to 255 (0377 octal). -This forces a unibyte string. - +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. @@ -909,20 +1014,20 @@ description of the read syntax for characters. However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that -a string can hold are the @sc{ASCII} control characters. Strings do not -distinguish case in @sc{ASCII} control characters. +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 allows the meta versions of @sc{ASCII} characters to be put in a -string. If you use the @samp{\M-} syntax to indicate a meta character -in a string constant, this sets the +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 -$2^{7}$ +@math{2^{7}} @end tex -@ifinfo +@ifnottex 2**7 -@end ifinfo +@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}. @@ -965,7 +1070,7 @@ that range. For example, 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 +@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.) @@ -1015,7 +1120,7 @@ Case tables (@pxref{Case Tables}). Character category tables (@pxref{Categories}). @item -Display Tables (@pxref{Display Tables}). +Display tables (@pxref{Display Tables}). @item Syntax tables (@pxref{Syntax Tables}). @@ -1027,26 +1132,48 @@ Syntax tables (@pxref{Syntax Tables}). 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 + 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 are -the lowest-numbered elements of the bool-vector. If the length is not a -multiple of 8, the printed representation shows extra elements, but -these extras really make no difference. +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"\007" + @result{} #&3"^G" (make-bool-vector 3 nil) - @result{} #&3"\0" -;; @r{These are equal since only the first 3 bits are used.} + @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}. + +@example +(make-hash-table) + @result{} # +@end example + @node Function Type @subsection Function Type @@ -1140,11 +1267,11 @@ opening @samp{[}. @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 @@ -1191,9 +1318,9 @@ 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, whereas ``inserting'' text into a string requires -concatenating substrings, and the result is an entirely new string -object. +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 @@ -1415,6 +1542,69 @@ positions. @xref{Overlays}, for how to create and use overlays. +@node Circular Objects +@section Read Syntax for Circular Objects +@cindex circular structure, read syntax +@cindex shared structure, read syntax +@cindex @samp{#@var{n}=} read syntax +@cindex @samp{#@var{n}#} read syntax + + 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}#}. + + Use @code{#@var{n}=} before an object to label it for later reference; +subsequently, you can use @code{#@var{n}#} to refer the same object in +another place. Here, @var{n} is some integer. For example, here is how +to make a list in which the first element recurs as the third element: + +@example +(#1=(a) b #1#) +@end example + +@noindent +This differs from ordinary syntax such as this + +@example +((a) b (a)) +@end example + +@noindent +which would result in a list whose first and third elements +look alike but are not the same Lisp object. This shows the difference: + +@example +(prog1 nil + (setq x '(#1=(a) b #1#))) +(eq (nth 0 x) (nth 2 x)) + @result{} t +(setq x '((a) b (a))) +(eq (nth 0 x) (nth 2 x)) + @result{} nil +@end example + + You can also use the same syntax to make a circular structure, which +appears as an ``element'' within itself. Here is an example: + +@example +#1=(a #1#) +@end example + +@noindent +This makes a list whose second element is the list itself. +Here's how you can see that it really works: + +@example +(prog1 nil + (setq x '#1=(a #1#))) +(eq x (cadr x)) + @result{} t +@end example + + The Lisp printer can produce this syntax to record circular and shared +structure in a Lisp object, if you bind the variable @code{print-circle} +to a non-@code{nil} value. @xref{Output Variables}. + @node Type Predicates @section Type Predicates @cindex predicates @@ -1464,7 +1654,6 @@ a list and @code{symbolp} to check for a symbol. ((listp x) ;; If X is a list, add its elements to LIST. (setq list (append x list))) -@need 3000 (t ;; We handle only symbols and lists. (error "Invalid argument %s in add-on" x)))) @@ -1522,6 +1711,9 @@ with references to further information. @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}. @@ -1531,6 +1723,9 @@ with references to further information. @item keymapp @xref{Creating Keymaps, keymapp}. +@item keywordp +@xref{Constant Variables}. + @item listp @xref{List-related Predicates, listp}. @@ -1597,7 +1792,7 @@ types. In most cases, it is more convenient to use type predicates than 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{subr}, +@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}. @@ -1625,8 +1820,7 @@ 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. +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 @@ -1634,7 +1828,8 @@ 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. +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 @@ -1694,8 +1889,9 @@ Symbols}. 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 @@ -1747,9 +1943,12 @@ arguments to see if their elements are the same. So, if two objects are @end example Comparison of strings is case-sensitive, but does not take account of -text properties---it compares only the characters in the strings. -A unibyte string never equals a multibyte string unless the -contents are entirely @sc{ASCII} (@pxref{Text Representations}). +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 @@ -1758,9 +1957,23 @@ contents are entirely @sc{ASCII} (@pxref{Text Representations}). @end group @end example -Two distinct buffers are never @code{equal}, even if their contents -are the same. +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