X-Git-Url: http://git.hcoop.net/bpt/emacs.git/blobdiff_plain/0fddfa7210f92c4c6bc3ff25ef44a894a54b29d2..e439b9255d56a820723e10fdf9131da9afee34bb:/lispref/objects.texi diff --git a/lispref/objects.texi b/lispref/objects.texi index 5d9762ee26..7c8eff0629 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 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}, 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. @@ -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 @@ -66,9 +70,12 @@ to use these types can be found in later chapters. 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. +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 @@ -78,14 +85,12 @@ mind, or you will occasionally be very confused. @cindex hash notation Every type has a printed representation. Some types have no read -syntax, since it may not make sense to enter objects of these types -directly in a Lisp program. For example, the buffer type does not have -a read syntax. Objects of these types are printed in @dfn{hash -notation}: the characters @samp{#<} followed by a descriptive string -(typically the type name followed by the name of the object), and closed -with a matching @samp{>}. Hash notation cannot be read at all, so the -Lisp reader signals the error @code{invalid-read-syntax} whenever it -encounters @samp{#<}. +syntax---for example, the buffer type has none. Objects of these types +are printed in @dfn{hash notation}: the characters @samp{#<} followed by +a descriptive string (typically the type name followed by the name of +the object), and closed with a matching @samp{>}. Hash notation cannot +be read at all, so the Lisp reader signals the error +@code{invalid-read-syntax} whenever it encounters @samp{#<}. @kindex invalid-read-syntax @example @@ -115,6 +120,11 @@ the end of line. The Lisp reader discards comments; they do not become part of the Lisp objects which represent the program within the Lisp system. + The @samp{#@@@var{count}} construct, which skips the next @var{count} +characters, is useful for program-generated comments containing binary +data. The Emacs Lisp byte compiler uses this in its output files +(@pxref{Byte Compilation}). It isn't meant for source files, however. + @xref{Comment Tips}, for conventions for formatting comments. @node Programming Types @@ -138,6 +148,9 @@ latter are unique to Emacs Lisp. * Array Type:: Arrays include strings and vectors. * String Type:: An (efficient) array of characters. * Vector Type:: One-dimensional arrays. +* Char-Table Type:: One-dimensional sparse arrays indexed by characters. +* Bool-Vector Type:: One-dimensional arrays of @code{t} or @code{nil}. +* Hash Table Type:: Super-fast lookup tables. * Function Type:: A piece of executable code you can call from elsewhere. * Macro Type:: A method of expanding an expression into another expression, more fundamental but less pretty. @@ -150,24 +163,24 @@ 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 @@ -179,10 +192,9 @@ 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!} - ; @r{ (on a 28-bit implementation)} +536870913 ; @r{Also the integer 1 on a 29-bit implementation.} @end group @end example @@ -191,9 +203,10 @@ leading @samp{+} or a final @samp{.}. @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. The precise number of significant figures and the range of +possible exponents is machine-specific; Emacs always uses the C data +type @code{double} to store the value. The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or @@ -205,7 +218,7 @@ 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 @@ -215,14 +228,19 @@ 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 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 @@ -233,7 +251,7 @@ 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: @@ -243,9 +261,9 @@ 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 @@ -264,28 +282,34 @@ 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. 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. +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 only in character +constants; in string constants, just write the space. @cindex control characters Control characters may be represented using yet another read syntax. @@ -301,38 +325,64 @@ equivalent to @samp{?\^I} and to @samp{?\^i}: ?\^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 @@ -341,35 +391,53 @@ syntax for a character. Thus, you can write @kbd{M-A} as @samp{?\M-A}, or as @samp{?\M-\101}. Likewise, you can write @kbd{C-M-b} as @samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}. - The case of an ordinary letter is indicated by its character code as -part of @sc{ASCII}, but @sc{ASCII} has no way to represent whether a -control character is upper case or lower case. Emacs uses the 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 @@ -378,15 +446,30 @@ important than the @sc{ASCII} representation. @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 @@ -402,6 +485,11 @@ 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 @@ -409,28 +497,27 @@ 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 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. +``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 seventh example because the rest of the name makes it invalid as a number. @example @@ -456,6 +543,18 @@ 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 @@ -464,48 +563,60 @@ elements. There are two kinds of sequence in Emacs Lisp, lists and arrays. Thus, an object of type list or of type array is also considered a sequence. - Arrays are further subdivided into strings 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}. + +@quotation +A note to C programmers: in Lisp, we do not distinguish between +``holding'' a value and ``pointing to'' the value, because pointers in +Lisp are implicit. +@end quotation A @dfn{list} is a series of cons cells, linked together so that the -@sc{cdr} of each cons cell points either to another cons cell or to the +@sc{cdr} slot of each cons cell holds either the next cons cell or the empty list. @xref{Lists}, for functions that work on lists. Because most cons cells are used as part of lists, the phrase @dfn{list structure} has come to refer to any structure made out of cons cells. - The names @sc{car} and @sc{cdr} have only historical meaning now. The + 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 is named +for the function @code{cons} that creates them, which in turn was named for its purpose, the construction of cells. @cindex atom @@ -520,41 +631,44 @@ right parenthesis. Upon reading, each object inside the parentheses becomes an element of the list. That is, a cons cell is made for each element. The -@sc{car} of the cons cell points to the element, and its @sc{cdr} points -to the next cons cell of the list, which holds the next element in the -list. The @sc{cdr} of the last cons cell is set to point to @code{nil}. +@sc{car} slot of the cons cell holds the element, and its @sc{cdr} +slot refers to the next cons cell of the list, which holds the next +element in the list. The @sc{cdr} slot of the last cons cell is set to +hold @code{nil}. @cindex box diagrams, for lists @cindex diagrams, boxed, for lists A list can be illustrated by a diagram in which the cons cells are -shown as pairs of boxes. (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 @@ -592,8 +706,9 @@ depicted with boxes and arrows: @example @group - ___ ___ ___ ___ - |___|___|--> |___|___|--> nil + --- --- --- --- + | | |--> | | |--> nil + --- --- --- --- | | | | --> A --> nil @@ -616,51 +731,57 @@ 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. +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 how box notation can illustrate dotted pairs. This example -shows the pair @code{(rose . violet)}: + 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 @@ -669,19 +790,20 @@ 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 @group - ___ ___ ___ ___ ___ ___ - |___|___|--> |___|___|--> |___|___|--> nil + --- --- --- --- --- --- + | | |--> | | |--> | | |--> nil + --- --- --- --- --- --- | | | | | | --> rose --> violet --> buttercup @end group @end example -@end ifinfo +@end ifnottex @node Association List Type @comment node-name, next, previous, up @@ -699,7 +821,7 @@ the list. @example (setq alist-of-colors - '((rose . red) (lily . white) (buttercup . yellow))) + '((rose . red) (lily . white) (buttercup . yellow))) @end example @noindent @@ -707,34 +829,43 @@ 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 -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 an array of arrays.) Each type of +array has its own read syntax; see the following sections for details. + + The array type is contained in the sequence type and +contains the string type, the vector type, the bool-vector type, and the +char-table type. @node String Type @subsection String Type @@ -745,58 +876,124 @@ the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string. + @xref{Strings and Characters}, for functions that operate on strings. + +@menu +* Syntax for Strings:: +* Non-ASCII in Strings:: +* Nonprinting Characters:: +* Text Props and Strings:: +@end menu + +@node Syntax for Strings +@subsubsection Syntax for Strings + @cindex @samp{"} in strings @cindex double-quote in strings @cindex @samp{\} in strings @cindex backslash in strings The read syntax for strings is a double-quote, an arbitrary number of -characters, and another double-quote, @code{"like this"}. 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}...) @@ -813,9 +1010,20 @@ of three as follows: @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, - @xref{Strings and Characters}, for functions that work on strings. +@example +#("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic)) +@end example + +@noindent +represents a string whose textual contents are @samp{foo bar}, in which +the first three characters have a @code{face} property with value +@code{bold}, and the last three have a @code{face} property with value +@code{italic}. (The fourth character has no text properties, so its +property list is @code{nil}. It is not actually necessary to mention +ranges with @code{nil} as the property list, since any characters not +mentioned in any range will default to having no properties.) @node Vector Type @subsection Vector Type @@ -837,6 +1045,85 @@ for evaluation. @xref{Vectors}, for functions that work with vectors. +@node Char-Table Type +@subsection Char-Table Type + + A @dfn{char-table} is a one-dimensional array of elements of any type, +indexed by character codes. Char-tables have certain extra features to +make them more useful for many jobs that involve assigning information +to character codes---for example, a char-table can have a parent to +inherit from, a default value, and a small number of extra slots to use for +special purposes. A char-table can also specify a single value for +a whole character set. + + The printed representation of a char-table is like a vector +except that there is an extra @samp{#^} at the beginning. + + @xref{Char-Tables}, for special functions to operate on char-tables. +Uses of char-tables include: + +@itemize @bullet +@item +Case tables (@pxref{Case Tables}). + +@item +Character category tables (@pxref{Categories}). + +@item +Display tables (@pxref{Display Tables}). + +@item +Syntax tables (@pxref{Syntax Tables}). +@end itemize + +@node Bool-Vector Type +@subsection Bool-Vector Type + + A @dfn{bool-vector} is a one-dimensional array of elements that +must be @code{t} or @code{nil}. + + The printed representation of a bool-vector is like a string, except +that it begins with @samp{#&} followed by the length. The string +constant that follows actually specifies the contents of the bool-vector +as a bitmap---each ``character'' in the string contains 8 bits, which +specify the next 8 elements of the bool-vector (1 stands for @code{t}, +and 0 for @code{nil}). The least significant bits of the character +correspond to the lowest indices in the bool-vector. + +@example +(make-bool-vector 3 t) + @result{} #&3"^G" +(make-bool-vector 3 nil) + @result{} #&3"^@@" +@end example + +@noindent +These results make sense, because the binary code for @samp{C-g} is +111 and @samp{C-@@} is the character with code 0. + + If the length is not a multiple of 8, the printed representation +shows extra elements, but these extras really make no difference. For +instance, in the next example, the two bool-vectors are equal, because +only the first 3 bits are used: + +@example +(equal #&3"\377" #&3"\007") + @result{} t +@end example + +@node Hash Table Type +@subsection Hash Table Type + + A hash table is a very fast kind of lookup table, somewhat like an +alist in that it maps keys to corresponding values, but much faster. +Hash tables are a new feature in Emacs 21; they have no read syntax, and +print using hash notation. @xref{Hash Tables}. + +@example +(make-hash-table) + @result{} # +@end example + @node Function Type @subsection Function Type @@ -863,7 +1150,7 @@ functions @code{funcall} and @code{apply}. @xref{Calling Functions}. 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. @@ -872,6 +1159,10 @@ 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 @@ -885,11 +1176,12 @@ not evaluate all its arguments is called a @dfn{special form} (@pxref{Special Forms}).@refill It does not matter to the caller of a function whether the function is -primitive. However, this does matter if you try to substitute a -function written in Lisp for a primitive of the same name. The reason -is that the primitive function may be called directly from C code. -Calls to the redefined function from Lisp will use the new definition, -but calls from C code may still use the built-in definition. +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 @@ -917,18 +1209,19 @@ the evaluator handles this data type specially when it appears as a function to be called. @xref{Byte Compilation}, for information about the byte compiler. -The printed representation for a 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 @@ -944,9 +1237,10 @@ symbol. @xref{Autoload}, for more details. @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. @@ -954,11 +1248,10 @@ with 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 @@ -974,10 +1267,10 @@ 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, 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 @@ -997,7 +1290,13 @@ a local syntax table (@pxref{Syntax Tables}); 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 @@ -1006,7 +1305,10 @@ override global bindings or values. These are used to customize the 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 @@ -1072,7 +1374,7 @@ in any given window can change frequently. @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. @@ -1084,7 +1386,7 @@ uniquely). @example @group (selected-frame) - @result{} # + @result{} # @end group @end example @@ -1098,9 +1400,23 @@ uniquely). 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{#}. @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{#}. @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 @@ -1163,44 +1479,81 @@ a list whose @sc{car} is the symbol @code{keymap}. @xref{Keymaps}, for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings. -@node Syntax Table Type -@subsection Syntax Table Type +@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 + In Emacs 21, 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 + +@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 - 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. + You can also use the same syntax to make a circular structure, which +appears as an ``element'' within itself. Here is an example: - @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 +#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 @@ -1223,17 +1576,40 @@ pass an argument to @code{+} that it cannot handle: @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 a table of predefined type predicates, in alphabetical order, + Here is an example which uses the predicates @code{listp} to check for +a list and @code{symbolp} to check for a symbol. + +@example +(defun add-on (x) + (cond ((symbolp x) + ;; If X is a symbol, put it on LIST. + (setq list (cons x list))) + ((listp x) + ;; If X is a list, add its elements to LIST. + (setq list (append x list))) + (t + ;; We handle only symbols and lists. + (error "Invalid argument %s in add-on" x)))) +@end example + + Here is a table of predefined type predicates, in alphabetical order, with references to further information. @table @code @@ -1243,6 +1619,9 @@ with references to further information. @item arrayp @xref{Array Functions, arrayp}. +@item bool-vector-p +@xref{Bool-Vectors, bool-vector-p}. + @item bufferp @xref{Buffer Basics, bufferp}. @@ -1250,26 +1629,38 @@ with references to further information. @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 integer-or-marker-p @xref{Predicates on Markers, integer-or-marker-p}. @@ -1279,6 +1670,9 @@ with references to further information. @item keymapp @xref{Creating Keymaps, keymapp}. +@item keywordp +@xref{Constant Variables}. + @item listp @xref{List-related Predicates, listp}. @@ -1334,6 +1728,34 @@ with references to further information. @xref{Basic Windows, windowp}. @end table + The most general way to check the type of an object is to call the +function @code{type-of}. Recall that each object belongs to one and +only one primitive type; @code{type-of} tells you which one (@pxref{Lisp +Data Types}). But @code{type-of} knows nothing about non-primitive +types. In most cases, it is more convenient to use type predicates than +@code{type-of}. + +@defun type-of object +This function returns a symbol naming the primitive type of +@var{object}. The value is one of the symbols @code{symbol}, +@code{integer}, @code{float}, @code{string}, @code{cons}, @code{vector}, +@code{char-table}, @code{bool-vector}, @code{hash-table}, @code{subr}, +@code{compiled-function}, @code{marker}, @code{overlay}, @code{window}, +@code{buffer}, @code{frame}, @code{process}, or +@code{window-configuration}. + +@example +(type-of 1) + @result{} integer +(type-of 'nil) + @result{} symbol +(type-of '()) ; @r{@code{()} is @code{nil}.} + @result{} symbol +(type-of '(x)) + @result{} cons +@end example +@end defun + @node Equality Predicates @section Equality Predicates @cindex equality @@ -1356,11 +1778,6 @@ 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}.) - @example @group (eq 'foo 'foo) @@ -1402,14 +1819,26 @@ the same name are not @code{eq}. @xref{Creating Symbols}.) @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 @@ -1460,7 +1889,13 @@ arguments to see if their elements are the same. So, if two objects are @end group @end example -Comparison of strings uses @code{string=}, and is case-sensitive. +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 @@ -1468,7 +1903,24 @@ Comparison of strings uses @code{string=}, and is case-sensitive. @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