Mercurial > hg > xemacs-beta
diff man/lispref/objects.texi @ 0:376386a54a3c r19-14
Import from CVS: tag r19-14
| author | cvs |
|---|---|
| date | Mon, 13 Aug 2007 08:45:50 +0200 |
| parents | |
| children | 850242ba4a81 |
line wrap: on
line diff
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/man/lispref/objects.texi Mon Aug 13 08:45:50 2007 +0200 @@ -0,0 +1,2362 @@ +@c -*-texinfo-*- +@c This is part of the XEmacs Lisp Reference Manual. +@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. +@c See the file lispref.texi for copying conditions. +@setfilename ../../info/objects.info +@node Lisp Data Types, Numbers, Introduction, Top +@chapter Lisp Data Types +@cindex object +@cindex Lisp object +@cindex type +@cindex data type + + A Lisp @dfn{object} is a piece of data used and manipulated by Lisp +programs. For our purposes, a @dfn{type} or @dfn{data type} is a set of +possible objects. + + Every object belongs to at least one type. Objects of the same type +have similar structures and may usually be used in the same contexts. +Types can overlap, and objects can belong to two or more types. +Consequently, we can ask whether an object belongs to a particular type, +but not for ``the'' type of an object. + +@cindex primitive type + A few fundamental object types are built into XEmacs. These, from +which all other types are constructed, are called @dfn{primitive types}. +Each object belongs to one and only one primitive type. These types +include @dfn{integer}, @dfn{character} (starting with XEmacs 20.0), +@dfn{float}, @dfn{cons}, @dfn{symbol}, @dfn{string}, @dfn{vector}, +@dfn{bit-vector}, @dfn{subr}, @dfn{compiled-function}, @dfn{hashtable}, +@dfn{range-table}, @dfn{char-table}, @dfn{weak-list}, and several +special types, such as @dfn{buffer}, that are related to editing. +(@xref{Editing Types}.) + + Each primitive type has a corresponding Lisp function that checks +whether an object is a member of that type. + + Note that Lisp is unlike many other languages in that Lisp objects are +@dfn{self-typing}: the primitive type of the object is implicit in the +object itself. For example, if an object is a vector, nothing can treat +it as a number; Lisp knows it is a vector, not a number. + + In most languages, the programmer must declare the data type of each +variable, and the type is known by the compiler but not represented in +the data. Such type declarations do not exist in XEmacs Lisp. A Lisp +variable can have any type of value, and it remembers whatever value +you store in it, type and all. + + This chapter describes the purpose, printed representation, and read +syntax of each of the standard types in Emacs Lisp. Details on how +to use these types can be found in later chapters. + +@menu +* Printed Representation:: How Lisp objects are represented as text. +* Comments:: Comments and their formatting conventions. +* Primitive Types:: List of all primitive types in XEmacs. +* Programming Types:: Types found in all Lisp systems. +* Editing Types:: Types specific to XEmacs. +* Window-System Types:: Types specific to windowing systems. +* Type Predicates:: Tests related to types. +* Equality Predicates:: Tests of equality between any two objects. +@end menu + +@node Printed Representation +@section Printed Representation and Read Syntax +@cindex printed representation +@cindex read syntax + + The @dfn{printed representation} of an object is the format of the +output generated by the Lisp printer (the function @code{prin1}) for +that object. The @dfn{read syntax} of an object is the format of the +input accepted by the Lisp reader (the function @code{read}) for that +object. Most objects have more than one possible read syntax. Some +types of object have no read syntax; except for these cases, the printed +representation of an object is also a read syntax for it. + + In other languages, an expression is text; it has no other form. In +Lisp, an expression is primarily a Lisp object and only secondarily the +text that is the object's read syntax. Often there is no need to +emphasize this distinction, but you must keep it in the back of your +mind, or you will occasionally be very confused. + +@cindex hash notation + Every type has a printed representation. Some types have no read +syntax, since it may not make sense to enter objects of these types +directly in a Lisp program. For example, the buffer type does not have +a read syntax. Objects of these types are printed in @dfn{hash +notation}: the characters @samp{#<} followed by a descriptive string +(typically the type name followed by the name of the object), and closed +with a matching @samp{>}. Hash notation cannot be read at all, so the +Lisp reader signals the error @code{invalid-read-syntax} whenever it +encounters @samp{#<}. +@kindex invalid-read-syntax + +@example +(current-buffer) + @result{} #<buffer "objects.texi"> +@end example + + When you evaluate an expression interactively, the Lisp interpreter +first reads the textual representation of it, producing a Lisp object, +and then evaluates that object (@pxref{Evaluation}). However, +evaluation and reading are separate activities. Reading returns the +Lisp object represented by the text that is read; the object may or may +not be evaluated later. @xref{Input Functions}, for a description of +@code{read}, the basic function for reading objects. + +@node Comments +@section Comments +@cindex comments +@cindex @samp{;} in comment + + A @dfn{comment} is text that is written in a program only for the sake +of humans that read the program, and that has no effect on the meaning +of the program. In Lisp, a semicolon (@samp{;}) starts a comment if it +is not within a string or character constant. The comment continues to +the end of line. The Lisp reader discards comments; they do not become +part of the Lisp objects which represent the program within the Lisp +system. + + The @samp{#@@@var{count}} construct, which skips the next @var{count} +characters, is useful for program-generated comments containing binary +data. The XEmacs Lisp byte compiler uses this in its output files +(@pxref{Byte Compilation}). It isn't meant for source files, however. + + @xref{Comment Tips}, for conventions for formatting comments. + +@node Primitive Types +@section Primitive Types +@cindex primitive types + + For reference, here is a list of all the primitive types that may +exist in XEmacs. Note that some of these types may not exist +in some XEmacs executables; that depends on the options that +XEmacs was configured with. + +@itemize @bullet +@item +bit-vector +@item +buffer +@item +char-table +@item +character +@item +charset +@item +coding-system +@item +cons +@item +color-instance +@item +compiled-function +@item +console +@item +database +@item +device +@item +event +@item +extent +@item +face +@item +float +@item +font-instance +@item +frame +@item +glyph +@item +hashtable +@item +image-instance +@item +integer +@item +keymap +@item +marker +@item +process +@item +range-table +@item +specifier +@item +string +@item +subr +@item +subwindow +@item +symbol +@item +toolbar-button +@item +tooltalk-message +@item +tooltalk-pattern +@item +vector +@item +weak-list +@item +window +@item +window-configuration +@item +x-resource +@end itemize + +In addition, the following special types are created internally +but will never be seen by Lisp code. You may encounter them, +however, if you are debugging XEmacs. The printed representation +of these objects begins @samp{#<INTERNAL EMACS BUG}, which indicates +to the Lisp programmer that he has found an internal bug in XEmacs +if he ever encounters any of these objects. + +@itemize @bullet +@item +char-table-entry +@item +command-builder +@item +extent-auxiliary +@item +extent-info +@item +lcrecord-list +@item +lstream +@item +opaque +@item +opaque-list +@item +popup-data +@item +symbol-value-buffer-local +@item +symbol-value-forward +@item +symbol-value-lisp-magic +@item +symbol-value-varalias +@item +toolbar-data +@end itemize + +@node Programming Types +@section Programming Types +@cindex programming types + + There are two general categories of types in XEmacs Lisp: those having +to do with Lisp programming, and those having to do with editing. The +former exist in many Lisp implementations, in one form or another. The +latter are unique to XEmacs Lisp. + +@menu +* Integer Type:: Numbers without fractional parts. +* Floating Point Type:: Numbers with fractional parts and with a large range. +* Character Type:: The representation of letters, numbers and + control characters. +* Symbol Type:: A multi-use object that refers to a function, + variable, or property list, and has a unique identity. +* Sequence Type:: Both lists and arrays are classified as sequences. +* Cons Cell Type:: Cons cells, and lists (which are made from cons cells). +* Array Type:: Arrays include strings and vectors. +* String Type:: An (efficient) array of characters. +* Vector Type:: One-dimensional arrays. +* Bit Vector Type:: An (efficient) array of bits. +* Function Type:: A piece of executable code you can call from elsewhere. +* Macro Type:: A method of expanding an expression into another + expression, more fundamental but less pretty. +* Primitive Function Type:: A function written in C, callable from Lisp. +* Compiled-Function Type:: A function written in Lisp, then compiled. +* Autoload Type:: A type used for automatically loading seldom-used + functions. +* Char Table Type:: A mapping from characters to Lisp objects. +* Hash Table Type:: A fast mapping between Lisp objects. +* Range Table Type:: A mapping from ranges of integers to Lisp objects. +* Weak List Type:: A list with special garbage-collection properties. +@end menu + +@node Integer Type +@subsection Integer Type + + The range of values for integers in XEmacs Lisp is @minus{}134217728 to +134217727 (28 bits; i.e., +@ifinfo +-2**27 +@end ifinfo +@tex +$-2^{27}$ +@end tex +to +@ifinfo +2**27 - 1) +@end ifinfo +@tex +$2^{28}-1$) +@end tex +on most machines. (Some machines, in particular 64-bit machines such as +the DEC Alpha, may provide a wider range.) It is important to note that +the XEmacs Lisp arithmetic functions do not check for overflow. Thus +@code{(1+ 134217727)} is @minus{}134217728 on most machines. (However, +you @emph{will} get an error if you attempt to read an out-of-range +number using the Lisp reader.) + + The read syntax for integers is a sequence of (base ten) digits with +an optional sign at the beginning. (The printed representation produced +by the Lisp interpreter never has a leading @samp{+}.) + +@example +@group +-1 ; @r{The integer -1.} +1 ; @r{The integer 1.} ++1 ; @r{Also the integer 1.} +268435457 ; @r{Causes an error on a 28-bit implementation.} +@end group +@end example + + @xref{Numbers}, for more information. + +@node Floating Point Type +@subsection Floating Point Type + + XEmacs supports floating point numbers. The precise range of floating +point numbers is machine-specific. + + The printed representation for floating point numbers requires either +a decimal point (with at least one digit following), an exponent, or +both. For example, @samp{1500.0}, @samp{15e2}, @samp{15.0e2}, +@samp{1.5e3}, and @samp{.15e4} are five ways of writing a floating point +number whose value is 1500. They are all equivalent. + + @xref{Numbers}, for more information. + +@node Character Type +@subsection Character Type +@cindex @sc{ASCII} character codes +@cindex char-int confoundance disease + + In XEmacs version 19, and in all versions of FSF GNU Emacs, a +@dfn{character} in XEmacs Lisp is nothing more than an integer. +This is yet another holdover from XEmacs Lisp's derivation from +vintage-1980 Lisps; modern versions of Lisp consider this equivalence +a bad idea, and have separate character types. In XEmacs version 20, +the modern convention is followed, and characters are their own +primitive types. (This change was necessary in order for @sc{MULE}, +i.e. Asian-language, support to be correctly implemented.) + + Even in XEmacs version 20, remnants of the equivalence between +characters and integers still exist; this is termed the @dfn{char-int +confoundance disease}. In particular, many functions such as @code{eq}, +@code{equal}, and @code{memq} have equivalent functions (@code{old-eq}, +@code{old-equal}, @code{old-memq}, etc.) that pretend like characters +are integers are the same. Byte code compiled under any version 19 +Emacs will have all such functions mapped to their @code{old-} equivalents +when the byte code is read into XEmacs 20. This is to preserve +compatibility -- Emacs 19 converts all constant characters to the equivalent +integer during byte-compilation, and thus there is no other way to preserve +byte-code compatibility even if the code has specifically been written +with the distinction between characters and integers in mind. + + Every character has an equivalent integer, called the @dfn{character +code}. For example, the character @kbd{A} is represented as the +@w{integer 65}, following the standard @sc{ASCII} representation of +characters. If XEmacs was not compiled with @sc{MULE} support, the +range of this integer will always be 0 to 255 -- eight bits, or one +byte. (Integers outside this range are accepted but silently truncated; +however, you should most decidedly @emph{not} rely on this, because it +will not work under XEmacs with @sc{MULE} support.) When @sc{MULE} +support is present, the range of character codes is much +larger. (Currently, 19 bits are used.) + + FSF GNU Emacs uses kludgy character codes above 255 to represent +keyboard input of @sc{ASCII} characters in combination with certain +modifiers. XEmacs does not use this (a more general mechanism is +used that does not distinguish between @sc{ASCII} keys and other +keys), so you will never find character codes above 255 in a +non-@sc{MULE} Xemacs. + + Individual characters are not often used in programs. It is far more +common to work with @emph{strings}, which are sequences composed of +characters. @xref{String Type}. + +@cindex read syntax for characters +@cindex printed representation for characters +@cindex syntax for characters + + The read syntax for characters begins with a question mark, followed +by the character (if it's printable) or some symbolic representation of +it. In XEmacs 20, where characters are their own type, this is also the +print representation. In XEmacs 19, however, where characters are +really integers, the printed representation of a character is a decimal +number. This is also a possible read syntax for a character, but +writing characters that way in Lisp programs is a very bad idea. You +should @emph{always} use the special read syntax formats that XEmacs Lisp +provides for characters. + + The usual read syntax for alphanumeric characters is a question mark +followed by the character; thus, @samp{?A} for the character +@kbd{A}, @samp{?B} for the character @kbd{B}, and @samp{?a} for the +character @kbd{a}. + + For example: + +@example +;; @r{Under XEmacs 20:} +?Q @result{} ?Q ?q @result{} ?q +(char-int ?Q) @result{} 81 +;; @r{Under XEmacs 19:} +?Q @result{} 81 ?q @result{} 113 +@end example + + You can use the same syntax for punctuation characters, but it is +often a good idea to add a @samp{\} so that the 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{?\\}. XEmacs 20 +always prints punctuation characters with a @samp{\} in front of them, +to avoid confusion. + +@cindex whitespace +@cindex bell character +@cindex @samp{\a} +@cindex backspace +@cindex @samp{\b} +@cindex tab +@cindex @samp{\t} +@cindex vertical tab +@cindex @samp{\v} +@cindex formfeed +@cindex @samp{\f} +@cindex newline +@cindex @samp{\n} +@cindex return +@cindex @samp{\r} +@cindex escape +@cindex @samp{\e} + You can express the characters Control-g, backspace, tab, newline, +vertical tab, formfeed, return, and escape as @samp{?\a}, @samp{?\b}, +@samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, @samp{?\r}, @samp{?\e}, +respectively. Their character codes are 7, 8, 9, 10, 11, 12, 13, and 27 +in decimal. Thus, + +@example +;; @r{Under XEmacs 20:} +?\a @result{} ?\^G ; @r{@kbd{C-g}} +(char-int ?\a) @result{} 7 +?\b @result{} ?\^H ; @r{backspace, @key{BS}, @kbd{C-h}} +(char-int ?\b) @result{} 8 +?\t @result{} ?\t ; @r{tab, @key{TAB}, @kbd{C-i}} +(char-int ?\t) @result{} 9 +?\n @result{} ?\n ; @r{newline, @key{LFD}, @kbd{C-j}} +?\v @result{} ?\^K ; @r{vertical tab, @kbd{C-k}} +?\f @result{} ?\^L ; @r{formfeed character, @kbd{C-l}} +?\r @result{} ?\r ; @r{carriage return, @key{RET}, @kbd{C-m}} +?\e @result{} ?\^[ ; @r{escape character, @key{ESC}, @kbd{C-[}} +?\\ @result{} ?\\ ; @r{backslash character, @kbd{\}} +;; @r{Under XEmacs 19:} +?\a @result{} 7 ; @r{@kbd{C-g}} +?\b @result{} 8 ; @r{backspace, @key{BS}, @kbd{C-h}} +?\t @result{} 9 ; @r{tab, @key{TAB}, @kbd{C-i}} +?\n @result{} 10 ; @r{newline, @key{LFD}, @kbd{C-j}} +?\v @result{} 11 ; @r{vertical tab, @kbd{C-k}} +?\f @result{} 12 ; @r{formfeed character, @kbd{C-l}} +?\r @result{} 13 ; @r{carriage return, @key{RET}, @kbd{C-m}} +?\e @result{} 27 ; @r{escape character, @key{ESC}, @kbd{C-[}} +?\\ @result{} 92 ; @r{backslash character, @kbd{\}} +@end example + +@cindex escape sequence + These sequences which start with backslash are also known as +@dfn{escape sequences}, because backslash plays the role of an escape +character; this usage has nothing to do with the character @key{ESC}. + +@cindex control characters + Control characters may be represented using yet another read syntax. +This consists of a question mark followed by a backslash, caret, and the +corresponding non-control character, in either upper or lower case. For +example, both @samp{?\^I} and @samp{?\^i} are valid read syntax for the +character @kbd{C-i}, the character whose value is 9. + + Instead of the @samp{^}, you can use @samp{C-}; thus, @samp{?\C-i} is +equivalent to @samp{?\^I} and to @samp{?\^i}: + +@example +;; @r{Under XEmacs 20:} +?\^I @result{} ?\t ?\C-I @result{} ?\t +(char-int ?\^I) @result{} 9 +;; @r{Under XEmacs 19:} +?\^I @result{} 9 ?\C-I @result{} 9 +@end example + + There is also a character read syntax beginning with @samp{\M-}. This +sets the high bit of the character code (same as adding 128 to the +character code). For example, @samp{?\M-A} stands for the character +with character code 193, or 128 plus 65. You should @emph{not} use this +syntax in your programs. It is a holdover of yet another confoundance +disease from earlier Emacsen. (This was used to represent keyboard input +with the @key{META} key set, thus the @samp{M}; however, it conflicts +with the legitimate @sc{ISO}-8859-1 interpretation of the character code. +For example, character code 193 is a lowercase @samp{a} with an acute +accent, in @sc{ISO}-8859-1.) + +@ignore @c None of this crap applies to XEmacs. + For use in strings and buffers, you are limited to the control +characters that exist in @sc{ASCII}, but for keyboard input purposes, +you can turn any character into a control character with @samp{C-}. The +character codes for these non-@sc{ASCII} control characters include the +@iftex +$2^{26}$ +@end iftex +@ifinfo +2**26 +@end ifinfo +bit as well as the code for the corresponding non-control +character. Ordinary terminals have no way of generating non-@sc{ASCII} +control characters, but you can generate them straightforwardly using an +X terminal. + + For historical reasons, Emacs treats the @key{DEL} character as +the control equivalent of @kbd{?}: + +@example +?\^? @result{} 127 ?\C-? @result{} 127 +@end example + +@noindent +As a result, it is currently not possible to represent the character +@kbd{Control-?}, which is a meaningful input character under X. It is +not easy to change this as various Lisp files refer to @key{DEL} in this +way. + + For representing control characters to be found in files or strings, +we recommend the @samp{^} syntax; for control characters in keyboard +input, we prefer the @samp{C-} syntax. This does not affect the meaning +of the program, but may guide the understanding of people who read it. + +@cindex meta characters + A @dfn{meta character} is a character typed with the @key{META} +modifier key. The integer that represents such a character has the +@iftex +$2^{27}$ +@end iftex +@ifinfo +2**27 +@end ifinfo +bit set (which on most machines makes it a negative number). We +use high bits for this and other modifiers to make possible a wide range +of basic character codes. + + In a string, the +@iftex +$2^{7}$ +@end iftex +@ifinfo +2**7 +@end ifinfo +bit indicates a meta character, so the meta +characters that can fit in a string have codes in the range from 128 to +255, and are the meta versions of the ordinary @sc{ASCII} characters. +(In Emacs versions 18 and older, this convention was used for characters +outside of strings as well.) + + The read syntax for meta characters uses @samp{\M-}. For example, +@samp{?\M-A} stands for @kbd{M-A}. You can use @samp{\M-} together with +octal character codes (see below), with @samp{\C-}, or with any other +syntax for a character. Thus, you can write @kbd{M-A} as @samp{?\M-A}, +or as @samp{?\M-\101}. Likewise, you can write @kbd{C-M-b} as +@samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}. + + The case of an ordinary letter is indicated by its character code as +part of @sc{ASCII}, but @sc{ASCII} has no way to represent whether a +control character is upper case or lower case. Emacs uses the +@iftex +$2^{25}$ +@end iftex +@ifinfo +2**25 +@end ifinfo +bit to indicate that the shift key was used for typing a control +character. This distinction is possible only when you use X terminals +or other special terminals; ordinary terminals do not indicate the +distinction to the computer in any way. + +@cindex hyper characters +@cindex super characters +@cindex alt characters + The X Window System defines three other modifier bits that can be set +in a character: @dfn{hyper}, @dfn{super} and @dfn{alt}. The syntaxes +for these bits are @samp{\H-}, @samp{\s-} and @samp{\A-}. Thus, +@samp{?\H-\M-\A-x} represents @kbd{Alt-Hyper-Meta-x}. +@iftex +Numerically, the +bit values are $2^{22}$ for alt, $2^{23}$ for super and $2^{24}$ for hyper. +@end iftex +@ifinfo +Numerically, the +bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper. +@end ifinfo +@end ignore + +@cindex @samp{?} in character constant +@cindex question mark in character constant +@cindex @samp{\} in character constant +@cindex backslash in character constant +@cindex octal character code + Finally, the most general read syntax consists of a question mark +followed by a backslash and the character code in octal (up to three +octal digits); thus, @samp{?\101} for the character @kbd{A}, +@samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the +character @kbd{C-b}. Although this syntax can represent any @sc{ASCII} +character, it is preferred only when the precise octal value is more +important than the @sc{ASCII} representation. + +@example +@group +;; @r{Under XEmacs 20:} +?\012 @result{} ?\n ?\n @result{} ?\n ?\C-j @result{} ?\n +?\101 @result{} ?A ?A @result{} ?A +@end group +@group +;; @r{Under XEmacs 19:} +?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10 +?\101 @result{} 65 ?A @result{} 65 +@end group +@end example + + A backslash is allowed, and harmless, preceding any character without +a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}. +There is no reason to add a backslash before most characters. However, +you should add a backslash before any of the characters +@samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing +Lisp code. Also add a backslash before whitespace characters such as +space, tab, newline and formfeed. However, it is cleaner to use one of +the easily readable escape sequences, such as @samp{\t}, instead of an +actual whitespace character such as a tab. + +@node Symbol Type +@subsection Symbol Type + + A @dfn{symbol} in XEmacs Lisp is an object with a name. The symbol +name serves as the printed representation of the symbol. In ordinary +use, the name is unique---no two symbols have the same name. + + A symbol can serve as a variable, as a function name, or to hold a +property list. Or it may serve only to be distinct from all other Lisp +objects, so that its presence in a data structure may be recognized +reliably. In a given context, usually only one of these uses is +intended. But you can use one symbol in all of these ways, +independently. + +@cindex @samp{\} in symbols +@cindex backslash in symbols + A symbol name can contain any characters whatever. Most symbol names +are written with letters, digits, and the punctuation characters +@samp{-+=*/}. Such names require no special punctuation; the characters +of the name suffice as long as the name does not look like a number. +(If it does, write a @samp{\} at the beginning of the name to force +interpretation as a symbol.) The characters @samp{_~!@@$%^&:<>@{@}} are +less often used but also require no special punctuation. Any other +characters may be included in a symbol's name by escaping them with a +backslash. In contrast to its use in strings, however, a backslash in +the name of a symbol simply quotes the single character that follows the +backslash. For example, in a string, @samp{\t} represents a tab +character; in the name of a symbol, however, @samp{\t} merely quotes the +letter @kbd{t}. To have a symbol with a tab character in its name, you +must actually use a tab (preceded with a backslash). But it's rare to +do such a thing. + +@cindex CL note---case of letters +@quotation +@b{Common Lisp note:} In Common Lisp, lower case letters are always +``folded'' to upper case, unless they are explicitly escaped. In 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 sixth example because the rest of the name +makes it invalid as a number. + +@example +@group +foo ; @r{A symbol named @samp{foo}.} +FOO ; @r{A symbol named @samp{FOO}, different from @samp{foo}.} +char-to-string ; @r{A symbol named @samp{char-to-string}.} +@end group +@group +1+ ; @r{A symbol named @samp{1+}} + ; @r{(not @samp{+1}, which is an integer).} +@end group +@group +\+1 ; @r{A symbol named @samp{+1}} + ; @r{(not a very readable name).} +@end group +@group +\(*\ 1\ 2\) ; @r{A symbol named @samp{(* 1 2)} (a worse name).} +@c the @'s in this next line use up three characters, hence the +@c apparent misalignment of the comment. ++-*/_~!@@$%^&=:<>@{@} ; @r{A symbol named @samp{+-*/_~!@@$%^&=:<>@{@}}.} + ; @r{These characters need not be escaped.} +@end group +@end example + +@node Sequence Type +@subsection Sequence Types + + A @dfn{sequence} is a Lisp object that represents an ordered set of +elements. There are two kinds of sequence in XEmacs Lisp, lists and +arrays. Thus, an object of type list or of type array is also +considered a sequence. + + Arrays are further subdivided into strings, vectors, and bit vectors. +Vectors can hold elements of any type, but string elements must be +characters, and bit vector elements must be either 0 or 1. However, the +characters in a string can have extents (@pxref{Extents}) and text +properties (@pxref{Text Properties}) like characters in a buffer; +vectors do not support extents or text properties even when their +elements happen to be characters. + + Lists, strings, vectors, and bit vectors are different, but they have +important similarities. For example, all have a length @var{l}, and all +have elements which can be indexed from zero to @var{l} minus one. +Also, several functions, called sequence functions, accept any kind of +sequence. For example, the function @code{elt} can be used to extract +an element of a sequence, given its index. @xref{Sequences Arrays +Vectors}. + + It is impossible to read the same sequence twice, since sequences are +always created anew upon reading. If you read the read syntax for a +sequence twice, you get two sequences with equal contents. There is one +exception: the empty list @code{()} always stands for the same object, +@code{nil}. + +@node Cons Cell Type +@subsection Cons Cell and List Types +@cindex address field of register +@cindex decrement field of register + + A @dfn{cons cell} is an object comprising two pointers named the +@sc{car} and the @sc{cdr}. Each of them can point to any Lisp object. + + A @dfn{list} is a series of cons cells, linked together so that the +@sc{cdr} of each cons cell points either to another cons cell or to the +empty list. @xref{Lists}, for functions that work on lists. Because +most cons cells are used as part of lists, the phrase @dfn{list +structure} has come to refer to any structure made out of cons cells. + + The names @sc{car} and @sc{cdr} have only historical meaning now. The +original Lisp implementation ran on an @w{IBM 704} computer which +divided words into two parts, called the ``address'' part and the +``decrement''; @sc{car} was an instruction to extract the contents of +the address part of a register, and @sc{cdr} an instruction to extract +the contents of the decrement. By contrast, ``cons cells'' are named +for the function @code{cons} that creates them, which in turn is named +for its purpose, the construction of cells. + +@cindex atom + Because cons cells are so central to Lisp, we also have a word for +``an object which is not a cons cell''. These objects are called +@dfn{atoms}. + +@cindex parenthesis + The read syntax and printed representation for lists are identical, and +consist of a left parenthesis, an arbitrary number of elements, and a +right parenthesis. + + Upon reading, each object inside the parentheses becomes an element +of the list. That is, a cons cell is made for each element. The +@sc{car} of the cons cell points to the element, and its @sc{cdr} points +to the next cons cell of the list, which holds the next element in the +list. The @sc{cdr} of the last cons cell is set to point to @code{nil}. + +@cindex box diagrams, for lists +@cindex diagrams, boxed, for lists + A list can be illustrated by a diagram in which the cons cells are +shown as pairs of boxes. (The Lisp reader cannot read such an +illustration; unlike the textual notation, which can be understood by +both humans and computers, the box illustrations can be understood only +by humans.) The following represents the three-element list @code{(rose +violet buttercup)}: + +@example +@group + ___ ___ ___ ___ ___ ___ + |___|___|--> |___|___|--> |___|___|--> nil + | | | + | | | + --> rose --> violet --> buttercup +@end group +@end example + + In this diagram, each box represents a slot that can refer to any Lisp +object. Each pair of boxes represents a cons cell. Each arrow is a +reference to a Lisp object, either an atom or another cons cell. + + In this example, the first box, the @sc{car} of the first cons cell, +refers to or ``contains'' @code{rose} (a symbol). The second box, the +@sc{cdr} of the first cons cell, refers to the next pair of boxes, the +second cons cell. The @sc{car} of the second cons cell refers to +@code{violet} and the @sc{cdr} refers to the third cons cell. The +@sc{cdr} of the third (and last) cons cell refers to @code{nil}. + +Here is another diagram of the same list, @code{(rose violet +buttercup)}, sketched in a different manner: + +@smallexample +@group + --------------- ---------------- ------------------- +| car | cdr | | car | cdr | | car | cdr | +| rose | o-------->| violet | o-------->| buttercup | nil | +| | | | | | | | | + --------------- ---------------- ------------------- +@end group +@end smallexample + +@cindex @samp{(@dots{})} in lists +@cindex @code{nil} in lists +@cindex empty list + A list with no elements in it is the @dfn{empty list}; it is identical +to the symbol @code{nil}. In other words, @code{nil} is both a symbol +and a list. + + Here are examples of lists written in Lisp syntax: + +@example +(A 2 "A") ; @r{A list of three elements.} +() ; @r{A list of no elements (the empty list).} +nil ; @r{A list of no elements (the empty list).} +("A ()") ; @r{A list of one element: the string @code{"A ()"}.} +(A ()) ; @r{A list of two elements: @code{A} and the empty list.} +(A nil) ; @r{Equivalent to the previous.} +((A B C)) ; @r{A list of one element} + ; @r{(which is a list of three elements).} +@end example + + Here is the list @code{(A ())}, or equivalently @code{(A nil)}, +depicted with boxes and arrows: + +@example +@group + ___ ___ ___ ___ + |___|___|--> |___|___|--> nil + | | + | | + --> A --> nil +@end group +@end example + +@menu +* Dotted Pair Notation:: An alternative syntax for lists. +* Association List Type:: A specially constructed list. +@end menu + +@node Dotted Pair Notation +@subsubsection Dotted Pair Notation +@cindex dotted pair notation +@cindex @samp{.} in lists + + @dfn{Dotted pair notation} is an alternative syntax for cons cells +that represents the @sc{car} and @sc{cdr} explicitly. In this syntax, +@code{(@var{a} .@: @var{b})} stands for a cons cell whose @sc{car} is +the object @var{a}, and whose @sc{cdr} is the object @var{b}. Dotted +pair notation is therefore more general than list syntax. In the dotted +pair notation, the list @samp{(1 2 3)} is written as @samp{(1 . (2 . (3 +. nil)))}. For @code{nil}-terminated lists, the two notations produce +the same result, but list notation is usually clearer and more +convenient when it is applicable. When printing a list, the dotted pair +notation is only used if the @sc{cdr} of a cell is not a list. + + Here's how box notation can illustrate dotted pairs. This example +shows the pair @code{(rose . violet)}: + +@example +@group + ___ ___ + |___|___|--> violet + | + | + --> rose +@end group +@end example + + Dotted pair notation can be combined with list notation to represent a +chain of cons cells with a non-@code{nil} final @sc{cdr}. For example, +@code{(rose violet . buttercup)} is equivalent to @code{(rose . (violet +. buttercup))}. The object looks like this: + +@example +@group + ___ ___ ___ ___ + |___|___|--> |___|___|--> buttercup + | | + | | + --> rose --> violet +@end group +@end example + + These diagrams make it evident why @w{@code{(rose .@: violet .@: +buttercup)}} is invalid syntax; it would require a cons cell that has +three parts rather than two. + + The list @code{(rose violet)} is equivalent to @code{(rose . (violet))} +and looks like this: + +@example +@group + ___ ___ ___ ___ + |___|___|--> |___|___|--> nil + | | + | | + --> rose --> violet +@end group +@end example + + Similarly, the three-element list @code{(rose violet buttercup)} +is equivalent to @code{(rose . (violet . (buttercup)))}. +@ifinfo +It looks like this: + +@example +@group + ___ ___ ___ ___ ___ ___ + |___|___|--> |___|___|--> |___|___|--> nil + | | | + | | | + --> rose --> violet --> buttercup +@end group +@end example +@end ifinfo + +@node Association List Type +@subsubsection Association List Type + + An @dfn{association list} or @dfn{alist} is a specially-constructed +list whose elements are cons cells. In each element, the @sc{car} is +considered a @dfn{key}, and the @sc{cdr} is considered an +@dfn{associated value}. (In some cases, the associated value is stored +in the @sc{car} of the @sc{cdr}.) Association lists are often used as +stacks, since it is easy to add or remove associations at the front of +the list. + + For example, + +@example +(setq alist-of-colors + '((rose . red) (lily . white) (buttercup . yellow))) +@end example + +@noindent +sets the variable @code{alist-of-colors} to an alist of three elements. In the +first element, @code{rose} is the key and @code{red} is the value. + + @xref{Association Lists}, for a further explanation of alists and for +functions that work on alists. + +@node Array Type +@subsection Array Type + + An @dfn{array} is composed of an arbitrary number of slots for +referring to other Lisp objects, arranged in a contiguous block of +memory. Accessing any element of an array takes the same amount of +time. In contrast, accessing an element of a list requires time +proportional to the position of the element in the list. (Elements at +the end of a list take longer to access than elements at the beginning +of a list.) + + XEmacs defines three types of array, strings, vectors, and bit +vectors. A string is an array of characters, a vector is an array of +arbitrary objects, and a bit vector is an array of 1's and 0's. All are +one-dimensional. (Most other programming languages support +multidimensional arrays, but they are not essential; you can get the +same effect with an array of arrays.) Each type of array has its own +read syntax; see @ref{String Type}, @ref{Vector Type}, and @ref{Bit +Vector Type}. + + An array may have any length up to the largest integer; but once +created, it has a fixed size. The first element of an array has index +zero, the second element has index 1, and so on. This is called +@dfn{zero-origin} indexing. For example, an array of four elements has +indices 0, 1, 2, @w{and 3}. + + The array type is contained in the sequence type and contains the +string type, the vector type, and the bit vector type. + +@node String Type +@subsection String Type + + A @dfn{string} is an array of characters. Strings are used for many +purposes in XEmacs, as can be expected in a text editor; for example, as +the names of Lisp symbols, as messages for the user, and to represent +text extracted from buffers. Strings in Lisp are constants: evaluation +of a string returns the same string. + +@cindex @samp{"} in strings +@cindex double-quote in strings +@cindex @samp{\} in strings +@cindex backslash in strings + The read syntax for strings is a double-quote, an arbitrary number of +characters, and another double-quote, @code{"like this"}. The Lisp +reader accepts the same formats for reading the characters of a string +as it does for reading single characters (without the question mark that +begins a character literal). You can enter a nonprinting character such +as tab or @kbd{C-a} using the convenient escape sequences, like this: +@code{"\t, \C-a"}. You can include a double-quote in a string by +preceding it with a backslash; thus, @code{"\""} is a string containing +just a single double-quote character. (@xref{Character Type}, for a +description of the read syntax for characters.) + +@ignore @c More ill-conceived FSF Emacs crap. + If you use the @samp{\M-} syntax to indicate a meta character in a +string constant, this sets the +@iftex +$2^{7}$ +@end iftex +@ifinfo +2**7 +@end ifinfo +bit of the character in the string. +This is not the same representation that the meta modifier has in a +character on its own (not inside a string). @xref{Character Type}. + + Strings cannot hold characters that have the hyper, super, or alt +modifiers; they can hold @sc{ASCII} control characters, but no others. +They do not distinguish case in @sc{ASCII} control characters. +@end ignore + + The printed representation of a string consists of a double-quote, the +characters it contains, and another double-quote. However, you must +escape any backslash or double-quote characters in the string with a +backslash, like this: @code{"this \" is an embedded quote"}. + + The newline character is not special in the read syntax for strings; +if you write a new line between the double-quotes, it becomes a +character in the string. But an escaped newline---one that is preceded +by @samp{\}---does not become part of the string; i.e., the Lisp reader +ignores an escaped newline while reading a string. +@cindex newline in strings + +@example +"It is useful to include newlines +in documentation strings, +but the newline is \ +ignored if escaped." + @result{} "It is useful to include newlines +in documentation strings, +but the newline is ignored if escaped." +@end example + + A string can hold extents and properties of the text it contains, in +addition to the characters themselves. This enables programs that copy +text between strings and buffers to preserve the extents and properties +with no special effort. @xref{Extents}; @xref{Text Properties}. + + Note that FSF GNU Emacs has a special read and print syntax for +strings with text properties, but XEmacs does not currently implement +this. It was judged better not to include this in XEmacs because it +entails that @code{equal} return @code{nil} when passed a string with +text properties and the equivalent string without text properties, which +is often counter-intuitive. + +@ignore @c Not in XEmacs +Strings with text +properties have a special read and print syntax: + +@example +#("@var{characters}" @var{property-data}...) +@end example + +@noindent +where @var{property-data} consists of zero or more elements, in groups +of three as follows: + +@example +@var{beg} @var{end} @var{plist} +@end example + +@noindent +The elements @var{beg} and @var{end} are integers, and together specify +a range of indices in the string; @var{plist} is the property list for +that range. +@end ignore + + @xref{Strings and Characters}, for functions that work on strings. + +@node Vector Type +@subsection Vector Type + + A @dfn{vector} is a one-dimensional array of elements of any type. It +takes a constant amount of time to access any element of a vector. (In +a list, the access time of an element is proportional to the distance of +the element from the beginning of the list.) + + The printed representation of a vector consists of a left square +bracket, the elements, and a right square bracket. This is also the +read syntax. Like numbers and strings, vectors are considered constants +for evaluation. + +@example +[1 "two" (three)] ; @r{A vector of three elements.} + @result{} [1 "two" (three)] +@end example + + @xref{Vectors}, for functions that work with vectors. + +@node Bit Vector Type +@subsection Bit Vector Type + + A @dfn{bit vector} is a one-dimensional array of 1's and 0's. It +takes a constant amount of time to access any element of a bit vector, +as for vectors. Bit vectors have an extremely compact internal +representation (one machine bit per element), which makes them ideal +for keeping track of unordered sets, large collections of boolean values, +etc. + + The printed representation of a bit vector consists of @samp{#*} +followed by the bits in the vector. This is also the read syntax. Like +numbers, strings, and vectors, bit vectors are considered constants for +evaluation. + +@example +#*00101000 ; @r{A bit vector of eight elements.} + @result{} #*00101000 +@end example + + @xref{Bit Vectors}, for functions that work with bit vectors. + +@node Function Type +@subsection Function Type + + Just as functions in other programming languages are executable, +@dfn{Lisp function} objects are pieces of executable code. However, +functions in Lisp are primarily Lisp objects, and only secondarily the +text which represents them. These Lisp objects are lambda expressions: +lists whose first element is the symbol @code{lambda} (@pxref{Lambda +Expressions}). + + In most programming languages, it is impossible to have a function +without a name. In Lisp, a function has no intrinsic name. A lambda +expression is also called an @dfn{anonymous function} (@pxref{Anonymous +Functions}). A named function in Lisp is actually a symbol with a valid +function in its function cell (@pxref{Defining Functions}). + + Most of the time, functions are called when their names are written in +Lisp expressions in Lisp programs. However, you can construct or obtain +a function object at run time and then call it with the primitive +functions @code{funcall} and @code{apply}. @xref{Calling Functions}. + +@node Macro Type +@subsection Macro Type + + A @dfn{Lisp macro} is a user-defined construct that extends the Lisp +language. It is represented as an object much like a function, but with +different parameter-passing semantics. A Lisp macro has the form of a +list whose first element is the symbol @code{macro} and whose @sc{cdr} +is a Lisp function object, including the @code{lambda} symbol. + + Lisp macro objects are usually defined with the built-in +@code{defmacro} function, but any list that begins with @code{macro} is +a macro as far as XEmacs is concerned. @xref{Macros}, for an explanation +of how to write a macro. + +@node Primitive Function Type +@subsection Primitive Function Type +@cindex special forms + + A @dfn{primitive function} is a function callable from Lisp but +written in the C programming language. Primitive functions are also +called @dfn{subrs} or @dfn{built-in functions}. (The word ``subr'' is +derived from ``subroutine''.) Most primitive functions evaluate all +their arguments when they are called. A primitive function that does +not evaluate all its arguments is called a @dfn{special form} +(@pxref{Special Forms}).@refill + + It does not matter to the caller of a function whether the function is +primitive. However, this does matter if you try to substitute a +function written in Lisp for a primitive of the same name. The reason +is that the primitive function may be called directly from C code. +Calls to the redefined function from Lisp will use the new definition, +but calls from C code may still use the built-in definition. + + The term @dfn{function} refers to all Emacs functions, whether written +in Lisp or C. @xref{Function Type}, for information about the +functions written in Lisp. + + Primitive functions have no read syntax and print in hash notation +with the name of the subroutine. + +@example +@group +(symbol-function 'car) ; @r{Access the function cell} + ; @r{of the symbol.} + @result{} #<subr car> +(subrp (symbol-function 'car)) ; @r{Is this a primitive function?} + @result{} t ; @r{Yes.} +@end group +@end example + +@node Compiled-Function Type +@subsection Compiled-Function Type + + The byte compiler produces @dfn{compiled-function objects}. The +evaluator handles this data type specially when it appears as a function +to be called. @xref{Byte Compilation}, for information about the byte +compiler. + + The printed representation for a compiled-function object is normally +@samp{#<compiled-function...>}. If @code{print-readably} is true, +however, it is @samp{#[...]}. + +@node Autoload Type +@subsection Autoload Type + + An @dfn{autoload object} is a list whose first element is the symbol +@code{autoload}. It is stored as the function definition of a symbol as +a placeholder for the real definition; it says that the real definition +is found in a file of Lisp code that should be loaded when necessary. +The autoload object contains the name of the file, plus some other +information about the real definition. + + After the file has been loaded, the symbol should have a new function +definition that is not an autoload object. The new definition is then +called as if it had been there to begin with. From the user's point of +view, the function call works as expected, using the function definition +in the loaded file. + + An autoload object is usually created with the function +@code{autoload}, which stores the object in the function cell of a +symbol. @xref{Autoload}, for more details. + +@node Char Table Type +@subsection Char Table Type +@cindex char table type + +(not yet documented) + +@node Hash Table Type +@subsection Hash Table Type +@cindex hash table type + + A @dfn{hash table} is a table providing an arbitrary mapping from +one Lisp object to another, using an internal indexing method +called @dfn{hashing}. Hash tables are very fast (much more efficient +that using an association list, when there are a large number of +elements in the table). + + Hash tables have no read syntax. They print in hash notation (The +``hash'' in ``hash notation'' has nothing to do with the ``hash'' in +``hash table''), giving the number of elements, total space allocated +for elements, and a unique number assigned at the time the hash table +was created. (Hash tables automatically resize as necessary so there +is no danger of running out of space for elements.) + +@example +@group +(make-hashtable 50) + @result{} #<hashtable 0/71 0x313a> +@end group +@end example + +@xref{Hash Tables}, for information on how to create and work with hash +tables. + +@node Range Table Type +@subsection Range Table Type +@cindex range table type + + A @dfn{range table} is a table that maps from ranges of integers to +arbitrary Lisp objects. Range tables automatically combine overlapping +ranges that map to the same Lisp object, and operations are provided +for mapping over all of the ranges in a range table. + + Range tables have a special read syntax beginning with +@samp{#s(range-table} (this is an example of @dfn{structure} read syntax, +which is also used for char tables and faces). + +@example +@group +(setq x (make-range-table)) +(put-range-table 20 50 'foo x) +(put-range-table 100 200 "bar" x) +x + @result{} #s(range-table data ((20 50) foo (100 200) "bar")) +@end group +@end example + +@xref{Range Tables}, for information on how to create and work with range +tables. + +@node Weak List Type +@subsection Weak List Type +@cindex weak list type + +(not yet documented) + +@node Editing Types +@section Editing Types +@cindex editing types + + The types in the previous section are common to many Lisp dialects. +XEmacs Lisp provides several additional data types for purposes connected +with editing. + +@menu +* Buffer Type:: The basic object of editing. +* Marker Type:: A position in a buffer. +* Extent Type:: A range in a buffer or string, maybe with properties. +* Window Type:: Buffers are displayed in windows. +* Frame Type:: Windows subdivide frames. +* Device Type:: Devices group all frames on a display. +* Console Type:: Consoles group all devices with the same keyboard. +* Window Configuration Type:: Recording the way a frame is subdivided. +* Event Type:: An interesting occurrence in the system. +* Process Type:: A process running on the underlying OS. +* Stream Type:: Receive or send characters. +* Keymap Type:: What function a keystroke invokes. +* Syntax Table Type:: What a character means. +* Display Table Type:: How display tables are represented. +* Database Type:: A connection to an external DBM or DB database. +* Charset Type:: A character set (e.g. all Kanji characters), + under XEmacs/MULE. +* Coding System Type:: An object encapsulating a way of converting between + different textual encodings, under XEmacs/MULE. +* ToolTalk Message Type:: A message, in the ToolTalk IPC protocol. +* ToolTalk Pattern Type:: A pattern, in the ToolTalk IPC protocol. +@end menu + +@node Buffer Type +@subsection Buffer Type + + A @dfn{buffer} is an object that holds text that can be edited +(@pxref{Buffers}). Most buffers hold the contents of a disk file +(@pxref{Files}) so they can be edited, but some are used for other +purposes. Most buffers are also meant to be seen by the user, and +therefore displayed, at some time, in a window (@pxref{Windows}). But a +buffer need not be displayed in any window. + + The contents of a buffer are much like a string, but buffers are not +used like strings in XEmacs Lisp, and the available operations are +different. For example, insertion of text into a buffer is very +efficient, whereas ``inserting'' text into a string requires +concatenating substrings, and the result is an entirely new string +object. + + Each buffer has a designated position called @dfn{point} +(@pxref{Positions}). At any time, one buffer is the @dfn{current +buffer}. Most editing commands act on the contents of the current +buffer in the neighborhood of point. Many of the standard Emacs +functions manipulate or test the characters in the current buffer; a +whole chapter in this manual is devoted to describing these functions +(@pxref{Text}). + + Several other data structures are associated with each buffer: + +@itemize @bullet +@item +a local syntax table (@pxref{Syntax Tables}); + +@item +a local keymap (@pxref{Keymaps}); + +@item +a local variable binding list (@pxref{Buffer-Local Variables}); + +@item +a list of extents (@pxref{Extents}); + +@item +and various other related properties. +@end itemize + +@noindent +The local keymap and variable list contain entries that individually +override global bindings or values. These are used to customize the +behavior of programs in different buffers, without actually changing the +programs. + + A buffer may be @dfn{indirect}, which means it shares the text +of another buffer. @xref{Indirect Buffers}. + + Buffers have no read syntax. They print in hash notation, showing the +buffer name. + +@example +@group +(current-buffer) + @result{} #<buffer "objects.texi"> +@end group +@end example + +@node Marker Type +@subsection Marker Type + + A @dfn{marker} denotes a position in a specific buffer. Markers +therefore have two components: one for the buffer, and one for the +position. Changes in the buffer's text automatically relocate the +position value as necessary to ensure that the marker always points +between the same two characters in the buffer. + + Markers have no read syntax. They print in hash notation, giving the +current character position and the name of the buffer. + +@example +@group +(point-marker) + @result{} #<marker at 50661 in objects.texi> +@end group +@end example + +@xref{Markers}, for information on how to test, create, copy, and move +markers. + +@node Extent Type +@subsection Extent Type + + An @dfn{extent} specifies temporary alteration of the display +appearance of a part of a buffer (or string). It contains markers +delimiting a range of the buffer, plus a property list (a list whose +elements are alternating property names and values). Extents are used +to present parts of the buffer temporarily in a different display style. +They have no read syntax, and print in hash notation, giving the buffer +name and range of positions. + + Extents can exist over strings as well as buffers; the primary use +of this is to preserve extent and text property information as text +is copied from one buffer to another or between different parts of +a buffer. + + Extents have no read syntax. They print in hash notation, giving the +range of text they cover, the name of the buffer or string they are in, +the address in core, and a summary of some of the properties attached to +the extent. + +@example +@group +(extent-at (point)) + @result{} #<extent [51742, 51748) font-lock text-prop 0x90121e0 in buffer objects.texi> +@end group +@end example + + @xref{Extents}, for how to create and use extents. + + Extents are used to implement text properties. @xref{Text Properties}. + +@node Window Type +@subsection Window Type + + A @dfn{window} describes the portion of the frame that XEmacs uses to +display a buffer. (In standard window-system usage, a @dfn{window} is +what XEmacs calls a @dfn{frame}; XEmacs confusingly uses the term +``window'' to refer to what is called a @dfn{pane} in standard +window-system usage.) Every window has one associated buffer, whose +contents appear in the window. By contrast, a given buffer may appear +in one window, no window, or several windows. + + Though many windows may exist simultaneously, at any time one window +is designated the @dfn{selected window}. This is the window where the +cursor is (usually) displayed when XEmacs is ready for a command. The +selected window usually displays the current buffer, but this is not +necessarily the case. + + Windows are grouped on the screen into frames; each window belongs to +one and only one frame. @xref{Frame Type}. + + Windows have no read syntax. They print in hash notation, giving the +name of the buffer being displayed and a unique number assigned at the +time the window was created. (This number can be useful because the +buffer displayed in any given window can change frequently.) + +@example +@group +(selected-window) + @result{} #<window on "objects.texi" 0x266c> +@end group +@end example + + @xref{Windows}, for a description of the functions that work on windows. + +@node Frame Type +@subsection Frame Type + + A @var{frame} is a rectangle on the screen (a @dfn{window} in standard +window-system terminology) that contains one or more non-overlapping +Emacs windows (@dfn{panes} in standard window-system terminology). A +frame initially contains a single main window (plus perhaps a minibuffer +window) which you can subdivide vertically or horizontally into smaller +windows. + + Frames have no read syntax. They print in hash notation, giving the +frame's type, name as used for resourcing, and a unique number assigned +at the time the frame was created. + +@example +@group +(selected-frame) + @result{} #<x-frame "emacs" 0x9db> +@end group +@end example + + @xref{Frames}, for a description of the functions that work on frames. + +@node Device Type +@subsection Device Type + + A @dfn{device} represents a single display on which frames exist. +Normally, there is only one device object, but there may be more +than one if XEmacs is being run on a multi-headed display (e.g. an +X server with attached color and mono screens) or if XEmacs is +simultaneously driving frames attached to different consoles, e.g. +an X display and a @sc{TTY} connection. + + Devices do not have a read syntax. They print in hash notation, +giving the device's type, connection name, and a unique number assigned +at the time the device was created. + +@example +@group +(selected-device) + @result{} #<x-device on ":0.0" 0x5b9> +@end group +@end example + + @xref{Consoles and Devices}, for a description of several functions +related to devices. + +@node Console Type +@subsection Console Type + + A @dfn{console} represents a single keyboard to which devices +(i.e. displays on which frames exist) are connected. Normally, there is +only one console object, but there may be more than one if XEmacs is +simultaneously driving frames attached to different X servers and/or +@sc{TTY} connections. (XEmacs is capable of driving multiple X and +@sc{TTY} connections at the same time, and provides a robust mechanism +for handling the differing display capabilities of such heterogeneous +environments. A buffer with embedded glyphs and multiple fonts and +colors, for example, will display reasonably if it simultaneously +appears on a frame on a color X display, a frame on a mono X display, +and a frame on a @sc{TTY} connection.) + + Consoles do not have a read syntax. They print in hash notation, +giving the console's type, connection name, and a unique number assigned +at the time the console was created. + +@example +@group +(selected-console) + @result{} #<x-console on "localhost:0" 0x5b7> +@end group +@end example + + @xref{Consoles and Devices}, for a description of several functions +related to consoles. + +@node Window Configuration Type +@subsection Window Configuration Type +@cindex screen layout + + A @dfn{window configuration} stores information about the positions, +sizes, and contents of the windows in a frame, so you can recreate the +same arrangement of windows later. + + Window configurations do not have a read syntax. They print in hash +notation, giving a unique number assigned at the time the window +configuration was created. + +@example +@group +(current-window-configuration) + @result{} #<window-configuration 0x2db4> +@end group +@end example + + @xref{Window Configurations}, for a description of several functions +related to window configurations. + +@node Event Type +@subsection Event Type + +(not yet documented) + +@node Process Type +@subsection Process Type + + The word @dfn{process} usually means a running program. XEmacs itself +runs in a process of this sort. However, in XEmacs Lisp, a process is a +Lisp object that designates a subprocess created by the XEmacs process. +Programs such as shells, GDB, ftp, and compilers, running in +subprocesses of XEmacs, extend the capabilities of XEmacs. + + An Emacs subprocess takes textual input from Emacs and returns textual +output to Emacs for further manipulation. Emacs can also send signals +to the subprocess. + + Process objects have no read syntax. They print in hash notation, +giving the name of the process, its associated process ID, and the +current state of the process: + +@example +@group +(process-list) + @result{} (#<process "shell" pid 2909 state:run>) +@end group +@end example + +@xref{Processes}, for information about functions that create, delete, +return information about, send input or signals to, and receive output +from processes. + +@node Stream Type +@subsection Stream Type + + A @dfn{stream} is an object that can be used as a source or sink for +characters---either to supply characters for input or to accept them as +output. Many different types can be used this way: markers, buffers, +strings, and functions. Most often, input streams (character sources) +obtain characters from the keyboard, a buffer, or a file, and output +streams (character sinks) send characters to a buffer, such as a +@file{*Help*} buffer, or to the echo area. + + The object @code{nil}, in addition to its other meanings, may be used +as a stream. It stands for the value of the variable +@code{standard-input} or @code{standard-output}. Also, the object +@code{t} as a stream specifies input using the minibuffer +(@pxref{Minibuffers}) or output in the echo area (@pxref{The Echo +Area}). + + Streams have no special printed representation or read syntax, and +print as whatever primitive type they are. + + @xref{Read and Print}, for a description of functions +related to streams, including parsing and printing functions. + +@node Keymap Type +@subsection Keymap Type + + A @dfn{keymap} maps keys typed by the user to commands. This mapping +controls how the user's command input is executed. + + NOTE: In XEmacs, a keymap is a separate primitive type. In FSF GNU +Emacs, a keymap is actually a list whose @sc{car} is the symbol +@code{keymap}. + + @xref{Keymaps}, for information about creating keymaps, handling prefix +keys, local as well as global keymaps, and changing key bindings. + +@node Syntax Table Type +@subsection Syntax Table Type + + Under XEmacs 20, a @dfn{syntax table} is a particular type of char +table. Under XEmacs 19, a syntax table a vector of 256 integers. In +both cases, each element defines how one character is interpreted when it +appears in a buffer. For example, in C mode (@pxref{Major Modes}), the +@samp{+} character is punctuation, but in Lisp mode it is a valid +character in a symbol. These modes specify different interpretations by +changing the syntax table entry for @samp{+}. + + Syntax tables are used only for scanning text in buffers, not for +reading Lisp expressions. The table the Lisp interpreter uses to read +expressions is built into the XEmacs source code and cannot be changed; +thus, to change the list delimiters to be @samp{@{} and @samp{@}} +instead of @samp{(} and @samp{)} would be impossible. + + @xref{Syntax Tables}, for details about syntax classes and how to make +and modify syntax tables. + +@node Display Table Type +@subsection Display Table Type + + A @dfn{display table} specifies how to display each character code. +Each buffer and each window can have its own display table. A display +table is actually a vector of length 256, although in XEmacs 20 this may +change to be a particular type of char table. @xref{Display Tables}. + +@node Database Type +@subsection Database Type +@cindex database type + +(not yet documented) + +@node Charset Type +@subsection Charset Type +@cindex charset type + +(not yet documented) + +@node Coding System Type +@subsection Coding System Type +@cindex coding system type + +(not yet documented) + +@node ToolTalk Message Type +@subsection ToolTalk Message Type + +(not yet documented) + +@node ToolTalk Pattern Type +@subsection ToolTalk Pattern Type + +(not yet documented) + +@node Window-System Types +@section Window-System Types +@cindex window system types + + XEmacs also has some types that represent objects such as faces +(collections of display characters), fonts, and pixmaps that are +commonly found in windowing systems. + +@menu +* Face Type:: A collection of display characteristics. +* Glyph Type:: An image appearing in a buffer or elsewhere. +* Specifier Type:: A way of controlling display characteristics on + a per-buffer, -frame, -window, or -device level. +* Font Instance Type:: The way a font appears on a particular device. +* Color Instance Type:: The way a color appears on a particular device. +* Image Instance Type:: The way an image appears on a particular device. +* Toolbar Button Type:: An object representing a button in a toolbar. +* Subwindow Type:: An externally-controlled window-system window + appearing in a buffer. +* X Resource Type:: A miscellaneous X resource, if Epoch support was + compiled into XEmacs. +@end menu + +@node Face Type +@subsection Face Type +@cindex face type + +(not yet documented) + +@node Glyph Type +@subsection Glyph Type +@cindex glyph type + +(not yet documented) + +@node Specifier Type +@subsection Specifier Type +@cindex specifier type + +(not yet documented) + +@node Font Instance Type +@subsection Font Instance Type +@cindex font instance type + +(not yet documented) + +@node Color Instance Type +@subsection Color Instance Type +@cindex color instance type + +(not yet documented) + +@node Image Instance Type +@subsection Image Instance Type +@cindex image instance type + +(not yet documented) + +@node Toolbar Button Type +@subsection Toolbar Button Type +@cindex toolbar button type + +(not yet documented) + +@node Subwindow Type +@subsection Subwindow Type +@cindex subwindow type + +(not yet documented) + +@node X Resource Type +@subsection X Resource Type +@cindex X resource type + +(not yet documented) + +@node Type Predicates +@section Type Predicates +@cindex predicates +@cindex type checking +@kindex wrong-type-argument + + The XEmacs Lisp interpreter itself does not perform type checking on +the actual arguments passed to functions when they are called. It could +not do so, since function arguments in Lisp do not have declared data +types, as they do in other programming languages. It is therefore up to +the individual function to test whether each actual argument belongs to +a type that the function can use. + + All built-in functions do check the types of their actual arguments +when appropriate, and signal a @code{wrong-type-argument} error if an +argument is of the wrong type. For example, here is what happens if you +pass an argument to @code{+} that it cannot handle: + +@example +@group +(+ 2 'a) + @error{} Wrong type argument: integer-or-marker-p, a +@end group +@end example + +@cindex type predicates +@cindex testing types + If you want your program to handle different types differently, you +must do explicit type checking. The most common way to check the type +of an object is to call a @dfn{type predicate} function. Emacs has a +type predicate for each type, as well as some predicates for +combinations of types. + + A type predicate function takes one argument; it returns @code{t} if +the argument belongs to the appropriate type, and @code{nil} otherwise. +Following a general Lisp convention for predicate functions, most type +predicates' names end with @samp{p}. + + Here is an example which uses the predicates @code{listp} to check for +a list and @code{symbolp} to check for a symbol. + +@example +(defun add-on (x) + (cond ((symbolp x) + ;; If X is a symbol, put it on LIST. + (setq list (cons x list))) + ((listp x) + ;; If X is a list, add its elements to LIST. + (setq list (append x list))) +@need 3000 + (t + ;; We only handle symbols and lists. + (error "Invalid argument %s in add-on" x)))) +@end example + + Here is a table of predefined type predicates, in alphabetical order, +with references to further information. + +@table @code +@item annotationp +@xref{Annotation Primitives, annotationp}. + +@item arrayp +@xref{Array Functions, arrayp}. + +@item atom +@xref{List-related Predicates, atom}. + +@item bit-vector-p +@xref{Bit Vector Functions, bit-vector-p}. + +@item bitp +@xref{Bit Vector Functions, bitp}. + +@item boolean-specifier-p +@xref{Specifier Types, boolean-specifier-p}. + +@item buffer-glyph-p +@xref{Glyph Types, buffer-glyph-p}. + +@item buffer-live-p +@xref{Killing Buffers, buffer-live-p}. + +@item bufferp +@xref{Buffer Basics, bufferp}. + +@item button-event-p +@xref{Event Predicates, button-event-p}. + +@item button-press-event-p +@xref{Event Predicates, button-press-event-p}. + +@item button-release-event-p +@xref{Event Predicates, button-release-event-p}. + +@item case-table-p +@xref{Case Tables, case-table-p}. + +@item char-int-p +@xref{Character Codes, char-int-p}. + +@item char-or-char-int-p +@xref{Character Codes, char-or-char-int-p}. + +@item char-or-string-p +@xref{Predicates for Strings, char-or-string-p}. + +@item char-table-p +@xref{Char Tables, char-table-p}. + +@item characterp +@xref{Predicates for Characters, characterp}. + +@item color-instance-p +@xref{Colors, color-instance-p}. + +@item color-pixmap-image-instance-p +@xref{Image Instance Types, color-pixmap-image-instance-p}. + +@item color-specifier-p +@xref{Specifier Types, color-specifier-p}. + +@item commandp +@xref{Interactive Call, commandp}. + +@item compiled-function-p +@xref{Compiled-Function Type, compiled-function-p}. + +@item console-live-p +@xref{Connecting to a Console or Device, console-live-p}. + +@item consolep +@xref{Consoles and Devices, consolep}. + +@item consp +@xref{List-related Predicates, consp}. + +@item database-live-p +@xref{Connecting to a Database, database-live-p}. + +@item databasep +@xref{Databases, databasep}. + +@item device-live-p +@xref{Connecting to a Console or Device, device-live-p}. + +@item device-or-frame-p +@xref{Basic Device Functions, device-or-frame-p}. + +@item devicep +@xref{Consoles and Devices, devicep}. + +@item eval-event-p +@xref{Event Predicates, eval-event-p}. + +@item event-live-p +@xref{Event Predicates, event-live-p}. + +@item eventp +@xref{Events, eventp}. + +@item extent-live-p +@xref{Creating and Modifying Extents, extent-live-p}. + +@item extentp +@xref{Extents, extentp}. + +@item face-boolean-specifier-p +@xref{Specifier Types, face-boolean-specifier-p}. + +@item facep +@xref{Basic Face Functions, facep}. + +@item floatp +@xref{Predicates on Numbers, floatp}. + +@item font-instance-p +@xref{Fonts, font-instance-p}. + +@item font-specifier-p +@xref{Specifier Types, font-specifier-p}. + +@item frame-live-p +@xref{Deleting Frames, frame-live-p}. + +@item framep +@xref{Frames, framep}. + +@item functionp +(not yet documented) + +@item generic-specifier-p +@xref{Specifier Types, generic-specifier-p}. + +@item glyphp +@xref{Glyphs, glyphp}. + +@item hashtablep +@xref{Hash Tables, hashtablep}. + +@item icon-glyph-p +@xref{Glyph Types, icon-glyph-p}. + +@item image-instance-p +@xref{Images, image-instance-p}. + +@item image-specifier-p +@xref{Specifier Types, image-specifier-p}. + +@item integer-char-or-marker-p +@xref{Predicates on Markers, integer-char-or-marker-p}. + +@item integer-or-char-p +@xref{Predicates for Characters, integer-or-char-p}. + +@item integer-or-marker-p +@xref{Predicates on Markers, integer-or-marker-p}. + +@item integer-specifier-p +@xref{Specifier Types, integer-specifier-p}. + +@item integerp +@xref{Predicates on Numbers, integerp}. + +@item itimerp +(not yet documented) + +@item key-press-event-p +@xref{Event Predicates, key-press-event-p}. + +@item keymapp +@xref{Creating Keymaps, keymapp}. + +@item keywordp +(not yet documented) + +@item listp +@xref{List-related Predicates, listp}. + +@item markerp +@xref{Predicates on Markers, markerp}. + +@item misc-user-event-p +@xref{Event Predicates, misc-user-event-p}. + +@item mono-pixmap-image-instance-p +@xref{Image Instance Types, mono-pixmap-image-instance-p}. + +@item motion-event-p +@xref{Event Predicates, motion-event-p}. + +@item mouse-event-p +@xref{Event Predicates, mouse-event-p}. + +@item natnum-specifier-p +@xref{Specifier Types, natnum-specifier-p}. + +@item natnump +@xref{Predicates on Numbers, natnump}. + +@item nlistp +@xref{List-related Predicates, nlistp}. + +@item nothing-image-instance-p +@xref{Image Instance Types, nothing-image-instance-p}. + +@item number-char-or-marker-p +@xref{Predicates on Markers, number-char-or-marker-p}. + +@item number-or-marker-p +@xref{Predicates on Markers, number-or-marker-p}. + +@item numberp +@xref{Predicates on Numbers, numberp}. + +@item pointer-glyph-p +@xref{Glyph Types, pointer-glyph-p}. + +@item pointer-image-instance-p +@xref{Image Instance Types, pointer-image-instance-p}. + +@item process-event-p +@xref{Event Predicates, process-event-p}. + +@item processp +@xref{Processes, processp}. + +@item range-table-p +@xref{Range Tables, range-table-p}. + +@item ringp +(not yet documented) + +@item sequencep +@xref{Sequence Functions, sequencep}. + +@item specifierp +@xref{Specifiers, specifierp}. + +@item stringp +@xref{Predicates for Strings, stringp}. + +@item subrp +@xref{Function Cells, subrp}. + +@item subwindow-image-instance-p +@xref{Image Instance Types, subwindow-image-instance-p}. + +@item subwindowp +@xref{Subwindows, subwindowp}. + +@item symbolp +@xref{Symbols, symbolp}. + +@item syntax-table-p +@xref{Syntax Tables, syntax-table-p}. + +@item text-image-instance-p +@xref{Image Instance Types, text-image-instance-p}. + +@item timeout-event-p +@xref{Event Predicates, timeout-event-p}. + +@item toolbar-button-p +@xref{Toolbar, toolbar-button-p}. + +@item toolbar-specifier-p +@xref{Toolbar, toolbar-specifier-p}. + +@item user-variable-p +@xref{Defining Variables, user-variable-p}. + +@item vectorp +@xref{Vectors, vectorp}. + +@item weak-list-p +@xref{Weak Lists, weak-list-p}. + +@ignore +@item wholenump +@xref{Predicates on Numbers, wholenump}. +@end ignore + +@item window-configuration-p +@xref{Window Configurations, window-configuration-p}. + +@item window-live-p +@xref{Deleting Windows, window-live-p}. + +@item windowp +@xref{Basic Windows, windowp}. +@end table + + The most general way to check the type of an object is to call the +function @code{type-of}. Recall that each object belongs to one and +only one primitive type; @code{type-of} tells you which one (@pxref{Lisp +Data Types}). But @code{type-of} knows nothing about non-primitive +types. In most cases, it is more convenient to use type predicates than +@code{type-of}. + +@defun type-of object +This function returns a symbol naming the primitive type of +@var{object}. The value is one of @code{bit-vector}, @code{buffer}, +@code{char-table}, @code{character}, @code{charset}, +@code{coding-system}, @code{cons}, @code{color-instance}, +@code{compiled-function}, @code{console}, @code{database}, +@code{device}, @code{event}, @code{extent}, @code{face}, @code{float}, +@code{font-instance}, @code{frame}, @code{glyph}, @code{hashtable}, +@code{image-instance}, @code{integer}, @code{keymap}, @code{marker}, +@code{process}, @code{range-table}, @code{specifier}, @code{string}, +@code{subr}, @code{subwindow}, @code{symbol}, @code{toolbar-button}, +@code{tooltalk-message}, @code{tooltalk-pattern}, @code{vector}, +@code{weak-list}, @code{window}, @code{window-configuration}, or +@code{x-resource}. + +@example +(type-of 1) + @result{} integer +(type-of 'nil) + @result{} symbol +(type-of '()) ; @r{@code{()} is @code{nil}.} + @result{} symbol +(type-of '(x)) + @result{} cons +@end example +@end defun + +@node Equality Predicates +@section Equality Predicates +@cindex equality + + Here we describe two functions that test for equality between any two +objects. Other functions test equality between objects of specific +types, e.g., strings. For these predicates, see the appropriate chapter +describing the data type. + +@defun eq object1 object2 +This function returns @code{t} if @var{object1} and @var{object2} are +the same object, @code{nil} otherwise. The ``same object'' means that a +change in one will be reflected by the same change in the other. + +@code{eq} returns @code{t} if @var{object1} and @var{object2} are +integers with the same value. Also, since symbol names are normally +unique, if the arguments are symbols with the same name, they are +@code{eq}. For other types (e.g., lists, vectors, strings), two +arguments with the same contents or elements are not necessarily +@code{eq} to each other: they are @code{eq} only if they are the same +object. + +(The @code{make-symbol} function returns an uninterned symbol that is +not interned in the standard @code{obarray}. When uninterned symbols +are in use, symbol names are no longer unique. Distinct symbols with +the same name are not @code{eq}. @xref{Creating Symbols}.) + +NOTE: Under XEmacs 19, characters are really just integers, and thus +characters and integers are @code{eq}. Under XEmacs 20, it was +necessary to preserve remants of this in function such as @code{old-eq} +in order to maintain byte-code compatibility. Byte code compiled +under any Emacs 19 will automatically have calls to @code{eq} mapped +to @code{old-eq} when executed under XEmacs 20. + +@example +@group +(eq 'foo 'foo) + @result{} t +@end group + +@group +(eq 456 456) + @result{} t +@end group + +@group +(eq "asdf" "asdf") + @result{} nil +@end group + +@group +(eq '(1 (2 (3))) '(1 (2 (3)))) + @result{} nil +@end group + +@group +(setq foo '(1 (2 (3)))) + @result{} (1 (2 (3))) +(eq foo foo) + @result{} t +(eq foo '(1 (2 (3)))) + @result{} nil +@end group + +@group +(eq [(1 2) 3] [(1 2) 3]) + @result{} nil +@end group + +@group +(eq (point-marker) (point-marker)) + @result{} nil +@end group +@end example + +@end defun + +@defun old-eq obj1 obj2 +This function exists under XEmacs 20 and is exactly like @code{eq} +except that it suffers from the char-int confoundance disease. +In other words, it returns @code{t} if given a character and the +equivalent integer, even though the objects are of different types! +You should @emph{not} ever call this function explicitly in your +code. However, be aware that all calls to @code{eq} in byte code +compiled under version 19 map to @code{old-eq} in XEmacs 20. +(Likewise for @code{old-equal}, @code{old-memq}, @code{old-member}, +@code{old-assq} and @code{old-assoc}.) + +@example +@group +;; @r{Remember, this does not apply under XEmacs 19.} +?A + @result{} ?A +(char-int ?A) + @result{} 65 +(old-eq ?A 65) + @result{} t ; @r{Eek, we've been infected.} +(eq ?A 65) + @result{} nil ; @r{We are still healthy.} +@end group +@end example +@end defun + +@defun equal object1 object2 +This function returns @code{t} if @var{object1} and @var{object2} have +equal components, @code{nil} otherwise. Whereas @code{eq} tests if its +arguments are the same object, @code{equal} looks inside nonidentical +arguments to see if their elements are the same. So, if two objects are +@code{eq}, they are @code{equal}, but the converse is not always true. + +@example +@group +(equal 'foo 'foo) + @result{} t +@end group + +@group +(equal 456 456) + @result{} t +@end group + +@group +(equal "asdf" "asdf") + @result{} t +@end group +@group +(eq "asdf" "asdf") + @result{} nil +@end group + +@group +(equal '(1 (2 (3))) '(1 (2 (3)))) + @result{} t +@end group +@group +(eq '(1 (2 (3))) '(1 (2 (3)))) + @result{} nil +@end group + +@group +(equal [(1 2) 3] [(1 2) 3]) + @result{} t +@end group +@group +(eq [(1 2) 3] [(1 2) 3]) + @result{} nil +@end group + +@group +(equal (point-marker) (point-marker)) + @result{} t +@end group + +@group +(eq (point-marker) (point-marker)) + @result{} nil +@end group +@end example + +Comparison of strings is case-sensitive. + +Note that in FSF GNU Emacs, comparison of strings takes into account +their text properties, and you have to use @code{string-equal} if you +want only the strings themselves compared. This difference does not +exist in XEmacs; @code{equal} and @code{string-equal} always return +the same value on the same strings. + +@ignore @c Not true in XEmacs +Comparison of strings is case-sensitive and takes account of text +properties as well as the characters in the strings. To compare +two strings' characters without comparing their text properties, +use @code{string=} (@pxref{Text Comparison}). +@end ignore + +@example +@group +(equal "asdf" "ASDF") + @result{} nil +@end group +@end example + +Two distinct buffers are never @code{equal}, even if their contents +are the same. +@end defun + + The test for equality is implemented recursively, and circular lists may +therefore cause infinite recursion (leading to an error).