X-Git-Url: https://git.hcoop.net/bpt/emacs.git/blobdiff_plain/03231f93f3fb4c78ad4ae1771f5c8f3f6376e486..0df333e5c914cd21820d87a8e4380af0ff28a926:/lispref/objects.texi diff --git a/lispref/objects.texi b/lispref/objects.texi index 9b862ae9dc..5665e5beee 100644 --- a/lispref/objects.texi +++ b/lispref/objects.texi @@ -1,7 +1,7 @@ @c -*-texinfo-*- @c This is part of the GNU Emacs Lisp Reference Manual. -@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999 -@c Free Software Foundation, Inc. +@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2002, 2003, +@c 2004, 2005, 2006 Free Software Foundation, Inc. @c See the file elisp.texi for copying conditions. @setfilename ../info/objects @node Lisp Data Types, Numbers, Introduction, Top @@ -42,7 +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 @@ -66,36 +68,37 @@ to use these types can be found in later chapters. The @dfn{printed representation} of an object is the format of the output generated by the Lisp printer (the function @code{prin1}) for -that object. The @dfn{read syntax} of an object is the format of the -input accepted by the Lisp reader (the function @code{read}) for that -object. @xref{Read and Print}. - - Most objects have more than one possible read syntax. Some types of -object have no read syntax, since it may not make sense to enter objects -of these types directly in a Lisp program. Except for these cases, the -printed representation of an object is also a read syntax for it. - - In other languages, an expression is text; it has no other form. In -Lisp, an expression is primarily a Lisp object and only secondarily the -text that is the object's read syntax. Often there is no need to -emphasize this distinction, but you must keep it in the back of your -mind, or you will occasionally be very confused. +that object. Every data type has a unique printed representation. +The @dfn{read syntax} of an object is the format of the input accepted +by the Lisp reader (the function @code{read}) for that object. This +is not necessarily unique; many kinds of object have more than one +syntax. @xref{Read and Print}. @cindex hash notation - Every type has a printed representation. Some types have no read -syntax---for example, the buffer type has none. Objects of these types -are printed in @dfn{hash notation}: the characters @samp{#<} followed by -a descriptive string (typically the type name followed by the name of -the object), and closed with a matching @samp{>}. Hash notation cannot -be read at all, so the Lisp reader signals the error -@code{invalid-read-syntax} whenever it encounters @samp{#<}. -@kindex invalid-read-syntax + In most cases, an object's printed representation is also a read +syntax for the object. However, some types have no read syntax, since +it does not make sense to enter objects of these types as constants in +a Lisp program. These objects are printed in @dfn{hash notation}, +which consists of the characters @samp{#<}, a descriptive string +(typically the type name followed by the name of the object), and a +closing @samp{>}. For example: @example (current-buffer) @result{} # @end example +@noindent +Hash notation cannot be read at all, so the Lisp reader signals the +error @code{invalid-read-syntax} whenever it encounters @samp{#<}. +@kindex invalid-read-syntax + + In other languages, an expression is text; it has no other form. In +Lisp, an expression is primarily a Lisp object and only secondarily the +text that is the object's read syntax. Often there is no need to +emphasize this distinction, but you must keep it in the back of your +mind, or you will occasionally be very confused. + When you evaluate an expression interactively, the Lisp interpreter first reads the textual representation of it, producing a Lisp object, and then evaluates that object (@pxref{Evaluation}). However, @@ -161,24 +164,24 @@ latter are unique to Emacs Lisp. @node Integer Type @subsection Integer Type - The range of values for integers in Emacs Lisp is @minus{}134217728 to -134217727 (28 bits; i.e., + The range of values for integers in Emacs Lisp is @minus{}268435456 to +268435455 (29 bits; i.e., @ifnottex --2**27 +-2**28 @end ifnottex @tex -@math{-2^{27}} +@math{-2^{28}} @end tex to @ifnottex -2**27 - 1) +2**28 - 1) @end ifnottex @tex @math{2^{28}-1}) @end tex on most machines. (Some machines may provide a wider range.) It is important to note that the Emacs Lisp arithmetic functions do not check -for overflow. Thus @code{(1+ 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 @@ -192,7 +195,7 @@ leading @samp{+} or a final @samp{.}. 1 ; @r{The integer 1.} 1. ; @r{Also the integer 1.} +1 ; @r{Also the integer 1.} -268435457 ; @r{Also the integer 1 on a 28-bit implementation.} +536870913 ; @r{Also the integer 1 on a 29-bit implementation.} @end group @end example @@ -201,9 +204,12 @@ leading @samp{+} or a final @samp{.}. @node Floating Point Type @subsection Floating Point Type - Emacs supports floating point numbers (though there is a compilation -option to disable them). The precise range of floating point numbers is -machine-specific. + Floating point numbers are the computer equivalent of scientific +notation; you can think of a floating point number as a fraction +together with a power of ten. The precise number of significant +figures and the range of possible exponents is machine-specific; Emacs +uses the C data type @code{double} to store the value, and internally +this records a power of 2 rather than a power of 10. The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or @@ -215,7 +221,7 @@ number whose value is 1500. They are all equivalent. @node Character Type @subsection Character Type -@cindex @sc{ascii} character codes +@cindex @acronym{ASCII} character codes A @dfn{character} in Emacs Lisp is nothing more than an integer. In other words, characters are represented by their character codes. For @@ -225,11 +231,12 @@ example, the character @kbd{A} is represented as the @w{integer 65}. common to work with @emph{strings}, which are sequences composed of characters. @xref{String Type}. - Characters in strings, buffers, and files are currently limited to the -range of 0 to 524287---nineteen bits. But not all values in that range -are valid character codes. Codes 0 through 127 are @sc{ascii} codes; the -rest are non-@sc{ascii} (@pxref{Non-ASCII Characters}). Characters that represent -keyboard input have a much wider range, to encode modifier keys such as + Characters in strings, buffers, and files are currently limited to +the range of 0 to 524287---nineteen bits. But not all values in that +range are valid character codes. Codes 0 through 127 are +@acronym{ASCII} codes; the rest are non-@acronym{ASCII} +(@pxref{Non-ASCII Characters}). Characters that represent keyboard +input have a much wider range, to encode modifier keys such as Control, Meta and Shift. @cindex read syntax for characters @@ -247,7 +254,7 @@ with a question mark. The usual read syntax for alphanumeric characters is a question mark followed by the character; thus, @samp{?A} for the character @kbd{A}, @samp{?B} for the character @kbd{B}, and @samp{?a} for the -character @kbd{a}. +character @kbd{a}. For example: @@ -257,9 +264,9 @@ character @kbd{a}. You can use the same syntax for punctuation characters, but it is often a good idea to add a @samp{\} so that the Emacs commands for -editing Lisp code don't get confused. For example, @samp{?\ } is the -way to write the space character. If the character is @samp{\}, you -@emph{must} use a second @samp{\} to quote it: @samp{?\\}. +editing Lisp code don't get confused. For example, @samp{?\(} is the +way to write the open-paren character. If the character is @samp{\}, +you @emph{must} use a second @samp{\} to quote it: @samp{?\\}. @cindex whitespace @cindex bell character @@ -278,13 +285,17 @@ way to write the space character. If the character is @samp{\}, you @cindex @samp{\r} @cindex escape @cindex @samp{\e} - You can express the characters Control-g, backspace, tab, newline, -vertical tab, formfeed, return, and escape as @samp{?\a}, @samp{?\b}, -@samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, @samp{?\r}, @samp{?\e}, -respectively. Thus, +@cindex space +@cindex @samp{\s} + You can express the characters control-g, backspace, tab, newline, +vertical tab, formfeed, space, return, del, and escape as @samp{?\a}, +@samp{?\b}, @samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, +@samp{?\s}, @samp{?\r}, @samp{?\d}, and @samp{?\e}, respectively. +(@samp{?\s} followed by a dash has a different meaning---it applies +the ``super'' modifier to the following character.) Thus, @example -?\a @result{} 7 ; @r{@kbd{C-g}} +?\a @result{} 7 ; @r{control-g, @kbd{C-g}} ?\b @result{} 8 ; @r{backspace, @key{BS}, @kbd{C-h}} ?\t @result{} 9 ; @r{tab, @key{TAB}, @kbd{C-i}} ?\n @result{} 10 ; @r{newline, @kbd{C-j}} @@ -292,14 +303,17 @@ respectively. Thus, ?\f @result{} 12 ; @r{formfeed character, @kbd{C-l}} ?\r @result{} 13 ; @r{carriage return, @key{RET}, @kbd{C-m}} ?\e @result{} 27 ; @r{escape character, @key{ESC}, @kbd{C-[}} +?\s @result{} 32 ; @r{space character, @key{SPC}} ?\\ @result{} 92 ; @r{backslash character, @kbd{\}} ?\d @result{} 127 ; @r{delete character, @key{DEL}} @end example @cindex escape sequence These sequences which start with backslash are also known as -@dfn{escape sequences}, because backslash plays the role of an escape -character; this usage has nothing to do with the character @key{ESC}. +@dfn{escape sequences}, because backslash plays the role of an +``escape character''; this terminology has nothing to do with the +character @key{ESC}. @samp{\s} is meant for use in character +constants; in string constants, just write the space. @cindex control characters Control characters may be represented using yet another read syntax. @@ -316,9 +330,9 @@ equivalent to @samp{?\^I} and to @samp{?\^i}: @end example In strings and buffers, the only control characters allowed are those -that exist in @sc{ascii}; but for keyboard input purposes, you can turn +that exist in @acronym{ASCII}; but for keyboard input purposes, you can turn any character into a control character with @samp{C-}. The character -codes for these non-@sc{ascii} control characters include the +codes for these non-@acronym{ASCII} control characters include the @tex @math{2^{26}} @end tex @@ -326,7 +340,7 @@ codes for these non-@sc{ascii} control characters include the 2**26 @end ifnottex bit as well as the code for the corresponding non-control -character. Ordinary terminals have no way of generating non-@sc{ascii} +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. @@ -358,9 +372,8 @@ modifier key. The integer that represents such a character has the @ifnottex 2**27 @end ifnottex -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. +bit set. We use high bits for this and other modifiers to make +possible a wide range of basic character codes. In a string, the @tex @@ -369,11 +382,11 @@ of basic character codes. @ifnottex 2**7 @end ifnottex -bit attached to an @sc{ascii} character indicates a meta character; thus, the -meta characters that can fit in a string have codes in the range from -128 to 255, and are the meta versions of the ordinary @sc{ascii} -characters. (In Emacs versions 18 and older, this convention was used -for characters outside of strings as well.) +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 @@ -383,8 +396,8 @@ or as @samp{?\M-\101}. Likewise, you can write @kbd{C-M-b} as @samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}. The case of a graphic character is indicated by its character code; -for example, @sc{ascii} distinguishes between the characters @samp{a} -and @samp{A}. But @sc{ascii} has no way to represent whether a control +for example, @acronym{ASCII} distinguishes between the characters @samp{a} +and @samp{A}. But @acronym{ASCII} has no way to represent whether a control character is upper case or lower case. Emacs uses the @tex @math{2^{25}} @@ -396,20 +409,22 @@ 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} +the shift bit is @samp{\S-}; thus, @samp{?\C-\S-o} or @samp{?\C-\S-O} represents the shifted-control-o character. @cindex hyper characters @cindex super characters @cindex alt characters - The X Window System defines three other modifier bits that can be set + The X Window System defines three other +@anchor{modifier bits}modifier bits that can be set in a character: @dfn{hyper}, @dfn{super} and @dfn{alt}. The syntaxes for these bits are @samp{\H-}, @samp{\s-} and @samp{\A-}. (Case is significant in these prefixes.) Thus, @samp{?\H-\M-\A-x} represents -@kbd{Alt-Hyper-Meta-x}. +@kbd{Alt-Hyper-Meta-x}. (Note that @samp{\s} with no following @samp{-} +represents the space character.) @tex -Numerically, the -bit values are @math{2^{22}} for alt, @math{2^{23}} for super and @math{2^{24}} for hyper. +Numerically, the bit values are @math{2^{22}} for alt, @math{2^{23}} +for super and @math{2^{24}} for hyper. @end tex @ifnottex Numerically, the @@ -424,9 +439,9 @@ character code in either octal or hex. To use octal, write a question mark followed by a backslash and the octal character code (up to three octal digits); thus, @samp{?\101} for the character @kbd{A}, @samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the -character @kbd{C-b}. Although this syntax can represent any @sc{ascii} +character @kbd{C-b}. Although this syntax can represent any @acronym{ASCII} character, it is preferred only when the precise octal value is more -important than the @sc{ascii} representation. +important than the @acronym{ASCII} representation. @example @group @@ -439,7 +454,7 @@ important than the @sc{ascii} representation. and the hexadecimal character code. You can use any number of hex digits, so you can represent any character code in this way. Thus, @samp{?\x41} for the character @kbd{A}, @samp{?\x1} for the -character @kbd{C-a}, and @code{?\x8e0} for the character +character @kbd{C-a}, and @code{?\x8e0} for the Latin-1 character @iftex @samp{@`a}. @end iftex @@ -452,17 +467,21 @@ a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}. There is no reason to add a backslash before most characters. However, you should add a backslash before any of the characters @samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing -Lisp code. Also add a backslash before whitespace characters such as +Lisp code. You can also add a backslash before whitespace characters such as space, tab, newline and formfeed. However, it is cleaner to use one of -the easily readable escape sequences, such as @samp{\t}, instead of an -actual whitespace character such as a tab. +the easily readable escape sequences, such as @samp{\t} or @samp{\s}, +instead of an actual whitespace character such as a tab or a space. +(If you do write backslash followed by a space, you should write +an extra space after the character constant to separate it from the +following text.) @node Symbol Type @subsection Symbol Type - A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The symbol -name serves as the printed representation of the symbol. In ordinary -use, the name is unique---no two symbols have the same name. + A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The +symbol name serves as the printed representation of the symbol. In +ordinary Lisp use, with one single obarray (@pxref{Creating Symbols}, +a symbol's name is unique---no two symbols have the same name. A symbol can serve as a variable, as a function name, or to hold a property list. Or it may serve only to be distinct from all other Lisp @@ -503,7 +522,7 @@ Lisp, upper case and lower case letters are distinct. Here are several examples of symbol names. Note that the @samp{+} in the fifth example is escaped to prevent it from being read as a number. -This is not necessary in the sixth example because the rest of the name +This is not necessary in the fourth example because the rest of the name makes it invalid as a number. @example @@ -529,7 +548,14 @@ 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. @@ -585,18 +611,10 @@ Lisp are implicit. A @dfn{list} is a series of cons cells, linked together so that the @sc{cdr} slot of each cons cell holds either the next cons cell or the -empty list. @xref{Lists}, for functions that work on lists. Because -most cons cells are used as part of lists, the phrase @dfn{list -structure} has come to refer to any structure made out of cons cells. - - The names @sc{car} and @sc{cdr} derive from the history of Lisp. The -original Lisp implementation ran on an @w{IBM 704} computer which -divided words into two parts, called the ``address'' part and the -``decrement''; @sc{car} was an instruction to extract the contents of -the address part of a register, and @sc{cdr} an instruction to extract -the contents of the decrement. By contrast, ``cons cells'' are named -for the function @code{cons} that creates them, which in turn was named -for its purpose, the construction of cells. +empty list. The empty list is actually the symbol @code{nil}. +@xref{Lists}, for functions that work on lists. Because most cons +cells are used as part of lists, the phrase @dfn{list structure} has +come to refer to any structure made out of cons cells. @cindex atom Because cons cells are so central to Lisp, we also have a word for @@ -604,9 +622,21 @@ for its purpose, the construction of cells. @dfn{atoms}. @cindex parenthesis +@cindex @samp{(@dots{})} in lists The read syntax and printed representation for lists are identical, and consist of a left parenthesis, an arbitrary number of elements, and a -right parenthesis. +right parenthesis. Here are examples of lists: + +@example +(A 2 "A") ; @r{A list of three elements.} +() ; @r{A list of no elements (the empty list).} +nil ; @r{A list of no elements (the empty list).} +("A ()") ; @r{A list of one element: the string @code{"A ()"}.} +(A ()) ; @r{A list of two elements: @code{A} and the empty list.} +(A nil) ; @r{Equivalent to the previous.} +((A B C)) ; @r{A list of one element} + ; @r{(which is a list of three elements).} +@end example Upon reading, each object inside the parentheses becomes an element of the list. That is, a cons cell is made for each element. The @@ -615,8 +645,26 @@ slot refers to the next cons cell of the list, which holds the next element in the list. The @sc{cdr} slot of the last cons cell is set to hold @code{nil}. + The names @sc{car} and @sc{cdr} derive from the history of Lisp. The +original Lisp implementation ran on an @w{IBM 704} computer which +divided words into two parts, called the ``address'' part and the +``decrement''; @sc{car} was an instruction to extract the contents of +the address part of a register, and @sc{cdr} an instruction to extract +the contents of the decrement. By contrast, ``cons cells'' are named +for the function @code{cons} that creates them, which in turn was named +for its purpose, the construction of cells. + +@menu +* Box Diagrams:: Drawing pictures of lists. +* Dotted Pair Notation:: A general syntax for cons cells. +* Association List Type:: A specially constructed list. +@end menu + +@node Box Diagrams +@subsubsection Drawing Lists as Box Diagrams @cindex box diagrams, for lists @cindex diagrams, boxed, for lists + A list can be illustrated by a diagram in which the cons cells are shown as pairs of boxes, like dominoes. (The Lisp reader cannot read such an illustration; unlike the textual notation, which can be @@ -660,26 +708,12 @@ buttercup)}, sketched in a different manner: @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: @@ -694,27 +728,64 @@ depicted with boxes and arrows: @end group @end example -@menu -* Dotted Pair Notation:: An alternative syntax for lists. -* Association List Type:: A specially constructed list. -@end menu + Here is a more complex illustration, showing the three-element list, +@code{((pine needles) oak maple)}, the first element of which is a +two-element list: + +@example +@group + --- --- --- --- --- --- + | | |--> | | |--> | | |--> nil + --- --- --- --- --- --- + | | | + | | | + | --> oak --> maple + | + | --- --- --- --- + --> | | |--> | | |--> nil + --- --- --- --- + | | + | | + --> pine --> needles +@end group +@end example + + The same list represented in the second box notation looks like this: + +@example +@group + -------------- -------------- -------------- +| car | cdr | | car | cdr | | car | cdr | +| o | o------->| oak | o------->| maple | nil | +| | | | | | | | | | + -- | --------- -------------- -------------- + | + | + | -------------- ---------------- + | | car | cdr | | car | cdr | + ------>| pine | o------->| needles | nil | + | | | | | | + -------------- ---------------- +@end group +@end example @node Dotted Pair Notation -@comment node-name, next, previous, up @subsubsection Dotted Pair Notation @cindex dotted pair notation @cindex @samp{.} in lists - @dfn{Dotted pair notation} is an alternative syntax for cons cells -that represents the @sc{car} and @sc{cdr} explicitly. In this syntax, + @dfn{Dotted pair notation} is a general syntax for cons cells that +represents the @sc{car} and @sc{cdr} explicitly. In this syntax, @code{(@var{a} .@: @var{b})} stands for a cons cell whose @sc{car} is -the object @var{a}, and whose @sc{cdr} is the object @var{b}. Dotted -pair notation is therefore more general than list syntax. In the dotted -pair notation, the list @samp{(1 2 3)} is written as @samp{(1 . (2 . (3 -. nil)))}. For @code{nil}-terminated lists, 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. +the object @var{a} and whose @sc{cdr} is the object @var{b}. Dotted +pair notation is more general than list syntax because the @sc{cdr} +does not have to be a list. However, it is more cumbersome in cases +where list syntax would work. In dotted pair notation, the list +@samp{(1 2 3)} is written as @samp{(1 . (2 . (3 . nil)))}. For +@code{nil}-terminated lists, you can use either notation, but list +notation is usually clearer and more convenient. When printing a +list, the dotted pair notation is only used if the @sc{cdr} of a cons +cell is not a list. Here's an example using boxes to illustrate dotted pair notation. This example shows the pair @code{(rose . violet)}: @@ -800,7 +871,7 @@ the list. @example (setq alist-of-colors - '((rose . red) (lily . white) (buttercup . yellow))) + '((rose . red) (lily . white) (buttercup . yellow))) @end example @noindent @@ -839,12 +910,13 @@ Once an array is created, its length is fixed. All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; -you can get the same effect with an array of arrays.) Each type of -array has its own read syntax; see the following sections for details. +you can get the same effect with nested one-dimensional arrays.) Each +type of array has its own read syntax; see the following sections for +details. - The array type is contained in the sequence type and -contains the string type, the vector type, the bool-vector type, and the -char-table type. + The array type is a subset of the sequence type, and contains the +string type, the vector type, the bool-vector type, and the char-table +type. @node String Type @subsection String Type @@ -891,17 +963,17 @@ ignores an escaped newline while reading a string. An escaped space in documentation strings, but the newline is \ ignored if escaped." - @result{} "It is useful to include newlines -in documentation strings, + @result{} "It is useful to include newlines +in documentation strings, but the newline is ignored if escaped." @end example @node Non-ASCII in Strings -@subsubsection Non-@sc{ascii} Characters in Strings +@subsubsection Non-@acronym{ASCII} Characters in Strings - You can include a non-@sc{ascii} international character in a string + You can include a non-@acronym{ASCII} international character in a string constant by writing it literally. There are two text representations -for non-@sc{ascii} characters in Emacs strings (and in buffers): unibyte +for non-@acronym{ASCII} characters in Emacs strings (and in buffers): unibyte and multibyte. If the string constant is read from a multibyte source, such as a multibyte buffer or string, or a file that would be visited as multibyte, then the character is read as a multibyte character, and that @@ -909,9 +981,9 @@ 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-@sc{ascii} character with its + 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-@sc{ascii} character codes are all +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 @@ -920,11 +992,14 @@ one character, @samp{a} with grave accent. @w{@samp{\ }} in a string constant is just like backslash-newline; it does not contribute any character to the string, but it does terminate the preceding hex escape. - Using a multibyte hex escape forces the string to multibyte. You can -represent a unibyte non-@sc{ascii} character with its character code, -which must be in the range from 128 (0200 octal) to 255 (0377 octal). -This forces a unibyte string. - + 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. @@ -940,14 +1015,14 @@ description of the read syntax for characters. However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that -a string can hold are the @sc{ascii} control characters. Strings do not -distinguish case in @sc{ascii} control characters. +a string can hold are the @acronym{ASCII} control characters. Strings do not +distinguish case in @acronym{ASCII} control characters. Properly speaking, strings cannot hold meta characters; but when a string is to be used as a key sequence, there is a special convention -that provides a way to represent meta versions of @sc{ascii} characters in a -string. If you use the @samp{\M-} syntax to indicate a meta character -in a string constant, this sets the +that provides a way to represent meta versions of @acronym{ASCII} +characters in a string. If you use the @samp{\M-} syntax to indicate +a meta character in a string constant, this sets the @tex @math{2^{7}} @end tex @@ -1046,7 +1121,7 @@ Case tables (@pxref{Case Tables}). Character category tables (@pxref{Categories}). @item -Display Tables (@pxref{Display Tables}). +Display tables (@pxref{Display Tables}). @item Syntax tables (@pxref{Syntax Tables}). @@ -1063,17 +1138,26 @@ 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. If the length is not a -multiple of 8, the printed representation shows extra elements, but -these extras really make no difference. +and 0 for @code{nil}). The least significant bits of the character +correspond to the lowest indices in the bool-vector. @example (make-bool-vector 3 t) - @result{} #&3"\007" + @result{} #&3"^G" (make-bool-vector 3 nil) - @result{} #&3"\0" -;; @r{These are equal since only the first 3 bits are used.} + @result{} #&3"^@@" +@end example + +@noindent +These results make sense, because the binary code for @samp{C-g} is +111 and @samp{C-@@} is the character with code 0. + + If the length is not a multiple of 8, the printed representation +shows extra elements, but these extras really make no difference. For +instance, in the next example, the two bool-vectors are equal, because +only the first 3 bits are used: + +@example (equal #&3"\377" #&3"\007") @result{} t @end example @@ -1083,8 +1167,8 @@ these extras really make no difference. 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}. +Hash tables have no read syntax, and print using hash notation. +@xref{Hash Tables}, for functions that operate on hash tables. @example (make-hash-table) @@ -1466,9 +1550,9 @@ positions. @cindex @samp{#@var{n}=} read syntax @cindex @samp{#@var{n}#} read syntax - In Emacs 21, to represent shared or circular structure within a -complex of Lisp objects, you can use the reader constructs -@samp{#@var{n}=} and @samp{#@var{n}#}. + To represent shared or circular structures within a complex of Lisp +objects, you can use the reader constructs @samp{#@var{n}=} and +@samp{#@var{n}#}. Use @code{#@var{n}=} before an object to label it for later reference; subsequently, you can use @code{#@var{n}#} to refer the same object in @@ -1524,7 +1608,6 @@ to a non-@code{nil} value. @xref{Output Variables}. @node Type Predicates @section Type Predicates -@cindex predicates @cindex type checking @kindex wrong-type-argument @@ -1628,6 +1711,9 @@ with references to further information. @item functionp @xref{Functions, functionp}. +@item hash-table-p +@xref{Other Hash, hash-table-p}. + @item integer-or-marker-p @xref{Predicates on Markers, integer-or-marker-p}. @@ -1693,6 +1779,12 @@ with references to further information. @item windowp @xref{Basic Windows, windowp}. + +@item booleanp +@xref{nil and t, booleanp}. + +@item string-or-null-p +@xref{Predicates for Strings, string-or-null-p}. @end table The most general way to check the type of an object is to call the @@ -1734,8 +1826,7 @@ describing the data type. @defun eq object1 object2 This function returns @code{t} if @var{object1} and @var{object2} are -the same object, @code{nil} otherwise. The ``same object'' means that a -change in one will be reflected by the same change in the other. +the same object, @code{nil} otherwise. @code{eq} returns @code{t} if @var{object1} and @var{object2} are integers with the same value. Also, since symbol names are normally @@ -1743,7 +1834,8 @@ unique, if the arguments are symbols with the same name, they are @code{eq}. For other types (e.g., lists, vectors, strings), two arguments with the same contents or elements are not necessarily @code{eq} to each other: they are @code{eq} only if they are the same -object. +object, meaning that a change in the contents of one will be reflected +by the same change in the contents of the other. @example @group @@ -1856,10 +1948,14 @@ always true. @end group @end example +@cindex equality of strings Comparison of strings is case-sensitive, but does not take account of -text properties---it compares only the characters in the strings. -A unibyte string never equals a multibyte string unless the -contents are entirely @sc{ascii} (@pxref{Text Representations}). +text properties---it compares only the characters in the strings. For +technical reasons, a unibyte string and a multibyte string are +@code{equal} if and only if they contain the same sequence of +character codes and all these codes are either in the range 0 through +127 (@acronym{ASCII}) or 160 through 255 (@code{eight-bit-graphic}). +(@pxref{Text Representations}). @example @group @@ -1884,3 +1980,7 @@ returns @code{t} if and only if both the expressions below return 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