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Diffstat (limited to 'lispref/objects.texi')
| -rw-r--r-- | lispref/objects.texi | 99 |
1 files changed, 54 insertions, 45 deletions
diff --git a/lispref/objects.texi b/lispref/objects.texi index f2c082b56b..ccdc1756fe 100644 --- a/lispref/objects.texi +++ b/lispref/objects.texi @@ -321,8 +321,8 @@ $2^{26}$ @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. +control characters, but you can generate them straightforwardly using X +and other window systems. For historical reasons, Emacs treats the @key{DEL} character as the control equivalent of @kbd{?}: @@ -433,7 +433,7 @@ 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{?\x8c0} for the character +character @kbd{C-a}, and @code{?\x8e0} for the character @iftex @samp{@`a}. @end iftex @@ -553,17 +553,21 @@ same object, @code{nil}. @subsection Cons Cell and List Types @cindex address field of register @cindex decrement field of register +@cindex pointers - A @dfn{cons cell} is an object comprising two pointers named the -@sc{car} and the @sc{cdr}. Each of them can point to any Lisp object. + A @dfn{cons cell} is an object that consists of two pointers or slots, +called the @sc{car} slot and the @sc{cdr} slot. Each slot can +@dfn{point to} or hold to any Lisp object. We also say that the ``the +@sc{car} of this cons cell is'' whatever object its @sc{car} slot +currently points to, and likewise for the @sc{cdr}. A @dfn{list} is a series of cons cells, linked together so that the -@sc{cdr} of each cons cell points either to another cons cell or to the +@sc{cdr} slot of each cons cell holds either the next cons cell or the empty list. @xref{Lists}, for functions that work on lists. Because most cons cells are used as part of lists, the phrase @dfn{list structure} has come to refer to any structure made out of cons cells. - The names @sc{car} and @sc{cdr} have only historical meaning now. The + The names @sc{car} and @sc{cdr} derive from the history of Lisp. The original Lisp implementation ran on an @w{IBM 704} computer which divided words into two parts, called the ``address'' part and the ``decrement''; @sc{car} was an instruction to extract the contents of @@ -584,18 +588,19 @@ right parenthesis. Upon reading, each object inside the parentheses becomes an element of the list. That is, a cons cell is made for each element. The -@sc{car} of the cons cell points to the element, and its @sc{cdr} points -to the next cons cell of the list, which holds the next element in the -list. The @sc{cdr} of the last cons cell is set to point to @code{nil}. +@sc{car} slot of the cons cell points to the element, and its @sc{cdr} +slot points 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 +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)}: +shown as pairs of boxes, like dominoes. (The Lisp reader cannot read +such an illustration; unlike the textual notation, which can be +understood by both humans and computers, the box illustrations can be +understood only by humans.) This picture represents the three-element +list @code{(rose violet buttercup)}: @example @group @@ -608,18 +613,18 @@ violet buttercup)}: @end group @end example - In this diagram, each box represents a slot that can refer to any Lisp + In this diagram, each box represents a slot that can point 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. +pointer to a Lisp object, either an atom or another cons cell. - In this example, the first box, the @sc{car} of the first cons cell, -refers to or ``contains'' @code{rose} (a symbol). The second box, the -@sc{cdr} of the first cons cell, refers to the next pair of boxes, the -second cons cell. The @sc{car} of the second cons cell refers to -@code{violet} and the @sc{cdr} refers to the third cons cell. The -@sc{cdr} of the third (and last) cons cell refers to @code{nil}. + In this example, the first box, which holds the @sc{car} of the first +cons cell, points to or ``contains'' @code{rose} (a symbol). The second +box, holding the @sc{cdr} of the first cons cell, points to the next +pair of boxes, the second cons cell. The @sc{car} of the second cons +cell is @code{violet}, and its @sc{cdr} is the third cons cell. The +@sc{cdr} of the third (and last) cons cell is @code{nil}. -Here is another diagram of the same list, @code{(rose violet + Here is another diagram of the same list, @code{(rose violet buttercup)}, sketched in a different manner: @smallexample @@ -683,13 +688,13 @@ that represents the @sc{car} and @sc{cdr} explicitly. In this syntax, 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. +. nil)))}. For @code{nil}-terminated lists, you can use either +notation, but list notation is usually clearer and more convenient. +When printing a list, the dotted pair notation is only used if the +@sc{cdr} of a cons cell is not a list. - Here's how box notation can illustrate dotted pairs. This example -shows the pair @code{(rose . violet)}: + Here's an example using boxes to illustrate dotted pair notation. +This example shows the pair @code{(rose . violet)}: @example @group @@ -702,10 +707,12 @@ shows the pair @code{(rose . violet)}: @end group @end example - Dotted pair notation can be combined with list notation to represent a -chain of cons cells with a non-@code{nil} final @sc{cdr}. For example, -@code{(rose violet . buttercup)} is equivalent to @code{(rose . (violet -. buttercup))}. The object looks like this: + You can combine dotted pair notation with list notation to represent +conveniently a chain of cons cells with a non-@code{nil} final @sc{cdr}. +You write a dot after the last element of the list, followed by the +@sc{cdr} of the final cons cell. For example, @code{(rose violet +. buttercup)} is equivalent to @code{(rose . (violet . buttercup))}. +The object looks like this: @example @group @@ -718,11 +725,12 @@ chain of cons cells with a non-@code{nil} final @sc{cdr}. For example, @end group @end example - These diagrams make it evident why @w{@code{(rose .@: violet .@: -buttercup)}} is invalid syntax; it would require a cons cell that has -three parts rather than two. + The syntax @code{(rose .@: violet .@: buttercup)} is invalid because +there is nothing that it could mean. If anything, it would say to put +@code{buttercup} in the @sc{cdr} of a cons cell whose @sc{cdr} is already +used for @code{violet}. - The list @code{(rose violet)} is equivalent to @code{(rose . (violet))} + The list @code{(rose violet)} is equivalent to @code{(rose . (violet))}, and looks like this: @example @@ -783,7 +791,7 @@ functions that work on alists. @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 +pointing to other Lisp objects, arranged in a contiguous block of memory. Accessing any element of an array takes approximately the same amount of time. In contrast, accessing an element of a list requires time proportional to the position of the element in the list. (Elements @@ -883,8 +891,9 @@ character code, using a hex escape, @samp{\x@var{nnnnnnn}}, with as many digits as necessary. (Multibyte non-@sc{ASCII} character codes are all greater than 256.) Any character which is not a valid hex digit terminates this construct. If the character that would follow is a hex -digit, write @w{@samp{\ }} to terminate the hex escape---for example, -@w{@samp{\x8c0\ }} represents one character, @samp{a} with grave accent. +digit, write @w{@samp{\ }} (backslash and space) +to terminate the hex escape---for example, +@w{@samp{\x8e0\ }} represents one character, @samp{a} with grave accent. @w{@samp{\ }} in a string constant is just like backslash-newline; it does not contribute any character to the string, but it does terminate the preceding hex escape. @@ -914,7 +923,7 @@ distinguish case in @sc{ASCII} control characters. Properly speaking, strings cannot hold meta characters; but when a string is to be used as a key sequence, there is a special convention -that allows the meta versions of @sc{ASCII} characters to be put in a +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 @tex @@ -965,7 +974,7 @@ that range. For example, represents a string whose textual contents are @samp{foo bar}, in which the first three characters have a @code{face} property with value @code{bold}, and the last three have a @code{face} property with value -@code{italic}. (The fourth character has no text properties so its +@code{italic}. (The fourth character has no text properties, so its property list is @code{nil}. It is not actually necessary to mention ranges with @code{nil} as the property list, since any characters not mentioned in any range will default to having no properties.) @@ -1032,8 +1041,8 @@ that it begins with @samp{#&} followed by the length. The string constant that follows actually specifies the contents of the bool-vector as a bitmap---each ``character'' in the string contains 8 bits, which specify the next 8 elements of the bool-vector (1 stands for @code{t}, -and 0 for @code{nil}). The least significant bits of the character are -the lowest-numbered elements of the bool-vector. If the length is not a +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. |