@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{?}:
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
@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
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
@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
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
@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
@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
@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
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.
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
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.)
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.
If @var{nosqueeze} is non-@code{nil}, that means to leave whitespace
other than line breaks untouched. If @var{to-eop} is non-@code{nil},
-that means to keep filling to the end of the paragraph---or next hard
+that means to keep filling to the end of the paragraph---or the next hard
newline, if @code{use-hard-newlines} is enabled (see below).
The variable @code{paragraph-separate} controls how to distinguish
If @var{nosqueeze} is non-@code{nil}, that means to leave whitespace
other than line breaks untouched. If @var{squeeze-after} is
-non-@code{nil}, specifies a position in the region, and means don't
+non-@code{nil}, it specifies a position in the region, and means don't
canonicalize spaces before that position.
In Adaptive Fill mode, this command calls @code{fill-context-prefix} to
@defopt adaptive-fill-regexp
This variable holds a regular expression to control Adaptive Fill mode.
-Whichever characters starting after the line's left margin match this
-regular expression, those are the candidate for the fill prefix.
+Adaptive Fill mode matches this regular expression against the text
+starting after the left margin whitespace (if any) on a line; the
+characters it matches are that line's candidate for the fill prefix.
@end defopt
@defopt adaptive-fill-first-line-regexp
-In a one-line paragraph, if the candidate fill prefix matches
-this regular expression, or if it matches @code{comment-start-skip},
-then it is used---otherwise, it is replaced with an equivalent
-number of spaces.
+In a one-line paragraph, if the candidate fill prefix matches this
+regular expression, or if it matches @code{comment-start-skip}, then it
+is used---otherwise, spaces amounting to the same width are used
+instead.
However, the fill prefix is never taken from a one-line paragraph
if it would act as a paragraph starter on subsequent lines.
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