ac26fe7554
Also, updates some of the mzlib files to point at `racket/*' libraries rather than to `scheme/*' ones.
117 lines
3.4 KiB
Racket
117 lines
3.4 KiB
Racket
#lang scribble/doc
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@(require scribble/manual scribble/eval "guide-utils.rkt")
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@title[#:tag "pairs"]{Pairs and Lists}
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A @deftech{pair} joins two arbitrary values. The @racket[cons]
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procedure constructs pairs, and the @racket[car] and @racket[cdr]
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procedures extract the first and second elements of the pair,
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respectively. The @racket[pair?] predicate recognizes pairs.
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Some pairs print by wrapping parentheses around the printed forms of
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the two pair elements, putting a @litchar{'} at the beginning and a
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@litchar{.} between the elements.
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@examples[
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(cons 1 2)
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(cons (cons 1 2) 3)
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(car (cons 1 2))
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(cdr (cons 1 2))
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(pair? (cons 1 2))
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]
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A @deftech{list} is a combination of pairs that creates a linked
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list. More precisely, a list is either the empty list @racket[null],
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or it is a pair whose first element is a list element and whose second
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element is a list. The @racket[list?] predicate recognizes lists. The
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@racket[null?] predicate recognizes the empty list.
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A list normally prints as a @litchar{'} followed by a pair of parentheses
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wrapped around the list elements.
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@examples[
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null
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(cons 0 (cons 1 (cons 2 null)))
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(list? null)
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(list? (cons 1 (cons 2 null)))
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(list? (cons 1 2))
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]
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A list or pair prints using @racketresult[list] or @racketresult[cons]
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when one of its elements cannot be written as a @racket[quote]d
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value. For example, a value constructed with @racket[srcloc] cannot be
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written using @racket[quote], and it prints using @racketresult[srcloc]:
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@interaction[
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(srcloc "file.rkt" 1 0 1 (+ 4 4))
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(list 'here (srcloc "file.rkt" 1 0 1 8) 'there)
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(cons 1 (srcloc "file.rkt" 1 0 1 8))
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(cons 1 (cons 2 (srcloc "file.rkt" 1 0 1 8)))
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]
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@margin-note{See also @racket[list*].}
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As shown in the last example, @racketresult[list*] is used to
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abbreviate a series of @racketresult[cons]es that cannot be
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abbreviated using @racketresult[list].
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The @racket[write] and @racket[display] functions print a pair or list
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without a leading @litchar{'}, @racketresult[cons],
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@racketresult[list], or @racketresult[list*]. There is no difference
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between @racket[write] and @racket[display] for a pair or list, except
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as they apply to elements of the list:
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@examples[
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(write (cons 1 2))
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(display (cons 1 2))
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(write null)
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(display null)
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(write (list 1 2 "3"))
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(display (list 1 2 "3"))
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]
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Among the most important predefined procedures on lists are those that
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iterate through the list's elements:
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@interaction[
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(map (lambda (i) (/ 1 i))
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'(1 2 3))
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(andmap (lambda (i) (i . < . 3))
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'(1 2 3))
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(ormap (lambda (i) (i . < . 3))
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'(1 2 3))
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(filter (lambda (i) (i . < . 3))
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'(1 2 3))
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(foldl (lambda (v i) (+ v i))
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10
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'(1 2 3))
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(for-each (lambda (i) (display i))
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'(1 2 3))
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(member "Keys"
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'("Florida" "Keys" "U.S.A."))
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(assoc 'where
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'((when "3:30") (where "Florida") (who "Mickey")))
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]
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@refdetails["pairs"]{pairs and lists}
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Pairs are immutable (contrary to Lisp tradition), and @racket[pair?]
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and @racket[list?] recognize immutable pairs and lists, only. The
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@racket[mcons] procedure creates a @deftech{mutable pair}, which works
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with @racket[set-mcar!] and @racket[set-mcdr!], as well as
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@racket[mcar] and @racket[mcdr]. A mutable pair prints using
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@racketresult[mcons], while @racket[write] and @racket[display] print
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mutable pairs with @litchar["{"] and @litchar["}"]:
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@examples[
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(define p (mcons 1 2))
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p
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(pair? p)
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(mpair? p)
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(set-mcar! p 0)
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p
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(write p)
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]
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@refdetails["mpairs"]{mutable pairs}
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