42b8163f8f
The Guide was was out-of-date in several ways, and the Reference didn't describe the dispatch-time guarantee.
47 lines
1.2 KiB
Racket
47 lines
1.2 KiB
Racket
#lang scribble/doc
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@(require scribble/manual scribble/eval "guide-utils.rkt"
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(for-label racket/match))
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@title[#:tag "case"]{Simple Dispatch: @racket[case]}
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The @racket[case] form dispatches to a clause by matching the result
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of an expression to the values for the clause:
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@specform[(case expr
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[(datum ...+) body ...+]
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...)]
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Each @racket[_datum] will be compared to the result of @racket[_expr]
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using @racket[equal?], and then the corresponding @racket[body]s are
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evaluated. The @racket[case] form can dispatch to the correct clause
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in @math{O(log N)} time for @math{N} @racket[datum]s.
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Multiple @racket[_datum]s can be supplied for each clause, and the
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corresponding @racket[_body]s are evaluated if any of the
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@racket[_datum]s match.
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@examples[
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(let ([v (random 6)])
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(printf "~a\n" v)
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(case v
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[(0) 'zero]
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[(1) 'one]
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[(2) 'two]
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[(3 4 5) 'many]))
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]
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The last clause of a @racket[case] form can use @racket[else], just
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like @racket[cond]:
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@examples[
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(case (random 6)
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[(0) 'zero]
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[(1) 'one]
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[(2) 'two]
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[else 'many])
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]
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For more general pattern matching (but without the dispatch-time
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guarantee), use @racket[match], which is introduced in
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@secref["match"].
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