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document #:when for match.

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Sam Tobin-Hochstadt 2013-10-22 15:28:36 -04:00
parent f9616959e9
commit da62a098de
1 changed files with 11 additions and 3 deletions
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14
pkgs/racket-pkgs/racket-doc/scribblings/reference/match.scrbl
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@ -17,7 +17,8 @@ on regular-expression matching on strings, bytes, and streams.
@defform/subs[(match val-expr clause ...)
([clause [pat body ...+]
[pat (=> id) body ...+]])]{
[pat (=> id) body ...+]
[pat #:when cond-expr body ...+]])]{
Finds the first @racket[pat] that matches the result of
@racket[val-expr], and evaluates the corresponding @racket[body]s with
@ -28,14 +29,21 @@ the @racket[match] expression.
To find a match, the @racket[clause]s are tried in order. If no
@racket[clause] matches, then the @exnraise[exn:misc:match?].
An optional @racket[#:when cond-expr] specifies that the pattern
should only match if @racket[cond-expr] produces a true value.
@racket[cond-expr] is in the scope of all of the variables bound in
@racket[pat]. @racket[cond-expr] must not mutate the object being
matched before calling the failure procedure, otherwise the behavior
of matching is unpredictable. See also @racket[failure-cont], which is
a lower-level mechanism achieving the same ends.
An optional @racket[(=> id)] between a @racket[pat] and the
@racket[body]s is bound to a @defterm{failure procedure} of zero
arguments. If this procedure is invoked, it escapes back to the
pattern matching expression, and resumes the matching process as if
the pattern had failed to match. The @racket[body]s must not mutate
the object being matched before calling the failure procedure,
otherwise the behavior of matching is unpredictable. See also
@racket[failure-cont].
otherwise the behavior of matching is unpredictable.
The grammar of @racket[pat] is as follows, where non-italicized
identifiers are recognized symbolically (i.e., not by binding).