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Clarify match's app pattern behavior (#2306)

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If I have understood the behavior of ```app``` correctly, the documentation needs to be clarified. Currently it implies that match expects a single result, and one could reasonably infer that the matcher will require that single result to satisfy all ```_pat```s given. The error message resulting from a reasonable-but-wrong pattern like ```(app string->number (? number?) x)``` is inscrutable, so a doc fix is important here.

I have also added an example that illustrates the use case that led me to submit this PR.
This commit is contained in:
Jordan Johnson 2018-10-10 06:27:21 -07:00 committed by Sam Tobin-Hochstadt
parent 2b9c0c4689
commit 5376a4b409
1 changed files with 9 additions and 2 deletions
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11
pkgs/racket-doc/scribblings/reference/match.scrbl
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@ -360,15 +360,22 @@ In more detail, patterns match as follows:
]}
@item{@racket[(#,(racketidfont "app") _expr _pats ...)] --- applies
@racket[_expr] to the value to be matched; the result of the
application is matched against @racket[_pats].
@racket[_expr] to the value to be matched; each result of the
application is matched against one of the @racket[_pats],
respectively.
@examples[
#:eval match-eval
(match '(1 2)
[(app length 2) 'yes])
(match "3.14"
[(app string->number (? number? pi))
`(I got ,pi)])
(match '(1 2)
[(app (lambda (v) (split-at v 1)) '(1) '(2)) 'yes])
(match '(1 2 3)
[(app (λ (ls) (apply values ls)) x y (? odd? z))
(list 'yes x y z)])
]}
@item{@racket[(#,(racketidfont "?") _expr _pat ...)] --- applies