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`syntax-id-rules' clarifications

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Closes PR 12232
This commit is contained in:
Matthew Flatt 2011-11-27 11:05:56 -07:00
parent fca5ceecc9
commit a707b0c5f5
1 changed files with 10 additions and 3 deletions
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13
collects/scribblings/guide/pattern-macros.scrbl
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@ -268,9 +268,11 @@ it creates a transformer that acts as an identifier macro:
...))]
Unlike a @racket[syntax-rules] form, the @racket[_pattern]s are not
required to start with an open parenthesis. Also, @racket[set!] is
typically used as a literal to match a use of @racket[set!] in the
pattern (as opposed to being a pattern variable.
required to start with an open parenthesis. In addition,
@racket[syntax-id-rules] cooperates specially with @racket[set!], so
that @racket[set!] invokes the macro when @racket[_id] is the target
of an assignment; consequently, @racket[set!] is typically used as a
literal with @racket[syntax-id-rules] to match such uses of @racket[set!].
@racketblock[
(define-syntax clock
@ -278,6 +280,11 @@ pattern (as opposed to being a pattern variable.
[(set! clock e) (put-clock! e)]
[(clock a ...) ((get-clock) a ...)]
[clock (get-clock)]))
(define-values (get-clock put-clock!)
(let ([private-clock 0])
(values (lambda () private-clock)
(lambda (v) (set! private-clock v)))))
]
The @racket[(clock a ...)] pattern is needed because, when an