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add library for creating generator sequences

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svn: r15803
This commit is contained in:
Jon Rafkind 2009-08-26 20:54:22 +00:00
parent a57b1e71a5
commit f61f933b8b
1 changed files with 56 additions and 0 deletions
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collects/scheme/generator.ss Normal file
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#lang scheme/base
(require (for-syntax scheme/base))
(require scheme/control)
(require scheme/stxparam)
(provide yield lambda-generator define-generator)
(define-syntax-parameter yield (lambda (stx)
(raise-syntax-error #f "yield is only bound inside a sequence generator")))
;; better version of shift/reset using continuation tags
;; also use a unique value to determine the end of the sequence instead of using #f
(define-syntax lambda-generator
(syntax-rules ()
[(_ (args ...) body0 bodies ...)
(lambda (args ...)
(let* ([last (lambda () (void))]
;; current is a function that invokes user code and produces values
[current (lambda ()
;; a unique tag to jump to
(define tag (make-continuation-prompt-tag))
;; give the value to the sequence
(define next (lambda (value)
(shift-at tag f (values value f))))
(syntax-parameterize ([yield (make-rename-transformer #'next)])
(reset-at tag
body0 bodies ...
(values #f last)
)))]
[seq (make-do-sequence (lambda ()
(values
;; produce a value and a continuation
(lambda (i) (let-values ([(value next) (current)])
;; set! is ugly but can we do better?
(set! current next)
value))
(lambda (x) (add1 x))
0
(lambda (x) (not (eq? last current)))
(lambda (v) (not (eq? last current)))
(lambda (x v) (not (eq? last current))))))])
seq))]))
(define-syntax define-generator
(syntax-rules ()
[(_ (name args ...) body0 bodies ...)
(define name (lambda-generator (args ...) body0 bodies ...))]))
#|
;; example
(define-generator (blah)
(for ([x (in-range 0 10)])
(yield x)))
|#