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Committing a change to move to an older force that doesn't infinite loop profj

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svn: r6933
This commit is contained in:
Kathy Gray 2007-07-18 21:31:40 +00:00
parent 54e020193e
commit b5861250f7
1 changed files with 27 additions and 1 deletions
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28
collects/lazy/promise.ss
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@ -143,7 +143,7 @@
;; actually it doesn't work with `lazy' holding `lazy' of multiple values, so
;; `lazy' works with multiple values unless rewrapped in `lazy'.)
;; #;
(define (force promise)
#;(define (force promise)
(if (promise? promise)
(let loop ([p (p:ref promise)])
(cond
@ -178,6 +178,32 @@
;; different from srfi-45: identity for non-promises
promise))
(define (force promise)
(if (promise? promise)
(let loop ([p (p:ref promise)])
(cond
[(procedure? p)
(p:set! promise #f) ; mark root for cycle detection
(let loop ([promise* (p)])
(if (promise? promise*)
(let ([p* (p:ref promise*)])
(p:set! promise* promise) ; share with root
(cond [(procedure? p*) (loop (p*))]
[(pair? p*) (p:set! promise p*) (car p*)]
[(promise? p*) (loop p*)]
[(not p*) (error 'force "reentrant promise")]
[else (error 'force
"invalid promise, contains ~e" p*)]))
(begin ; error here for "library approach" (see above URL)
(p:set! promise (list promise*))
promise*)))]
[(pair? p) (car p)]
[(promise? p) (loop (p:ref p))]
[(not p) (error 'force "reentrant promise")]
[else (error 'force "invalid promise, contains ~e" p)]))
;; different from srfi-45: identity for non-promises
promise))
#|
Timing results (#1, #2, #3 are the above versions), in Lazy Scheme: