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included a commented version of multiple-value force

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svn: r6917
This commit is contained in:
Eli Barzilay 2007-07-15 04:08:27 +00:00
parent 3b6e85aad0
commit 9ffbae2a7c
1 changed files with 42 additions and 6 deletions
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48
collects/lazy/promise.ss
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@ -3,11 +3,11 @@
;; This is similar to the *new* version of srfi-45 -- see the post-finalization
;; discussion at http://srfi.schemers.org/srfi-45/ for more details;
;; specifically, this version is the `lazy2' version from
;; http://srfi.schemers.org/srfi-45/post-mail-archive/msg00013.html and `lazy3'
;; in that post shows how to extend it to multiple values. Note: if you use
;; only `force'+`delay' it behaves as in Scheme (except that `force' is
;; identity for non promise values), and `force'+`lazy' are sufficient for
;; implementing the lazy language.
;; http://srfi.schemers.org/srfi-45/post-mail-archive/msg00013.html and (a
;; `lazy3' variant of `force' that deals with multiple values is included and
;; commented). Note: if you use only `force'+`delay' it behaves as in Scheme
;; (except that `force' is identity for non promise values), and `force'+`lazy'
;; are sufficient for implementing the lazy language.
(module promise "mz-without-promises.ss"
(provide lazy delay force promise?)
@ -63,7 +63,12 @@
;; but provided for completeness.)
(define-syntax (delay stx)
(syntax-case stx ()
[(delay expr) (syntax/loc stx (lazy (promise (list expr))))]))
[(delay expr)
(syntax/loc stx
(lazy (promise (list expr)))
;; for use with the multiple-values variant:
;; (lazy (promise (call-with-values (lambda () expr) list)))
)]))
;; iterates on lazy promises (forbid dependency cycles)
;; * (force X) = X for non promises
@ -92,4 +97,35 @@
[(not p) (error 'force "reentrant promise")]
[else (error 'force "invalid promise, contains ~e" p)]))
;; different from srfi-45: identity for non-promises
promise))
#; ; this version deals with multiple values
(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 loop1 ([vals* (call-with-values p list)])
(if (and (pair? vals*)
(null? (cdr vals*))
(promise? (car vals*)))
(let loop2 ([promise* (car vals*)])
(let ([p* (p:ref promise*)])
(p:set! promise* promise) ; share with root
(cond [(procedure? p*) (loop1 (call-with-values p* list))]
[(or (pair? p*) (null? p*))
(p:set! promise p*)
(apply values p*)]
[(promise? p*) (loop2 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 vals*)
(apply values vals*))))]
[(or (pair? p) (null? p)) (apply values 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)))