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delay can deal with multiple values, lazy cannot

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svn: r6918
This commit is contained in:
Eli Barzilay 2007-07-15 06:33:47 +00:00
parent 9ffbae2a7c
commit 8196473712
2 changed files with 54 additions and 8 deletions
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5
collects/lazy/doc.txt
View File

@ -136,6 +136,11 @@ for example,
requires three `force's to evaluate E.
Note: `lazy' cannot be used with an expression that evaluates to
multiple values. `delay' is, however, is fine with multiple values.
(This is for efficiency in the lazy language, where multiple values
are avoided.)
As mentioned above, using `delay' and `force' is as in Scheme, except
for two differences. The first is a technicality -- force is an
identity for non-promise values. This makes it more convenient in

57
collects/lazy/promise.ss
View File

@ -64,16 +64,16 @@
(define-syntax (delay stx)
(syntax-case stx ()
[(delay expr)
;; see below for using multiple-values:
(syntax/loc stx
(lazy (promise (list expr)))
;; for use with the multiple-values variant:
;; (lazy (promise (call-with-values (lambda () expr) list)))
)]))
(lazy (promise (call-with-values (lambda () expr) list))))]))
;; iterates on lazy promises (forbid dependency cycles)
;; force iterates on lazy promises (forbid dependency cycles)
;; * (force X) = X for non promises
;; * does not deal with multiple values, since they're not used by the lazy
;; language (but would be easy to add them)
#; ; this version cannot handle multiple values
(define (force promise)
(if (promise? promise)
(let loop ([p (p:ref promise)])
@ -88,7 +88,8 @@
[(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*)]))
[else (error 'force
"invalid promise, contains ~e" p*)]))
(begin ; error here for "library approach" (see above URL)
(p:set! promise (list promise*))
promise*)))]
@ -99,7 +100,7 @@
;; different from srfi-45: identity for non-promises
promise))
#; ; this version deals with multiple values
#; ; this version works properly with multiple values
(define (force promise)
(if (promise? promise)
(let loop ([p (p:ref promise)])
@ -119,7 +120,47 @@
(apply values p*)]
[(promise? p*) (loop2 p*)]
[(not p*) (error 'force "reentrant promise")]
[else (error 'force "invalid promise, contains ~e" p*)])))
[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))
;; finally: this version deals with multiple values only in `delay'
;; (technicality: 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)
(if (promise? promise)
(let loop ([p (p:ref promise)])
(cond
[(procedure? p)
(p:set! promise #f) ; mark root for cycle detection
(let ([vals* (call-with-values p list)])
(if (and (pair? vals*)
(null? (cdr vals*)))
(let loop1 ([val* (car vals*)])
(if (promise? val*)
(let loop2 ([promise* val*])
(let ([p* (p:ref promise*)])
(p:set! promise* promise) ; share with root
(cond [(procedure? p*) (loop1 (p*))]
[(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*)
val*)))
(begin ; error here for "library approach" (see above URL)
(p:set! promise vals*)
(apply values vals*))))]