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svn: r11001
This commit is contained in:
Eli Barzilay 2008-07-31 04:03:28 +00:00
parent fceb977f7b
commit b93cdc5803
1 changed files with 13 additions and 17 deletions
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30
collects/scheme/promise.ss
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@ -4,19 +4,16 @@
(for-syntax '#%kernel "private/stxcase-scheme.ss"))
(#%provide lazy delay force promise?)
;; This module implements "lazy promises" and a `force' that is iterated
;; through them.
;; This module implements "lazy" (composable) promises and a `force'
;; that is iterated through them.
;; 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 (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.
;; 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.
;; 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.
(define (promise-printer promise port write?)
(let loop ([p (promise-val promise)])
@ -70,7 +67,7 @@
;; Creates a promise that does not compose
;; X = (force (delay X)) = (force (lazy (delay X)))
;; = (force (lazy^n (delay X)))
;; X = (force (force (delay (delay X)))) =/= (force (delay (delay X)))
;; X = (force (force (delay (delay X)))) != (force (delay (delay X)))
;; so each sequence of `(lazy^n o delay)^m' requires m `force's and a
;; sequence of `(lazy^n o delay)^m o lazy^k' requires m+1 `force's (for k>0)
;; (This is not needed with a lazy language (see the above URL for details),
@ -105,12 +102,11 @@
;; this is uuused durinc computation to avoid reentrant loops (which makes it
;; non-r5rs, but there's no good uses for allowing that)
(define (running proc)
;; important: be careful not to close over the thunk!
(let ([name (object-name proc)])
;; important: be careful not to close over the thunk!
(lambda ()
(if name
(error 'force "reentrant promise ~v" name)
(error 'force "reentrant promise")))))
(if name
(lambda () (error 'force "reentrant promise ~v" name))
(lambda () (error 'force "reentrant promise")))))
(define (force promise)
(if (promise? promise)