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* USe a thunk for caught values, not just the exception value, since

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(as Mathew notes as a bug) any value can be raised
*  Actually use an applicable struct, so we can printout something
   sensible.  The current printout is:
     #<promise!exn!...exn-message...> for exceptions
     #<promise!(raise val)> for other values

svn: r10423
This commit is contained in:
Eli Barzilay 2008-06-23 13:57:18 +00:00
parent b1c561f917
commit 3619ab2062
1 changed files with 23 additions and 10 deletions
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33
collects/scheme/promise.ss
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@ -19,12 +19,17 @@
(define (promise-printer promise port write?)
(let loop ([p (promise-val promise)])
(cond [(procedure? p)
(cond [(reraise? p)
(let ([v (reraise-val p)])
(if (exn? v)
(fprintf port "#<promise!exn!~a>" (exn-message v))
(fprintf port (if write? "#<promise!~a>" "#<promise!~s>")
`(raise ,v))))]
[(procedure? p)
(cond [(object-name p)
=> (lambda (n) (fprintf port "#<promise:~a>" n))]
[else (display "#<promise>" port)])]
[(promise? p) (loop (promise-val p))] ; hide sharing
[(exn? p) (display "#<promise!exception>" port)] ; exn when forced
;; values
[(null? p) (fprintf port "#<promise!(values)>")]
[(null? (cdr p))
@ -39,11 +44,12 @@
#:mutable
#:property prop:custom-write promise-printer)
;; A promise value can hold
;; - <thunk>: usually a delayed promise, but can also hold a `running' thunk
;; - <promise>: a shared (redirected) promise that points at another one
;; - (list <value> ...): forced promise (possibly multiple-values, usually one)
;; - <exn>: a forced promise, where an exception happened when forcing
;; - <promise>: a shared (redirected) promise that points at another one
;; - <thunk>: usually a delayed promise,
;; - can also hold a `running' thunk that will throw a reentrant error
;; - can also hold a raising-a-value thunk on exceptions and other
;; `rais'ed values (actually, applicable structs for printouts)
;; Creates a `composable' promise
;; X = (force (lazy X)) = (force (lazy (lazy X))) = (force (lazy^n X))
(define-syntax (lazy stx)
@ -53,6 +59,13 @@
'inferred-name (syntax-local-name))])
(syntax/loc stx (make-promise proc)))]))
;; use this to create a value to be raised, make it a procedure so no other
;; code need to change (we could just use the exceptions -- but any value can
;; be raised); also make it have a proper printer so we can show such promises
;; properly.
(define-struct reraise (val)
#:property prop:procedure (lambda (this) (raise (reraise-val this))))
;; Creates a promise that does not compose
;; X = (force (delay X)) = (force (lazy (delay X)))
;; = (force (lazy^n (delay X)))
@ -81,14 +94,15 @@
(cond [(procedure? p*) (loop1 (p*))]
[(promise? p*) (loop2 p*)]
[else (set-promise-val! root p*)
(cond [(exn? p*) (raise p*)]
[(null? p*) (values)]
(cond [(null? p*) (values)]
[(null? (cdr p*)) (car p*)]
[else (apply values p*)])])))
(begin ; error here for "library approach" (see above URL)
(set-promise-val! root (list v))
v))))
;; 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)
(let ([name (object-name proc)])
;; important: be careful not to close over the thunk!
@ -104,10 +118,9 @@
;; "mark" root as running (avoids cycles)
(set-promise-val! promise (running p))
(call-with-exception-handler
(lambda (exn) (set-promise-val! promise exn) exn)
(lambda (e) (set-promise-val! promise (make-reraise e)) e)
(lambda () (force-proc p promise)))]
[(promise? p) (loop (promise-val p))]
[(exn? p) (raise p)]
[(null? p) (values)]
[(null? (cdr p)) (car p)]
[else (apply values p)]))