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by default, raise an error when HOLD (or a derived construct like ACCUM-B or COLLECT-B) is used to create a higher-order behavior

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svn: r11071
This commit is contained in:
Greg Cooper 2008-08-04 19:20:31 +00:00
parent 93334db6f5
commit 5daed4abaa
1 changed files with 11 additions and 3 deletions
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14
collects/frtime/lang-ext.ss
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@ -123,7 +123,7 @@
(define switch
(opt-lambda (e [init undefined])
(let* ([init (box init)]
[e-b (hold e (unbox init))]
[e-b (hold e (unbox init) #t)]
[ret (proc->signal:switching
(case-lambda [() (value-now (unbox init))]
[(msg) e])
@ -281,12 +281,20 @@
; hold : a event[a] -> behavior[a]
(define hold
(opt-lambda (e [init undefined])
(let ([val init])
(opt-lambda (e [init undefined] [allow-behaviors? #f])
(let ([val init]
[warn-about-behaviors? #t])
(lift #t (lambda (es) (let ([events (event-set-events es)])
(when (and (= (current-logical-time) (event-set-time es))
(cons? events))
(set! val (first (last-pair (event-set-events es)))))
(when (and (behavior? val) (not allow-behaviors?))
(set! val (value-now val))
(when warn-about-behaviors?
(thread
(lambda ()
(error "hold: input event had a behavior; snapshotting to prevent nested behavior")))
(set! warn-about-behaviors? #f)))
val))
e))))