Re: proposed support for lazy values

[Per Bothner <per at bothner dot com>]

On 06/03/2011 03:25 PM, Jamison Hope wrote:
> So the mechanism for creating an object of type lazy[T] would be to call
> (delay EXP), where EXP is an expression that would evaluate to an instance
> of T?
>
> So for instance:
> (define the-answer ::lazy[integer] (delay (* 2 3 7)))
> ;;; Right now, the multiplication hasn't happened yet
> (+ the-answer 0) ;; <- implicit force here as context requires an integer

Correct.

> By the way, I notice that force() in kawa.lang.Promise checks result==null
> twice, presumably as a lockless mechanism to make sure the result is only
> memoized once in case two threads both force it simultaneously. It seems
> like
> this could still lead to trouble, if the promised computation has side
> effects.
> Even though the result seen by all threads will be the same (usually,
> though I
> think you'd need an AtomicReference and compareAndSet in there to be
> absolutely
> sure), there's still nothing preventing the promise from being evaluated N
> times:
>
> #|kawa:14|# (define my-promise (delay (begin (display "Ding!") (newline))))
> #|kawa:15|# (do ((i 0 (+ i 1))) ((= i 10)) ((java.lang.Thread (runnable
> (lambda () (force my-promise)))):start))
> Ding!
> Ding!
> Ding!
> Ding!
> Ding!
> Ding!
>
>
> That may be an acceptable trade-off to avoid using a synchronized block,
> but it seems
> like something that should be documented.

Could you try the attached patch?  A review would be appreciated as well.
--
	--Per Bothner
per@bothner.com   http://per.bothner.com/

Attachment: Promise.patch
Description: Text document