Re: proposed support for lazy values

[Per Bothner <per at bothner dot com>]

On 06/06/2011 07:28 AM, Jamison Hope wrote:
> On Jun 4, 2011, at 3:15 AM, Per Bothner wrote:
>
> > Could you try the attached patch? A review would be appreciated as well.
>
> That works for me. Good catch with initializing result to something
> other than null. I suppose someone could still write a promise that
> evaluates to Location.UNBOUND, but that's probably unlikely enough to
> ignore. There's still a minor deficiency in print():
>
> #|kawa:1|# (define my-promise (delay (throw (java.lang.Exception))))
> #|kawa:2|# my-promise
> #<promise - not forced yet>
> #|kawa:3|# (force my-promise)
> java.lang.Exception
>
> I would argue that the print at line 4 should say something like
> "#<promise - force threw a java.lang.Exception>" instead. How about the
> attached update to your patch?

I took your suggestion, but added synchronization.  See the attachment.

Note I also changed force to add an extra check after the apply0 call, to
match RnRS semantics for "nested" forcing.

Note also that after the grand plan calls for auto-force by display and 
the REPL;
at that point the change to print will primarily affect calls to write.
--
	--Per Bothner
per@bothner.com   http://per.bothner.com/

Attachment: Promise3.patch
Description: Text document