Re: proposed support for lazy values

[Jamison Hope <jrh at theptrgroup dot com>]

On Jun 7, 2011, at 2:50 PM, Per Bothner wrote:

> 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.

Looks good to me.

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

I presume by "RnRS semantics" you're referring to the sample  
implementation
of make-promise in the R5RS and R6RS reports?

> 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.

Right. When that happens, then we'll see output like this, right?

#|kawa:1|# (define a (delay 1))
#|kawa:2|# (define b (delay 2))
#|kawa:3|# (define c (delay 3))
#|kawa:4|# (write a) ; no implicit force
#<promise - not forced yet>
#|kawa:5|# (display b) ; implicit force
2
#|kawa:6|# c ; implicit display, which does implicit force
3


Calls to format will also implicitly force, right? The documentation  
for the
~A directive says "Any (print as display does)." So,
 (format #f "~A~%" (delay 4)) => "4"



--
Jamison Hope
The PTR Group
www.theptrgroup.com