Re: proposed support for lazy values

[Jamison Hope <jrh at theptrgroup dot com>]

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
	at atInteractiveLevel$1.lambda1(stdin:1)
	at atInteractiveLevel$1.apply0(stdin)
	at gnu.expr.ModuleMethod.apply0(ModuleMethod.java:186)
	at kawa.lang.Promise.force(Promise.java:36)
	at kawa.lang.Promise.force(Promise.java:53)
	at kawa.lib.misc.force(misc.scm:99)
	at atInteractiveLevel$3.run(stdin:3)
	at gnu.expr.ModuleExp.evalModule2(ModuleExp.java:286)
	at gnu.expr.ModuleExp.evalModule(ModuleExp.java:187)
	at kawa.Shell.run(Shell.java:281)
	at kawa.Shell.run(Shell.java:194)
	at kawa.Shell.run(Shell.java:175)
	at kawa.repl.main(repl.java:852)
#|kawa:4|# my-promise
#<promise - not forced yet>

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?

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

Attachment: Promise2.patch
Description: Binary data