[Vm-dev] A possible Cog bug (was: Re: [squeak-dev] Direct implementation of shift/reset in Smalltalk)

[Levente Uzonyi]

On Thu, 21 Apr 2011, Frank Shearar wrote:

> I finished a preliminary version of the shift/reset control operator for 
> creating and using partial continuations:
>
> http://www.squeaksource.com/Control.html
>
> It's pretty easy to use:
>

snip

> Comments welcome!

I thought it's a bit easier to use notifications instead of control 
operators. Here's an example:

c := [ 3 + PartialContinuationNotification signal ]
 	on: PartialContinuationNotification
 	do: [ :not | not continuation ].
c value: 4. "==> 7"

So I implemented it and uploaded it to 
http://leves.web.elte.hu/squeak/Control-ul.5.mcz . I also refactored the 
code a bit. Even though some Pharo folks always wanted to be able to use 
streams with OrderedCollection (which works in Squeak btw), it's kinda 
pointless thing to use #streamContents: with them, because
OrderedCollections can grow by themselves, unlike Arrays or Strings.
The code works, the tests are green, etc.

But I wasn't happy with the results, because ContextPart already 
implements #copyTo: (which I think can also be simplified by removing 
BlockContext support), which can be used to serialize the stack. So I 
wrote the code, but it didn't work for some reason. The execution always 
ran into the error 'computation has been terminated' in MethodContext >> 
#cannotReturn: and the method context (which stitches the continuation 
onto the stack) was broken (nil in temporaries where should be non nil 
values). Finally I tried it in SqueakVM instead of Cog and it worked. The 
tests are green, the example code works, etc. The code with these changes 
is available at http://leves.web.elte.hu/squeak/Control-ul.6.mcz . I 
prepared an image to ease the debugging process, which is available at 
http://leves.web.elte.hu/squeak/ContextBug.zip .


Levente

P.S.: Feel free to add an issue to the Cog tracker. :)

>
> frank
>
> [1] 
> http://www.lshift.net/blog/2011/04/20/direct-implementation-of-shiftreset-in-smalltalk
>
>