[Sablevm-developer] Continuations

[Etienne M. G. <eg...@j-...>]

For the record.

-------- Original Message --------
Date: Wed, 19 Jul 2000 21:29:12 -0700
From: Brent Fulgham <bfu...@de...>
To: "Etienne M. Gagnon" <eg...@j-...>

...
On another subject:  Continuations.

I was thinking that it would probably be possible to simulate
continuations at the JVM level by using the same mechanisms that
are used for the exception handling.  I'm not sure if this is an
efficient way to go about this, but in theory it should work.

I'm not really sure how I could even pseudo-code this in Java,
but in a pseudo-code:

...
1. Execute some statements
2. Source code calls for a "call-with-current-continuation"
3. JVM creates an exception that holds the current state of 
   this routine (i.e., must make a copy of the current variables)
4. JVM starts a "Try" block
4. Execution continues
5. Eventually, a condition is reached where we want to jump 
   back to the continuation.   So the JVM "Throws" that
   exception
6. We unwind until we hit the top of the "Try" block.  This
   is also coded as the "Catch", which restores state to 
   where we were when we hit the start of the continuation.

In fact, I'm not sure how much of this might be automatic
with respect to the stack unwinding features.  Is it possible
we might find ourselves back where we started with the 'stack'
in the proper state just automatically?  I suppose it's too
much to hope for...

The nice thing about this idea is that, again, we don't have to
do anything new -- we make use of existing JVM features.  The
Scheme (or whatever) compiler that targets the JVM would just
have to know how to generate the code that starts / ends the
continuation...

What do you think?  Terrible idea? :-)

Regards,

-Brent