Re: [Jscheme-devel] Patch to allow multiple independent JScheme instances

[Toby A. <tob...@pe...>]

On Tue, May 11, 2004 at 11:35:51PM -0400, Timothy John Hickey wrote:
> On May 10, 2004, at 6:35 PM, Toby Allsopp wrote:
> > In this example, the variable called 'x is looked up in three
> > different, nested evaluators.  As each .eval finishes, the current
> > evaluator must be restored to what it was when the .eval was called.
> 
> So the stacks are needed because the implementation relies on
> threadlocal variables and you need to reset the values of the
> threadlocal variables when the evaluator is complete?

Yes, that sums it up exactly.  The implementation needs thread-local
variables because the existing code relies on calls to static
jsint.Scheme methods and I didn't see a clear direction for changing
that.

> > > Perhaps there are static methods in Scheme.java and other classes
> > > that should simply be removed.  For example, Scheme.runJscheme()
> > > should no longer be static, so the currentEvaluator() call in that
> > > method wouldn't be necessary.  runJscheme() would have to be moved
> > > to Evaluator, i guess.  I think getting rid of the Scheme class
> > > might be the way to go.
> 
> I haven't looked at Toby's code yet, but I wonder if we can get by
> without threadlocal variables? Perhaps a major revision (of
> Scheme.java, Primitives.java, ...) would eliminate the need for this
> complication (and the stacks of interpreters....)

I actually think that a stack of thread-local variables will turn out to
be the best solution, but I'm a big fan of Common Lisp-style special
variables.  The alternative is to pass a reference to the appropriate
Evaluator instance through every method call, which seems a bit
inelegant.  Something like AspectJ might help here, although I don't
imagine that fits in with other goals of the project.

Toby.