Dynamic scoping (was: Proposal: Squeak-E = Squeak x Kernel-E)

Stephen Pair spair at acm.org
Mon Jan 27 19:41:18 UTC 2003

Anthony wrote:

> Sorry, Stephen, I forgot to mention your work.  Our two 
> implementations are basically equivalent.  I think mine is a 
> little faster since it reuses the context sender chain 
> (hierarchy) instead of creating a new parallel environment 
> hierarchy.  

Is your stuff available?  Will it work on a stock VM?

You mentioned using resumable exceptions directly.  I consider that to
be a poor way to getting a runtime environment (mainly because you'd
need a separate class for getting each variable).  But, I guess you were
speaking of how to do it in the absence of a dynamic context mechanism.

Actually, I've been considering enhancing RuntimeEnvironments use the
context stack as well (it is redundant to manage the parallel
hierarchy).  I didn't do that initially because of some legacy code that
I wasn't sure would map well to a context stack based approach...as it
turns out, it does.  But, the usage above would remain the same.

> This allows me to re-implement Exceptions to use 
> my general context handlers/environments, without loss of 
> efficiency.  Your point about environments being inherited by 
> forked processes is a nice option, but I wouldn't leave out 
> exception handlers from being inherited also.  I believe 
> exception handlers and dynamic variables should be treated 
> the same, not differently.  

Agreed.  The exception handlers should be maintained as runtime
environment values.  And runtime environment lookup should be using the
VMs handler context lookup machinery.

- Stephen

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