Dynamic variables

Alaric B. Williams alaric@abwillms.demon.co.uk
Fri, 16 May 1997 20:27:21 +0000

> >Note that recently bound dynamic variables are found very fast,
> >they are at the top of the stack.
> If I start talking about dynamic variables, I'll probably be talking
> nonsense, but here goes...
> Can't dynamic variables be implemented efficiently by storing the
> current value in the global place, and pushing the old global value on
> the stack. When the local binding of the variable goes out of scope
> you pop the old value off the stack and replace the global position
> with the original value. Thus lookups are always constant and fast.

There's a problem with parallel threads there...
> It's a bit the opposite of what most languages do, by making the
> global storage the local lookup, and pushing the old values on the
> stack, rather than the new ones.
Well, I'm toying with the idea of a reified stack as an extension to
continuations. Basically, if a function asks for it's continuation,
it receives a list of call frames, each containing the continuation
of that call, and some optional extra data.

Something like catch/throw would be written by making a call with a special
marker in the stack, which throw looks for and recontinues from there.

Something like dynamic binding would be written by putting alists in
the special markers, and having a (dyna-get '<symbol>) to read them

Many other interesting tricks are also doable with this mechanism...

Alaric B. Williams (alaric@abwillms.demon.co.uk)

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