In what seems to have become an autumn tradition in the Parrot
community, I am about to make my third annual attempt to implement
Parrot support for what Common Lisp calls "special variable binding."
Most of the rest of the world calls it "dynamic binding" or "dynamic
scoping" [1]; after last fall's email discussion [2], I am hoping that
"dynamic variable scoping" will capture the, ah, scope of this
particular project in a way that is satisfactory to everyone.
In the aforementioned thread [2], we seem to have reached a number of
conclusions about what "dynamic variable scoping" should mean. I have
summarized these below, to be sure we're all on the same page. If I
don't hear objections in the next week, I will start updating the
proposal.
FWIW, I am (re-)starting this project a few weeks earlier than I did
last year, so I am supremely confident that I will have the third
implementation ready by Christmas. At which time, if tradition holds,
the Powers That Be will veto it. ;-)
-- Bob Rogers
http://rgrjr.dyndns.org/
Project summary:
1. The dynamic variable scoping mechanism will address only global
variables, not registers or slots in aggreggates (e.g. arrays).
2. This mechanism will do binding, rather than assignment, where
"binding" is defined thus: "A binding replaces the PMC stored in the
namespace or lexical pad with another PMC [3]."
3. It may be possible to use dynamic variable scoping to implement
Perl 5 "local" and Perl 6 "temporization" in the case of binding of
global variables. However, full implementation of these features would
have to go far beyond this, so support for "temporization" is not a
design goal.
4. Dynamic variable binding state must be stored on the dynamic
environment stack, so that dynamic values are available to called subs,
and can be captured and restored by continuations. This in turn
requires new instructions to manipulate dynamic bindings on the dynamic
environment stack.
5. Dynamic variable bindings must be visible only within that
thread. Besides being useful in its own right, this makes it possible
to use thread-scoped dynamic variables to implement thread-scoped
dynamic control structures (e.g. coroutines, Lisp CATCH/THROW).
6. The implementation should not multiply PMC classes beyond
necessity.
References:
[1] http://en.wikipedia.org/wiki/Dynamic_variable_scoping#Dynamic_scoping
[2]
http://groups.google.com/group/perl.perl6.internals/browse_thread/thread/1fd0d3b432c2e425/fb9071f9697552a3?hl=en&lnk=st
[3] Quoting Allison's message of 7-Dec-2006 from thread [2].
