On Tue, 18 Mar 2003, Matthijs van Duin wrote: > and maybe also: > What is the current plan? > > although I got the impression earlier that there isn't any yet for invoking > subrules :-)
See line 1014, languages/perl6/P6C/rule.pm. The hack I used was to call rules like ordinary subs, and have them push marks onto the regex stack before they return. I'm not sure if this can be made to work with hypotheticals, and I'm sure it won't interact kindly with continuation-taking, but there's _something_. As for the interaction with continuations, I was about to post some of my concerns when I received your long and well-thought-out mail. I need to think about the discussion so far a bit more, but briefly: (1) There's more than one way to go when combining dynamically-scoped variables with continuations: for example, do you use dynamic bindings from where the continuation was taken, or from where it's invoked? (see e.g. Scheme's "dynamic-wind"). (2) (internals) The functional-language people have found that full continuations are slow, and put a lot of effort into avoiding them where possible. Backtracking languages like Icon and Prolog are implemented by special mechanisms rather than general continuations, probably for this reason. So if we're forced to do a regex engine using full continuations, it will probably be dog-slow (3) On the other hand, we probably want people to intermix regex backtracking, continuation-taking, and hypothetical/dynamic variables, and have it "do the right thing", where "right" means something like "mind-bendingly difficult to reason about, but consistent". How do we want these features to play with each other? (4) (internals) Given that Parrot has so many different control mechanisms (call/ret, exceptions, closures, continuations, ...), how do we maintain consistency? And how much of that is parrot's responsibility (versus the perl6 compiler's)? /s
