Hi Stefan, On Tue 06 Jul 2010 22:52, stefan <stefan.ta...@spray.se> writes:
> So here is what I would like to use > > (match #:tag pr > Z > ((a X) (begin (do-something X pr) > (abort-to-prompt pr))) > ((b X) (b-it X))) > > And this translate to something like > > (call-with-prompt > *prompt* > (lambda () > (let ((pr *prompt*)) > (ma-it Z (a X) > (begin (do-something X pr) ...) > (abort-to-prompt pr)))) > (lambda (s) ...)) > > Now this works if we stay in the function, but if do-something contains > again a match constructed from the *prompt* prompt, then it get confused > so I was thinking like *prompt* beeing like a fluid let but it's not as it > seams. At the repo unify-iso I instead construct a fresh new prompt at each > match invocation which of cause is very costly but correct. I suspect that > there is a quick hack to fix this and therefore I ask if anyone can > help? Let me see if I understand you: you want to be able to do a match, and if it fails, abort the match. If that is the case, you will need N prompts, with N unique prompt tags. Otherwise you could just use one tag, and pass some auxiliary identifier to the abort handlers; but that is silly, unless you need to unwind somehow, and in that case you could handle it with dynamic-wind anyway. So why not have the second argument to `do-something' not be a tag, but instead a procedure, e.g. (lambda () (abort-to-prompt pr)). You generate the prompt tag local to the invocation of `match', and pass the thunk encapsulating that tag to nested match helper procedures. If the helper fails, it invokes the thunk. Otherwise you could bind the thunk to a fluid using with-fluids, as is done by `catch' et al -- but that would make it be *the* abort handler for the dynamic extent of that invocation to match. That's one way to do it, anyway. Let me know if I'm misunderstanding the issue. Cheers, Andy -- http://wingolog.org/
