Scheme Programming Language :: How does dynamic-wind work?


	How does dynamic-wind work? As I learn Scheme, I am sort of mentally figuring out what the minimum set of primitives is. Initially, I thought that dynamic-wind must be a primitive, but now I'm not so sure. If call/cc were never invented, you could make dynamic-wind by saying (define dynamic-wind (lambda (a b c) (begin a b c))) Since the only way into b is by calling it, this ensures that a and c are called before and after it. The problem, as so often, is that call/cc means we can jump into b from anywhere. So what if you masked call/cc somehow? If b calls call/cc then it really calls our fake call/cc, which returns a continuation that calls a first and c after. I'm not entirely sure how to fake it. fluid-let would only work if they haven't made another name point at call/cc, which means it's probably impossible. Am I on the right track? Do I completely misunderstand some feature of dynamic-wind? --Ross - Hide quoted text - Ross Andrews <randr@geekfu.org> writes: > As I learn Scheme, I am sort of mentally figuring out what the minimum > set of primitives is. Initially, I thought that dynamic-wind must be a > primitive, but now I'm not so sure. > If call/cc were never invented, you could make dynamic-wind by saying > (define dynamic-wind > (lambda (a b c) > (begin a b c))) > Since the only way into b is by calling it, this ensures that a and c > are called before and after it. The problem, as so often, is that > call/cc means we can jump into b from anywhere. > So what if you masked call/cc somehow? If b calls call/cc then it > really calls our fake call/cc, which returns a continuation that calls > a first and c after. That's the essence of it: CALL/CC needs to cooperate with DYNAMIC-WIND so as not to destroy its invariants. As to how it works, you might find http://mumble.net/~jar/pubs/scheme-of-things/june-92-meeting.ps or http://www-pu.informatik.uni-tuebingen.de/users/sperber/papers/adding... helpful in understanding the technicalities. -- Cheers =8-} Mike Friede, Vlkerverstndigung und berhaupt blabla