Gregory Crosswhite
On 09/08/10 19:14, Ertugrul Soeylemez wrote:
Have you looked at ContT from monadLib? It's not just a goto, but in fact a setjmp/longjmp, i.e. a goto with value. I haven't used it for anything yet, but it might come in handy for some algorithms:
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Whoa, that's cool! I glanced at monadLib but I didn't realize that it let you create labels that you could return to like that. :-) (I know of callCC, but that isn't quite the same as this.) Thanks for the pointer!
It is, in fact, callCC. ;)
The limitation with continuation-based approaches to goto, though, is that you can only jump back to points that you've seen before. The reason why I don't use a continuation-based approach in GotoT is because I wanted the user (i.e., me, and maybe one or two other people if I'm lucky :-) ) to be able to jump to an arbitrary point outside the calculation that has never been visited before, rather than returning a previously visited point of the same calculation.
Of course, if someone can prove to me that I am wrong and that GotoT semantics can be implemented with continuations, then I would welcome this information. :-)
I don't think you need 'goto' to implement jumps in Haskell. Note that functions as well as computations are first class: myComp :: ContT () IO () myComp = do input <- inBase $ putStrLn "Print something (y/n)?" >> getLine unless ("y" `isPrefixOf` input) exit inBase $ putStrLn "Something." input <- inBase $ putStrLn "Print more (y/n)?" >> getLine unless ("y" `isPrefixOf` input) exit inBase $ putStrLn "More." where exit = do inBase $ putStrLn "Ok, I'm exiting." abort () You can interpret 'exit' as a label. Binding can be interpreted as a jump. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/