Re: [Haskell-cafe] Re: ANNOUNCE: GotoT-transformers version 1.0

On Wed, Sep 8, 2010 at 11:54 PM, Ertugrul Soeylemez wrote:

Gregory Crosswhite wrote:

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 On 09/08/10 19:14, Ertugrul Soeylemez wrote:

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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. ;)

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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:

To recover from my overly complex previous post, here is a much simply goto based on existing monad transformers:

goto :: Monad m => ContT r m r -> ContT r m a goto (ContT m) = ContT $ \_ -> m return

Reading your post, Ertugrul, made something click for me Antoine