Re: [Haskell-cafe] Re: How to abort a computation within Continuation Monad?
gleb.alexeev: Derek Elkins wrote: As Derek Elkins has written, one of the options is to use delimited continuations, see http://research.microsoft.com/~simonpj/papers/control/ for Haskell implementation. I made no such suggestion. I didn't mean that you suggested using implementation referenced above. But you suggested using 'control' which is obviously a delimited control operator and that is what I was trying to say. While we're here, it might be a good time to remind people also that we've got: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/CC-delcont-0.1 An implementation of multi-prompt delimited continuations based on the paper, A Monadic Framework for Delimited Continuations, by R. Kent Dybvig, Simon Peyton Jones and Amr Sabry It also includes a corresponding implementation of dynamically scoped variables, as implemented in the paper, Delimited Dynamic Binding, by Oleg Kiselyov, Chung-chieh Shan and Amr Sabry sitting on hackage, thanks to the work of Dan Doel, who jumped in to fill a gap in the libraries. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: How to abort a computation within Continuation Monad?
On Tue, 2007-11-20 at 13:22 +0200, Gleb Alexeyev wrote: Dimitry Golubovsky wrote: If I have callCC $ \exit - do foo ... I cannot jump to `exit' from within foo unless `exit' is given to foo as an argument. As Derek Elkins has written, one of the options is to use delimited continuations, see http://research.microsoft.com/~simonpj/papers/control/ for Haskell implementation. I made no such suggestion. I simply suggested using instead of callCC f = Cont (\k - runCont (f (\a - Cont $ \_ - k a)) k) with control f = Cont (\k - runCont (f (\a - Cont $ \_ - k a)) id) abort is then simply abort = control . const . return callCC is obviously just callCC f = control (\k - f k = k) If you didn't want to break abstraction like this, you can implement this in terms of just callCC awkwardly. To do so requires having a top-level continuation and is exactly what is below. The above and below is (part of) why I prefer control. But in this case Cont may be enough. If you don't like passing `exit' explicitly, you can put in into Reader monad. This is the idea: import Control.Monad.Cont import Control.Monad.Reader type Abortable r a = ReaderT (r - Cont r r) (Cont r) a runAbortable :: Abortable a a - a runAbortable m = runCont (callCC $ \exit - runReaderT m exit) id abort :: r - Abortable r a abort x = do exit - ask lift (exit x) undefined -- this hack is needed to make abort polymorphic test a b c = do x - if a then abort a else return 1 y - if b then abort b else return False z - foo c -- calling foo without explicit abort continuation return $ show (x, y, z) where foo True = abort c foo False = return 5.39 run m = putStrLn (runAbortable m) main = do run (test False False False) run (test False False True) run (test False True False) run (test True False False) -- This implementation is a bit hackish, since it uses undefined to make abort polymorphic in return type. You can use rank-2 types to avoid it, see http://www.vex.net/~trebla/tmp/ContMonad.lhs by Albert C. Lai. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
