Tomasz Zielonka wrote:
Some time ago I wanted to return the escape continuation out of the callCC block, like this: getCC = callCC (\c -> return c)
patterns like this are characteristic of shift/reset -- From http://www.haskell.org/hawiki/MonadCont reset :: (Monad m) => ContT a m a -> ContT r m a reset e = ContT $ \k -> runContT e return >>= k shift :: (Monad m) => ((a -> ContT r m b) -> ContT b m b) -> ContT b m a shift e = ContT $ \k -> runContT (e $ \v -> ContT $ \c -> k v >>= c) return with them, we can implement the same tests, in a slightly de-sugared way, for clarity:
newtype H r m = H (H r m -> ContT r m r) unH (H x) = x
-- prints "hello!" in an endless loop test :: IO () test = (`runContT` return) $ reset (do jump <- shift (\f -> f (H f)) lift (putStrLn "hello!") unH jump jump)
newtype H' r m a = H' ((a,H' r m a) -> ContT r m ()) unH' (H' x) = x
-- prints integers from 0 to 10, then prints finish and ends test' :: IO () test' = (`runContT` return) $ do reset (do (x, jumpWith) <- (\x -> shift (\f -> f (x,(H' f)))) 0 lift (print x) when (x < 10) (unH' jumpWith ((x + 1),jumpWith))) lift (putStrLn "finish")
Delimited continuations are really cool. The lack of the answer-type polymorphism in ContT will come to bite us in the end: we can't use reset several times in differently-typed contexts (which often means that we can use reset only once in our program). The CC monad transformer (derived from the CC library by Sabry, Dybvig, Peyton-Jones), freely available from http://pobox.com/~oleg/ftp/packages/LogicT.tar.gz is free from that drawback. BTW, with that monad transformer, the example looks as follows (again, in a de-sugared way, for clarity)
import CC_2CPST newtype H'' r m a = H'' (CC r m (a,H'' r m a) -> CC r m ()) unH'' (H'' x) = x test'' :: IO () test'' = runCC ( do p <- newPrompt pushPrompt p ( do (x, jumpWith) <- (\x -> shiftP p (\f -> f (return (x,(H'' f))))) 0 lift (print x) when (x < 10) (unH'' jumpWith (return ((x + 1),jumpWith)))) lift (putStrLn "finish"))
shiftP p f = letSubCont p $ \sk -> pushPrompt p (f (\c -> pushPrompt p (pushSubCont sk c)))