On Thu, Jul 07, 2005 at 07:08:23PM +0200, 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)
It seems using shift/reset is better not only in principle but in practice as well.
module Foo where
import Control.Monad.Cont
-- 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
We can define a more general getCC' form simply as follows:
getCC' init = shift (\f -> f (init,jump f)) where jump f x = f (x,jump f)
The compiler figures out the types. Here's the usage example from the original Tomasz's message
test1' :: IO () test1' = (`runContT` return) $ do reset (do (x, jump) <- getCC' 0 lift (print x) when (x < 10) $ jump (x + 1)) lift (putStrLn "finish")
That was using the ContT monad exactly as it comes with GHC 6.4. With the CC_CPST monad transformer (with multiple polymorphic prompts) we can do better:
getCCP' p init = shiftP p (\f -> f (return (init,jump f))) where jump f x = f (return (x,jump f))
The following code (written a couple of weeks) prints 1 through 10, then again 4 through 10 and then again 5 through 10, and then "finish". So we can not only note a label to jump to: we can return the label so to jump to it from different `functions'. In a sense, we emulate multiple entry points to a block and cross-block jumps.
test1'' :: IO () test1'' = runCC ( do p <- newPrompt p1 <- newPrompt j <- pushPrompt p1 ( pushPrompt p ( do (x, jump) <- getCCP' p 0 lift (print x) when (x < 10) $ jump (x+1) shiftP p1 (const (return jump))) >> undefined) lift (putStrLn "again") pushPrompt p1 (j 4 >> undefined) lift (putStrLn "and again") pushPrompt p1 (j 5 >> undefined) lift (putStrLn "finish"))
In this terminating code, undefined plays a great role.