MonadCont done right

The Cont class MonadCont defined in the monad template library could be improved if you are willing to use rank two polymorphism. Notice the change in the signature of callCC. This allows one to use the passed continuation in different situations inside a callCC block. However, you will have to provide an explicit signature for the function you are calling callCC with.

Possible Implementation

newtype Cont r a = Cont { runCont :: ((a -> r) -> r) } -- r is the final result type of the whole computation

class (Monad m) => MonadCont m where
    callCC :: ((a -> (forall b. m b)) -> m a) -> m a

instance Monad (Cont r) where
    return a = Cont (\k -> k a)                             -- i.e. return a = \k -> k a
    (Cont c) >>= f = Cont (\k -> c (\a -> runCont (f a) k)) -- i.e. c >>= f = \k -> c (\a -> f a k)

instance MonadCont (Cont r) where
    callCC f = Cont (\k -> runCont (f (\a -> Cont (\_ -> k a))) k)

Alternative Implementation

This implementation has the advantage that it provides a polymorphic version of callCC for all instances of MonadCont from Control.Monad.Cont. I also added shift and reset functions for using ComposableContinuations.

{-# OPTIONS -fglasgow-exts -fno-warn-unused-binds -cpp #-}
module ContExts (
    callCC',
    shift, reset, shiftT, resetT,
  ) where

import Control.Monad.Cont

-- Cont' m a is the type of a continuation expecting an a within the 
-- continuation monad Cont m
type Cont' m a = forall r. a -> m r

callCC' :: forall a m. MonadCont m => (Cont' m a -> m a) -> m a
#if __GLASGOW_HASKELL__ > 602
callCC' f = callCC f' where
#else
callCC' (f :: ((a -> (forall b. m b)) -> m a) ) = callCC f' where
#endif
  f' :: (a -> m (EmptyMonad m)) -> m a
  f' g = f g' where
    g' :: a -> m b
    g' = (=<<) runEmptyMonad . g

-- ghc doesn't allow something like m (forall c. m c)
newtype EmptyMonad m = EmptyMonad { runEmptyMonad :: forall c. m c }

-- shift/reset for the Cont monad
shift :: ((a -> Cont s r) -> Cont r r) -> Cont r a
shift e = Cont $ \k -> e (return . k) `runCont` id

reset :: Cont a a -> Cont r a 
reset e = return $ e `runCont` id

-- shiftT/resetT for the ContT monad transformer
shiftT :: Monad m => ((a -> ContT r m s) -> ContT s m s) -> ContT s m a
shiftT e = ContT $ \k -> e (lift . k) `runContT` return

resetT :: Monad m => ContT a m a -> ContT r m a
resetT e = lift $ e `runContT` return

All of this is presumably meant to be under the MIT license, since it wants to be in the library. --SamB 22:44, 30 October 2006 (UTC)