# MonadCont done right

### From HaskellWiki

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 ofcallCC

callCC

callCC

## 1 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)

## 2 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)

[Category:Monad]