contagain.hs

import Control.Applicative
import Control.Monad

main = putStrLn "Hello, World!"

data Void
data Cont r a = Cont ((a -> r) -> r)

unCont (Cont m) = m

instance Functor (Cont r) where
  fmap f (Cont ma) = Cont (\k -> ma (\a -> k (f a)))

instance Applicative (Cont r) where
  pure a = Cont (\k -> k a)
  (Cont mf) <*> (Cont ma) = Cont (\k -> mf (\f -> ma (\a -> k (f a))))

instance Monad (Cont r) where
  (Cont ma) >>= amb = Cont (\k -> ma (\a -> unCont (amb a) k))

exc0 :: Cont r (Either a (a -> r))
exc0 = Cont (\k -> k (Right (\a -> k (Left a))))

exc :: Cont r (Either a (a -> Cont r b))
exc = Cont (\k -> k (Right (\a -> Cont (\k' -> k (Left a)))))