Cont.hs

newtype Cont r a = Cont { runCont :: (a -> r) -> r }

instance Functor (Cont c) where
  fmap f (Cont c) = Cont $ \k -> c (\x -> k (f x))

instance Applicative (Cont c) where
  Cont cf <*> Cont cx = Cont $ \k -> cf (\fn -> cx (\x -> k (fn x)))
  pure x = Cont $ \k -> k x

instance Monad (Cont r) where
  return a = Cont ($ a)
  f >>= g  = Cont $ \k -> runCont f (\a -> runCont (g a) k)

f :: Int -> Cont r Int
f x = Cont $ \c -> c (x * 2)
g :: Int -> Cont r Int
g x = Cont $ \c -> c (x + 10)

h :: Int -> Cont r Int
h x = if x == 5 then f x else g x

compos :: Cont r Int
compos = return 5 >>= g >>= f

finale :: Show a => a -> String
finale x = "Done: " ++ show(x)

main = print $ runCont compos finale