gistfile1.txt

{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}


class Dual f g | f -> g, g -> f where
    zap :: (a -> b -> c) -> f a -> g b -> c

data PrintF a = PrintF String a deriving Functor
data CoPrintF a = CoPrintF { coPrint :: String -> a } deriving Functor

instance Dual PrintF CoPrintF where
    zap f (PrintF msg k) (CoPrintF prn) = f k (prn msg)

pr = PrintF "Hello world" id
copr = CoPrintF print

main = zap ($) pr copr