-- Haskell only has covariant functors of the following form:
--  class Functor f where
--      fmap:: (a->b) -> f a -> f b
-- Contra/Co-variant : 
--  http://www.reddit.com/r/haskell/comments/1cht4v/what_does_coalgebra_mean_in_the_context_of/c9gzulr

class Contravariant f where
    contramap :: (a->b) -> f b -> f a

data Cont a = Cont (a -> Int)

instance Contravariant Cont where
    contramap f (Cont x) = Cont (x.f)

-- The following however is covariant
data H t = H1 t | H2 ((t -> Int) -> Int)

instance Functor H where
    fmap f (H1 x) = H1 (f x)
    fmap f (H2 g) = H2 (\h -> g (h . f))