{-# LANGUAGE DeriveFunctor #-}

data F a = F a
  deriving Functor
data G a = G a
  deriving Functor

counit :: F (G a) -> a
counit (F (G a)) = a

unit :: a -> G (F a)
unit a = G (F a)

{-
*Main> :t unit
unit :: a -> G (F a)

*Main> :t fmap counit
fmap counit :: Functor f => f (F (G b)) -> f b

*Main> :t fmap counit . unit
fmap counit . unit :: G b -> G b  -- WTF? Shouldn't the type be G (F (G b)) -> G b ?
-}