First try (not working): Functor over Functor as Functor

gistfile1.txt

Prelude> data TwoD f g a = TwoD (f (g a)) deriving Show
Prelude> TwoD [Just 3]
TwoD [Just 3]
Prelude> :t TwoD [Just 3]
TwoD [Just 1] :: Num a => TwoD [] Maybe a
Prelude> :{
Prelude| instance (Functor f, Functor g) => Functor (TwoD f g) where
Prelude|   fmap o = fmap (fmap o)
Prelude| :}

<interactive>:20:18:
    Couldn't match type ‘a’ with ‘f0 a’
      ‘a’ is a rigid type variable bound by
          the type signature for fmap :: (a -> b) -> TwoD f g a -> TwoD f g b
          at <interactive>:20:3
    Expected type: a -> b
      Actual type: f0 a -> f0 b
    Relevant bindings include
      o :: a -> b (bound at <interactive>:20:8)
      fmap :: (a -> b) -> TwoD f g a -> TwoD f g b
        (bound at <interactive>:20:3)
    In the first argument of ‘fmap’, namely ‘(fmap o)’
    In the expression: fmap (fmap o)

<interactive>:20:18:
    Couldn't match type ‘b’ with ‘f0 b’
      ‘b’ is a rigid type variable bound by
          the type signature for fmap :: (a -> b) -> TwoD f g a -> TwoD f g b
          at <interactive>:20:3
    Expected type: a -> b
      Actual type: f0 a -> f0 b
    Relevant bindings include
      o :: a -> b (bound at <interactive>:20:8)
      fmap :: (a -> b) -> TwoD f g a -> TwoD f g b
        (bound at <interactive>:20:3)
    In the first argument of ‘fmap’, namely ‘(fmap o)’
    In the expression: fmap (fmap o)

I had just forgot the unpacking of TwoD

There is also -XDeriveFunctor which does this by deriving (Functor)