Pair has two functors

PairFunctors.hs

module PairFunctors where

data Fst b a = Fst (a, b)
  deriving Show
data Snd a b = Snd (a, b)
  deriving Show

instance Functor (Fst b) where
  fmap f (Fst (x, y)) = Fst (f x, y)

instance Functor (Snd a) where
  fmap f (Snd (x, y)) = Snd (x, f y)

main = do
  let pair = (3, 4)
  print $ fmap (*2) $ Fst pair -- Fst (6, 4)
  print $ fmap (*2) $ Snd pair -- Fst (3, 8)