data Strict a = Strict !a
  deriving (Show)

instance Functor Strict where
  -- A lawful instance would have ~(Strict a)
  fmap f (Strict a) = Strict (f a)

-- Before optimization:
fun0 :: Functor f => f a -> f Int
fun0 = fmap (const 7) . fmap (const undefined)

-- After optimization:
fun1 :: Functor f => f a -> f Int
fun1 = fmap (const 7 . const undefined)

-- Compare fun0 vs. fun1:
main = print $ fun0 (Strict 5)