data Identity a = Identity {runIdentity :: a}

instance Functor Identity where
  fmap f = Identity . f . runIdentity

data Const a b = Const {getConst :: a}

instance Functor (Const a) where
  fmap _ (Const a) = Const a

type Lens s t a b = forall f . Functor f => (a -> f b) -> s -> f t

view :: Lens s t a b -> s -> a
view l = getConst . l Const

over :: Lens s t a b -> (a -> b) -> s -> t
over l f = runIdentity . l (Identity . f)

set :: Lens s t a b -> b -> s -> t
set l = over l . const

_1 :: Lens (a, x) (b, x) a b
_1 f (a, b) = (\a' -> (a', b)) <$> f a

_2 :: Lens (x, a) (x, b) a b
_2 f (a, b) = (\b' -> (a, b')) <$> f b

main :: IO ()
main = putStrLn . show . view _1 . over _1 (++", World!") $ set _1 "Hello" ("ABC", "DEF")