Storeを眺めていたらLensが出てきそうな気がしたのですけれど、なにか違う気がしますです＞＜

lens.hs

import Prelude hiding ((.))
import Control.Category

data Store s a = Store {
  set' :: s -> a, view' :: s}

newtype Lens' b a = Lens {
  runLens :: a -> Store b a}

type Lens a b = Lens' b a

set :: Lens a b -> b -> a -> a
set l x c = set' (runLens l c) x

over :: Lens a b -> (b -> b) -> a -> a
over l f c = set l (f $ view l c) c

view :: Lens a b -> a -> b
view l c = view' $ runLens l c

_1 :: Lens (a, b) a
_1 = Lens $ \(a, b)-> Store (\a'-> (a', b)) a

_2 :: Lens (a, b) b
_2 = Lens $ \(a, b)-> Store (\b'-> (a, b')) b

compose :: Lens a b -> Lens b c -> Lens a c
compose l r = Lens $ \a-> Store (\c'-> set l (set r c' (view l a)) a) (view r (view l a))

instance Category Lens' where
  id = Lens $ \a-> Store (const a) a
  (.) = compose

main = do
  print $ view (_1 . _2) ((1, 2), 3)
  print $ set  (_1 . _2) 4 ((1, 2), 3)
  print $ over (_1 . _2) (* 2) ((1, 2), 3)