A simple lens presentation, saving for posterities sake.

SimpleLens.hs

{-# LANGUAGE RankNTypes #-}
module SimpleLens where

import Data.Functor.Identity
import Data.Functor.Constant

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

type Lens s a = LensP s s a a

get :: Lens s a -> s -> a
get ln s = getConstant $ ln Constant s

set :: Lens s a -> a -> s -> s
set ln x = runIdentity . ln (Identity . const x)

_1 :: Lens (a, b) a
_1 f (x, y) = fmap (\x' -> (x', y)) (f x) 

_2 :: Lens (a, b) b
_2 f (x, y) = fmap (\y' -> (x, y')) (f y)