lens.hs

{-# LANGUAGE RankNTypes #-}

type Lens t i =  Functor f => (i -> f i) -> t -> f t

data Person = Person { _name :: String, _age :: Int } deriving Show

nameLens :: Lens Person String
nameLens f (Person name age) = fmap (\newName -> Person newName age) (f name)

ageLens :: Lens Person Int
ageLens f (Person name age) = fmap (\newAge -> Person name newAge) (f age)


set :: Lens t i -> t -> i -> t
set l x v  = l (flip const) x v

get :: Lens t i -> t -> i
get l x = val
  where (Left val) = l Left x