module Lens.Family.Identical where
import Control.Applicative
import Control.Applicative.Backwards (Backwards(..))
import Data.Functor.Identity (Identity(..))
import Data.Functor.Compose (Compose(..))
class Applicative f => Identical f where
extract :: f a -> a
instance Identical Identity where
extract (Identity x) = x
instance Identical f => Identical (Backwards f) where
extract (Backwards x) = extract x
instance (Identical f, Identical g) => Identical (Compose f g) where
extract (Compose x) = extract (extract x)