LensRef.hs

{-# LANGUAGE Rank2Types #-}

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

type Ref s t a b = s -> Rep a b t
data Rep a b t = Rep a (b -> t)

instance Functor (Rep a b) where
  fmap f (Rep a g) = Rep a (f . g)

lens2Ref :: Lens s t a b -> Ref s t a b
lens2Ref r = r (\b -> Rep b id)

ref2Lens :: Ref s t a b -> Lens s t a b
ref2Lens l k x = case l x of
  Rep s f -> fmap f (k s)