Data.Functor.NormalizedConstrained.hs

Data.Functor.NormalizedConstrained.hs

{-# LANGUAGE RankNTypes, InstanceSigs, KindSignatures, GADTs, ConstraintKinds #-}

-- Paper: http://neilsculthorpe.com/publications/constrained-monad-problem.pdf
-- Talk: https://vimeo.com/69261960

import GHC.Exts

data NF :: (* -> Constraint) -> (* -> *) -> * -> * where
  FMap :: c x => (x -> a) -> t x -> NF c t a

instance Functor (NF c t) where
  fmap :: (a -> b) -> NF c t a -> NF c t b
  fmap g (FMap h tx) = FMap (g . h) tx

liftNF :: c a => t a -> NF c t a
liftNF ta = FMap id ta

lowerNF :: (forall x. c x => (x -> a) -> t x -> t a) -> NF c t a -> t a
lowerNF fmp (FMap g tx) = fmp g tx