:~>の例

NatTrans.hs

{-# LANGUAGE RankNTypes    #-}
{-# LANGUAGE TypeOperators #-}

module NatTrans where

import Data.Foldable
import Data.Functor.Identity


newtype f :~> g = NT
  { unNT :: forall a. f a -> g a
  }

type f ~> g = forall a. f a -> g a

-- | Alias of 'toList :: forall f a. Foldable f => f a -> [a]'
--
-- >>> toListNt (Just True)
-- [True]
--
toListNt :: Foldable f => f ~> []
toListNt = toList

-- | Alias of 'toListNt'
--
-- >>> unNT toListNT (Just True)
-- [True]
--
toListNT :: Foldable f => f :~> []
toListNT = NT toListNt


-- |
--
-- >>> unNT (toRightIdentNT Just) (Identity True)
-- Just True
--
toRightIdentNT :: (forall a. a -> f a) -> Identity :~> f
toRightIdentNT f = NT (f . runIdentity)