Opening datakinds

OpenKinds.hs

{-# LANGUAGE DataKinds    #-}
{-# LANGUAGE PolyKinds    #-}
{-# LANGUAGE TypeFamilies #-}

module OpenKinds where

import GHC.TypeLits (Nat)

type family Wrap :: k -> k

type family Match (b :: Bool) :: Nat where
  Match 'True = 0
  Match 'False = 1
  Match (w 'True) = 2
  Match (w 'False) = 3

type Woo = Match (Wrap 'True)
  -- = 2

Temp.hs

{-# LANGUAGE DataKinds    #-}
{-# LANGUAGE PolyKinds    #-}
{-# LANGUAGE TypeFamilies #-}

module OpenKinds where

import GHC.TypeLits (Nat)

type family Wrap :: WrapTag -> k -> k

data WrapTag = WrapTag

type family Match (b :: Bool) :: Nat where
  Match 'True = 0
  Match 'False = 1
  Match (w 'WrapTag 'True) = 2
  Match (w 'WrapTag 'False) = 3

type Woo = Match (Wrap 'WrapTag 'False)
  -- = 3