typeord.hs

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

module Data.Type.Ord
  ( Compare
  , OrdCase
  , IsEQ
  , IsLT
  , IsGT
  )
import Data.Bool (Bool(..))
import Data.Ord (Ordering(..))

type family Compare (l :: k) (r :: k) :: Ordering

type family OrdCase (x :: Ordering) (a :: k) (b :: k) (c :: k) :: k where
  OrdCase LT a b c = a
  OrdCase EQ a b c = b
  OrdCase GT a b c = c

type IsLT t = OrdCase t True False False
type IsEQ t = OrdCase t False True False
type IsGT t = OrdCase t False False True