tyset.hs

{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
module TySet where


type InsertRes x set = If (Member x set) set (x ': set)

type family If f a b where
  If True  a b = a
  If False a b = b
                                              
type family Member (x :: α) (set :: [α]) :: Bool where
  Member x '[]       = False
  Member x (x ': xs) = True
  Member x (y ': xs) = Member x xs