Equality on Types

TypeEq.hs

-- based on http://okmij.org/ftp/Haskell/de-typechecker.lhs

{-# LANGUAGE IncoherentInstances,UndecidableInstances #-}

data HTrue
data HFalse

instance Show HTrue  where show _ = "HTrue"
instance Show HFalse where show _ = "HFalse"

class TypeEq x y b | x y -> b where
    type'eq :: x -> y -> b
    type'eq _ _ = undefined
instance (b ~ HTrue)  => TypeEq x x b
instance (b ~ HFalse) => TypeEq x y b

-- *Main> type'eq True "True"
-- HFalse
-- *Main> type'eq True False
-- HTrue
-- *Main> type'eq id id
-- HFalse
-- *Main> type'eq putStr putStrLn
-- HTrue