-- http://cgi.cse.unsw.edu.au/~dons/blog/2008/05/12

-- http://www.haskell.org/ghc/docs/latest/html/users_guide/rewrite-rules.html
-- Required to enable the rewrite rules
{-# OPTIONS_GHC -fglasgow-exts -O #-}

-- Don't like the warnings
{-# OPTIONS_GHC -w #-}

-- For debugging, we can -ddump-rules, and -ddump-simpl-stats
{-# RULES
"rule1"   (=-=) = eq_Bool :: Bool -> Bool -> Bool
"rule2"   (=-=) = eq_Unit :: ()   -> ()   -> Bool
"bad"     (=-=) = (=-=)
 #-}

class  MEEq a  where
        (=-=)   :: a -> a -> Bool

instance MEEq ()
instance MEEq Bool
instance MEEq Int

eq_Bool :: Bool -> Bool -> Bool
eq_Bool True  True  = True
eq_Bool False False = True
eq_Bool _     _     = False

eq_Unit :: () -> () -> Bool
eq_Unit () () = True

main = do
        print $ True =-= False
        print $ ()   =-= ()
--        print $ 7    =-= (8 :: Int)   -- INFINITE compile-time loop