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Haskell static dispatch rewrite rules

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Haskell uses static dispatch for overloaded methods, in addition it has an extension to write rules that optimize Haskell dispatching code.

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  1. -- Enabling the extension
  2. {-# OPTIONS -fglasgow-exts #-} 
  3.  
  4. --  Equal like class
  5. class  MEEq a  where
  6.    (=-=)   :: a -> a -> Bool
  7.  
  8. -- Instances with no implementation
  9. instance MEEq ()
  10. instance MEEq Bool
  11. instance MEEq Int
  12.  
  13. -- The rewrite rule
  14. {-# RULES
  15.     "eq/Bool"   (=-=) = eq_Bool :: Bool -> Bool -> Bool
  16.     "eq/Unit"   (=-=) = eq_Unit :: ()   -> ()   -> Bool
  17.     #-}
  18.  
  19. main = do
  20.        print $ True =-= False
  21.        print $ ()   =-= ()
  22.        -- This fails (at runtime!) since we didn't define a rule for it
  23.        print $ 7    =-= (8 :: Int)
  24.  
  25. -- Makes sure to fail any instance that doesn't have an implementation on compile time
  26. {-# RULES
  27.     "Unable to resolve instance resolution" (=-=) = (=-=)
  28.       #-}

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