test.hs

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE MultiParamTypeClasses #-}

class Add a b where
  type Ret a b :: *
  add :: a -> b -> Ret a b

instance Add Int Double where
  type Ret Int Double = Double
  add a b = (fromIntegral a) + b

instance Add Int Int where
  type Ret Int Int = Integer
  add a b = (toInteger a) + (toInteger b)

main = ((>>) (putStrLn (show (add (1 :: Int) (2 :: Int))))
             (putStrLn (show (add (1 :: Int) (2 :: Double)))))