TypeFamilies - Intro

{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}

-- Generalised Num class

class GNum a b where

type SumTy a b :: *

(+>) :: a -> b -> SumTy a b

instance GNum Int Int where

type SumTy Int Int = Int

x +> y = x + y

instance GNum Int Float where

type SumTy Int Float = Float

x +> y = fromIntegral x + y

instance GNum Float Int where

type SumTy Float Int = Float

x +> y = x + fromIntegral y

main = do

print $ (1 :: Int) +> (5 :: Int)

print $ (1 :: Int) +> (4.2 :: Float)

print $ (1.24 :: Float) +> (2 :: Int)

-- http://research.microsoft.com/en-us/um/people/simonpj/papers/assoc-types/fun-with-type-funs/FunWithTypeFuns-Apr09.pdf