[Haskell-cafe] Newbie question: "multi-methods" in Haskell

[Brian Hulley]

peterv wrote:
> In de book Modern C++ design, Andrei Alexandrescu writes that Haskell
> supports “multi-methods”

> Using multi-methods, I could write (in pseudo code)
> collide (Asteroid, Planet) = "an asteroid hit a planet"
> collide (Asteroid, Earth) = "the end of the dinos"
> ...
> collide (Planet, Asteroid) = collide (Asteroid, Planet)
> collide (Earth, Asteroid)  = collide (Earth, Asteroid)

Hi, In Haskell you can use multi parameter type classes to solve this 
problem:

{-# OPTIONS_GHC -fglasgow-exts
        -fallow-undecidable-instances
        -fallow-overlapping-instances #-}

module Collide where

class Collide a b where
    collide :: (a,b) -> String

data Solid = Solid
data Asteroid = Asteroid
data Planet = Planet
data Jupiter = Jupiter
data Earth = Earth

instance Collide Asteroid Planet where
    collide (Asteroid, Planet) = "an asteroid hit a planet"

instance Collide Asteroid Earth where
    collide (Asteroid, Earth) = "the end of the dinos"

-- Needs overlapping and undecidable instances
instance Collide a b => Collide b a where
    collide (a,b) = collide (b, a)

-- ghci output
*Collide> collide (Asteroid, Earth)
"the end of the dinos"
*Collide> collide (Earth, Asteroid)
"the end of the dinos"

Best regards, Brian.