This is very nice, but it does not really solve the original problem. In your code, evaluating collide (Jupiter, Asteroid) will result in an endless loop. This is expected in your code, because no "inheritance" relation is present between e.g Jupiter and Planet. With multi-dispatch, it should pick the "best" matching collide function based on inheritance, or raise an error when ambiguous types. I could fix that be just keeping the "leafs" (Earth, Jupiter, Asteroid) as datatypes, and adding type classes for the "super classes" (Planet, Solid), like the code below, but I could not check Asteroid-Asteroid collision with that, GHCi gives an error. {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances #-} module Collide where class Collide a where collide :: a -> String data Asteroid = Asteroid data Jupiter = Jupiter data Earth = Earth class IsSolid a class IsSolid a => IsPlanet a instance IsSolid Asteroid instance IsSolid Jupiter instance IsSolid Earth instance IsPlanet Earth instance IsPlanet Jupiter instance (IsSolid a, IsSolid b) => Collide (a, b) where collide (x,y) = "generic collision" instance (IsPlanet a) => Collide (Asteroid, a) where collide (x,y) = "an asteroid hit a planet" instance (IsPlanet a) => Collide (a, Asteroid) where collide (x, y) = "an asteroid hit a planet" instance Collide (Asteroid, Earth) where collide (_,_) = "the end of the dinos" instance Collide (Earth, Asteroid) where collide (_,_) = "the end of the dinos" -- This is how you get dynamic dispatch in Haskell data Collision = forall a. Collide a => Collision a instance Collide Collision where collide (Collision a) = collide a ae = collide (Asteroid, Earth) ea = collide (Earth, Asteroid) ja = collide (Jupiter, Asteroid) aj = collide (Asteroid, Jupiter) -- However, this one gives an error? --aa = collide (Asteroid, Asteroid) -----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Brian Hulley Sent: Monday, August 06, 2007 9:15 PM To: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Newbie question: "multi-methods" in Haskell Dan Weston wrote:
Remember that type classes do not provide object-oriented functionality. The dispatch is static, not dynamic. Although OOP can be simulated in Haskell, it is not a natural idiom. If you need dynamic dispatch (including multiple dispatch), you may want to reconsider your solution. Dynamic dispatch is easily added to Haskell code by using an existential to represent any collision:
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances #-} module Collide where -- Changed to a single param to make life easier... class Collide a where collide :: a -> 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) -- This is how you get dynamic dispatch in Haskell data Collision = forall a. Collide a => Collision a instance Collide Collision where collide (Collision a) = collide a -- ghci output *Collide> let ae = Collision (Asteroid, Earth) *Collide> let pa = Collision (Planet, Asteroid) *Collide> collide ae "the end of the dinos" *Collide> collide pa "an asteroid hit a planet" *Collide> map collide [ae, pa] ["the end of the dinos","an asteroid hit a planet"] Best regards, Brian. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe