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.
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