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.
Dan Weston
Brian Hulley wrote:
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.
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