peterv wrote:
This is very nice, but it does not really solve the original problem.
To get Haskell to choose the best fit it's necessary to encode the location of each element in the hierarchy, so that elements deeper in the hierarchy are more instantiated than those at the top. Then instance selection chooses the best fit by just choosing the most instantiated match. Encoding can be done using phantom types, so a generic solid has the path IsSolid s a planet has IsSolid (IsPlanet p) and a specific planet eg Earth has path IsSolid (IsPlanet Earth) A newtype can be used to associate the path with the actual object: newtype InH path body = InH body so Earth is represented by InH Earth :: InH (IsSolid (IsPlanet Earth)) Earth A class with a functional dependency gives us the mapping between concrete objects and the objects as viewed by the hierarchy: class ToH body path | body -> path where toH :: body -> InH path body toH = InH The functional dependency means that the path (location in the hierarchy) is uniquely determined by the body, and instance decls then define this relationship: instance ToH Asteroid (IsSolid Asteroid) instance ToH Jupiter (IsSolid (IsPlanet Jupiter)) instance ToH Earth (IsSolid (IsPlanet Earth)) The code is below but as you can see the OOP encoding in Haskell becomes quite heavy and clunky so this style is probably not ideal for a real program - Tillmann's suggestion to use algebraic datatypes instead is more idiomatic - but anyway here goes: {-# 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 data IsSolid a data IsPlanet a newtype InH path body = InH body class ToH body path | body -> path where toH :: body -> InH path body toH = InH instance ToH Asteroid (IsSolid Asteroid) instance ToH Jupiter (IsSolid (IsPlanet Jupiter)) instance ToH Earth (IsSolid (IsPlanet Earth)) data Collision = forall a. Collide a => Collision a mkCollision :: (ToH a pa, ToH b pb, Collide (InH pa a, InH pb b)) => a -> b -> Collision mkCollision a b = Collision (toH a, toH b) instance Collide (InH (IsSolid a) aa, InH (IsSolid b) bb) where collide _ = "generic collision" instance Collide (InH (IsSolid Asteroid) Asteroid, InH (IsSolid (IsPlanet bb)) cc) where collide _ = "an asteroid hit a planet" instance Collide (InH (IsSolid (IsPlanet a)) aa, InH (IsSolid Asteroid) Asteroid) where collide _ = "an asteroid hit a planet" instance Collide (InH (IsSolid Asteroid) Asteroid, InH (IsSolid (IsPlanet Earth)) Earth) where collide _ = "the end of the dinos" instance Collide (InH (IsSolid (IsPlanet Earth)) Earth, InH (IsSolid Asteroid) Asteroid) where collide _ = "the end of the dinos" instance Collide Collision where collide (Collision a) = collide a ----------------------- ghci output *Collide> mapM_ putStrLn (map collide [ mkCollision Asteroid Earth , mkCollision Earth Asteroid , mkCollision Jupiter Asteroid , mkCollision Asteroid Jupiter , mkCollision Asteroid Asteroid ]) the end of the dinos the end of the dinos an asteroid hit a planet an asteroid hit a planet generic collision *Collide> Best regards, Brian.