I'm not exactly sure what you're trying to do, but the problem is that you're trying to return a specific value where the type signature is polymorphic. getParticleI returns a p, (with the constraint that p is a type in the class Particle) This means that getParticleI can be called in any context that needs a Particle p, but your getParticleI returns (Double, Double, Double) so it would only work in a context that needed a (Double, Double, Double), and the type signature doesn't reflect that, so you get an error. To emphasize the problem, say I make a ParticleD type ParticleD = (Int, Int) instance Particle ParticleD let (a, b) = getParticleI myConfig 5 -- this is perfectly valid since ParticleD is a Particle, but doesn't work with your getParticleI definition because it returns a specific type (Double, Double, Double). Do you see what I mean? You can fix it by either fixing the type of getParticleI: getParticleI :: c -> Int -> ParticleC or by using multiparameter type classes class Configuration c p where getParticleI :: (Particle p) => c -> Int -> p depending on what you're actually trying to do. - Job On Mon, Aug 17, 2009 at 2:35 AM, Grigory Sarnitskiy <sargrigory@ya.ru>wrote:
Hello! I can't understand why the following dummy example doesn't work.
{-# OPTIONS -XTypeSynonymInstances #-} {-# OPTIONS -XFlexibleInstances #-} module Main where import Data.Array.Unboxed
class Particle p
type ParticleC = (Double, Double, Double) instance Particle ParticleC
class Configuration c where getParticleI :: (Particle p) => c -> Int -> p
type Collection p = UArray (Int,Int) Double instance Configuration (Collection p) where getParticleI config i = (1,1,1) :: ParticleC _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe