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
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