Hi,
One reason (there may be more) is as follows:
Grigory Sarnitskiy wrote:
class Configuration c where
getParticleI :: (Particle p) => c -> Int -> p
This type signature declares that for any type c that has a
Configuration instance (and an Int), you can give me back something that
is of *any type* p, provided I have a Particle instance for p.
So if I was to declare:
instance Particle Int
Then I should be able to do:
x :: Int
x = getParticleI someConfigurationItem 6
But...
type Collection p = UArray (Int,Int) Double
instance Configuration (Collection p) where
getParticleI config i = (1,1,1) :: ParticleC
What you are doing in your instance, however, is always returning a
ParticleC, which is a specific type rather than any type that belongs to
Particle. There are several ways to solve this. A few examples:
1. Make getParticleI specifically return a ParticleC, rather than the
type p.
2. Add a makeParticle function to the particle type-class. If you had:
class Particle p where
makeParticle :: (Double, Double, Double) -> p
Then you could rewrite that last line as:
getParticleI config i = makeParticle (1, 1, 1)
And then the return would be of any type p that has a Particle instance.
3. Parameterise the collection over the particle, e.g.
class Configuration c where
getParticleI :: Particle p => c p -> Int -> p
But currently Collection is not actually parameterised using the p
parameter (the UArray has Double, not Particle), so I can't properly
adjust your example for that.
Hope that helps,
Neil.
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