[Haskell-cafe] Got problems with classes

Neil Brown nccb2 at kent.ac.uk
Mon Aug 17 10:49:51 EDT 2009


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

> 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,


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