Type design question

[Dylan Thurston]

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On Fri, Jul 25, 2003 at 08:31:26AM -0700, Hal Daume wrote:
> However, once we fix this, we can see the real problem.  Your Universe
> class has a method, distanceVector, of type:
>=20
> | distanceVector :: Universe u, Floating a =3D> u -> Vector a -> Vector a
> -> Vector a
>=20
> And here's the problem.  When 'u' is 'OrthorhombicUniverse x', it
> doesn't know that this 'x' is supposed to be the same as the 'a'.  One
> way to fix this is to parameterize the Universe data type on the
> element, something like:
>=20
> class Universe u where
>   distanceVector :: (RealFrac a, Floating a) =3D> u a -> (Vector a) ->
> (Vector a) -> (Vector a)
> ...

Another approach is to make Universe a multi-parameter type class:

class (RealFrac a, Floating a) =3D> Universe u a | u -> a where
  distanceVector :: u -> Vector a -> Vector a -> Vector a
=2E..

You need to use ghc with '-fglasgow-exts' for this.

Peace,
	Dylan

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