Bug in type equality constraints?
Conal Elliott
conal at conal.net
Tue Jul 15 10:59:25 EDT 2008
I'm converting some code from functionally dependencies to associated types,
and I've run into a problem with equality constraints and subclasses. The
classes:
class AdditiveGroup v => VectorSpace v where
type Scalar v :: *
(*^) :: Scalar v -> v -> v
class VectorSpace v => InnerSpace v where
(<.>) :: v -> v -> Scalar v
Products of vector spaces are vector spaces *if* over the same scalar
field. Hence:
instance ( VectorSpace u,VectorSpace v
, Scalar u ~ Scalar v ) => VectorSpace (u,v) where
type Scalar (u,v) = Scalar u
s *^ (u,v) = (s*^u,s*^v)
Similarly for inner product spaces:
instance ( InnerSpace u,InnerSpace v, Scalar u ~ Scalar v
, AdditiveGroup (Scalar v) ) => InnerSpace (u,v) where
(u,v) <.> (u',v') = (u <.> u') ^+^ (v <.> v')
But here's where ghc-6.9.20080622 balks:
Data\VectorSpace.hs:106:0:
Couldn't match expected type `Scalar v'
against inferred type `Scalar u'
When checking the super-classes of an instance declaration
In the instance declaration for `InnerSpace (u, v)'
Any ideas?
- Conal
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