This works: -- Tensor product of two vector spaces type family Tensor u v :: * type instance Tensor (Vect k a) (Vect k b) = Vect k (TensorBasis a b)
On Aug 21, 2010, at 9:15 AM, DavidA wrote: > Brandon S Allbery KF8NH <allbery <at> ece.cmu.edu> writes: > >> >>> type Tensor u v = >>> (u ~ Vect k a, v ~ Vect k b) => Vect k (TensorBasis a b) -- ** >> >> IIRC this actually substitutes as >> >> (forall k a b. (u ~ Vect k a, v ~ Vect k b) => Vect k (TensorBasis a b)) >> >> and the implicit forall will generally mess things up because it won't be >> floated out to the top level. (Or in other words, constraints in type >> declarations don't generally do what you intend.) >> > > Thanks. Is it possible to elaborate a bit - I still don't really understand > what goes wrong. And is there any way to fix it? > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Sjoerd Visscher http://w3future.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
