I would suggest defining types for unit and functions... data Scalar a = Scalar a data Function c v = Function c v instance VSpace a (Scalar a) ... -- replaces 'a' instance VSpace a (Function c a) ... -- replaces c -> a instance VSpace a v => a (Function c v) ... -- replaces c -> v Here Scalar is just a 'special' identity, and 'Function' is a 'special' arrow (->) ... Keean. karczma@info.unicaen.fr wrote:
Ashley Yakeley writes:
GHCi is correct to complain:
class Vspace a v | v -> a
OK, the first parameter ("a") depends on the second ("v").
This is what I want. For a given set of vectors, the associated scalars are unique, otherwise I would have problems with norm. But I have problems anyway...
instance Vspace a a where
And this determines it: the first parameter must always be the same as the second.
instance (Vspace a v) => Vspace a (c->v) where
This is incompatible with the previous instance declaration, since "a" is not the same as "c -> v".
Why "always the same"? This is just *this* instance. If I eliminate Vspace a a, and I write instance (Num a)=>Vspace a (c->a) where (f <+> g) x = f x + g x (a *> f) x = a * f x and then I tried to generalize, by instance (Vspace a v) => Vspace a (c->v) where (f <+> g) x = f x <+> g x -- ... I get another error, even less understandable. Can you guess it without testing?... I permitted all extensions, overlapping instances, etc.
Jerzy Karczmarczuk _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe