On Tue, 30 Mar 2004, Keith Wansbrough wrote:
My question was why he doesn't believe that. My definition data (Num a) => VList a = VList [a] clearly states that VLists will ever get types of class Num as parameters.
Ah. Constraints on datatype declarations are a misfeature of Haskell, and have no useful effect. You shouldn't use them.
Ok. Btw. ghc suggests: VectorSpace.lhs:37: Could not deduce (Num a) from the context (VectorSpace VList) arising from the literal `0' at VectorSpace.lhs:37 Probable fix: Add (Num a) to the class or instance method `zero' ^^^^^^^^^^^^^^^ I know how to add context information to the signature in a class definition. But how can I add context information at the point of instantation of a method?