[Haskell-cafe] Context for type parameters of type constructors

Henning Thielemann iakd0 at clusterf.urz.uni-halle.de
Tue Mar 30 14:32:20 EST 2004

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.


Btw. ghc suggests:
    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?

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