[Haskell-cafe] FGL instance constraint

[Kevin Quick]

I need help understanding how to express the following:


>  data (Cls a) => B a = B [a]

>  data GrB a b = GrB (B a)

>  instance Graph GrB where ...

In the methods for the instance specification, I need to perform Cls a operations on a.

  * As shown, the compiler complains that it cannot deduce (Cls a) from the context () on those methods.
  * I can't redefine the Graph methods to introduce the (Cls a) constraint [reasonable]
  * If I try to express the constraint as part of the Graph instance: "instance (Cls a) => Graph GrB where ..." then it says it's an ambiguous constraint because 'a' isn't mentioned.
  * I've tried specifying a functional constraint: "instance (Cls a) => Graph GrB | GrB -> a where ..." but that's not valid for an instance declaration.
  * I can't include a in the GrB instance: "instance (Cls a) => Graph (GrB a b) where ..." because that's a kind conflict.

Suggestions/solutions are appreciated.

Thanks!

-- 
-KQ