"Kevin Quick"
I need help understanding how to express the following:
data (Cls a) => B a = B [a]
I think this only works if you have a forall in there.
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.
You need to explicitly state them again (which is why putting the constraint in the data definition is pointless).
* I can't redefine the Graph methods to introduce the (Cls a) constraint [reasonable]
Not sure if you can.
* 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.
Right, because the instance is on GrB _only_, not on the values it contains.
* 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.
You're putting the constraint in the wrong places: put the "(Cls a) => " in the actual functions where you need it. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com