Question

I have a parametric variable in a type class which I would like to represent any of the number types (Float, Integer, Double etc).

How I do declare this in the type and instance?

Answer 1

If you mean the type parameter for the class itself, you can add a constraint to the context of the class definition:

class (Num a) => Foo a where
    -- stuff

On the other hand, if it’s a parameter in a function’s type that isn’t the class parameter, then you can add a constraint in the usual fashion:

class Foo a where
    foo :: (Num b) => a -> b -> b

If you want to constrain it to only certain specific types, that’s not really possible–type classes are “open”, i.e., you can always add instances. The best you could do is have your own class (FooNumbers or such), write only the instances you want, and then require that instead of Num. But that’s usually not helpful.