How to put constraints on the associated data?

Question

I would like to state that the associated data is always an instance of a certain class.

class (Context (Associated a b)) => Class a where
  data Associated a         


        

Answer 1

One idiomatic way is to create a Context1 class. Supposing we have

class Context a where
    func :: a -> String

we might generalize as:

class Context1 f where
    func1 :: Context a => f a -> String

Then you give a single instance for all Associateds:

instance (Context1 (Associated a), Context b) => Context (Associated a b) where
    func = func1

Now it is easy to write the class you want as

instance Context1 (Associated a) => Class a where
    data Associated a :: * -> *

and you can be sure that the given Context1 (Associated a) context ensures the desired forall b. Context b => Context (Associated a b) context.

There are many examples of this pattern on Hackage, like Show1, Foldable1, and Traversable1.