Hello

Consider the following data, class and instance:

class PP m where
create :: a -> m a

data A a = A a

instance PP A where
create a = A a

And then:

class CB a where
fb :: a -> Int

data B m a = B (m a)

If I try to define an instance of PP for B with the following implementation:

instance (PP m) => PP (B m) where
create a = let _ = fb a
in B (create a)

GHC issues the (expected) error: Could not deduce (CB a) arising from a use of 'fb' from the context (PP m).

So the workaround I am using is to define a new class PP' where the type a is made more explicit. That allows the constraint (CB a) to be stated in an instance declaration.

class PP' m a where
create' :: a -> m a

instance (PP m) => PP' m a where
create' = create

instance (PP m, CB a) => PP' (B m) a where
create' a = let _ = fb a
in B (create a)

So, my question is: is there a way to achieve the same results without having to create the new class PP'?

Thank you for your help

J-C