Now that we have roles, it might be helpful to be able to give a role signature for a data/type family.

At the moment, data/type family constructors have all parameters conservatively assigned to be role N. Thus

data family D a    -- Parameter conservatively assumed to be N

class C a where
  op :: D a -> a

instance C Int where ....

newtype N a = MkN a deriving( C )  -- Rejected

The generalised-newtype-deriving clause deriving( C ) is rejected because D might use its parameter at role N thus:

data instance D [b] = MkD (F b)   -- F is a type function

It would be strictly more expressive if we could

• Declare the roles of D's arguments (as we can declare their kinds). E.g.

data family D a@R

• Check that each family instance obeys that role signature. E.g. given the preceding role signature, reject this instance:

data instance D [b] = MkD (F b)   -- F is a type function

I think there is no technical difficulty here. Just a question of doing it.