Closed type classes

Question

Is it possible to create a typeclass that can no longer admit new members (perhaps by using module boundaries)? I can refuse to export a function necessary for a complete instan

Answer 1

Since GHC 7.8.1, closed type families can be declared, and I think with the help of them, and ConstraintKinds, you can do this:

type family SecretClass (a :: *) :: Constraint where
  SecretClass Int = ()

SecretClass a forms a constraint, equivalent to a type class, and since the family can't be extended by anyone, no other instances of the "class" can be defined.

(This is actually just speculation, since I can't test it, but the code in this interesting link makes it look like it would work.)