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

When all you are interested in that you have an enumertated set of instances, then this trick might help:

class (Elem t '[Int, Integer, Bool] ~ True) => Closed t where

type family Elem (t :: k) (ts :: [k]) :: Bool where
  Elem a '[] = False
  Elem a (a ': as) = True
  Elem a (b ': bs) = Elem a bs

instance Closed Int
instance Closed Integer
-- instance Closed Float -- ERROR