Functor/Foldable/Traversable/Applicative instances for Union?

[spacekitteh]

It would be amazing if you could provide instances of these classes for Data.OpenUnion. Here's how I implemented Functor:

type family ConstrainedMembers constr l = (r :: Constraint) where
  ConstrainedMembers constr (t ': c) = (constr t, ConstrainedMembers constr c)
  ConstrainedMembers constr '[] = ()

instance {-#OVERLAPPING#-} Functor (Union '[]) where
  fmap f a = error "Absurd?"
instance {-#OVERLAPPING#-} (ConstrainedMembers Functor f, r ~ (h : tail), Functor (Union tail)) => Functor (Union r) where
  fmap f a = case decomp a of
                       Right t -> inj (fmap f t)
                       Left remainder -> weaken (fmap f remainder)

As you can see, it's pretty ugly and it took quite a while to get everything just right. I'm scared of doing the rest!