Raising a singleton effect row? - Polysemy

inject :: forall eff r a. Member eff r => Sem '[eff] a -> Sem r a
inject (Sem a) = Sem $ \k -> a $ \case
  Union Here (Weaving e s f n v) ->
    k $ Union (membership @eff) $ Weaving e s (inject . f) n v

inject :: Inject effs r => Sem effs a -> Sem r a
inject (Sem a) = a $ liftSem . deject . hoist inject

class Inject effs r where
  deject :: Union effs (Sem r) a -> Union r (Sem r) a

instance Inject '[] r where
  deject = absurdU

instance (Member eff r, Inject effs r) => Inject (eff ': effs) r where
  deject u =
    case decomp u of
      Left  u' -> deject u'
      Right w  -> Union membership w

    • Could not deduce: Members effs effs arising from a use of ‘gen’

type family Members2 es r :: Constraint where
  Members2 r         r = ()
  Members2 '[]       r = ()
  Members2 (e ': es) r = (Member e r, Members2 es r)

works great