subscribeWithLast :: forall a b eff.
      (b -> a -> Eff (frp :: FRP | eff) b)
  ->  b
  ->  Event a
  ->  Eff (frp :: FRP | eff) (Eff (frp :: FRP | eff) b)
subscribeWithLast f z evt = dropRefEff $ do
  r <- newRef z
  unsub <-
    FRP.subscribe evt
    (\e -> readRef r >>= \a -> addRefEff (f a e) >>= writeRef r)
  pure (dropRefEff (unsub *> readRef r))
  where
  dropRefEff :: forall e x. Eff (ref :: REF | e) x -> Eff e x
  dropRefEff = unsafeCoerceEff
  addRefEff :: forall e x. Eff e x -> Eff (ref :: REF | e) x
  addRefEff = unsafeCoerceEff
--