advance.hs

-- What I am struggling to do is sequence to looping things. First I have a loop where the output is feed into the 
-- next step as input until it completes. Then the next loop starts.

-- I doubt this works ... I'm trying to test it now ... anyway I don't like it and feel like
-- there must be an easier way.

advance :: (Adjustable t m, MonadHold t m, Monad m, MonadFix m)
  => m (Event t (Maybe a))
  -- ^ initial
  -> (a -> m (Event t (Maybe a)))
  -- loop step
  -> (forall void. a -> m void)
  -- next loop ... probably never returns
  -> m ()
advance initial loopStep next = do
  rec out <- networkHold initial $ ffor (updated out) $ \e -> do
        rec delayed <- networkHold (delayOne e) $ ffor (switch $ current delayed) $ \case
              (Just x , _) -> fmap (, Just x) <$> loopStep x
              (Nothing, Just x) -> next x
              (Nothing, Nothing) -> error "impossible"
        pure never
  pure ()