Looking for `flatten :: Event [a] -> Event a` in Reactive Banana

Question

I'm looking for something like flatten :: Event [a] -> Event a (swap [] with Foldable f => f if you want) which would generate a separate event for each a in an Event's list, like split in an old version of sodium.

I suspect that this is somehow possible with switchE, but then I'd need a function of type [a] -> Event a.

I could possibly craft that myself with newEvent, but is there a function built-in to reactive-banana?

Edit:

Actually, I'm not so sure I can implement that with newEvent after all.

flatten :: Foldable f => f a -> Banana.MomentIO (Banana.Event a)
flatten xs = do
  (event, fire) <- Banana.newEvent
  liftIO $ forkIO $ mapM_ fire xs
  return event 

Will fire block until there are subscribers or will it just return immediately if there aren't any?

Edit 2:

Looking at the implementation of newAddHandler my implementation above won't work, because all events are possibly fired before any handlers can register.

Answer 1

This appears to be impossible. According to the notes in Heinrich Apfelmus's blog, Event doesn't support simultaneous occurrences. This is a relatively recent change; the post is dated August last year and v1.0 was released in October. It certainly wasn't the case when I originally learned Reactive Banana a few years ago.

But Event [a] seems like a reasonable way to represent a set of coincidental events in the first place. Why do you need to flatten it?