Hook into event/behavior life cycle?

[mitchellwrosen]

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Hello! Is it possible to detect when an Event is no longer relevant to the network (garbage collected)? My use-case is killing a thread that's spawned in a MomentIO computation. I can provide more details if necessary.

Thanks!

[HeinrichApfelmus]

This is currently not part of the API, but I think it should be possible to implement. Could you provide a short overview of your use case for why this is a good thing to have? Note that the whole garbage collection / finalizer thing can be rather tricky, so it's always a good idea to avoid it if possible.

[mitchellwrosen]

Sure, I was playing around with implementing a sort of arcade, in which some minigames had a concept of a timer and others did not.

Rather than have a global ticking thread, I wanted to spawn a background thread in the Moment that some relevant game is switched to, and kill it when it's no longer necessary to have a tick event:

makeTickEvent :: MomentIO (Event ())
makeTickEvent = do
  (etick, tick) <- newEvent
  
  tid <- liftIO . forkIO . forever $ do
    threadDelay 1000000
    tick ()

  -- Here I want to attach a finalizer to `etick` that kills thread `tid`

  pure etick

[ocharles]

While this is obviously a fine thing to want to do, can you explain why you'd want to do it without a global tick event? It's easier to add new features if it's clear that they enable functionality that is otherwise impossible.

[mitchellwrosen]

It's just the first solution that came to mind, and it'd actually make the code a bit cleaner.

The real type signature for makeTickEvent is

makeTickEvent 
  :: Int -- Initial delay
  -> Event (Int -> Int) -- Change delay
  -> MomentIO (Event ())

So, the ticks can be sped up or slowed down (in some games) by pressing some keys. If I had to use a global ticking event:

• I'd have an additional behavior to determine whether or not speeding up/slowing down events is possible
• I'd have to reset the delay to the default when switching
• I couldn't guarantee that a tick happens in the first moment of a game's existence

None of these are really game-changers (ha), so a global tick event will actually be fine for my purposes :)

However, now I'm imagining some polling behavior that is truly costly to perform, like periodically hitting some rate-limited API, where only paying for what you use is much more desirable.