That's very interesting, thanks! It gave me a brainwave and I wondered I could implement that in Bluefin. I'm pretty sure Bluefin's Request[1] is a second class stackful coroutine, and sure enough it turns out to be possible, so I'm pleased about that.

    -- ghci> example
    -- Hello
    -- World
    -- Timed out
    example = runEff $ \io -> awaitYield (receiver io) sender
    
    receiver ::
      (e1 <: es, e2 <: es) =>
      IOE e1 ->
      Await String e2 ->
      Eff es ()
    receiver io a = do
      r1 <- await a
      effIO io (putStrLn r1)
    
      r2 <- await a
      effIO io (putStrLn r2)
    
      mr3 <- timeout io 0 (await a)
      effIO io $ case mr3 of
        Nothing -> putStrLn "Timed out"
        Just r3 -> putStrLn r3
    
    sender ::
      e1 <: es =>
      Yield String e1 ->
      Eff es ()
    sender y = do
      yield y "Hello"
      yield y "World"
      yield y "More"
    
    timeout ::
      e1 <: es =>
      IOE e1 ->
      Int ->
      Eff es r ->
      Eff es (Maybe r)
    timeout io t m = withEffToIO
      (\effToIO -> System.Timeout.timeout t (effToIO (\_ -> useImpl m)))
      io

Or in Lua you'd wrap the initial call in a coroutine, possibly with coronest[a] or something similar to make handling the effects at the right layer easier.

And so then the outer code is a loop around coroutine.resume, and the inner code uses coroutine.yield to perform an effect.

[a]: https://github.com/saucisson/lua-coronest