Event loop implementation?

This has come up at least a couple times in different contexts and I think this would be a cool idea with some interesting applications. Plus @keynmol may have volunteered himself to take this on 😉

Why:

• JVM: IO#unsafeRunSync in single-threaded environments (e.g. AWS lambda) without the penalty of creating threads or shifting.
• JS: unsafeRunSync (!!) via a new “TemporalSyncIO” data type that implements Temporal and Sync. @djspiewak pointed out this would be similar to the microtask loop (with the addition of timers).
• Native: when Native rolls around, IIUC this should give us a single-threaded runtime essentially for free.

[armanbilge]

@ritschwumm actually it seems like it is already possible to run tasks on the AWT event dispatch thread without any changes in CE.

I see there is a method SwingUtilities.doLater that causes a Runnable “to be executed asynchronously on the AWT event dispatching thread.”

So all you need is something like:

object AwtEventDispatchEC extends ExecutionContext {
  def execute(r: Runnable): Unit = SwingUtilities.doLater(r)
  def reportFailure(t: Throwable): Unit = t.printStackTrace()
}

IO(whatever).evalOn(AwtEventDispatchEC)

Since the main thread is special, it requires the changes proposed in https://github.com/typelevel/cats-effect/pull/2732.

[ritschwumm]

@bilki it’s not just the main thread, with e.g. AWT (and by extension, swing) you sometimes need to run things synchronously in the event dispatch thread