Why are there no functions for building Events out of non-events in reactive-banana?

Question

I'm in the process of teaching myself FRP and Reactive-banana while writing what I hope will be a more useful tutorial for those that follow me. You can check out my progress on the tutorial here.

I'm stuck at trying to implement the simple beepy noise examples using events. I know I need to do something like this:

reactimate $ fmap (uncurry playNote) myEvent

in my NetworkDescription, but I can't figure out how to just have the network do the same thing repeatedly, or do something once. Ideally, I'm looking for things like this:

once :: a -> Event t a
repeatWithDelay :: Event t a -> Time -> Event t a
concatWithDelay :: Event t a -> Event t a -> Time -> Event t a

The Time type above is just a stand-in for whatever measurement of time we end up using. Do I need to hook up the system time as a Behavior to drive the "delay" functions? That seems more complicated than necessary.

Thanks in advance,

Echo Nolan

EDIT: Okay the types for repeatWithDelay and concatWithDelay don't make sense. Here's what I actually meant.

repeatWithDelay :: a -> Time -> Event t a
concatWithDelay :: a -> a -> Time -> Event t a

Answer 1

I have chosen not to include such functions in the core model for now, because time raises various challenges for consistency. For instance, if two events are scheduled to happen 5 seconds from now, should they be simultaneous? If not, which one should come first? I think the core model should be amenable to formal proof, but this does not work with actual, physical time measurements.

That said, I plan to include such functions in a "they work, but no guarantees" fashion. The main reason that I have not already done so is that there is no canonical choice for time measurement. Different applications have different needs, sometimes you want nanosecond resolution, sometimes you want to use timers from your GUI framework, and sometimes you want to synchronize to an external MIDI clock. In other words, you want the time-based function to work generically with many timer implementation, and it is only with reactive-banana-0.7.0 that I have found a nice API design for this.

Of course, it is already possible to implement your own time-based function by using timers. The Wave.hs example demonstrates how to do that. Another example is Henning Thielemann's reactive-balsa library, which implements various time-based combinators to process MIDI data in real time.