Hi Ivan,
The easiest way to see the difference is by looking at some of the combinators. Notice that things like 'hold', 'scan'/'accum', and 'tag' are real functions. In a first-class FRP system these would have types like the following:
hold :: a -> Event a -> Moment (Behaviour a) scan :: a -> Event (a -> a) -> Moment (Event a) tag :: Behaviour (a -> b) -> Event a -> Event b
The Moment monad is not inherent to the way the underlying state machine is constructed, but acts merely as a provider for the notion of "now". Since 'tag' doesn't need that notion, it's a completely pure function.
Well, in a way. Yes, it can be a pure function, and an event can somehow be a delayed computation of how/when it is actually produced, computed/consumed the moment you want to actually evaluate the network.
Saying that they are pure would be just fine if Behaviours did not depend on the outside world (that is, if they were "calculated" from pure haskell functions). But I don't think they are. Not always. Not if you want to depend on any external user input.
Behaviours are actually pure values. They don't really depend on time or any effects. Their values may very well be generated from effects, for example the "current cursor position", but conceptually the behaviour that represents the whole timeline of values is indeed a pure value. There is a caveat of course: We like to think of behaviours as functions of time, but that's not the whole truth, because our capability to observe the value of a behaviour is very limited, in most implementations to an abstract notion of "now". The same is true for events: we can only ever ask whether an event is happening "now". That's how effects and a pure API can be compatible. We can think of behaviours as pure timelines (or functions of time), but the API cannot possibly give us full access to it.
In Reflex (and I'm not trying to discuss the particularities of this implementation), yes, Behaviour and Event are types in a family, but the actual definitions in Spider I can seee are records of IORefs with bangs. Far from pure.
Yes, of course. The implementation is shockingly impure and hacky, which is why there is such a massive test suite. =) There are much less hacky ways to implement it, but unfortunately some impurity is inevitable. The reason for Spider's hacikness is efficiency: Reflex is incredibly fast, and a lot of effort went into only ever computing things that matter, and never computing them twice. In my benchmarks it comes very close to wires, which is quite impressive, if you consider what thin an abstraction layer Wire (or MSF) is.
You can have that function in AFRP as well:
fmap :: (a -> b) -> Event a -> Event b
However, unlike 'fmap', 'tag' makes sense in a pure context. You can pass an Event and a Behaviour to a different thread via an MVar, combine them there, then send the result back, and it will still work in the context of the greater application (no isolated state machines).
I don't see how you cannot do that with wires. For instance, you can send a Wire m () (Event b), and a Wire m () (a -> b), and compose them in a pure context. Then you can bring that back and use it.
Right. The difference is that you need to be very careful about context. If you have a "main wire", you must make sure to communicate that result back into it *or* run two wires concurrently. This caution is not necessary with first-class FRP, because it does not have that context-sensitivity.
You can hold an event in any concurrent thread, etc.
Can you use it without doing IO and executing the computation associated to calculating/polling the behaviour? If so, it must be because the FRP evaluation method has some inherent thread-safety (I you need IO + more for that). Wouldn't you be able to put that thread safety in your monad, and then use it with MSFs/Wires?
Thread safety is a different matter, and yes, the implementation must be thread-safe for that to work. This is the reason why I was investigating an FRP implementation based on STM to see how fine-grained regions would pan out, but it was so slow that i abandoned that approach. Reflex does global locking, which sucks, but I can't think of a better way. To answer your question: it depends on the controller API of the framework. For example in Reflex the frame boundary is created by 'fireEventsAndRead'. This is the only action that can "advance time". You can use it from multiple threads, and it will have a timeline-global effect (you can have multiple timelines in Reflex, but if that doesn't make sense to you, just think of "timeline-global" as "global"). In reactive-banana the frame boundery is created by registered callbacks. R-b registers callbacks for events that matter (that's where 'fromAddHandler' and 'reactimate' meet), and whenever one of them is invoked, a new frame begins. In both cases the clock ticks as events fire.
Another example is that if the underlying monad is nontrivial (say IO) you can't easily split behaviours in a pure context in AFRP.
You can, but you need a monad such that: (,) <$> ma <*> ma == (\x -> (x,x)) <$> ma.
Is this called idempotent?
But to implement any form of Classic FRP or Reactive Programming on top of MSFs, you want that kind of monad.
Not sure if idempotency is the right term, but in any case you have that monad in fist-class FRP. It's called Behavio(u)r. =) Note: The Monad instance for Behavior is not implemented yet in Reflex 0.4.0, but you can easily achieve the same by using 'pull' and 'sample': pull (liftA2 (,) (sample b1) (sample b2)) The instance is implemented in the git version.
This restriction does not exist in first-class FRP:
Well, it is not exposed to the user, but someone must have thought about it and solved it. Duplication of effects is inherent to having monadic computations associated to obtaining the values of behaviours. If you don't cache for a given timestamp, you duplicate effects.
This is only really inherent to the mealy-machine approach (i.e. "what AFRP does"). The monads involved in first-class FRP really only serve to tie reactive combinators to "now". Their implementations only control when exactly (in which frame) you hold an event, which is usually a simple matter of effect sequencing, i.e. "having a monad". In other words: moment monads are generally just IO in disguise.
I cannot say I like arrow notation, or inputs based on tuples. We need more work on this.
However, I decided to embrace the A and I am finding a lot of extensions and guarantees that are possible, or easier, thanks to that.
Cale Gibbard has done some work on desugaring arrow notation in smarter ways than the tuple-based approach we have now, but ultimately the whole arrow approach was abandoned (and eventually Reflex was born). My original approach with Netwire was to provide higher-level composition capabilities to reduce the amount of "side channels" necessary, which lead to an interesting Alternative instance for Netwire's version of Wire. One of the defining features of Netwire is the ability to "inhibit", which facilitates a form of switching that eliminates most use cases of Yampa's event-based switches. The following is a string-valued wire that displays "---", but every five seconds it switches to "Ding!" temporarily for one second: ("Ding!" . holdFor 1 <|> "---") . periodic 5 However, nowadays I think first-class FRP is the superior approach. Greets ertes