Bartosz Milewski wrote:Ok. As I mentioned, if you just want to use the library there is no need to understand the implementation.
Thanks, Heinrich. I looked at the examples and at the references you
provided. I understand the semantic model, so I guess I'm mostly trying to
understand the implementation.
Essentially, Conal implements events as
Conal's paper was mostly about refining data
structures in order to provide better implementation. It's all beautiful up
to the point where he introduces the unamb hack. How did you manage to work
around this problem and implement event merging efficiently?
type Event a = [(Time,a)]
The trouble is that when merging events, this representation forces you to wait for both events. In other words, the pattern match
union ((t1,x1):e1) ((t2,x2):e2) = ...
needs to know the times of occurrences of both events before it can return the earlier one. The trouble is that the merge function should have returned the earlier one right away, before knowing exactly when the later one happens. The purpose of the unamb hack is circumvent that problem.
Reactive-banana's very simple solution to this problem is to represent events as
type Event a = [(Time, Maybe a)]
and impose the additional invariant that all events in your program are "synchronized", in the sense that they indicate their occurrences at the same times^1. If they don't occur at that time, they use Nothing . Then, you can implement merge simply as
union ((t1,x1):e1) ((t2,x2):e2) = -- we always have t1 = t2
(t1, combine x1 x2) : union e1 e2
where
combine (Just x) Nothing = Just x -- only left occurs
combine Nothing (Just y) = Just y -- only right occurs
combine (Just x) (Just y) = Just x -- simultaneous occurrence
combine Nothing Nothing = Nothing -- neither occurs
Since the times are given globally, we can also remove them and obtain
type Event a = [Maybe a]
This is how Reactive.Banana.Model does it.
Of course, keeping track of a lot of Nothing is something that can be optimized. The optimization to apply here is to transform the implementation into a push-driven style. I haven't published the details yet, but some design notes can be found here.
http://apfelmus.nfshost.com/blog/2011/04/24-frp-push-driven-sharing.html
^1: Note that the times do not need to follow a uniform time step.
Best regards,
Heinrich Apfelmus
--
http://apfelmus.nfshost.com
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