Lack of a filter that simultaneously reveals information about the value of a Behavior about reactive-banana HOT 2 CLOSED

Thanks for your feedback!

Fortunately, the semantics of reactive-banana are awesome and do everything you want already. :-) In particular,

First, it is not clear to me that this is guaranteed to work all the time because it isn't clear to me that the semantics of the network allow me to be sure that the value of b hasn't changed between the isJust and the fromJust --- say, in a concurrent setting.

it is guaranteed that the value of the behavior b "doesn't change in between". I'm using parentheses here because the notion of something "changing in between" is a actually concept that simply cannot occur in reactive-banana (since version 0.2, that is).

The semantics of reactive-banana are given by the [model implementation in the module Reactive.Banana.Model]. I have kept it very simple, it should be easy to understand. These semantics are law. Concurrency or not, if the efficient implementation does not satisfy them, I'm going to jail! :-O

In particular, this means that your implementation is indeed correct. Here a slightly tidied up version:

filterJust :: Event (Maybe a) -> Event a
filterJust = fmap fromJust . filterE isJust

e2a = filterJust    $ b <@ e1
e2b = filterNothing $ b <@ e1
    where filterNothing e = () <$ filterE isNothing e

That said, I agree that the filterJust function is a useful shortcut and should probably be added to the API.

from reactive-banana.

I have now added the filterJust function to the API. See issue #18.

from reactive-banana.