Your suggested case is contrived and doesn't apply to JavaScript (the examples were in JavaScript where I assume all these streams are in the same thread). I am not claiming withLatestFrom is the correct mechanism for determining order of events in multi-threaded assumptions. That's an orthogonal concern to the issue at hand.
> I strongly suspect that combining async streams will prove to be a problem in practice
Combining async results is a true problem and people often attempt to solve them with callbacks or heavy usage of Futures/Promises. Rx solves these problems easily, and I've witnessed a lot of developers confirm this when learning Rx. It's not hard to teach the difference between withLatestFrom and combineLatest and their intended use cases. Much easier than managing callback hell, which is what RxJS intends to replace.
I didn't recognise that you were talking chiefly about javascript. It is true that my example doesn't apply here.
Ok, then I would make one comment (and edit my first comment).
I would avoid the discussion of simultaneity at the start
'Events in parentheses happen “simultaneously”.
In practice they happen at slightly different times,
but separated by only a couple of nanoseconds, so
people understand them to be simultaneous. Events (c1c2)
are called glitches and sometimes considered a problem
because one would expect only c2 to happen.'
errors ---e1-------e2---------------
userActions -u1------u2------------u3----
analyticsMessages ---e1u1--(e1u2 e2u2)---e2u3--
errors ---e1----e2------------------
userActions -u1----------u2--------u3----
analyticsMessages ---e1u1--(e2u1 e2u2)---e2u3--
Anyway - that is nitpicking. I enjoyed your article. I agree that the scenario described at top does not apply in javascript.
I too strongly dislike callbacks :)
userActions(combined)
\
\--u1-----------------e1--X
\--u2---------------------------e2--X
\--u3--...
\--u4--...
\--u5--...
\...
e1
\
X
(At least as far as I recall. If there's some documentation/papers that suggest otherwise, please link!)
EDIT: I should say: FRPNow! seems to have cracked a bit of this puzzle and Neel Krishnaswami had a recent-ish (2014?) paper on avoiding both space and time leaks with static checking, but I don't think we have anything approaching the mainstream, unfortunately.
I strongly suspect that combining async streams will prove to be a problem in practice, whether we resolve glitches or not. This article requires quite subtle reasoning about async systems. In my experience this type of reasoning is rarely done effectively on real life projects.
I am not criticising Rx, I am interested in Rx. But I am always concerned when a project/community starts producing articles on very subtle topics and claims 'x isn't a problem if you just understand it properly'. Here properly probably means deeply, and deep knowledge in software is scarce. There are too many things to know, to know many things deeply.
Let me look at one example (and please comment if I'm really not understanding something).
So we consume, asynchronously, two streams. Because they are asynchronous we might hit some point where it's a bit uncertain where two outputs are related.
That's a good example, and we fixed that problem with 'errors.withLatestFrom(...'. But, since we are asynchronous don't we also have to worry about this scenario. Now, let me clarify what happened here. The error occurred after u2, but the errors thread got descheduled and the userActions thread kept on processing. I am not certain exactly what to expect from 'errors.withLatestFrom(userActions,...' here but I suspect that 'e2u8' may be the expected output.So this error reporting system is fundamentally unreliable. If this is correct then I would say that combining Rx streams is not appropriate for this use case. This also suggests that although we managed to get rid of our glitches we still produced a terrible piece of software.
Do I have to wrong end of the stick? (The answer was yes, but it was good anyway)