combine on synchronous sources from array

Given this code

const a$ = most.from([1,2])
const b$ = most.from([10]);

const c$ = most.combine((a,b)=>a+b, a$, b$);

c$.subscribe({next: c => console.log(c)});

The log shows

12

While most people would expect it to display

11
12

By the way, I’m raising these issues also for xstream https://github.com/staltz/xstream/issues/173 and RxJS https://github.com/ReactiveX/rxjs/issues/2414. Is this is a bug or a feature? I’m not sure. What are people’s ideas and opinions on this case?

[briancavalier]

I’m ok with saying that the current behavior is correct for 1.x. Like both @trxcllnt and @TylorS have pointed out, values from an array (or other synchronously available data structure) are not really events, they’re simply data, and as such they have no time dimension. There is an implicit time imposed on them based on the instant of observation, but it’s artificial. They’re simultaneous, but indexed.

For now, I think we can say that when given a bunch of simultaneous indexed values, combine() observes the last (“last” as in index order) one.

What I don’t like is that the API allows a user to do get into this situation in the first place. The only way to prevent it currently is to tell users not to do it. Let’s think about ways we can prevent it in the future. The best solution may turn out to be @TylorS’s option 3.

[TylorS]

I think there are basically 4 options for these issues (across each library) after some discussion with @briancavalier.

1. “Solve” the confusion via documentation. Ensure it is clearly marked that synchronously emitting “events” are not just synchronous but simultaneous, and that combine always observes the last (as in order not time) simultaneous event.
2. Do lots of buffering (large performance impact)
3. Remove these APIs that don’t produce “true” events. Instead provide a kind of overlay for arrays of values that operate on top of streams that do produce predictable and sane timing behaviors.
4. Asynchronously emit each value one after another, which also feels very unexpected just like the current behavior.