flatMapLatest spawns next substream before the previous is disposed

[Matthias Seemann]

Sometimes flatMapLatest(fn) creates the substream fn(x) before the past substream got disposed.

I did not expect this behaviour. When I used tasks.flatMapLatest(sendTaskToWebWorkerAndWaitForCancellationOrResult) to feed tasks into a single WebWorker thread, the webworker did sometimes receive a new task before the previous got cancelled.

Also easy to reproduce using fromArray.

The reason is that in the implementation, which is roughly

flatMapLatest(obs, fn) = obs.flatMap(x => fn(x).takeUntil(obs));

It is unclear if a new x value first cancels the previous substream fn(x).takeUntil(obs) or first calls fn(x) to create the next substream.

An implementation

flatMapLatest(obs, fn) = obs.flatMap(x => B.later(0, x).map(fn).takeUntil(obs));

might fix it.

However, I don't know if I am aware of all use cases. Otherwise I might attempt a PR.

Do you think it's worth fixing?

[Matthias Seemann]

One could also delegate this problem to user-land.

For example, if there is this unwanted behaviour:

var subStream = x => { 
   console.log(`Creating substream ${x}...`); 
   return B.fromBinder(sink => {
      const interval = setInterval(sink, 1000, x);
      return () => {
         console.log(`disposing substream ${x}...`);
         clearInterval(interval);
      };
   });
 };
B.sequentially(2500, ["A", "B", "C"])
.flatMapLatest(subStream)
.take(7).onValue(console.log);
/*
Creating substream A...
A
A
Creating substream B...    ↑  incorrect order
disposing substream A...   ↓
B
B
Creating substream C...    ↑  incorrect order
disposing substream B...   ↓
C
C
disposing substream C...
C
*/

the user can enforce the correct order by delaying substream creation:

var callNextTick = f => x => B.later(0, x).flatMap(f);

B.sequentially(2500, ["A", "B", "C"])
.flatMapLatest(callNextTick(subStream))
.take(7).onValue(console.log);
/*
Creating substream A...
A
A
disposing substream A...
Creating substream B...
B
B
disposing substream B...
Creating substream C...
C
C
disposing substream C...
C
*/

If this would be fixed in the library, problems with synchronous substreams (e.g. x => B.once(x)) as in #719 would reappear. On the other hand I don't see much value in synchronous events, since in v2.0 they are discontinued anyway.

So either add a comment to the documentation of flatMapLatest or fix in the library possible breaking stuff?