Rxjs: Deprecate the partition operator

[benlesh]

I think we should deprecate the partition operator and remove it for v7.

Reasons:

1. Not really an operator: partition isn't really an "operator" in that it returns [Observable<T>, Observable<T>] rather than Observable<T>. This means it doesn't compose via pipe like the others.
2. Easy to replace with filter: partition is easily replaced with the much more widely known filter operator. As partition is effectively the same thing as: const partition = (predicate) => [source.pipe(filter(predicate)), source.pipe(filter((x, i) => !predicate(x, i)))]
3. Rarely used: It's little used, by any code survey I've taken (within thousands of lines of code that I know use RxJS)

[kwonoj]

I actually have quite lot of frequent usage around partition in my codebases. Rationale for partition is usually I want to have 2 streams at once instead of applying filter twice, i.e simple example like

[ onlineObservable, offlineObservable ] = partition((x) => x.isOnline));. Though this could be achieved as suggested snippet via filter.

[cartant]

I think I've used it only once. However, I also think I'd be more likely to use it it if were to be a static function.

[kwonoj]

Having it as static makes sense to me too.

[cartant]

I was thinking about this, this morning. I'd like to keep partition, but make it a static function - as suggested, by Ben, here - and also extend it so that more than one predicate can be passed. Something like this.

[benlesh]

I did put together a static version of it in experimental, which you can see here... but that's what spurred me to think "this is so simple, and so little used, I'm not sure it's worth it"

[realappie]

I do actually use it quite often, but destructuring two filtered sources actually does the exact same.

[realappie]

@benlesh I think in codebases where its used quite often, it would be nice to have so you are not constantly repeating yourself.

[wmaurer]

Now that I know that partition exists, I could see nice use cases for it. Please do make it static operator.

[bdadsetan]

@benlesh I understand that partition is rather simple, and I understand it might not be used as often as filter or map, however, it does clean up the code and express intent quite well.

https://github.com/ReactiveX/rxjs/issues/3807 would move us away from the lodash terminology known by many developers. I still believe that it should be called partition, and if it does not get the special treatment that publish also needs (as publish also does not return an Observable), then I would also argue for your initial idea of making it a static.

[benlesh]

Now that I know that partition exists, I could see nice use cases for it. Please do make it static operator.

That's just the thing... it's not an operator as-is. It returns (source: Observable<T>) => [Observable<T>, Observable<T>], not (source: Observable<T>) => Observable<T>, so it doesn't compose via pipe. :man_shrugging:

It's going to have to change one way or another. It's either:

1. have a function : partition(source: Observable<T>, fn: (value: T, index: number) => boolean): [Observable<T>, Observable<T>
2. or create a totally new operator: partitionThing(fn: (value: T, index: number) => boolean): Observable<[Observable<T>, Observable<T>]>
3. or both?

[bdadsetan]

Should this discussion maybe wait until the discussion [1] about publish(), refCount() and so on are sorted, as they are probably also not composable by pipe? The partition() issue should probably not drive the design decision.

[1] https://github.com/ReactiveX/rxjs/issues/3833

[realappie]

@benlesh I would go with the second one, as its quite similar to how the original partition worked. I think the first one is nice too, but having two different approaches that effectively solve the same problem can in my opinion be confusing.

[ivan7237d]

I recently suggested a simpler grouping operator (https://github.com/ReactiveX/rxjs/issues/3922) and maybe that operator would cover the partition's use-case well enough for partition to be deprecated.

[mattiLeBlanc]

I actually wanted to use partition in my NGRX Effects, I want to separate an httpEvent stream that sends event.type 0 to 4 and I want to throttle the type 3 events and also handle the type 4 (success).
I thought partition was ideal but I don't think I can throttle and merge 2 streams back into one. So when do you use partition?

[spearmootz]

I was using partition before and it worked great. i recently migrated to rxjs 6 and had a really weird bug when i switched to using two separate filter operators to achieve the result. essentially both filters received true even though the condition function were opposite.

so the issue happened because one of the filters receiving and object and resolving to true, it then ran a side effect function and because the value received was an object when the second filter ran the side effect function had mutated the original value and this second filter also resolved to true. this wouldnt have happened with partition.

as far as the two options above, i like 1

[maartentibau]

https://github.com/ReactiveX/rxjs/issues/3797#issuecomment-399180980

I personally also would go for option 2

[martinsik]

I agree partition() isn't very useful. I've answered around 750 questions on StackOverflow tagged as "rxjs" and I think I've never seen a good use-case for partition().

[baamenabar]

I actually wanted to use partition in my NGRX Effects, I want to separate an httpEvent stream that sends event.type 0 to 4 and I want to throttle the type 3 events and also handle the type 4 (success).
I thought partition was ideal but I don't think I can throttle and merge 2 streams back into one. So when do you use partition?

I have a simmilar use case. Handling a stream with diferent outcomes, or different error responses from an http connections is a great use case.

I suspect many people have tried to use partition, but given that it's not pipeable they give up on it and end up using filter or something else. I throw errors and filter them, but dislike throwing errors when the flow is actually a possible expected event in the stream.

[AodhanHayter]

I actually wanted to use partition in my NGRX Effects, I want to separate an httpEvent stream that sends event.type 0 to 4 and I want to throttle the type 3 events and also handle the type 4 (success).
I thought partition was ideal but I don't think I can throttle and merge 2 streams back into one. So when do you use partition?

I have a simmilar use case. Handling a stream with diferent outcomes, or different error responses from an http connections is a great use case.

I suspect many people have tried to use partition, but given that it's not pipeable they give up on it and end up using filter or something else. I throw errors and filter them, but dislike throwing errors when the flow is actually a possible expected event in the stream.

I'm one of those folks who was trying to use partition, then slowly discovering that it is not pipeable. In fact it's also on an httpEvent stream, would be very useful, but just awkward with its current implementation.

[spearmootz]

I think that in the new rxjs it's clear that if it is not part of operators
it is not pipeable

On Fri, Jan 18, 2019, 12:04 PM Aodhan Hayter <[email protected]
wrote:

I actually wanted to use partition in my NGRX Effects, I want to separate
an httpEvent stream that sends event.type 0 to 4 and I want to throttle the
type 3 events and also handle the type 4 (success).
I thought partition was ideal but I don't think I can throttle and merge 2
streams back into one. So when do you use partition?

I have a simmilar use case. Handling a stream with diferent outcomes, or
different error responses from an http connections is a great use case.

I suspect many people have tried to use partition, but given that it's
not pipeable they give up on it and end up using filter or something else.
I throw errors and filter them, but dislike throwing errors when the flow
is actually a possible expected event in the stream.

I'm one of those folks who was trying to use partition, then slowly
discovering that it is not pipeable. In fact it's also on an httpEvent
stream, would be very useful, but just awkward with its current
implementation.

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[BioPhoton]

@benlesh
As partition operator is deprecated: https://rxjs.dev/api/operators/partition
and implemented as static: https://rxjs.dev/api/index/function/partition
we can close this issue

[doggy8088]

@BioPhoton The partition looks like not exist in some RxJS 6 versions.