I have a List<I> and a mapping function `sus...

# coroutines

[David Glasser]

I have a List<I> and a mapping function suspend (I)->O. I’d like to get the List<O> out, running a bounded number of copies of the mapping function in parallel. I feel like this should be straightforward, eg with Flow, but I can’t quite get it to work

[David Glasser]

eg, it’s not in.asFlow().buffer(N).map(f) or in.asFlow().flatMapMerge{ flow { emit (f(it)) } }

[David Glasser]

I guess there’s

map { async { f(it) } }.awaitAll()

which seems “unbounded” but that’s fine i suppose

[bezrukov]

there is concurrency param in flatMapMerge

[bezrukov]

you can find more info here: https://github.com/Kotlin/kotlinx.coroutines/issues/1147

[David Glasser]

@bezrukov but flatMapMerge doesn’t preserve order, right?

[bezrukov]

yes

[David Glasser]

yeah, i do want to preserve order, so flatMapMerge is no good.

[David Glasser]

(i mean i can do some sort of flatMapMerge with indices and sort at the end I guess but that seems overkill)

[bezrukov]

you can create dispatcher with your concurrency level and then use

map { async(yourDispatcher) { f(it) } }.awaitAll()

but it looks like overkill as well.

there was a ticket for re-implementation fixedThreadPoolContext to avoid creation real dispatcher, but there is no eta.

[Dominaezzz]

Or use a Semaphore !