The Coroutines docs discourage the use of `flatMap...

# coroutines

[Lukas Lechner]

The Coroutines docs discourage the use of flatMapMerge and flatMapConcat in regular application-specific flows, see attached image. What should we used instead when flatMapping two flows?

[Trevor Stone]

flatMapLatest so prior work is canceled if that's what you want it. Otherwise what they're saying is that you could use a transform or map and collect from the child flow and emit downstream.

If you feel that you and your team are comfortable using merge and concat though I personally wouldn't see it as an issue. I think the docs are written from someone that knows coroutines and is learning flow vs someone who knows something like rxJava and learning flow

[Lukas Lechner]

Thanks! Interesting, that flatMapLatest is basically only using transformLatest under the hood...

[Lukas Lechner]

I now know how I can replace flatMapX with transform, but how can I replace it with map ?

I can only map a single value and cannot emit multiple emissions from a map, right?

[Trevor Stone]

so map is just

public inline fun <T, R> Flow<T>.map(crossinline transform: suspend (value: T) -> R): Flow<R> = transform { value ->
    return@transform emit(transform(value))
}

[Trevor Stone]

so you're right. It would be for 1 item in 1 item out transforms

[Trevor Stone]

possibly when they mention it looks familiar it might be referencing flatmap on collections https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/flat-map.html which I could see being confusing for people

[Raheel Naz [Amex]]

the one key difference between the two is if you use flattenConcat a suspended collect will actually block other flows from emitting data downstream because order matters