Continuing my exploration of `Concurrent` - I trie...

# arrow

[Marcin Gryszko]

Continuing my exploration of Concurrent - I tried in my program replacing Concurrent.parMapN with a ParApplicative instance (obtained by Concurrent.parApplicative(ctx)). Then, I replaced parMapN with mapN and checked which threads effects passed to mapN were executed in. To my surprise, they were executed in the caller's thread:

C.parApplicative(dispatchers().io()).run {
    mapN(getAthleteActivities(), getAthlete()) { (apiActivities, apiAthlete) -> /* transform */ }
}

I checked quickly the implementation; to have a concurrent execution, I have to use map2 extension function:

C.parApplicative(dispatchers().io()).run {
    getAthleteActivities().map2(getAthlete()) { (apiActivities, apiAthlete) -> /* transform */ }
}

Is this a conscious design decision? Should mapN be delegated to parMapN?

[simon.vergauwen]

That’s strange since it does delegated to parMapN.
mapN is derived from product, which delegates to parTupledN.

[Marcin Gryszko]

map2 delegates to parMapN (source)
mapN, defined in Apply typeclass, is not overridden by ParApplicative.

[Marcin Gryszko]

Should I file an issue for that?

[simon.vergauwen]

Yes please! This needs some investigation and a fix.
What I find strange is that mapN is derived from product, and product does delegate to parMapN in ParApplicative.

https://github.com/arrow-kt/arrow-fx/blob/3ac53b1eaa3ed744e97a13ea201b1e9589237d5b/arrow-fx/src/main/kotlin/arrow/fx/typeclasses/ParApplicative.kt#L27
https://github.com/arrow-kt/arrow-core/blob/master/arrow-core-data/src/main/kotlin/arrow/typeclasses/Apply.kt#L70

[simon.vergauwen]

Thanks for bringing this up!

[Marcin Gryszko]

https://github.com/arrow-kt/arrow/issues/2149