Add arity 4,5,6 to parMapN

[unaryops]

I had a use case for using parMapN 4. I've added support for arity 4, 5, 6.

[nomisRev]

Hey @unaryops, Thanks for your contribution!

We've recently merged a PR to make KotlinX Coroutines a base dependency for Arrow Fx Coroutines to get rid of the friction there was today in terms of cancellation interop etc

This would require a couple changes in the parMapN implementation, it should be easy to write concrete implementation using coroutineScope and async like in parMapN 2-3 which prevents additional schedules/dispatching.

https://github.com/arrow-kt/arrow-fx/pull/317 https://github.com/arrow-kt/arrow-fx/blob/master/arrow-fx-coroutines/src/main/kotlin/arrow/fx/coroutines/ParMapN.kt#L78

[unaryops]

@nomisRev got it. I will update my PR.

[unaryops]

@nomisRev I've noticed the following TODO comment https://github.com/arrow-kt/arrow-fx/blob/aff2bec2c4d67cb5dcbae819c098a9076fb103b4/arrow-fx-coroutines/src/main/kotlin/arrow/fx/coroutines/ParMapN.kt#L151

Is that something I should be taking into consideration?

[nomisRev]

@unaryops yes, in the existing IO implementation and the implementations you added we derived implementations like below. This however requires scheduling 3 parMapN inside parMapN which means that it requires 3 additional schedules/dispatches.

 parMapN(
    ctx,
    suspend { parMapN(ctx, fa, fb, ::Pair) },
    suspend { parMapN(ctx, fc, fd, ::Pair) },
    suspend { parMapN(ctx, fe, ff, ::Pair) }
  ) { ab, cd, ef ->
    val (a, b) = ab
    val (c, d) = cd
    val (e, f) = ef
    f(a, b, c, d, e, f)
  }

With the simpler implementation that delegates to KotlinX we can easily avoid that without requiring large implementations like was required before for parMap2 and parMap3. Since the desired behavior is already ensured by coroutineScope.

[unaryops]

I'm not sure why Check Previous Build Integration is failing.

[nomisRev]

Thanks @unaryops! 🎉