Adding sequenceM method to Traverse

[zainab-ali]

No description provided.

[codecov-io]

Current coverage is 91.74% (diff: 100%)

Merging #1394 into master will increase coverage by 0.06%

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[adelbertc]

sequenceA? since applicative

[kailuowang]

Well this one is corresponding traverseM, maybe both should be renamed? (if we ended up doing that we need to add the "breaking change" label. ) Also, maybe implementing this with traverseM would make the connection more obvious?

[zainab-ali]

What about flatTraverse or flatSequence, taking the convention from Scala's flatMap? I realize that this isn't well established in other languages, but both traverseM and sequenceM aren't as self explanatory

[peterneyens]

There was a similar conversation when I added traverseM in #1020.

[kailuowang]

Sorry for the delayed response. I like flatTraverse and flatSequence, they make sense to me.

[kailuowang]

Thanks very much! @zainab-ali 👍 LGTM.

[travisbrown]

👍

[peterneyens]

I was not convinced about the name change at first, but after playing around with it I actually like the analogy between flatMap and flatTraverse :

def smallerNaturals(i: Int): List[Int] = 
  List.range(1, Math.max(i, 1))

def smallerNaturalsOpt(i: Int): Option[List[Int]] = 
  if (i <= 1) None else Some(List.range(1, i))

List(2,3,4).flatMap(smallerNaturals)
// List[Int] = List(1, 1, 2, 1, 2, 3)

List(2,3,4).flatTraverse(smallerNaturalsOpt)
// Option[List[Int]] = Some(List(1, 1, 2, 1, 2, 3))

So 👍 , thanks @zainab-ali.