Add NonEmptyList.fromListGroupBy

[chuwy]

Name probably can be improved (fromListGroupedBy?), implementation needs to be improved for sure - I and will be happy to work on it if feature looks useful for someone. Basically idea is following:

def fromListGroupBy[A, K](as: List[A], f: A => K): Map[K, NonEmptyList[A]] =
  as.groupBy(f).map { case (k, list) => list match {
    case h :: t => (k, NonEmptyList(h, t)).some
    case Nil => none
  }}.toList.sequence.map(_.toMap).getOrElse(Map.empty)

We always know that if List[A] is not empty then values in Map will be NonEmptyList.

This also can be implemented along with #2141 as there's clear connection between two structures.

[LukaJCB]

That function already exists here: https://github.com/typelevel/cats/blob/master/core/src/main/scala/cats/syntax/list.scala

:)

[chuwy]

Ah, thank you very much, @LukaJCB. I was looking in NonEmptyList. Closing.

[chuwy]

...and even in 0.9.0. SIgh.

[ShahOdin]

groupByNel needs an Order implementation and returns a SortedMap. Would be been really nice if we also had the above version which doesn't need all that.