Scala: filtering list of tuples to get a nonEmptyList

Question

I have a list of the type List[(A, List[B])]. I want to flatten this structure and get:

• NonEmptyList[A] consisting of all the Acorresponding to a nonEmpty List[B].

• all those Bs combined: NonEmptyList[B]

ie, I want to get Option[(NonEmptyList[A], NonEmptyList[B])]. What is the most concise way of doing this.

Answer 1

Untested, but using definitions from the comments, this should work:

for {
  a <- NonEmptyList.fromList(list.collect{ case (a, _::_) => a })
  b <- NonEmptyList.fromList(list.flatMap(_._2))
} yield (a, b)

This also has the advantage of avoiding the second computation if the first returns None.

---

Previous versions:

val listA = list.collect{ case (a, _::_) => a }
val listB = list.flatMap(_._2)

listA.headOption.flatMap(listB.headOption.map(_ => (NonEmptyList(listA), NonEmptyList(listB))))

Another version might be

(listA, listB) match {
  case (_::_, _::_) =>
    Some(NonEmptyList(listA), NonEmptyList(listB))
  case _ =>
    None
}