Question

I would like to convert a List[Box[T]] into a Box[List[T]].

I know that I could use foldRight, but I can’t find an elegant way into doing so.

EDIT I would like to keep the properties of Box, that is to say, if there is any failure, return a Box with this failure.

Answer 1

It’s really easier to do this with a tail-recursive function than with a fold:

final def flip[T](l: List[Option[T]], found: List[T] = Nil): Option[List[T]] = l match {
  case Nil => if (found.isEmpty) None else Some(found.reverse)
  case None :: rest => None
  case Some(x) :: rest => flip(rest, x :: found)
}

This works as expected:

scala> flip(List(Some(3),Some(5),Some(2)))
res3: Option[List[Int]] = Some(List(3, 5, 2))

scala> flip(List(Some(1),None,Some(-1)))
res4: Option[List[Int]] = None

One can also do this with Iterator.iterate, but it’s more awkward and slower, so I would avoid that approach in this case.

(See also my answer in the question 4e6 linked to.)