Convert a List of Options to an Option of List using Scalaz

Starting Scala 2.13, and the addition of the Option::unless builder to the standard library, a variant to Rex Kerr's answer would be:

Option.unless(list contains None)(list.flatten)
// val list = List(Some(1), Some(2))          =>    Some(List(1, 2))
// val list = List(Some(1), None, Some(2))    =>    None

or, if performance is at stake (in order to avoid flatten's implicit conversion from Option to List):

Option.unless(list contains None)(list.map(_.get))

For some reason you dislike

if (lo.exists(_ isEmpty)) None else Some(lo.map(_.get))

? That's probably the shortest in Scala without Scalaz.

There's a function that turns a List[Option[A]] into an Option[List[A]] in Scalaz. It's sequence. To get None in case any of the elements are None and a Some[List[A]] in case all the elements are Some, you can just do this:

import scalaz.syntax.traverse._
import scalaz.std.list._     
import scalaz.std.option._

lo.sequence

This method actually turns F[G[A] into G[F[A]] given that there exists an implementation of Traverse[F], and of Applicative[G] (Option and List happen to satisfy both and are provided by those imports).

The semantics of Applicative[Option] are such that if any of the elements of a List of Options are None, then the sequence will be None as well. If you want to get a list of all the Some values regardless of whether any other values are None, you can do this:

lo flatMap (_.toList)

You can generalize that for any Monad that also forms a Monoid (List happens to be one of these):

import scalaz.syntax.monad._

def somes[F[_],A](x: F[Option[A]])
                 (implicit m: Monad[F], z: Monoid[F[A]]) =
  x flatMap (o => o.fold(_.pure[F])(z.zero))