List of options: equivalent of sequence in Scala?

Question

What is the equivalent of Haskell\'s sequence in Scala? I want to turn list of options into an option of list. It should come out as None if any of the options

Answer 1

Scalaz defines sequence.

Here's an example:

scala> import scalaz._
import scalaz._

scala> import Scalaz._
import Scalaz._

scala> List(Some(1), None, Some(2)).sequence
res0: Option[List[Int]] = None

scala> List(some(1), some(2), some(3)).sequence
res1: Option[List[Int]] = Some(List(1, 2, 3))

Note that in the second example, you have to use Scalaz's some function to create a Some -- otherwise, the List is constructed as List[Some[Int]], which results in this error:

scala> List(Some(1), Some(2), Some(3)).sequence
:14: error: could not find implicit value for parameter n: scalaz.Applicative[N]
       List(Some(1), Some(2), Some(3)).sequence

The Scalaz some(a) and none functions create Some and None values of type Option[A].