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

First off, I recommend that you check out the API docs for List.

As for a solution, this may not be the most graceful way to do it, but it'll work (and with no external dependencies):

// a function that checks if an option is a None
def isNone(opt:Option[_]) = opt match {
  case None => true
  case _ => false
}

//templated for type T so you can use whatever Options
def optionifyList[T](list:List[Option[T]]) = list.exists(isNone) match {
  case true => None
  case false => Some(list.flatten)
}

And a test just to be sure...

scala> val hasNone = Some(1) :: None :: Some(2) :: Nil
hasNone: List[Option[Int]] = List(Some(1), None, Some(2))

scala> val hasSome = Some(1) :: Some(2) :: Some(3) :: Nil
hasSome: List[Some[Int]] = List(Some(1), Some(2), Some(3))

scala> optionifyList(hasSome)
res2: Option[List[Int]] = Some(List(1, 2, 3))

scala> optionifyList(hasNone)
res3: Option[List[Int]] = None