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

Since you need to flatten anyway, just do it first...

def sequence(lo: List[Option[A]]): Option[List[A]] = lo.flatten match {
    la: List[A] if(la.length == lo.length) => Some(la)
    _ => None
}

tail recursion might be quickest