Question

In Scala I have a List with an optional Option. This arises for example when you use for comprehension on a List and your yield returns an Option. In my case I was processing a JSON object and using for comprehension on the list of fields (List[JField]).

What’s the best way to open up the list and map List() to None and List(Some(a)) to Some(a)?

A first approach would be

def headOrNone[A](list:List[Option[A]]) = 
    list match {
        case Nil => None
        case a::Nil => a
    }

Another approach

def headOrNone[A](list:List[Option[A]]) = list.headOption.getOrElse(None)

A third approach (a variation on the headOption implementation)

def headOrNone[A](list:List[Option[A]]) = if (list.isEmpty) None else list.head

I personally prefer the third approach. Is there a better name for this function than headOrNone and what is the idiomatic scala way to write it?

Answer 1

How about this:

def headOrNone[A](list: List[Option[A]]) = list.flatten.headOption

headOrNone(List(Some(4)))   // Some(4)
headOrNone(List())          // None

Though the first choice has the advantage of giving you an error if you happen to have list with more than one item, which, according to your description, seems like an error condition.

But personally, I would re-evaluate the code that produces the List[Option[A]] and see if there’s a way to just have it return the right thing in the first place!