Folding over a list of Options to Find First or Last Some

Question

I'm trying to fold over a list of Options in order return the first(or last) Some value or None if there aren't any Some values.

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

scala> opts foldLeft(None)((a,io) => a match { case None => io; case Some(i) =>
a})
<console>:9: error: object None does not take parameters
              opts foldLeft(None)((a,io) => a match { case None => io; case Some
(i) => a})
                                 ^

Not sure what I'm doing wrong. Also there is probably a way to do this simpler using a higher order function but nothing from here caught my eye.

Answer 1

Maybe this can solve your problem - the first element:

opts.flatten.headOption

And the last element:

opts.flatten.lastOption

flatten method will unbox all Option values in the list and drop all None values. headOption/lastOption will return either Some for the first/last element in the list or None if list is empty.

Answer 2

tenshi’s answer is pretty straightforward but for long lists it will attempt to flatten everything as it isn’t lazy. (I think view won’t help us here either but I’m not quite sure.)

In that case, you could use:

opts.dropWhile(_.isEmpty).headOption.flatMap(identity)

Unfortunately, we cannot use flatten here as this will return a generic Iterable[Int] and no Option, so we have to chose the longer idiom flatMap(identity).

Edit: As dave noticed:

opts.find(_.isDefined).flatMap(identity)

would be even better.