Optimize Option[A] as A | Null

[sir-wabbit]

As discussed with @Odomontois on Telegram, in non-polymorphic context, Option[A] can be replaced with A | Null (primitives would have to be replaced with their Java counterparts in Scala2).

Consider the following function, that computes the maximum value of all odd numbers in a list:

def maxOdd(l: List[Int]): Option[Int] = {
  def go(l: List[Int], sum: Option[Int]): Option[Int] = l match {
    case Nil => sum
    case x :: xs if x % 2 == 0 => go(xs, sum)
    case x :: xs => sum match {
      case None => go(xs, Some(x))
      case Some(y) => go(xs, Some(x max y))
    }
  }

  go(l, None)
}

We can optimize it as (Integer here plays the role of Int | Null):

def maxOdd(l: List[Int]): Option[Int] = {
  def go(l: List[Int], sum: Integer): Integer = l match {
    case Nil => sum
    case x :: xs if x % 2 == 0 => go(xs, sum)
    case x :: xs => sum match {
      case null => go(xs, x)
      case y => go(xs, x max y)
    }
  }

  Option(go(l, null))
}

This sort of rewriting should only be applied locally and to private functions.

[NeQuissimus]

Just have to keep in mind that as soon as I do anything with the Option[A], this can go horribly wrong :)

Option(x).map(_ max y).getOrElse(y) is a little more difficult to re-write :)