Nullable.scala

/**
 * Scala 2 encoding for the union type `T | Null`
 *
 * This encoding implementation is copied from the implementation of [[scala.Option]]
 *
 * See tweets to see the explored encodings: https://twitter.com/guizmaii/status/1481530910290100227?s=20
 *
 * Why do we need this weird type? See https://twitter.com/guizmaii/status/1481543904734617601?s=20
 */
sealed abstract class Nullable[+A] extends Product with Serializable {
  def toOption: Option[A]
  def orNull: A
  def map[B](f: A => B): Nullable[B]
}
object Nullable {
  case object Empty                    extends Nullable[Null] {
    override def toOption: Option[Null]            = None
    override def orNull: Null                      = null
    override def map[B](f: Null => B): Nullable[B] = empty
  }
  final case class Value[+A](value: A) extends Nullable[A]    {
    override def toOption: Option[A]            = Some(value)
    override def orNull: A                      = value
    override def map[B](f: A => B): Nullable[B] = Nullable(f(value))
  }

  def empty[T]: Nullable[T] = Empty.asInstanceOf[Nullable[T]]

  def apply[A](a: A): Nullable[A] = if (a == null) empty else Value(a)
}