Minimal Either sum type in Scala

Either.scala

sealed trait Either[A, B]
final case class Left[A, B](a: A) extends Either[A, B]
final case class Right[A, B](b: B) extends Either[A, B]
object Either {
  def left[A, B](a: A): Either[A, B] = Left(a)
  def right[A, B](b: B): Either[A, B] = Right(b)
}