Union monoidal functors

UnionMonoidal.scala

trait Functor[F[_]] {
  def map[A, B](fa: F[A])(f: A => B): F[B]
}
trait UnionMonoidal[F[_]] extends Functor[F] {
  def union[A, B](fa: F[A], fb: F[B]): F[A | B]
  def empty[A]: F[A]
  
  def combineK[A](x: F[A], y: F[A]): F[A] = union(x, y)
  def sum[A, B](fa: F[A], fb: F[B]): F[Either[A, B]] = 
    combineK(map(fa)(Left(_)), map(fb)(Right(_)))
}

implicit val optionUnionMonoidal: UnionMonoidal[Option] = new UnionMonoidal[Option] {
  def union[A, B](fa: Option[A], fb: Option[B]): Option[A | B] = fa.orElse(fb)
  def empty[A]: Option[A] = Option.empty
  def map[A, B](fa: Option[A])(f: A => B): Option[B] = fa.map(f)
}