PlayJsonEitherFormat.scala

implicit def eitherFormat[A, B](implicit af: Format[A], bf: Format[B]) = new Format[Either[A, B]] {
  override def writes(o: Either[A, B]): JsValue = o match {
    case Left(a) => af.writes(a)
    case Right(b) => bf.writes(b)
  }
  override def reads(json: JsValue): JsResult[Either[A, B]] =
  json.validate[B].map(Right.apply).orElse(json.validate[A].map(Left.apply))
}