Scala で JSON のための Enum をシールドクラスで作る

sealed abstract class BloodType(val code: Int)
object BloodType {
    case object A extends BloodType(0)
    case object B extends BloodType(1)
    case object AB extends BloodType(2)
    case object O extends BloodType(3)
    val map: Map[Int, BloodType] = getClass.getClasses
        .map(_.getField("MODULE$").get(null).asInstanceOf[BloodType])
        .map(x => x.code -> x).toMap
}

import play.api.libs.json._

implicit val format = new Format[BloodType] {
    override def writes(o: BloodType): JsValue = JsNumber(o.code)

    override def reads(json: JsValue): JsResult[BloodType] = json.asOpt[Int]
        .flatMap(code => BloodType.map.get(code))
        .fold(JsError("error"))(v => JsSuccess(v))
}