Json Serialization for Trait with Multiple Case Classes (Sum Types) in Scala's Play

Question

I often have to serialize/deserialize sum types (like Either[S,T]), and I haven't yet found a general or elegant way to do it. Here's an example type (essentially equivalent to Either)

sealed trait OutcomeType
case class NumericOutcome(units: String)              extends OutcomeType
case class QualitativeOutcome(outcomes: List[String]) extends OutcomeType

Here's my best effort at a companion object that implements serialization. It works, but it's very tiresome to write these sorts of things over and over for every sum type. Are there any suggestions for making it nicer and/or more general?

import play.api.libs.json._
import play.api.libs.functional.syntax._

object OutcomeType {

  val fmtNumeric     = Json.format[NumericOutcome]
  val fmtQualitative = Json.format[QualitativeOutcome]

  implicit object FormatOutcomeType extends Format[OutcomeType] {
    def writes(o: OutcomeType) = o match {
      case n@NumericOutcome(_)     => Json.obj("NumericOutcome"     -> Json.toJson(n)(fmtNumeric))
      case q@QualitativeOutcome(_) => Json.obj("QualitativeOutcome" -> Json.toJson(q)(fmtQualitative))
    }

    def reads(json: JsValue) = (
      Json.fromJson(json \ "NumericOutcome")(fmtNumeric) orElse
      Json.fromJson(json \ "QualitativeOutcome")(fmtQualitative)
    )
  }
}

Answer 1

I think that is about as simple as you can make it, if you want to avoid writing the code for each explicit subtype maybe you could do it with reflection, use jackson directly or some other json library with reflection support. Or write your own macro to generate the Format from a list of subtypes.