Noise free JSON format for sealed traits with Play 2.2 library

AMENDED 2015-09-22

The library play-json-extra includes the play-json-variants strategy, but also the [play-json-extensions] strategy (flat string for case objects mixed with objects for case classes no extra $variant or $type unless needed). It also provides serializers and deserializers for macramé based enums.

Previous answer There is now a library called play-json-variants which allows you to write :

implicit val format: Format[Foo] = Variants.format[Foo]

This will generate the corresponding formats automatically, it will also handle disambiguation of the following case by adding a $variant attribute (the equivalent of 0__ 's class attribute)

sealed trait Foo
case class Bar(x: Int) extends Foo
case class Baz(s: String) extends Foo
case class Bah(s: String) extends Foo

would generate

val bahJson = Json.obj("s" -> "hello", "$variant" -> "Bah") // This is a `Bah`
val bazJson = Json.obj("s" -> "bye", "$variant" -> "Baz") // This is a `Baz`
val barJson = Json.obj("x" -> "42", "$variant" -> "Bar") // And this is a `Bar`

Here is a manual implementation of the Foo companion object:

implicit val barFmt = Json.format[Bar]
implicit val bazFmt = Json.format[Baz]

object Foo {
  def unapply(foo: Foo): Option[(String, JsValue)] = {
    val (prod: Product, sub) = foo match {
      case b: Bar => (b, Json.toJson(b)(barFmt))
      case b: Baz => (b, Json.toJson(b)(bazFmt))
    }
    Some(prod.productPrefix -> sub)
  }

  def apply(`class`: String, data: JsValue): Foo = {
    (`class` match {
      case "Bar" => Json.fromJson[Bar](data)(barFmt)
      case "Baz" => Json.fromJson[Baz](data)(bazFmt)
    }).get
  }
}
sealed trait Foo
case class Bar(i: Int  ) extends Foo
case class Baz(f: Float) extends Foo

implicit val fooFmt = Json.format[Foo]   // ça marche!

Verification:

val in: Foo = Bar(33)
val js  = Json.toJson(in)
println(Json.prettyPrint(js))

val out = Json.fromJson[Foo](js).getOrElse(sys.error("Oh no!"))
assert(in == out)

Alternatively the direct format definition:

implicit val fooFmt: Format[Foo] = new Format[Foo] {
  def reads(json: JsValue): JsResult[Foo] = json match {
    case JsObject(Seq(("class", JsString(name)), ("data", data))) =>
      name match {
        case "Bar"  => Json.fromJson[Bar](data)(barFmt)
        case "Baz"  => Json.fromJson[Baz](data)(bazFmt)
        case _      => JsError(s"Unknown class '$name'")
      }

    case _ => JsError(s"Unexpected JSON value $json")
  }

  def writes(foo: Foo): JsValue = {
    val (prod: Product, sub) = foo match {
      case b: Bar => (b, Json.toJson(b)(barFmt))
      case b: Baz => (b, Json.toJson(b)(bazFmt))
    }
    JsObject(Seq("class" -> JsString(prod.productPrefix), "data" -> sub))
  }
}

Now ideally I would like to automatically generate the apply and unapply methods. It seems I will need to use either reflection or dive into macros.

A small fix for the previous answer by 0__ regarding the direct format definition - the reads method didn't work, and here is my refactor to it, to also become more idiomatic -

def reads(json: JsValue): JsResult[Foo] = {

  def from(name: String, data: JsObject): JsResult[Foo] = name match {
    case "Bar"  => Json.fromJson[Bar](data)(barFmt)
    case "Baz"  => Json.fromJson[Baz](data)(bazFmt)
    case _ => JsError(s"Unknown class '$name'")
  }

  for {
    name <- (json \ "class").validate[String]
    data <- (json \ "data").validate[JsObject]
    result <- from(name, data)
  } yield result
}