#Json #Reads/#Writes for a sealed #trait & inheriting caseclasses without type indication in Json #Play2.1

__LICENSE.txt

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                    Version 2, December 2004

 Copyright (C) 2013 YOUR_NAME_HERE <YOUR_URL_HERE>

 Everyone is permitted to copy and distribute verbatim or modified
 copies of this license document, and changing it is allowed as long
 as the name is changed.

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. You just DO WHAT THE FUCK YOU WANT TO.

CODE.scala

import play.api.libs.json._
import play.api.libs.functional.syntax._
 
sealed trait Shape

case class Circle(c: (Float, Float), r: Float) extends Shape
object Circle {
  // reader is covariant and can be implicit in inheriting caseclasses
  implicit val reader = Json.reads[Circle]
  // writer is contravariant and can't be implicit in inheriting caseclasses
  val writer = Json.writes[Circle]
}

case class Polygon(points: Seq[(Float, Float)]) extends Shape
object Polygon {
  // reader is covariant and can be implicit in inheriting caseclasses
  implicit val reader = Json.reads[Polygon]
  // writer is contravariant and can't be implicit in inheriting caseclasses
  val writer = Json.writes[Polygon]
}

object Shape {
  // Note the map to  convert into Shape because we want a Reads[Shape]
  // As we don't have any type indication, we can just use try/fail strategy
  implicit val shapeReads = 
    __.read[Rect].map(x => x:Shape) orElse __.read[Circle].map(x => x:Shape)
 
  implicit val shapeWrites = Writes[Shape]{
    case circle: Circle => Circle.writer.writes(circle)
    case poly: Polygon => Polygon.writer.writes(poly)
  }
}

Thanks man!