Single field json serialization using Play's JSON

gistfile1.scala

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

object Main {
  implicit val jsonFormat =
    (__ \ "v").write[String].contramap((fb: FizzBuzz) => fb.v)

  def main(args: Array[String]) {
    print(Json.toJson(FizzBuzz.Fizz))
    print(Json.toJson(FizzBuzz.Buzz))
  }
}

object FizzBuzz {
  case object Fizz extends FizzBuzz("Fizz")
  case object Buzz extends FizzBuzz("Buzz")
}

sealed abstract class FizzBuzz(val v: String)

これはダメ