Using tagged type with play-json

TaggedScalazSuite.scala

package taggettest

import org.scalatest.FlatSpec
import org.scalatest.Matchers
import play.api.libs.json._

import scalaz.{Tag, @@}

sealed trait UserTag

case class User(id: Long @@ UserTag, name: String)

class TaggedSuite extends FlatSpec with Matchers {

  implicit def taggedLongFormat[T]: Format[Long @@ T] = new Format[Long @@ T] {
    def reads(json: JsValue): JsResult[Long @@ T] = json match {
      case JsNumber(v) => JsSuccess(Tag.of[T](v.toLong))
      case unknown => JsError(s"Number value expected, got: $unknown")
    }
    def writes(v: Long @@ T): JsValue = JsNumber(Tag.unwrap(v))
  }

  implicit def taggedStringFormat[T]: Format[String @@ T] = new Format[String @@ T] {
    def reads(json: JsValue): JsResult[String @@ T] = json match {
      case JsString(v) => JsSuccess(Tag.of[T](v))
      case unknown => JsError(s"String value expected, got: $unknown")
    }
    def writes(v: String @@ T): JsValue = JsString(Tag.unwrap(v))
  }

  implicit val userFormat = Json.format[User]

  "User containing tagged type" should "be convertible to String from case class" in {

    val user = User(Tag.of[UserTag](1), "Josef")

    val userJson = Json.toJson(user)
    val userString = Json.prettyPrint(userJson)

    userString should be(
      """|{
        |  "id" : 1,
        |  "name" : "Josef"
        |}""".stripMargin)
  }

  it should "be convertible to case class from String" in {
    val userString = """|{
      |  "id" : 1,
      |  "name" : "Josef"
      |}""".stripMargin

    val user = Json.fromJson[User](Json.parse(userString))
    user.get should be (User(Tag.of[UserTag](1), "Josef"))
  }
}