json,scala,playframework,scala-macros , Providing implicit value for singletons in Play Json library


Providing implicit value for singletons in Play Json library

Question:

Tag: json,scala,playframework,scala-macros

I have following configuration:

sealed trait Status
case object Edited extends Status
case object NotEdited extends Status

case class Tweet(content:String, status:Status)

I want to use Play Json format, so I guess I have to have something like this(I don't want to do it in companion object):

trait JsonImpl{
    implicit val TweetFormat = Json.format[Tweet]
    implicit val statusFormat = Json.format[Status]
    implicit val StatusFormat = Json.format[Edited.type]
    implicit val NotEditedFormat = Json.format[NotEdited.type]
}

but compiler complains and says:

No implicit format for Tweet available.

Also it says I cannot use Edited.type because it needs apply and unapply functions. What should I do?

Edit1:

I can think of something like this:

implicit object StatusFormat extends Format[Status] {
    def reads(json: JsValue) =
      (json \ "type").get.as[String] match {
        case "" => Edited
        case _ => UnEdited
      }

    def writes(stat: Status) = JsObject(Seq(
      stat match {
        case Edited => "type" -> JsString("Edited")
        case NotEdited => "type" -> JsString("UnEdited")
      }
    ))
  }

but the read part has problem, the compiler complains that it needs JsonResult not Edited.type


Answer:

For doing that I should define an implicit object like this:

  implicit object StatusFormat extends Format[Status] {
    def reads(json: JsValue) =
      json match {
        case JsString("Edited") => JsSuccess(Edited)
        case JsString("NotEdited") => JsSuccess(NotEdited)
        case _ => JsError("cannot parse it")
      }

    def writes(stat: Status) = JsString(stat.toString)
}

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