Scala play Json - reads converter with type parameter

Question

I would like to create a JSON reads converter with type parameter like this:



case class AClass(name: String, age: Int)
case class AClass(name: String)

object GeneralReadsType extends Enumeration {
  type GeneralReadsType = Value
  val AReadType = Value("atype")
  val BReadType = Value("btype")
}

implicit val repoReads: Reads[GeneralReadsType.GeneralReadsType] = {

  GeneralReadsType match {
    case GeneralReadsType.AReadType => (
      (JsPath \ "name").read[String] and
        (JsPath \ "age").read[Int]
      ) (GeneralReadsType.AReadType.apply _)


    case GeneralReadsType.BReadType => (
      (JsPath \ "name").read[String]
      ) (GeneralReadsType.BReadType.apply _)

  }
}

But when I try to use it in

def getResponse[GeneralReadsType](request: WSRequest): Future[List[GeneralReadsType]] = {
      request.get().map {
        case response if response.status == 200 => {
          (response.json).validate[List[GeneralReadsType]] match {
            case JsSuccess(items: List[GeneralReadsType], _) => items
            case _: JsError => List[GeneralReadsType]()
          }
        }
        case _ => List[GeneralReadsType]()
      }
    }


getResponse[AReadType](request)

I get a compile error

No Json deserializer found for type List[GeneralReadsType]. Try to implement an implicit Reads or Format for this type.

So, how should this be achieved?

Thanks!

Make sure your instance is in the implicit scope. Alsoif your are testing in REPL, the behaviour is not exactly the same, so take care. — cchantep
You defined a Reads for GeneralReadsType.GeneralReadsType but getResponse needs a Reads for GeneralReadsType. — Dylan

Marcin Bablok Marcin Bablok · Accepted Answer · 2019-10-02T20:17:05

I find it best to place format objects as an implicit field in the object itself (or in companion object of a case class). Also when using Enumeration plural form for the Enumeration object and singular for the value make it read better. So:

object GeneralReadsTypes extends Enumeration {
  type GeneralReadsType = Value
  val AReadType = Value("atype")
  val BReadType = Value("btype")

  implicit val repoReads: Reads[GeneralReadsType] = {
    ((JsPath \ "name").read[String] and
      (JsPath \ "age").readNullable[Int]).apply { (_, maybeAge) =>
      maybeAge.fold(BReadType)( _ => AReadType)
    }
  }  
}

or if you find functional style too foreign:

object GeneralReadsTypes extends Enumeration {
  type GeneralReadsType = Value
  val AReadType = Value("atype")
  val BReadType = Value("btype")

  implicit val repoReads: Reads[GeneralReadsType] = {
    ((JsPath \ "name").read[String] and
      (JsPath \ "age").readNullable[Int]).apply { (_, maybeAge) =>
      if (maybeAge.isDefined) AReadType
      else BReadType
    }
  }
}

Scala play Json - reads converter with type parameter

1 Answers