How to obtain implicit val/def via parameterized types in scala?

Question

I'm using Playframework 2.5 and trying to make a method that all Json request/response controller method calls to avoid writing implicit val foo = Json.writes[Bar] or reads[Bar] inside of each controller method as well as handling basic validation and return the error code.

BaseController which will be inherited by each controller

class BaseController @Inject()(implicit exec: ExecutionContext) extends Controller {
  def handleJson[T <: RequestModel,U <: ResponseModel](request: Request[JsValue])(cb: (T) => Future[U]):Future[Result] = {
    implicit val convertReq = Json.reads[T]
    implicit val convertRes = Json.writes[U]
    val reqOpt = request.body.asOpt[T]
    reqOpt match {
      case Some(data) =>
        cb(data).map{x => Ok(Json.toJson(x))}
      case None =>
        Future.successful(Ok(Json.obj("foo" -> "bar")))
    }
  }
}

controller

  def send = Action.async(parse.json) { req =>
    handleJson[RequestCaseClass,ResponseCaseClass](req)( (x)  =>
      injectedClass.foo(x.bar).map{ case (success: Boolean,mes: String) =>
        if(success) SendRes(SomethingForSuccess)
        else SendRes(SomethingForError)
      }
    )
  }

Codes above can't be compiled because handleJson's implicit val convertReq = Json.reads[T] and implicit val convertRes = Json.writes[U] says

No Json deserializer found for type T. Try to implement an implicit Reads or Format for this type.

I've already tried making case class's companion object and put the implicit in it though this does not solve anything.

My question is, How can I use implicit value of parameterized types?

Thanks in advance.

Łukasz Łukasz · Accepted Answer · 2016-04-01T10:36:29

I would suggest doing it like this

class Application @Inject()(implicit exec: ExecutionContext) extends Controller {
  def handleJson[A : Reads, B : Writes](request: Request[JsValue])(cb: A => Future[B]): Future[Result] = {
    val reqOpt = request.body.asOpt[A]
    reqOpt match {
      case Some(data) =>
        cb(data).map { x => Ok(Json.toJson(x)) }
      case None =>
        Future.successful(Ok(Json.obj("foo" -> "bar")))
    }
  }

  def index = Action.async(parse.json) { req =>
    handleJson[MyReq, MyResp](req)(x => Future.successful(MyResp((x.i to x.i + 4).toList)))
  }
}

You don't have to constrain your model with any trait, just enough that implicit Reads and Writes exists.

To ensure that, it is nice to have your Reads and Writes defined in companion object of your class. You will bring them to implicit scope when you import your model classes.

object MyReq {
  implicit val myReqReads: Reads[MyReq] = Json.reads[MyReq]
}
case class MyReq(i: Int)

object MyResp {
  implicit val myRespWrites: Writes[MyResp] = Json.writes[MyResp]
}
case class MyResp(l: List[Int])

How to obtain implicit val/def via parameterized types in scala?

2 Answers