Scala Generics：无法将一个T实例写入HTTP响应。 尝试定义一个可写[T]

Question

Following code when written using generic give a compilation error.

Without Generic

def getData(id: String) = Action.async {

    val items = getItems(id)

    sendResult(items)
  }

  private def sendResult(result: Future[Any]) = {


    result.map {
      items => {
        try {
          val itemStr = items.asInstanceOf[String]
          Ok(itemStr)
        } catch {
          case t: ClassCastException => InternalServerError(s"Casting Exception while processing output $t")
        }
      }
    }.recover {
      case t:TimeoutException => InternalServerError("Api Timed out")
      case t: Throwable => InternalServerError(s"Exception in the api $t")
    }
  }

With Generic

  def getData(id: String) = Action.async {

    val items = getItems(id)

    sendResult[String](items)
  }

  private def sendResult[T](result: Future[Any]) = {


    result.map {
      items => {
        try {
          val itemStr = items.asInstanceOf[T]
          Ok(itemStr)
        } catch {
          case t: ClassCastException => InternalServerError(s"Casting Exception while processing output $t")
        }
      }
    }.recover {
      case t:TimeoutException => InternalServerError("Api Timed out")
      case t: Throwable => InternalServerError(s"Exception in the api $t")
    }
  }

The code is part of play app's contorller method. First one works fine. Second one gives following compilation error

Cannot write an instance of T to HTTP response. Try to define a Writeable[T] [error] Ok(itemStr) [error]

Answer 1

Using Any with a generic function doesn't make much sense.

private def sendResult[T](result: Future[Any])
// Should better be
private def sendResult[T](result: Future[T])
// ... also remove the unsafe cast

Then this T needs to be provided an instance of Writeable , so it can be written a Array[Byte] over network.

// Either ...
private def sendResult[T: Writeable](result: Future[T])
// ... or ...
private def sendResult[T](result: Future[T])(implicit w: Writeable[T])

Answer 2

The Ok(...) is calling the method apply[C](content: C)(implicit writeable: Writeable[C]): Result on Status class. You need to have an implicit value for Writeable[T] in scope.

As a side note, rather than using Future[Any], you may as well use Future[T] and remove the cast. Also you can use Writes for JSON serialization.

private def sendResult[T](result: Future[T])(implicit writeable: Writes[T]) = {
         result.map {
          items => {
            Ok(Json.toJson(items))                
          }
        }.recover {
          case t:TimeoutException => InternalServerError("Api Timed out")
          case t: Throwable => InternalServerError(s"Exception in the api $t")
        }
      }