如何使清单可用于Scala中的泛型类型？

Question

给定一个通过json4s解析的scala http客户端，我有以下工作代码将HttpResponse的Future转换为case类的Future：

  def getJobIds(): Future[Seq[Job]] = {
    ... 
    var req = HttpRequest(
      method = HttpMethods.GET,
      uri = uri
    )

    val res: Future[HttpResponse] = Http().singleRequest(req)

    res.flatMap {
      case response@HttpResponse(StatusCodes.OK, _, _, _) => {
        val buffer = ByteString("")

        response.entity.dataBytes
          .runFold(buffer)(_ ++ _)
          .map(_.utf8String)
          .map(
            (body: String) => {
              parse(body).as[Seq[Job]]
            }
          )
      }
      case _: Any => throw new Throwable("Error!")
    }
  }

现在，我想提取逻辑以使用泛型将HttpResponse转换为案例类，以便不再重复其他调用：

  def getJobIds(): Future[Seq[Job]] = {
    ...
    val res: Future[HttpResponse] = Http().singleRequest(req)
    transformResponse[Seq[Job]](res)
  }

  private def transformResponse[T](r: Future[HttpResponse]): Future[T] = {
    r.flatMap {
      case response@HttpResponse(StatusCodes.OK, _, _, _) => {
        val buffer = ByteString("")

        response.entity.dataBytes
          .runFold(buffer)(_ ++ _)
          .map(_.utf8String)
          .map(parse(_).extract[T])
      }
      case _: Any => throw new Throwable("Error!")
    }
  }

但是现在抛出：

 Metronome.scala:46:32: No Manifest available for T

为线

.map(parse(_).extract[T])

如何使代码与泛型一起使用？

Answer 1

问题来自json4s extract方法。

如果您查找它，将会发现它的定义是：

  def extract[A](implicit formats: Formats, mf: scala.reflect.Manifest[A]): A =
    Extraction.extract(jv)(formats, mf)

由于某种原因，隐式清单查找适用于具体类型，但是对于一般情况，您必须为T类型定义隐式清单：

  private def transformResponse[T](r: Future[HttpResponse])
                                  (implicit ev: scala.reflect.Manifest[T]): Future[T] = {
    r.flatMap {
      case response@HttpResponse(StatusCodes.OK, _, _, _) => {
        val buffer = ByteString("")

        response.entity.dataBytes
          .runFold(buffer)(_ ++ _)
          .map(_.utf8String)
          .map(parse(_).extract[T])
      }
      case _: Any => throw new Throwable("Error!")
    }

现在，您的代码将编译并运行。