中断 Future 映射链并返回一个 Left

【问题标题】：Interrupt Future map chain and return a Left中断 Future 映射链并返回一个 Left
【发布时间】：2020-04-26 04:41:43
【问题描述】：

如何在未来的地图链中间返回一个 Left，然后让未来失败？我发送一个例子来澄清一下。

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

object TestApp extends App {

  final case class Message(statusCode: Int, data: String)
  final case class Success(data: String)
  final case class Failure()

  val msg = Message(500, "My data")
  val future = Future { msg }

  def getMessage(future: Future[Message]): Future[Either[Failure, Success]] = {
    future.map { msg =>
      // Evaluate msg here - if failure, make this feature fail and then return Left(Failure), otherwise, continue in the chain
      msg
    }.map(_.data).map(data => Right(Success(data)))
  }
}

【问题讨论】：

标签： scala future either

【解决方案1】：

一种选择是抛出一个异常，该异常将使Future 失败，然后recover 失败为Left，就像这样

  def getMessage(future: Future[Message]): Future[Either[Failure, Success]] =
    future
      .map(msg => if (msg.statusCode == 500) throw new RuntimeException("boom") else msg)
      .map(_.data)
      .map(data => Right(Success(data)))
      .recover{case ex => Left(Failure())}

另一种选择是transform 像这样

  def getMessage(future: Future[Message]): Future[Either[Failure, Success]] = {
    future
      .map(msg => if (msg.statusCode == 500) throw new RuntimeException("boom") else msg)
      .map(_.data)
      .transform(v => scala.util.Success {
        v.toEither
          .map(v => Success(v))
          .left.map(e => Failure())
      })
  }

警告，您已经定义了自己的 Success/Failure，它会影响 Scala 的 Success/Failure。

【讨论】：