结合期货,Eithers和期权进行理解

Question

我有一组返回不同类型的方法:

Either[ErrorResponse, X]
Future[Either[ErrorResponse, X]]
Option[ErrorResponse]

这些方法需要先前方法的结果来执行它们的计算.方法:

type Parameters = Map[String, String]

// allows me to flatmap on an either
implicit def toRightProjection[Failure, Success](e: Either[Failure, Success]) =
  e.right

// converts anything to a future
implicit def toFuture[T](t: T) =
  Future.successful(t)

// retrieves the request paramters from the given request
def requestParameters(request: RequestHeader): Either[ErrorResponse, Parameters] = ???

// retrieves the response type from the given parameters
def responseType(p: Parameters): Either[ErrorResponse, String] = ???

// retrieves the client id from the given parameters
def clientId(p: Parameters): Either[ErrorResponse, String] = ???

// retrieves the client using the given client id
def client(clientId: String): Future[Either[ErrorResponse, Client]] = ???

// validates the response type of the client
def validateResponseType(client: Client, responseType: String): Option[ErrorResponse] = ???

我可以将它们与以下内容连接起来进行理解(请注意,我写下了一些类型来澄清计算特定部分的内容).

val result: Either[ErrorResponse, Future[Either[ErrorResponse, Client]]] =
  for {
    parameters <- requestParameters(request)
    clientId <- clientId(parameters)
    responseType <- responseType(parameters)
  } yield {
    val result: Future[Either[ErrorResponse, Either[ErrorResponse, Client]]] =
      for {
        errorOrClient <- client(clientId)
        client <- errorOrClient
      } yield validateResponseType(client, responseType).toLeft(client)

    result.map(_.joinRight)
  }

val wantedResult: Future[Either[ErrorResponse, Client]] =
  result.left.map(Future successful Left(_)).merge

上面的代码非常混乱,我觉得这可以用不同的方式完成.我读过monad和monad变形金刚.这些概念对我来说很新鲜,我无法理解它.

大多数示例仅处理两种类型的结果:Either[X, Y]和Future[Either[X, Y]].我仍然觉得很难绕过它.

我怎样才能写出一个很好的和干净的理解来取代上面的理解？

像这样的东西会很棒(我不确定这是否可能):

val result: Future[Either[ErrorResponse, Client]] =
  for {
    parameters <- requestParameters(request)
    clientId <- clientId(parameters)
    responseType <- responseType(parameters)
    client <- client(clientId)
    _ <- validateResponseType(client, responseType)
  }

Answer 1

好的,这是我的尝试:

import scalaz._, Scalaz._

implicit val futureMonad = new Monad[Future] {
  override def point[A](a: ? A): Future[A] = future(a)

  override def bind[A, B](fa: Future[A])(f: A ? Future[B]): Future[B] =
    fa.flatMap(f)
}

import EitherT._
val result: EitherT[Future, ErrorResponse, Client] =
  for {
    parameters <- fromEither(Future(requestParameters(request)))
    clientId <- fromEither(Future(clientId(parameters)))
    responseType <- fromEither(Future(responseType(parameters)))
    client <- fromEither(client(clientId))
    response <- fromEither[Future, ErrorResponse, Client](Future(validateResponseType(client, responseType).toLeft(client)))
  } yield response

val x: Future[\/[ErrorResponse, Client]] = result.run