将阻塞代码转换为使用scala期货

Question

我的旧代码看起来像下面这样，所有的db调用都是阻塞的。

我需要帮助将此转换为使用Futures。

def getUserPoints(username: String): Option[Long]
    db.getUserPoints(username) match {
        case Some(userPoints) => Some(userPoints.total)
        case None => {
            if (db.getSomething("abc").isEmpty) {
                db.somethingElse("asdf") match {
                    case Some(pointId) => {
                        db.setPoints(pointId, username)
                        db.findPointsForUser(username)
                    }
                    case _ => None
                }
            } else {
                db.findPointsForUser(username)
            }
        }       
    }
}

我的新API位于我返回Futures的地方。

db.getUserPoints(username: String): Future[Option[UserPoints]]
db.getSomething(s: String): Future[Option[Long]]
db.setPoints(pointId, username): Future[Unit]
db.findPointsForUser(username): Future[Option[Long]]

我怎样才能将上面的代码转换为使用我的新API呢？

我尝试使用for-compr，但开始遇到像FutureNothing这样奇怪的错误。

var userPointsFut: Future[Long] = for {
  userPointsOpt <- db.getUserPoints(username)
  userPoints <- userPointsOpt
} yield userPoints.total

但所有的分支和if子句以及试图将其转换为期货，都会变得有点棘手。

Answer 1

我认为这种设计的第一个问题是，对Future的阻塞调用的端口不应该包装Option类型：

阻塞调用：def giveMeSomethingBlocking(for:Id): Option[T]应该变成：def giveMeSomethingBlocking(for:Id): Future[T]，而不是：def giveMeSomethingBlocking(for:Id): Future[Option[T]]

阻塞调用提供了一个值Some(value)或None，非阻塞未来版本提供了一个Success(value)或Failure(exception)，它以非阻塞方式完全保留了Option语义。

考虑到这一点，我们可以在Future上使用组合子对所讨论的过程进行建模。让我们看看如何：

首先，让我们将API重构为我们可以使用的东西：

type UserPoints = Long
object db {
  def getUserPoints(username: String): Future[UserPoints] = ???
  def getSomething(s: String): Future[UserPoints] = ???
  def setPoints(pointId:UserPoints, username: String): Future[Unit] = ???
  def findPointsForUser(username: String): Future[UserPoints] = ???
}
class PointsNotFound extends Exception("bonk")
class StuffNotFound extends Exception("sthing not found")

然后，该过程将如下所示：

def getUserPoints(username:String): Future[UserPoints] = {
  db.getUserPoints(username)
  .map(userPoints => userPoints /*.total*/)
  .recoverWith{ 
    case ex:PointsNotFound => 
    (for {
      sthingElse <- db.getSomething("abc")
      _ <- db.setPoints(sthingElse, username)
      points <- db.findPointsForUser(username)
    } yield (points))
    .recoverWith{
      case ex: StuffNotFound => db.findPointsForUser(username)
    }
  }
}

哪种类型检查正确。

编辑

由于API是一成不变的，处理嵌套的一元类型的一种方法是定义一个MonadTransformer。简而言之，让我们让Future[Option[T]]成为一个新的单子，让我们称之为FutureO，它可以与同类产品组合在一起。1

case class FutureO[+A](future: Future[Option[A]]) {
  def flatMap[B](f: A => FutureO[B])(implicit ec: ExecutionContext): FutureO[B] = {
    val newFuture = future.flatMap{
      case Some(a) => f(a).future
      case None => Future.successful(None)
    }
    FutureO(newFuture)
  }

  def map[B](f: A => B)(implicit ec: ExecutionContext): FutureO[B] = {
    FutureO(future.map(option => option map f))
  }
  def recoverWith[U >: A](pf: PartialFunction[Throwable, FutureO[U]])(implicit executor: ExecutionContext): FutureO[U] = {
    val futOtoFut: FutureO[U] => Future[Option[U]] = _.future
    FutureO(future.recoverWith(pf andThen futOtoFut))
  }

  def orElse[U >: A](other: => FutureO[U])(implicit executor: ExecutionContext): FutureO[U] = {
      FutureO(future.flatMap{
        case None => other.future
        case _ => this.future
      }) 
    }
  }

现在我们可以重写我们的流程，保留与基于未来的组合相同的结构。

type UserPoints = Long 
object db { 
  def getUserPoints(username: String): Future[Option[UserPoints]] = ???
  def getSomething(s: String): Future[Option[Long]] = ???
  def setPoints(pointId: UserPoints, username:String): Future[Unit] = ???
  def findPointsForUser(username: String): Future[Option[Long]] = ???
}
class PointsNotFound extends Exception("bonk")
class StuffNotFound extends Exception("sthing not found")

def getUserPoints2(username:String): Future[Option[UserPoints]] = {
  val futureOpt = FutureO(db.getUserPoints(username))
  .map(userPoints => userPoints /*.total*/)
  .orElse{ 
    (for {
      sthingElse <- FutureO(db.getSomething("abc"))
      _ <- FutureO(db.setPoints(sthingElse, username).map(_ => Some(())))
      points <- FutureO(db.findPointsForUser(username))
    } yield (points))
    .orElse{
      FutureO(db.findPointsForUser(username))
    }
  }
  futureOpt.future
}

1，并向http://loicdescotte.github.io/posts/scala-compose-option-future/致谢