隐式 class 有时无法工作

Question

Scala 版本：2.11.8

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global
import play.api.mvc._

object foooo {

  type ActionFilter[R] = R => Future[Either[Int, Unit]]

  case class FooRequest[A](request: Request[A]) extends WrappedRequest(request)

  implicit class ActionFilterOps[R](val f: ActionFilter[R]) {
    def * (f1: ActionFilter[R]): ActionFilter[R] = { (r: R) =>
      f(r).flatMap {
        case Left(r) => Future.successful(Left(r))
        case Right(_) => f1(r)
      }
    }
  }

  def test[A]() = {
    val f0: ActionFilter[FooRequest[A]] = { (r: FooRequest[A]) =>
      Future.successful(Left(1))
    }
    val f1 = f0
    ActionFilterOps(f1) * f0 // Ok
    f1 * f0 // Won't compile
  }


}

如您所见，隐式 class 不起作用。我必须更新 ActionFilterOps

Answer 1

附带说明：如果可能的话，如果您有一个不依赖于外部依赖项（例如 Play）的自包含示例，它会有所帮助。

现在开始回答，或者至少修复错误...如果您将类型别名 ActionFilter 更改为包装函数的 class，它就可以工作。

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

case class ActionFilter[R](f: R => Future[Either[Int, Unit]]) { def apply(r: R) = f(r) }
type Request[A] = Option[A]

case class FooRequest[A](request: Request[A])

implicit class ActionFilterOps[R](val f: ActionFilter[R]) {
  def * (f1: ActionFilter[R]): ActionFilter[R] = ActionFilter{ (r: R) =>
    f(r).flatMap {
      case Left(r) => Future.successful(Left(r))
      case Right(_) => f1(r)
    }
  }
}

def test[A]() = {
  val f0: ActionFilter[FooRequest[A]] = ActionFilter{ (r: FooRequest[A]) =>
    Future.successful(Left(1))
  }
  val f1 = f0
  ActionFilterOps(f1) * f0 // Ok
  f1 * f0 // Ok
}

如果您使用原始函数类型而不是类型别名，它也有效。

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

type Request[A] = Option[A]

case class FooRequest[A](request: Request[A])

implicit class ActionFilterOps[R](val f: R => Future[Either[Int, Unit]]) {
  def * (f1: R => Future[Either[Int, Unit]]): R => Future[Either[Int, Unit]] = { (r: R) =>
    f(r).flatMap {
      case Left(r) => Future.successful(Left(r))
      case Right(_) => f1(r)
    }
  }
}

def test[A]() = {
  val f0: FooRequest[A] => Future[Either[Int, Unit]] = { (r: FooRequest[A]) =>
    Future.successful(Left(1))
  }
  val f1 = f0
  ActionFilterOps(f1) * f0 // Ok
  f1 * f0 // Ok
}

特别是第二个 "solution" 让我觉得这是一个错误。

隐式 class 有时无法工作

Implicit class sometimes cannot work

scala

implicit-conversion