Scala 对带有两个隐式参数的重载定义的模糊引用答案

【问题标题】：Scala ambiguous reference to overloaded definition with two implicit parametersScala 对带有两个隐式参数的重载定义的模糊引用
【发布时间】：2013-06-28 14:02:24
【问题描述】：

lazy val productService = BeanLookup[ProductDataService]

object  BeanLookup {

    def apply[T](implicit manifest: Manifest[T], context: ActorContext) =    {
    val beanLookup =
      context.actorFor("/user/spring/beanLookup")
    Await.result(
      (beanLookup.ask(LookupBean(manifest.erasure))(timeout)).mapTo[T](manifest),
      timeout.duration)   }

  def apply[T](implicit manifest: Manifest[T], system: ActorSystem) =   {
    val beanLookup =
      system.actorFor("/user/spring/beanLookup")
    Await.result(
      (beanLookup.ask(LookupBean(manifest.erasure))(timeout)).mapTo[T](manifest),
      timeout.duration)   } }

scalac 抱怨：

 scala: ambiguous reference to overloaded definition,
both method apply in object BeanLookup of type (implicit manifest: Manifest[com.tooe.core.service.LocationCategoryDataService], implicit system: akka.actor.ActorSystem)com.tooe.core.service.LocationCategoryDataService
and  method apply in object BeanLookup of type (implicit manifest: Manifest[com.tooe.core.service.LocationCategoryDataService], implicit context: akka.actor.ActorContext)com.tooe.core.service.LocationCategoryDataService

【问题讨论】：

标签： scala overloading implicit erasure

【解决方案1】：

总之

trait Foo; trait Bar

object Test {
  def apply(implicit foo: Foo) {}
  def apply(implicit bar: Bar) {}
}

Test.apply   // ambiguous

Scala 不会通过判断两个隐式中是否只有一个可用来解决重载问题。

如果你想在这种情况下重载，你应该使用magnet pattern。

object FooOrBar {
  implicit def fromFoo(implicit f: Foo) = FooOrBar(Left(f))
  implicit def fromBar(implicit b: Bar) = FooOrBar(Right(b))
}
case class FooOrBar(e: Either[Foo, Bar])

object Test {
  def apply(implicit i: FooOrBar) = i.e match {
    case Left (f) => "foo"
    case Right(b) => "bar"
  }
}

Test.apply  // could not find implicit value for parameter i: FooOrBar

implicit val f = new Foo {}
Test.apply  // "foo"

【讨论】：