Scala 模式匹配与 Option[Any] 混淆

Question

我有以下 Scala 代码。

import scala.actors.Actor

object Alice extends Actor {
  this.start
  def act{
    loop{
      react {
        case "Hello" => sender ! "Hi"
        case i:Int => sender ! 0
      }
    }
  }
}
object Test {
  def test = {
    (Alice !? (100, "Hello")) match {
      case i:Some[Int] => println ("Int received "+i)
      case s:Some[String] => println ("String received "+s)
      case _ =>
    }
    (Alice !? (100, 1)) match {
      case i:Some[Int] => println ("Int received "+i)
      case s:Some[String] => println ("String received "+s)
      case _ =>
    }
  }
}

做完 Test.test ，我得到输出:

scala> Test.test
Int received Some(Hi)
Int received Some(0)

我期待输出

String received Some(Hi)
Int received Some(0)

解释是什么？

作为第二个问题，我得到 unchecked上述警告如下:

C:\scalac -unchecked a.scala
a.scala:17: warning: non variable type-argument Int in type pattern Some[Int] is unchecked since it is eliminated by erasure
      case i:Some[Int] => println ("Int received "+i)
             ^
a.scala:18: warning: non variable type-argument String in type pattern Some[String] is unchecked since it is eliminated by erasure
      case s:Some[String] => println ("String received "+s)
             ^
a.scala:22: warning: non variable type-argument Int in type pattern Some[Int] is unchecked since it is eliminated by erasure
      case i:Some[Int] => println ("Int received "+i)
             ^
a.scala:23: warning: non variable type-argument String in type pattern Some[String] is unchecked since it is eliminated by erasure
      case s:Some[String] => println ("String received "+s)
             ^
four warnings found

我怎样才能避免这些警告？

编辑:感谢您的建议。 Daniel 的想法不错，但似乎不适用于泛型类型，如下例所示

def test[T] = (Alice !? (100, "Hello")) match { 
   case Some(i: Int) => println ("Int received "+i) 
   case Some(t: T) => println ("T received ") 
   case _ =>  
}

遇到以下错误警告: warning: abstract type T in type pattern T is unchecked since it is eliminated by erasure

Answer 1

这是由于类型删除。 JVM 不知道任何类型参数，除了数组。因此，Scala 代码无法检查 Option是 Option[Int]或 Option[String] ——该信息已被删除。

不过，您可以通过这种方式修复您的代码:

object Test {
  def test = {
    (Alice !? (100, "Hello")) match {
      case Some(i: Int) => println ("Int received "+i)
      case Some(s: String) => println ("String received "+s)
      case _ =>
    }
    (Alice !? (100, 1)) match {
      case Some(i: Int) => println ("Int received "+i)
      case Some(s: String) => println ("String received "+s)
      case _ =>
    }
  }
}

这样你就不会测试 Option 的类型。是，但其内容的类型是什么——假设有任何内容。一个 None将落入默认情况。