选项匹配上的 Scala 类型删除

Question

object Test {
  def main(args: Array[String]) {
    val list: List[Double] = List(1.0, 2.0, 3.0, 4.0)
    val none = None

    case class Test()

    val test = Test()

    def f(x: Any) = x match {
        case _: Some[Test] => println("_ matched")
        case None => println("None matched")
    }

    f(list)
    f(none)
    f(test)
  }
}

尝试编译上述代码会导致“被删除消除”编译时警告。

   $>scalac Test.scala
    Test.scala:11: warning: non-variable type argument Test in type pattern
 Some[Test] is unchecked since it is eliminated by erasure
            case _: Some[Test] => println("_ matched")
                    ^
    one warning found

我读了这篇备受推崇的 Stackoverflow post ，但我不明白这里的类型删除。

Answer 1

在运行时 jvm没有Some[Test]或 Some[String]只有Some[Any] .所以你不能匹配 Some[Test] .

在这种情况下，您可以匹配 Some 的内容像这样:

case Some(e: Test) => println(s"$e matched")