匹配类型删除类型的部分函数

Question

在下面的代码中，我希望仅当 i 的类型为 A 时才调用 hitA。要检查的类型作为类型参数提供，因此它被类型擦除并且match 失败，给我一个编译器警告：

抽象类型模式 T 未被选中，因为它被擦除消除了

代码的期望输出是：

点击A

未命中

在当前版本中，当我运行代码时，我得到 Hit A，后跟 ClassCastException。

我了解正在发生的事情以及为什么会出现警告和异常，但我不知道如何解决这个问题。我已阅读有关 TypeTags 的基本文章，并且了解如何在基本情况下使用它们，但我看不到如何使用 TypeTags 创建部分函数。

import scala.reflect.runtime.universe._


object TestMatch extends App {
  abstract class I

  class A extends I {
    def hitA() = println("Hit A")
  }

  class B extends I

  def processOpGen[T <: I : TypeTag](op: PartialFunction[I, Unit])(i: I) = {
    val fallback : PartialFunction[I, Unit] = {case _ => println("Not hit")}
    (op orElse fallback)(i)
  }

  def partialMatchGen[T <: I : TypeTag](op: T => Unit)(i: I) = {
    processOpGen[T] {
      case c: T => op(c) // can TypeTag be used here for matching somehow?
    }(i)
  }


  partialMatchGen[A](a => a.hitA())(new A)
  partialMatchGen[A](a => a.hitA())(new B)

}

Answer 1

只要replace TypeTag with ClassTag:

import scala.reflect._

object Main extends App {
  abstract class I

  class A extends I {
    def hitA() = println("Hit A")
  }

  class B extends I

  def processOpGen[T <: I : ClassTag](op: PartialFunction[I, Unit])(i: I) = {
    val fallback : PartialFunction[I, Unit] = {case _ => println("Not hit")}
    (op orElse fallback)(i)
  }

  def partialMatchGen[T <: I : ClassTag](op: T => Unit)(i: I) = {
    processOpGen[T] {
      case c: T => op(c) // can TypeTag be used here for matching somehow?
    }(i)
  }

  partialMatchGen[A](a => a.hitA())(new A)
  partialMatchGen[A](a => a.hitA())(new B)
}

Processing...
Reused last reload result
[info] Loading project definition from /tmp/rendererL3zBdh8HOA/project/project
[info] Loading project definition from /tmp/rendererL3zBdh8HOA/project
[info] Set current project to rendererWorker (in build file:/tmp/rendererL3zBdh8HOA/)
[info] Reapplying settings...
[info] Set current project to rendererWorker (in build file:/tmp/rendererL3zBdh8HOA/)
[info] Formatting 1 Scala source {file:/tmp/rendererL3zBdh8HOA/}rendererWorker(compile) ...
[warn] Scalariform parser error for /tmp/rendererL3zBdh8HOA/src/main/scala/test.scala: Expected token RBRACKET but got Token(XML_START_OPEN,<,335,<)
[info] Compiling 1 Scala source to /tmp/rendererL3zBdh8HOA/target/classes...
[success] Total time: 11 s, completed Oct 13, 2015 5:16:45 PM
Now running...
[info] Running Main 
Hit A
Not hit
[success] Total time: 0 s, completed Oct 13, 2015 5:16:45 PM