scala - 在运行时检查 Scala 类型和类型删除

Question

假设我们有几个这样的类:

abstract class Throw {
    def winsOver(t2: Throw): Boolean
}

class Rock extends Throw {
    override def winsOver(t2: Throw): Boolean = t2 match {
        case _: Scissors => true
        case _           => false
    }
}

class Scissors extends Throw {
    override def winsOver(t2: Throw): Boolean = t2 match {
        case _: Paper => true
        case _        => false
    }
}

class Paper extends Throw {
    override def winsOver(t2: Throw): Boolean = t2 match {
        case _: Rock => true
        case _       => false
    }
}

这行得通

scala>new Paper winsOver new Rock
res0: Boolean = true
scala>new Rock winsOver new Rock
res1: Boolean = false

但是，代码有很多重复。由于唯一不同的是他们击败的类型，我们可以尝试将其排除在外

abstract class Throw {

    type Beats <: Throw

    def winsOver(t2: Throw): Boolean = t2 match {
        case _: Beats => true
        case _        => false
    }

}

class Rock {
    type Beats = Scissors
}

class Scissors {
    type Beats = Paper
}

class Paper {
    type Beats = Rock
}

但是编译器开始报错

warning: abstract type pattern Throw.this.Beats is unchecked since it is eliminated by erasure
    case _: Beats => true

果然，它不起作用。 winsOver 突然总是返回 true

scala>new Rock winsOver new Rock
res0: Boolean = true

我一直在努力解决这个问题，根据我的发现，这是因为 JVM 没有尽可能多地携带类型信息。这会导致一些信息丢失(“删除”)，并且有一些方法可以在 Scala 中解决这个问题，以前使用 list ，现在使用类标签和类型标签。

我还没有真正能够更具体地弄清楚这是如何工作的，虽然有时我能够从互联网上复制代码片段来做类似的事情，但我真的不明白这些代码是如何工作的，而且我不能适应这个例子。我还注意到有一个 shapeless 库对这种东西有很多支持，但我也想了解它是如何工作的。

Answer 1

你不应该在运行时检查类型信息。 Type erasure is a good thing Scala 应该删除比它更多的类型。

相反，使用代数数据类型和模式匹配:

sealed abstract class Throw {
  def winsOver(t2: Throw): Boolean
}

case object Rock extends Throw {
  def winsOver(t2: Throw): Boolean = t2 match {
    case Scissors => true
    case _        => false
  }
}

case object Scissors extends Throw {
  def winsOver(t2: Throw): Boolean = t2 match {
    case Paper => true
    case _     => false
  }
}

case object Paper extends Throw {
  def winsOver(t2: Throw): Boolean = t2 match {
    case Rock => true
    case _    => false
  }
}

这有一些重复，所以我们可以把它分解出来:

sealed abstract class Throw {
  def winsOver(t2: Throw): Boolean = (this,t2) match {
    case (Paper, Rock) | (Rock, Scissors) | (Scissors,Paper) => true
    case _ => false 
  }
}

case object Rock extends Throw
case object Scissors extends Throw
case object Paper extends Throw

这按预期工作:

scala> Rock winsOver Scissors
res0: Boolean = true

scala> Paper winsOver Scissors
res1: Boolean = false