在 Scala 中将 null 转换为 case class 的实例

Question

我有一段代码依赖于特定案例 class 的任意元素的存在，以便对 class 的字段进行操作。有some options out there, and even though pretty much every Scala blog recommends not using null ever, it seems like not a terrible idea in type level programming (e.g. this answer: ）。但是，此代码直接将 null 强制转换为特定情况 class 不起作用并且不会引发错误，我对原因很感兴趣。

trait V {}
case class W(x: Int) extends V

val y = null.asInstanceOf[W]

y match {
  case w:W => println("null cast to W is a W")
  case _ => println("null cast to W is NOT a W")
}

// prints "null cast to W is NOT a W"

y match {
  case v:V => println("null cast to W is a V")
  case _ => println("null cast to W is NOT a V")
}

// prints "null cast to W is NOT a V"

val z = W(1)

z match {
  case w:W => println("instantiated W is a W")
  case _ =>  println("instantiated W is NOT a W")
  }

// prints "instantiated W is a W"

z match {
  case v:V =>  println("instantiated W is a V")
  case _ =>  println("instantiated W is NOT a V")
}

// prints "instantiated W is a V"

Answer 1

转换会更改编译时类型，而不是运行时类型。模式匹配检查运行时类型。事实上，case class 模式匹配甚至对 null 有显式检查；请参阅 Why is there a `null` check in the implementation of the `unapply` method for case classes?（尽管该检查不会影响您的情况，因为您使用的是类型模式）。

此外，即使不是模式匹配问题，您也无法 "operate on the class's fields" 得到 NullPointerException。

Answer 2

因为 Type Patterns 是这样定义的：

A reference to a class C, p.C, or T#C. This type pattern matches any non-null instance of the given class.

因此：null 是类型 W 的完全有效值；但它不会与模式 w: W.

匹配

而不匹配的主要原因恰恰是

I have a piece of code that relies on the existence of an arbitrary element of a certain case class in order to operate on the class's fields.

所以当你匹配 w: W 时你想知道它的字段和方法是否可用。但是 null 他们不是。

在 Scala 中将 null 转换为 case class 的实例

Casting null to an instance of a case class in Scala

generics

scala

scala-2.11