如何匹配 Scala 中的类“匹配"?陈述? [英] How can I match classes in a Scala "match" statement?

问题描述

如何使用匹配"语句来识别类变量的值?以下是无效的，我找不到可接受的变体——除了 if ... else if ... else ...

How can I use a "match" statement to identify the value of a class variable? The following is invalid, and I can't find an acceptable variant -- other than if ... else if ... else ...

val c: Class[_] = classOf[Int]
val what = c match { case classOf[Int] => "int!"; case classOf[Float] => "float!" }

编译器抱怨:error: not found: type classOf

当然，我不能使用 Class[Int] 因为该类型信息已被删除:

And of course, I can't use Class[Int] because that type information is erased:

c match { case Class[Int] => "int!"; case Class[Float] => "float!" }
error: type Class of type Class does not take type parameters.

我也尝试过像 Int.class 这样的变体，但都无济于事.(而且我真的不想转换为字符串:我觉得让编译器捕获重命名/移动类很重要.)

I've also tried variants like Int.class, all to no avail. (And I don't really want to convert to strings: I feel it's important to have the compiler catch renamed/moved classes.)

我是不是太笨了，还是偶然发现了 Scala 盲点?

Am I being dense, or have I stumbled into a Scala blind spot?

推荐答案

如果为类值创建稳定的标识符(即 val)，则可以匹配它们，

You can match on class values if you create a stable identifier (ie. a val) for them,

scala> val c: Class[_] = classOf[Int]
c: Class[_] = int

scala> val ClassOfInt = classOf[Int]
ClassOfInt: java.lang.Class[Int] = int

scala> val ClassOfFloat = classOf[Float]
ClassOfFloat: java.lang.Class[Float] = float

scala> val what = c match {
     |     case ClassOfInt => "int!"
     |     case ClassOfFloat => "float!"
     | }
what: String = int!

请注意，您不能匹配类型(即 Class[Int])，因为擦除意味着 Class[T] 的不同类型实例化在运行时无法区分......因此下面的警告

Note that you can't match on type (ie. Class[Int]) because erasure means that the different type instantiations of Class[T] are indistinguishable at runtime ... hence the warning below

scala> val what = c match {
     |     case _: Class[Int] => "int!"
     |     case _: Class[Float] => "float!"
     | }
warning: there were 2 unchecked warnings; re-run with -unchecked for details
what: java.lang.String = int!

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