理解Scala枚举 [英] Understanding scala enumerations

问题描述

我不得不说我不懂 Scala 枚举类.我可以从文档中复制粘贴示例，但我不知道发生了什么.

I have to say I don't understand Scala enumeration classes. I can copy-paste the example from documentation, but I have no idea what is going on.

object WeekDay extends Enumeration {
  type WeekDay = Value
  val Mon, Tue, Wed, Thu, Fri, Sat, Sun = Value
}
import WeekDay._

• type WeekDay = Value 是什么意思，我为什么要写这个?
• 为什么val Mon = Value?这到底是什么意思?
• 为什么我必须导入WeekDay目的?而且，
• 当我写val day = WeekDay.Mon时，为什么输入的是WeekDay.Value，而不是WeekDay?
• What means type WeekDay = Value and why do I have to write that?
• Why is val Mon = Value? What does that even mean?
• Why do I have to import the WeekDay object? And,
• when I write val day = WeekDay.Mon, why is it type WeekDay.Value, not type WeekDay?

推荐答案

Enumeration trait 有一个类型成员 Value 表示枚举的各个元素(它实际上是一个内部类，但这里的区别并不重要).

the Enumeration trait has a type member Value representing the individual elements of the enumeration (it's actually an inner class, but the difference doesn't matter here).

因此 object WeekDay 继承了该类型成员.type WeekDay = Value 行只是一个类型别名.它很有用，因为在你用 import WeekDay._ 将它导入到别处之后，你可以使用那个类型，例如:

Thus object WeekDay inherits that type member. The line type WeekDay = Value is just a type alias. It is useful, because after you import it elsewhere with import WeekDay._, you can use that type, e.g.:

def isWorkingDay(d: WeekDay) = ! (d == Sat || d == Sun)

相反，最小版本将是:

object WeekDay extends Enumeration {
  val Mon, Tue, Wed, Thu, Fri, Sat, Sun = Value
}

并且您不必必须导入object WeekDay的内容，但是您需要使用类型WeekDay.Value并个人会员资格.所以这个例子会变成

and you do not have to import the contents of object WeekDay, but then you would need to use type WeekDay.Value and to qualify individual members. So the example would become

def isWorkingDay(d: WeekDay.Value) = ! (d == WeekDay.Sat || d == WeekDay.Sun)

<小时>

第二个问题是关于val Mon, ... = Value 的含义.如果你不研究 Enumeration 的实现，这确实很令人困惑.这不是类型的赋值！相反，它调用了一个受保护的同名方法，Value，它返回一个 Value 类型的具体实例.

---

The second question is about the meaning of val Mon, ... = Value. This is indeed very confusing if you don't look into the implementation of Enumeration. This is not the assignment of a type! It is instead calling a protected method of the same name, Value, which returns a concrete instance of type Value.

碰巧您可以在 Scala 中编写 val a, b, c = foo，并且对于每个值 a, b,并且 c 方法 foo 将被一次又一次地调用.Enumeration 使用这个技巧来增加一个内部计数器，以便每个值都是独立的.

It so happens that you can write val a, b, c = foo in Scala, and for each value a, b, and c the method foo will be called again and again. Enumeration uses this trick to increment an internal counter so that each value is individual.

如果您打开 Enumeration 的 Scala API 文档并单击 Visibility: All，您将看到该方法出现.

If you open the Scala API docs for Enumeration and click on Visibility: All, you will see that method appearing.

这篇关于理解Scala枚举的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！