究竟什么是单​​例类型? [英] What is a Singleton Type exactly?

问题描述

什么是单例类型?有哪些应用和影响?

What is a singleton type? what are the applications, the implications ?

示例不仅受欢迎，外行术语更受欢迎！

Examples are more than welcome and layman terms are even more welcome !

推荐答案

如果你把一个类型看作一组值，一个值的单例类型 x 就是只包含这个的类型值 ({x}).用法示例:

If you think of a type as a set of values, the singleton type of a value x is the type which only contains this value ({x}). Usage examples:

1. 模式匹配:case _: Foo.type 使用eq检查匹配的对象是否与Foo相同，其中case Foo 仅使用 equals 检查它是否等于 Foo.

1. Pattern matching: case _: Foo.type checks that the matched object is the same as Foo using eq, where case Foo only checks that it's equal to Foo using equals.

需要写下object的类型(作为类型参数、实参等):

It's needed to write down the type of an object (as a type parameter, an argument, etc.):

object A
def method(): A.type = A

为了保证方法的返回值是它被调用的对象(对于方法链很有用，例如 从这里):

class A { def method1: this.type = { ...; this } }
class B extends A { def method2: this.type = { ...; this } }

您现在可以调用 new B.method1.method2，如果没有 this.type，您将无法调用，因为 method1 将返回 A.

You can now call new B.method1.method2, which you couldn't without this.type because method1 would return A.

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