问题描述
Scala 代码:
trait T
class C
type W = C with T
class X extends W
W 是一个类型别名，但我想定义一个类来扩展它.为什么以及如何修复它?
W is a type alias, but I want define a class to extend it. Why and how to fix it?
推荐答案
我很难以一种很好的方式构建我的答案，但这里仍然试图解释正在发生的事情:
I have difficulty structuring my answer in a nice way, but here is nevertheless an attempt at explaining what's going on:
您会收到编译错误，因为 extends 子句需要类和特征，而不是类型，并且您提供的是类型.类和特征不能与类型混淆.
You get a compilation error because the extends clause requires class and traits, not types, and you're giving a type. Classes and traits must not be confused with types.
肯定有更好的解释.但基本上，类型指定了可以应用于某物(有时是其他属性)的操作.类和特征定义了它们实例的行为.
There are certainly better explanations of this out there. But basically a type specifies the operations that can be applied to something (and sometimes other properties). Classes and traits define the behavior of their instances.
在大多数静态类型的面向对象语言中，每个类/接口/特征也有一个关联的类型.然而，倒数通常不是真的:并非所有类型都有相应的类/接口/特征.例如，你的 C with T 是一个类型，但不是一个类，也不是一个特征(甚至不是它们的组合).
In most statically typed OO languages, every class/interface/trait also has an associated type. However, the reciprocal is typically not true: not all types have a corresponding class/interface/trait. For example, your C with T is a type, but not a class nor a trait (nor even a combination thereof).
extends 子句需要类和特征(由 with 分隔)，但不是一种类型.这是因为extends 的意思是:扩展这个东西的行为.正如我所说，类型不定义行为.
The extends clauses expects classes and traits (separated by with), but not one type. This is because extends means: extend the behavior of this thing. As I said, types do not define behavior.
在大多数地方，语法 A with B 代表一个 type，它是 A 类型和 类型的子类型>B.然而，在 extends 子句中，with 具有不同的含义，它只是作为 extends 参数的分隔符(很像 >, 作为方法调用参数的分隔符).
In most places, the syntax A with B represents a type which is a subtype of both the type A and the type B. In the extends clause, however, with takes a different meaning, and simply acts as a separator for the arguments of extends (much like , acts as separator for arguments to a method call).
如果你写class X extends C with T，它会起作用，因为它意味着class X extends C, T，如果你愿意的话.
If you write class X extends C with T, it will work, because it means class X extends C, T, if you want.
HTH
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