Scala 特征语法 [英] Scala trait syntax

问题描述

我正在阅读 Odersky 的书，并且有一个包含以下代码部分的电子表格示例:

I am reading Odersky's book and there is a speadsheet example with the follwowing code part:

package org.stairwaybook.scells
    trait Arithmetic { this: Evaluator =>
      operations += (
        "add"  -> { case List(x, y) => x + y },
        "sub"  -> { case List(x, y) => x - y },
        "div"  -> { case List(x, y) => x / y },
        "mul"  -> { case List(x, y) => x * y },
        "mod"  -> { case List(x, y) => x % y },
        "sum"  -> { xs => (0.0 /: xs)(_ + _) },
        "prod" -> { xs => (1.0 /: xs)(_ * _) }
      )
    }

this: Evaluator"指的是什么?有人可以帮助理解这个特征吗?正如我所见，它定义了不同的操作，它们是函数，但我看不到大图...

what does "this: Evaluator" refer to? Could someone help to understand this trait? As I see it defines different operations which are functions, but I do not see the big picture...

推荐答案

你在这里看到的

this:Evaluator =>

是 self-type 对 trait 的用法.它基本上强制将要混合特征算术的类也混合特征评估器.

is the usage of self-type for a trait. It basically forces class which are going to mix the trait Arithmetic to also mix the trait Evaluator.

如果您尝试创建如下所示的类:

If you try to create a class such as the following:

class ArithmeticClass extends Arithmetic

你会得到一个编译时错误，而如果你尝试这样做:

you'll get a compile time error, while if you try to do:

class ArithmeticClass extends Arithmetic with Evaluator

这会奏效.如您所见，Arithmetic 类修改了向操作添加的内容，这可能是 Evaluator trait 中定义的集合.

this is going to work. As you can see, Arithmetic class modifies add something to operations, which is probably a collection defined in the Evaluator trait.

请注意，与简单继承相比，自类型可以让您设计更清晰的类层次结构:

Please note that self-types let you design a cleaner class hierarchy compared to simple inheritance:

如果你使用 self 类型，你可以考虑如下:

If you use self types, you can think about something like the following:

trait Evaluator { def runEvaluation : Int }
trait Arithmetic { self: Evaluator => def evaluate: Int = runEvaluation }
trait NullEvaluator extends Evaluator { def runEvaluation: Int = 0 }

class MyClass1 extends Arithmetic with Evaluator {... concrete methods .... }
class MyClass2 extends Arithmetic with NullEvaluator { ....concrete methods ..... }

所以 self 类型可以让你表达与继承不同的东西.

So self types let you express something different from inheritance.

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