案例类继承有什么问题? [英] What is *so* wrong with case class inheritance?

问题描述

在寻找其他东西时，完全出于巧合，我偶然发现了一些关于案例类继承是多么邪恶的评论.有一个叫做 ProductN 的东西，可怜的和国王的，精灵和巫师的，以及某种非常理想的属性是如何随着案例类继承而丢失的.那么案例类继承有什么问题?

While looking for something else, quite out of mere coincidence I stumbled upon few comments about how diabolical case class inheritance is. There was this thing called ProductN , wretches and kings, elves and wizards and how some kind of a very desirable property is lost with case classes inheritance. So what is so wrong with case class inheritance ?

推荐答案

一句话:平等

case 类提供了 equals 和 hashCode 的实现.被称为 equals 的等价关系的工作原理如下(即必须具有以下属性):

case classes come with a supplied implementation of equals and hashCode. The equivalence relation, known as equals works like this (i.e. must have the following properties):

1. 对于所有x；x 等于 x 是 true(自反)
2. 对于x、y、z；如果 x 等于 y 并且 y 等于 z 那么 x 等于 z(传递)
3. 对于x，y；如果 x 等于 y 那么 y 等于 x(对称)

1. For all x; x equals x is true (reflexive)
2. For x, y, z; if x equals y and y equals z then x equals z (transitive)
3. For x, y; if x equals y then y equals x (symmetric)

只要您允许继承层次结构中的相等，您就可以打破 2 和 3.以下示例简单地证明了这一点:

As soon as you allow for equality within an inheritance hierarchy you can break 2 and 3. this is trivially demonstrated by the following example:

case class Point(x: Int, y: Int)
case class ColoredPoint(x: Int, y: Int, c: Color) extends Point(x, y) 

然后我们有:

Point(0, 0) equals ColoredPoint(0, 0, RED)

但不是

ColoredPoint(0, 0, RED) equals Point(0, 0)

你可能会争辩说所有的类层次结构都可能有这个问题，这是真的.但是案例类的存在是为了从开发人员的角度(以及其他原因)简化平等，因此让它们的行为非直观将是一个自己的目标的定义！

You might argue that all class hierarchies may have this problem, and this is true. But case classes exist specifically to simplify equality from a developer's perspective (among other reasons), so having them behave non-intuitively would be the definition of an own goal!

还有其他原因；值得注意的是 copy 没有按预期工作 和 与模式匹配器的交互.

There were other reasons as well; notably the fact that copy did not work as expected and interaction with the pattern matcher.

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