案例类继承的* so *错误是什么？ [英] What is *so* wrong with case class inheritance?

问题描述

在寻找别的东西时，我只是偶然发现了一些关于恶魔案件类继承的评论。有一个名为 ProductN 的东西，wretches和国王，精灵和向导，以及如何通过case类继承丢失某种非常理想的属性。那么case case继承有什么问题呢？

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

一个词：平等

case 类提供的实现等于和的hashCode 。等价关系，称为等于就像这样（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 （反身）


2. 对于 x ， y ， z ; if x等于y 和 y等于z 然后 x等于z （及物）


3. 对于 x ， y ; if x等于y 然后 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)

只要在继承层次结构中允许相等就可以中断2和3.这可以通过以下示例轻松证明：

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

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

您可能认为所有类层次结构都可能存在此问题，这是事实。但是案例类专门用于从开发人员的角度（以及其他原因）简化相等性，因此让它们非直观地行为

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 未按预期工作和与模式匹配器的互动。

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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