为数据类型定义自己的Ord [英] Defining own Ord for a data type

问题描述

我正在尝试建立一些数据结构来解决图形难题。我正在尝试定义边缘的比较标准，但不确定如何。到目前为止：

I am attempting to make some data structures to solve a graph puzzle. I am trying to define an edge's comparison criteria, but I am not sure how. So far:

data Edge = Edge (Set String) Bool

如果它们具有相同的字符串集，但不具有相等性，我如何通知编译器我希望将边声明为相等，则与布尔值有任何关系值？

How can I 'inform' the compiler that I want edges to be declared equal if they have identical sets of strings, and not have equality have anything to do with the boolean value?

推荐答案

虽然我不确定为什么要忽略布尔值（我很好奇），但是因此您必须定义自己的 Eq 实例；默认设置无效，因为它会比较每个字段。幸运的是，这很容易：

Although I'm not sure why you want to ignore the boolean value (I'm curious), to do so you'll have to define your own Eq instance; the default one won't work, as it compares every field. Luckily, this is easy:

instance Eq Edge where
  (Edge s1 _) == (Edge s2 _) = s1 == s2

如果您想订购边缘，并且希望订购仅比较集合，您的实现也非常相似：

If you want to be able to order edges, and you want the ordering to compare just the sets too, your implementation is very similar:

instance Ord Edge where
  (Edge s1 _) `compare` (Edge s2 _) = s1 `compare` s2

每个类型类都定义了一组需要实施的方法； Eq 需要 == 或 / = 和 Ord 需要< = 或 compare 。 （要了解哪些功能是必需的，哪些是可选的，可以检查文档。）

Each type class defines a certain set of methods which need to be implemented; Eq requires == or /=, and Ord requires <= or compare. (To find out which functions are required and which are optional, you can check the docs.)

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