斯卡拉的平等关系 [英] Equality relations in Scala

问题描述

我偶然发现了Tony Morris的博客有关Java的帖子和该语言的一个基本问题：为集合定义定制的平等关系。我认为这是一件大事，想知道是否存在一些scala解决方案。

I just stumbled on one of Tony Morris' blog-posts about Java and a fundamental problem with the language: that of defining a bespoke equality-relation for a collection. This is something that I think is a big deal and wondered whether there was some scala solution.

经典问题体现在考虑交易等方面。假设我以150p的价格交易了两笔100股沃达丰股票。这两个行业是平等的，是吗？除非它们不是同一行业。对于具有持久性或序列化功能的常规真实系统，我不能依靠 identity 来告诉我两个引用是否指向同一交易！

The classic issue manifests itself in thinking about, say, a trade. Let's say I make two trades of +100 vodafone shares @150p. The two trades are equal, yes? Except they are not the same trade. In the case of a normal real-world system, with persistence or serialization, I cannot rely on identity to tell me whether two references are to the same trade!

所以我想要的是能够创建一个集合，我可以将一个平等关系传递给：

So what I want is to be able to create a collection which I can pass an Equality-relation to:

val as = CleverSet[Trade](IdEquality)
val bs = CleverSet[Trade](EconomicsEquality)

如何我是否可以高效地实现我的集合（除非 EqualityRelation 也定义了 hash 机制）？

How would I implement my set in an efficient manner (unless the EqualityRelation also defines a hash mechanism)?

trait EqualityRelation[T] {
  def equal(t1: T, t2: T) : Boolean
  def hash(t: T) : Int
}

所以问题是：

• 是否有一个提供此功能的库？


• 在Scala中有某种方法可以很好地做到这一点吗？

使用隐式函数，将它添加到现有的scala Set

It seems that with implicits, it would be quite an easy thing to add to the existing scala Set type.

推荐答案

这可以通过Java的TreeSet和Comparator实现来实现：

This can already be achieved with Java's TreeSet and a Comparator implementation:

TreeSet<String> ignoreCase = new TreeSet<String>(new Comparator<String>(){
    @Override
    public int compare(String o1, String o2) {
        return o1.compareToIgnoreCase(o2);
    }});

TreeSet<String> withCase = new TreeSet<String>();

List<String> values = asList("A", "a");
ignoreCase.addAll(values);
withCase.addAll(values);

输出：

ignoreCase -> [A]
withCase -> [A, a]

这样做的缺点是比较器实现的功能比所需的功能强大，并且您只能使用支持比较器的集合。如oxbow_lakes所指出的，Comparator实现破坏了Set合同（对于！a.equals（b）可能是 new Set（）; set .add（a）== true&& set.add（b）== false ）。

This has the drawbacks that the Comparator to implement is more powerful than needed and that you're restricted to collections that support Comparators. As pointed out by oxbow_lakes the Comparator implementation breaks the Set contract (for !a.equals(b) it could be that new Set(); set.add(a) == true && set.add(b) == false).

Scala支持使用

scala> new scala.collection.immutable.TreeSet[String]()(x=> x.toLowerCase) + "a"
 + "A"
res0: scala.collection.immutable.TreeSet[String] = Set(A)

这篇关于斯卡拉的平等关系的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！