如何使用伴随工厂对象作为策略? [英] How to use companion factory objects as strategy?

问题描述

当我遍历 Map 中的值时，由于排序问题，我有一个测试间歇性失败.

I have a test which fails intermittently because of ordering issues when I iterate over the values in a Map.

Scala 提供了一个 ListMap ，它以牺牲性能为代价使测试稳定.所以我将 ImmutableMapFactory 抽象为一个 val 并在我的代码中使用它.

Scala helpfully provides a ListMap which makes the tests stable, at the expense of performance. So I abstracted the ImmutableMapFactory as a val and use it in my code.

class C {
  val immutableMapFactory = scala.collection.immutable.Map

  def func = {
    ...
    immutableMapFactory(pairs :_*)
  }
}

现在我的计划是扩展 C 并覆盖 immutableMapFactory 以进行测试

Now my plan was to extend C and override immutableMapFactory for tests

class TestableC extends C {
  override val immutableMapFactory = scala.collection.immutable.ListMap
}

不出所料，这会失败，因为 ListMap 与 Map 的类型不同.我应该如何指定 val(或 def)的类型，以便我可以在需要创建 Map 的任何地方使用工厂?

Unsurprising this fails as ListMap does not have the same type as Map. How should I specify the type of the val (or a def) so that I can use the factory wherever I need to create a Map?

推荐答案

你的问题在这一行:

val immutableMapFactory = scala.collection.immutable.Map

这使得 immutableMapFactory 等于单例对象 Map.ListMap(单例)不是Map(单例)的子类，因此后续覆盖失败.

This makes immutableMapFactory equal to the singleton object Map. ListMap (the singleton) is not a subclass of Map (the singleton), so the subsequent override fails.

如果您改为从 Map 中获取 apply 方法，并将其部分应用以形成第一类函数(类型 (A, B)* => immutable.Map[A,B]) 然后可以使该技术起作用:

If you instead take the apply method from Map, and partially apply it to form a first class function (of type (A, B)* => immutable.Map[A,B]) then the technique can be made to work:

import collection.immutable

class Bip {
  def fac[A,B] = immutable.Map.apply[A,B] _
}

class Bop extends Bip {
  override def fac[A,B] = immutable.ListMap.apply[A,B] _
}

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