扩展 Scala 集合

问题描述

我想派生一个 Scala 内置集合的版本，它扩展了特定泛型类型的功能，例如，

I would like to derive a version of a Scala built-in collection that expands on the functionality for a particular generic type e.g.,

import scala.collection.immutable._
class Tuple2Set[T1,T2] extends HashSet[Tuple2[T1,T2]] {
 def left = map ( _._1 )
 def right = map ( _._2 )
}

但是当我尝试将其用于以下测试时

However when I try to use it with the following test

  new Tuple2Set[String,String]() + (("x","y")) left

我收到以下编译错误

error: value left is not a member of scala.collection.immutable.HashSet[(String, String)]

如何更改课程以使其有效?

How can I change the class so that this works?

推荐答案

正如Kevin Wright所说，+操作会返回HashSet.类型类CanBuildFrom 用于在map 等操作期间构建新集合.因此，如果您希望 + 返回 Tuple2Set 而不是 HashSet，您应该实现 CanBuildFrom 并使其在伴随中隐式可用像这样的对象:

As Kevin Wright said, + operation will return back HashSet. Type class CanBuildFrom is used to build new collections during operations like map. So if you want + to return Tuple2Set instead of HashSet you should implement CanBuildFrom and make it implicitly available in companion object like this:

object Tuple2Set {
    implicit def canBuildFrom[T1, T2] =
        new CanBuildFrom[Tuple2Set[T1, T2], (T1, T2), Tuple2Set[T1, T2]] {...}
}

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