scala - 创建一个自定义 scala 集合，其中地图默认返回自定义集合？

Question

该特征TraversableLike[+A, +Repr]允许创建一个集合，其中一些函数将返回 a Repr，而其他函数继续返回That函数的类型参数。有没有办法定义一个CustomCollection[A]where 函数，如map,++和其他函数将默认That，就Repr好像没有以其他方式推断一样？

这是一个代码片段，希望能描述我想要的内容：

case class CustomCollection[A](list: List[A]) extends TraversableLike[A, CustomCollection[A]] {
  protected[this] def newBuilder = new CustomCollectionBuilder[A]
  def foreach[U](f: (A) => U) {list foreach f}
  def seq = list
}

class CustomCollectionBuilder[A] extends mutable.Builder[A, CustomCollection[A]] {
  private val list = new mutable.ListBuffer[A]()
  def += (elem: A): this.type = {
    list += elem
    this
  }
  def clear() {list.clear()}
  def result(): CustomCollection[A] = CustomCollection(list.result())
}

object CustomCollection extends App {
  val customCollection = CustomCollection(List(1, 2, 3))
  println(customCollection filter {x => x == 1}) // CustomCollection(1)
  println(customCollection map {x => x + 1}) // non-empty iterator
}

我希望最后一行是CustomCollection(2, 3, 4).

Answer 1

您需要设置一个提供精炼CanBuildFrom实例的伴随对象：

import collection.TraversableLike
import collection.generic.{CanBuildFrom, GenericCompanion, GenericTraversableTemplate,
  TraversableFactory}
import collection.mutable.{Builder, ListBuffer}

object CustomCollection extends TraversableFactory[CustomCollection] {
  def newBuilder[A] = new CustomCollectionBuilder[A]
  implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, CustomCollection[A]] = 
    new CanBuildFrom[Coll, A, CustomCollection[A]] {
       def apply(): Builder[A, CustomCollection[A]] = new CustomCollectionBuilder()
       def apply(from: Coll): Builder[A, CustomCollection[A]] = apply()
    }
}
case class CustomCollection[A](list: List[A]) extends Traversable[A]
with TraversableLike[A, CustomCollection[A]]
with GenericTraversableTemplate[A, CustomCollection] {
  override def companion: GenericCompanion[CustomCollection] = CustomCollection
  def foreach[U](f: A => U) { list foreach f }
  override def seq = list
}

class CustomCollectionBuilder[A] extends Builder[A, CustomCollection[A]] {
  private val list = new ListBuffer[A]()
  def += (elem: A): this.type = {
    list += elem
    this
  }
  def clear() {list.clear()}
  def result(): CustomCollection[A] = CustomCollection(list.result())
}

val customCollection = CustomCollection(List(1, 2, 3))
val f = customCollection filter {x => x == 1} // CustomCollection[Int]
val m = customCollection map {x => x + 1}     // CustomCollection[Int]