创建自定义scala集合,其中map默认为返回自定义集合？

特征TraversableLike [A,Repr]允许一个集合,其中一些函数将返回一个Repr,而其他函数继续返回函数上的类型参数That.有没有办法定义一个CustomCollection [A],其中像map,和其他函数将默认如果没有推断,那么作为Repr？

这是一个代码片段,希望能够描述我想要的内容：

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
}

我想最后一行是CustomCollection(2,3,4).

解决方法

您需要设置一个伴随对象,该对象提供精炼的CanBuildFrom实例：

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