特征TraversableLike[+A, +Repr]
允许用户创建一个集合,其中某些函数将返回Repr
,而其他人继续返回类型参数That
功能。有没有办法定义CustomCollection[A]
,其中map
,++
和其他函数将That
默认为Repr
,如果不另外推断的那样?
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
}
我希望最后一行是CustomCollection(2, 3, 4)
。
答案 0 :(得分:4)
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, 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]