如何为自定义 Scala collection（具有正确的方差）实现 newBuilder？

Question

我正在尝试实现一个遵循相同习语的新 collection 类型作为标准库，但我无法弄清楚如何处理 Builder 机械师。我已经通读了优秀的 "Architecture of Scala Collections" 文档第页，但它不包括我的情况。

这是我正在尝试做的事情的简化版本：

import scala.collection.TraversableLike
import scala.concurrent.Future

trait AsyncMap[A, +B]
  extends Traversable[(A, B)]
  with TraversableLike[(A, B), AsyncMap[A, B]]
{

  def empty: AsyncMap[A, B]

  // This is the main difference from scala.collection.Map (an AsyncMap doesn't
  // block while it checks if it contains an element for a given key).
  def get(key: A): Future[Option[B]]

  def +[B1 >: B](kv: (A, B1)): AsyncMap[A, B1]

}

编译上面的代码报错：

error: overriding method newBuilder in trait TraversableLike of type => scala.collection.mutable.Builder[(A, B),AsyncMap[A,B]];
 method newBuilder in trait GenericTraversableTemplate of type => scala.collection.mutable.Builder[(A, B),Traversable[(A, B)]] has incompatible type
trait AsyncMap[A, +B]
      ^

我认为这是在抱怨GenericTraversableTemplate 有一个具体的 newBuilder 实现，其签名与 TraversableLike 正在寻找的那个。我不明白的是我是如何可以解决这个问题。

实施newBuilder: Builder[(A, B), Traversable[(A, B)]]产生这个错误：

error: overriding method newBuilder in trait TraversableLike of type => scala.collection.mutable.Builder[(A, B),AsyncMap[A,B]];
 method newBuilder has incompatible type
  override def newBuilder: Builder[(A, B), Traversable[(A, B)]] = {
               ^

在执行 newBuilder: Builder[(A, B), AsyncMap[A, B]] 时会产生这个错误：

error: covariant type B occurs in contravariant position in type => scala.collection.mutable.Builder[(A, B),AsyncMap[A,B]] of method newBuilder
  override def newBuilder: Builder[(A, B), AsyncMap[A, B]] = {
               ^

我认为我采用后一种方法是正确的，但我不确定如何在此处指定方差。

我也试过让这个看起来更像内部 collections 实施一个 trait AsyncMapLike[A, +B, +This <: AsyncMapLike[A, B, This] with AsyncMap[A, B]], 但这种方法没有取得任何成果。

我应该承认我是 Scala 的新手，虽然我认为我理解它类型系统我可能不知道某些类型运算符或简单的设计模式解决了这个问题。

如有任何帮助，我们将不胜感激。

Possibly-related 个问题：

Creating typed collection
Specific Builder for Parameterized Type
Scala Inheritance; Builder Trouble; Non-Generic IterableLike
How do I specify a newBuilder for a scala set?
Error: Covariant type A occurs in contravariant position
covariant type T occurs in contravariant position

Answer 1

As mucaho helped me discover in the comments above, it turns out that my problem was caused by a missing access modifier. The error message about variance still doesn't make sense to me (I've opened a new question about it: Why does scalac only emit variance errors with certain access modifiers?)，但是当我用访问权限为 protected[this] 的具体实现重写 newBuilder 时，一切都按预期工作（之前我一直试图使它成为 public).

import scala.collection.mutable.Builder
import scala.collection.TraversableLike
import scala.concurrent.Future

trait AsyncMap[A, +B]
  extends Traversable[(A, B)]
  with TraversableLike[(A, B), AsyncMap[A, B]]
{
  def empty: AsyncMap[A, B]

  def get(key: A): Future[Option[B]]

  def +[B1 >: B](kv: (A, B1)): AsyncMap[A, B1]

  // This works!
  override protected[this] def newBuilder: Builder[(A, B), AsyncMap[A, B]] = ???
}

如何为自定义 Scala collection（具有正确的方差）实现 newBuilder？

How to implement newBuilder for a custom Scala collection (with correct variance)?

generics

collections

types

scala

builder