具有路径相关类型和协变参数的 Scala 副本 class

Question

我在路径相关类型和协变参数方面遇到了一些问题。我需要创建一个新的 SomeFancyCollection 实例，但由于路径依赖类型，它不会编译。当我从 class 中拉出所有像 Node 和 Branch 这样的东西时，我必须将根参数声明为私有 [this] 并且我也无法获得 class 的新实例。我的代码如下所示：

class SomeFancyCollection[+A] {

  private var root: Node = Branch(0x0, Vector[Node]())

  trait Node {
    val id: Byte
    def put[A1 >: A](ids: Seq[Byte], item: A1): Node
    def remove[A1 >: A](ids: Seq[Byte], item: A1): Node
  }

  case class Branch(id: Byte, subs: Vector[Node]) extends Node {
      ...
  }

  case class Leaf[A1 >: A](id: Byte, subs: Vector[A1]) extends Node {
      ...
  }

  def add[A1 >: A](item: A1): SomeFancyCollection[A1] = {
    val ids: Seq[Byte] = getIds() // doesn't matter
    val newRoot = root.put(ids, item)
    val newInstance = new SomeFancyCollection[A1]()
    newInstance.root = newRoot
    newInstance
  }
}

Answer 1

在您的代码中，没有明显的理由使 Node 和 Leaf 嵌套路径依赖类。只需让它们独立协变类，然后路径依赖的所有问题都会消失：

trait Node[+A] {
  val id: Byte
  def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1]
  def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1]
}

case class Branch[+A](id: Byte, subs: Vector[Node[A]]) extends Node[A] {

  def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
  def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
}

case class Leaf[+A](id: Byte, subs: Vector[A]) extends Node[A] {

  def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
  def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
}



class SomeFancyCollection[+A](val root: Node[A] = Branch(0x0, Vector[Node[A]]())) {
  def add[A1 >: A](item: A1): SomeFancyCollection[A1] = {
    val ids: Seq[Byte] = ???// getIds() // doesn't matter
    val newRoot = root.put(ids, item)
    new SomeFancyCollection[A1](newRoot)
  }
}

如果你不想污染命名空间，只需将 Node 类包声明为私有，甚至将所有辅助实现细节类隐藏在 companion 中SomeFancyCollection 的对象：

class SomeFancyCollection[+A] private[SomeFancyCollection](
  val root: SomeFancyCollection.AnnoyingDetails.Node[A]
) {
  def add[A1 >: A](item: A1): SomeFancyCollection[A1] = {
    val ids: Seq[Byte] = ???// getIds() // doesn't matter
    val newRoot = root.put(ids, item)
    new SomeFancyCollection[A1](newRoot)
  }
}

object SomeFancyCollection {

  def empty[A]: SomeFancyCollection[A] = new SomeFancyCollection[A](
    AnnoyingDetails.Branch(0x0, Vector[AnnoyingDetails.Node[A]]())
  )

  private[SomeFancyCollection] object AnnoyingDetails {
    trait Node[+A] {
      val id: Byte
      def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1]
      def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1]
    }

    case class Branch[+A](id: Byte, subs: Vector[Node[A]]) extends Node[A] {
      def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
      def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
    }

    case class Leaf[+A](id: Byte, subs: Vector[A]) extends Node[A] {
      def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
      def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
    }
  }
}

具有路径相关类型和协变参数的 Scala 副本 class

Scala copy class with path dependent types and covariant parameter

scala

covariance

path-dependent-type