Scala copy class with path dependent types and covariant parameter

Question

I have some trouble with path dependent types and covariant parameters. I need to create a new Instance of SomeFancyCollection but it won't compile because of the Path dependent types. When I pull all the stuff like Node and Branch out of the class i have to declare the root paramter as private[this] and Iam also not able to get a new Instance of the class. My code looks like:

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

In your code, there is no obvious reason to make Node and Leaf nested path-dependent classes. Just make them covariant stand-alone classes, then all the problems with path-dependence go away:

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)
  }
}

If you don't want to pollute the namespace, just declare the Node classes package-private, or even hide all those auxiliary implementation-detail classes inside the companion object of SomeFancyCollection:

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] = ???
    }
  }
}