Rose-tree in Scala, demonstrating comonad implementation

Tree.scala

case class Tree[A](root: A, forest: List[Tree[A]]) {
  def map[B](f: A => B): Tree[B] =
    coflatMap(t => f(t.copoint))

  // categorical dual to canonical "flatMap"
  def coflatMap[B](f: Tree[A] => B): Tree[B] = {
    import Tree.unfoldTree
    unfoldTree(this, (a: Tree[A]) => (f(a), a.forest))
  }

  // categorical dual to canonical "flatten"
  def duplicate: Tree[Tree[A]] =
    coflatMap(identity)

  // identity for coflatMap
  def copoint: A =
    root
}

object Tree {
  // caution: mutual recursion
  def unfoldForest[A, B](x: List[A], f: A => (B, List[A])): List[Tree[B]] =
    x map (unfoldTree(_, f))

  // caution: mutual recursion
  def unfoldTree[A, B](x: A, f: A => (B, List[A])): Tree[B] =
    f(x) match {
      case (b, as) => Tree(b, unfoldForest(as, f))
    }
}