https://gist.github.com/wheaties/6613325

scala-specialized-bug.scala

package scalaz

import Free.Trampoline

object Main extends App{

  def fff = CTree(1,Map(2 -> CTree(3, Map()))).map(identity)
  println(fff)
}

final case class CTree[@specialized(Double) V](root: V,
                               children: Map[Int, CTree[V]] = Map.empty[Int, CTree[V]]){

  def map[A](f: V => A): CTree[A] = walk(this, f).run

  protected def walk[A](tree: CTree[V], f: V => A): Trampoline[CTree[A]] = tree match {
    case CTree(value, childs) if childs.isEmpty => Trampoline.done(CTree(f(value)))
    case CTree(value, childs)                   => step(childs.toList, f) map (CTree(f(value), _))
  }

  protected def step[A](trees: List[(Int, CTree[V])],
                        f: V => A,
                        acc: List[(Int, CTree[A])] = Nil): Trampoline[Map[Int, CTree[A]]] = trees match {
    case (id, head) :: tail => for {
      that <- Trampoline.suspend(walk(head, f))
      remainder <- Trampoline.suspend(step(tail, f, (id, that) :: acc))
    } yield remainder
    case Nil => Trampoline.done(acc.toMap)
  }
}