package.scala

package object {
    implicit class VarTraversablePML[M[AA] <: Traversable[AA],A](val self: Var[M[A]]) extends AnyVal {
    def ++=(more: M[A]) : Unit = {
      self.toOption match {
        case Some(ma) =>
          self := ma.++[A,M[A]](more)
        case None =>
          self := more
      }
    }
  }

  implicit class VarSetPML[A](val self: Var[Set[A]]) extends AnyVal {
    def contains(a: A) : Boolean = {
      self.toOption match {
        case Some(ma) => ma.contains(a)
        case None => false
      }
    }

    def +=(more: A) : Unit = {
      self.toOption match {
        case Some(ma) =>
          self := ma + more
        case None =>
          self := Set(more)
      }
    }
  }

  implicit class VarSeqPML[M[AA] <: Seq[AA],A](val self: Var[M[A]]) extends AnyVal {
    def +=(more: A)(implicit cbf:CanBuildFrom[Nothing,A,M[A]]) : Unit = {
      self.toOption match {
        case Some(ma) =>
          self := ma.:+[A,M[A]](more)
        case None =>
          val builder = cbf()
          builder += more
          self := builder.result()
      }
    }
  }

}

Var.scala

import scala.languageFeature.higherKinds

trait Var[A] {
  def isSet : Boolean
  def :=(a: A) : Unit
  def unset() : Unit
  def toOption: Option[A]
  def assumeSetAndGet : A
}

object Var {
  class Set[A](
    var value: Option[A] = None
  ) extends Var[A] {
    override def isSet = value.isDefined
    override def :=(a: A): Unit =
      value = Some(a)
    override def unset(): Unit =
      value = None
    override def toOption: Option[A] = value
    override def assumeSetAndGet: A = value.get
  }
  def Unset[A] = new Set[A]
}