Attempt at something to constrain setting of var. Still requires a significant amount of discipline to ensure var does not escape, but at least you can make sure (potentially large) parts of your system can not set the var. Not quite, set once, but if you have a specific flow it can sometimes help. Can be extended to ensure var is only read when…

Phantom.scala

object IsEmpty

class Holder[A, B](private var value: A) {
  def set(a: A)(implicit ev: B =:= IsEmpty.type) {
    value = a
  }
}

object Holder {
  def empty[A]: Holder[A, IsEmpty.type] =
    new Holder[A, IsEmpty.type](null.asInstanceOf[A])

  def full[A](a: A): Holder[A, Nothing] =
    new Holder[A, Nothing](a)
}

object Example extends App {
  def somePartOfSystem(h: Holder[Int, _]) {
    // doesn't compile
//    h.set(1)
  }

  def somePartOfSystemThatKnowsItIsNotSet(h: Holder[Int, IsEmpty.type]) {
    // does compile
    h.set(1)
  }

  somePartOfSystem(Holder.empty[Int])
  somePartOfSystemThatKnowsItIsNotSet(Holder.empty[Int])

}