// An immutable version of Signal[A]. Based on the concept of state transformations.

case class SignalImmutable[A](value: A, listeners: Seq[Function[A, Unit]] = Seq())

object SignalImmutable {
  def set[A](newValue: A)(signal: SignalImmutable[A]): SignalImmutable[A] = {
    signal.listeners.foreach(_(newValue))
    signal.copy(value = newValue)
  }

  def get[A](signal: SignalImmutable[A]): A = signal.value

  def listen[A](listener: Function[A, Unit])(signal: SignalImmutable[A]): SignalImmutable[A] = {
    signal.copy(listeners = signal.listeners :+ listener)
  }
}

object Foo extends App {
  // New instances after every state change. Ugly, yes.
  val sig = SignalImmutable(1)
  val sig2 = SignalImmutable.listen({v: Int => println(v)})(sig)
  val sig3 = SignalImmutable.set(5)(sig2)
}