WriteOnce

class WriteOnce[T] {
  private val ref = new java.util.concurrent.atomic.AtomicReference[Option[T]](None)
  def init(t: T) { if(!ref.compareAndSet(None, Some(t))) sys.error("Already written"); }
  def get: Option[T] = ref.get
}

scala> val w = new WriteOnce[Int]
w: WriteOnce[Int] = WriteOnce@215d55cb

scala> w.get
res4: Option[Int] = None

scala> w.init(42)

scala> w.get
res6: Option[Int] = Some(42)

scala> w.init(43)
java.lang.RuntimeException: Already written