Scala atom

Atom.scala

/*
    atoms
    ------------------------------------
    non local lazy val
    "write once var"
    externally pure, interanally mutable
    remote attributes
*/
class Atom[A] {
  private var value: A = _
  private var isSet = false
  private var isForced = false
    
  protected def populate(){
    sys.error("cannont self-populate atom")
  }
    
  def update(a: A) {
    if (!isSet || !isForced){
      value = a
      isSet = true
    }
  }
    
  def apply(): A {
    isForced = true
       
    if (isSet) {
      value
    } else {
      populate()
            
      if (!isSet) {
        sys.error("value not set")
      }
      value
    }
  }      
}