sclass OKOrError<A> implements IF0<A> {
  A value;
  gettable Throwable error;
  
  *() {}
  *(A *value) {}
  *(bool dummy, Throwable *error) {}
  
  bool ok aka isOK() { ret error == null; }
  
  toString {
    ret ok()
      ? "ok: " + str(value)
      : "Error: " + str(error);
  }
  
  public A get() { ret !ok() ? null : value; }
  
  public A getMandatory() { if (!ok()) throw rethrow(error); ret value; }
}