sclass OKOrError<A> implements IF0<A> {
  A value;
  gettable Throwable error;
  
  *() {}
  *(A *value) {}
  *(bool dummy, Throwable *error) { assertNotNull(error); }
  
  bool ok aka isOK() { ret error == null; }
  bool isError() { 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; }
  
  static <A> OKOrError<A> ok(A a) { ret new OKOrError(a); }
  static OKOrError error(Throwable error) { ret new OKOrError(false, error); }
  
  void setValue(A value) {
    this.value = value;
    error = null;
  }
  
  void setError(Throwable error) {
    this.value = null;
    this.error = error;
  }
  
  void copyFrom(OKOrError<A> x) {
    value = x.value;
    error = x.error;
  }
}