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;

  }

}