!7

sclass Calculation {

  O f;

  Calculation deferredTo;

  OKOrError result;

  *() {}

  *(O *f) {}

  void calc() {

    if (done()) ret;

    try {

      result = OKOrError_ok(callF(f));

    } catch e {

      Throwable inner = getInnerException(e);

      if (inner instanceof DeferCalculation)

        deferredTo = inner/DeferCalculation.calculation();

      else

        result = OKOrError_error(e);

    }

  }

  bool done() { ret result != null || deferredTo != null; }

  toString {

    S s = "Calculation of " + f + ".";

    if (deferredTo != null) s += " Deferred to: " + deferredTo;

    else if (result != null) s += " " + result;

    else s += " Not run";

    ret s;

  }

  Calculation resolve() {

    Calculation c = this;

    while (c.deferredTo != null) c = c.deferredTo;

    ret c;

  }

}

p {

  Calculation c = new Calculation(f now);

  print(c);

  c.calc();

  print(c);

  c = new Calculation(f fail);

  c.calc();

  print(c);

  c = new Calculation(r { throw DeferCalculation(Calculation(f now)) });

  print(c);

  c.calc();

  print(c);

  c = c.resolve();

  c.calc();

  print(c);

}