!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);
}

Began life as a copy of #1012583