Ass-Backwards Compatibility

Wangohan, a monk of the Spider Clan, stumbled upon a bug in a utility class provided by the Laughing Monkey Clan. He called upon the junior monk who was its author.

“Your last refactoring of isHexNumber was incorrect,” said Wangohan. “Although there is no documentation, the name implies that it will only match strings of one or more hex digits. Yet your regular expression uses no anchors and the wrong quantifer. You will return true for any string in which zero or more hex digits are present anywhere—which is to say, any string at all.”

“I ensured that the existing unit tests passed,” protested the junior monk. “Null is rejected, and legitimate hex numbers are accepted.”

Wangohan scowled. “I tell you that a cardboard box is not a fit meal for the Emperor’s daughter, and you reply, a goat did not choke on it.”

Wangohan called on the senior monk who tended the shared library and explained the problem.

“We cannot change the implementation,” said the senior monk. “This method is now used throughout the Temple. If it suddenly rejects strings it once accepted, disaster might ensue.”

Wangohan pounded his fist on the monk’s desk. “I tell you that a cardboard box is not a fit meal for the Emperor’s daughter, and you reply, she may dislike the taste of lamb and figs.”

Wangohan called upon the head abbot of Laughing Monkey and explained the problem.

“I will attend to the matter,” sighed the abbot. “There must be a solution which will satisfy all parties.”

That evening Wangohan received an email from the abbot, stating that the method should now perform exactly as expected. Wangohan brought up the code and found it to be completely unchanged, save for a new comment at the top:

  /**
   * isHexNumber returns true for any string
   * in which zero or more hex digits are present
   * anywhere.
   */

Wangohan’s head fell to his desk. “Clearly a cardboard box must be a fit meal for the Emperor’s daughter, for here it is on the Imperial menu.”