Question

Retry

Say you have a function primitiveMultiply that, in 50 percent of cases, multiplies two numbers, and in the other 50 percent, raises an exception of type MultiplicatorUnitFailure . Write a function that wraps this clunky function and just keeps trying until a call succeeds, after which it returns the result.

Make sure you handle only the exceptions you are trying to handle.

The locked box

Consider the following (rather contrived) object:

var box = {
  locked: true,
  unlock: function() { this.locked = false; },
  lock: function() { this.locked = true;  },
  _content: [],
  get content() {
    if (this.locked) throw new Error("Locked!");
    return this._content;
  }
};

It is a box with a lock. Inside is an array, but you can get at it only when the box is unlocked. Directly accessing the _content property is not allowed.

Write a function called withBoxUnlocked that takes a function value as argument, unlocks the box, runs the function, and then ensures that the box is locked again before returning, regardless of whether the argument function returned normally or threw an exception.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.