Past exam of the year 1 of the computer science course of the University of Oxford / 2014 / Imperative programming / 7 / d / Solution

The rightmost trait intercepts insertion first. In A, capitalization runs before counting, so the counter receives uppercase A and does not increment the lowercase count. In B, counting receives lowercase a, delegates to capitalization, and then increments the original lowercase argument's counter. Both concrete documents store uppercase A.

The exact output isThis illustrates an order-sensitive stackable trait modification. In the second composition the cached count describes intercepted insertion arguments, and no longer the actual lowercase characters in the stored document. For example, deleting its uppercase A would not decrement the lowercase counter. A counter required to track actual contents under arbitrary transforming mixins would need a stronger composition contract or would inspect the stored character after insertion; such a change would alter the intended behavior of this example.