“Norvig: I see tests more as a way of correcting errors rather than as a way of design. This extreme approach of saying, “Well, the first thing you do is write a test that says I get the right answer at the end,” and then you run it and see that it fails, and then you say, “What do I need next?”—that doesn't seem like the right way to design something to me. It seems like only if it was so simple that the solution was preordained would that make sense. I think you have to think about it first. You have to say, “What are the pieces? How can I write tests for pieces until I know what some of them are?” And then, once you've done that, then it is good discipline to have tests for each of those pieces and to understand well how they interact with each other and the boundary cases and so on. Those should all have tests. But I don't think you drive the whole design by saying, “This test has failed.”