Seibel: Though your job now doesn’t entail a lot of programming you still write programs for the essays on your web site. When you’re writing these little programs, how do you approach it?

Norvig: I think one of the most important things is being able to keep everything in your head at once. If you can do that you have a much better chance of being successful. That makes a small program easier. For a bigger program, you need extra tools to be able to handle that.

It’s also important to know what you’re doing. When I wrote my Sudoku solver, some bloggers commented on that. They said, “Look at the contrast—here’s Norvig’s Sudoku thing and then there’s this other guy, whose name I’ve forgotten, one of these test-driven design gurus. He starts off and he says, “Well, I’m going to do Sudoku and I’m going to have this class and first thing I’m going to do is write a bunch of tests.” But then he never got anywhere. He had five different blog posts and in each one he wrote a little bit more and wrote lots of tests but he never got anything working because he didn’t know how to solve the problem.

I actually knew—from AI—that, well, there’s this field of constraint propagation—I know how that works. There’s this field of recursive search—I know how that works. And I could see, right from the start, you put these two together, and you could solve this Sudoku thing. He didn’t know that so he was sort of blundering in the dark even though all his code “worked” because he had all these test cases.

Then bloggers were arguing back and forth about what this means. I don’t think it means much of anything—I think test-driven design is great. I do that a lot more than I used to do. But you can test all you want and if you don’t know how to approach the problem, you’re not going to get a
solution.

Seibel: So then the question is, how should he have known that? Should he have gone and gotten a PhD and specialized in artificial intelligence? You can’t know every algorithm. These days you have Google, but finding the right approach to a problem is a little different than finding a web framework.

Norvig: How do you know what you don’t know?

Seibel: Exactly.

Norvig: So I guess it’s two parts. One is to recognize that maybe there is a known solution to this. You could say, “Well, nobody could possibly know how to do this, so just exploring randomly is as good as everything else.” That’s one possibility. The other possibility is, “Well, probably somebody does know how to do this. I just don’t know what the words are for it, so I have to discover those.” I guess that’s partly just intuition and saying, “It seems like the kind of thing that should be in the body of knowledge from AI.” And then you have to figure out, how do I find it? And probably he could’ve done a search on Sudoku and found it that way. Maybe he thought that was cheating. I don’t know.

Coders at Work – Page 301 – Peter Norvig