After my recent posting about what some of the folks I interviewed for Coders at Work had to say about unit testing and TDD, I’ve seen comments various places from people who seem to think that I must never have really tried TDD or that I think unit testing is not useful. Neither of which is really true.

In the last two jobs I had before I quit full-time work to write books and do a bit of consulting I was, among other things, the guy who wrote the test framework used for testing all our Java code. My first framework, which I wrote in the early days of Weblogic, predated JUnit but was similar in intent—my goal was to make it as easy as possible for developers to write fine grained tests and to pinpoint as precisely as possible the cause of test failures so that we could run tests in a continuous build-and-test system that tested every check-in and reported the results. (”Build and test monkeys” we called them at Weblogic and a stuffed monkey made the rounds of the developers’ desks, sitting on the desk of whoever had last broken something.)

In addition to working on the framework itself, I spent a lot of my time at Weblogic trying to figure out how we could have finer-grained tests that would run more quickly and provide easier to track down failures than the rather coarse-grained system tests we had a lot of. (At one point, when I was working on our EJB implementation, I came up with a trick that I’m still a bit proud of: to test our implementation of the state machine implementing the EJB life-cycle, we wrote an EJB that collected a list of tokens for each of the methods called on it by the EJB container and the test, after putting the EJB through its paces, fed that sequence of tokens to a parser generated from the grammar of legal paths through the state machine.)

At my next job, at another start-up founded by one of the Weblogic founders, I was hired early on not only to work on the server part of our product but to be in charge of our software development process. We set up another battery of build-and-test monkeys, using a test framework I had written after leaving Weblogic. And having experienced the benefits of pair programming and lots of testing at Weblogic, I tried to push our process toward something like XP though I don’t think we ever did enough of the practices to really count as an XP shop. Now, with six years of hindsight, I’m still not sure whether I’m glad or sad about that.

In my role as a developer at Kenamea, one of the biggest projects I worked on was implementing a transactional object store. Other than some proof of concept code I wrote by myself, that part of the system was almost entirely pair programmed and was possibly one of the most extensively unit tested parts of our product. I don’t actually recall whether we ever did test-first programming on it but my pair and I tried to be quite strict about writing unit tests for all the code we wrote. And the tests were definitely valuable. They gave us—as unit testing advocates always promise—confidence to dramatically refactor things when necessary and they told us when we had slipped up and broken something. On the other hand, the system was multithreaded which, as Joshua Bloch points out in Coders, always makes things much harder to test. Often the tests were sufficient to show the presence of a bug without pinpointing its location; hours or days of hard thinking were often required to track down those concurrency bugs and when we found them it was usually not at all clear that there were any unit tests we could have written that would have uncovered them.

When I quit Kenamea in order to spend some time hacking Lisp, I was still interested in unit testing and TDD. (There’s a reason that one of the first “practical” chapters in Practical Common Lisp is a simple test framework.) Shortly before I started work on PCL I read Kent Beck’s book Test Driven Development and did a few experiments with “pure” TDD which I recorded in a coding diary. The first was an implementation of an algorithm for generating starting positions for Fischer Random Chess, a version of chess where the pieces in the back row are positioned randomly, subject to a few constraints. Here’s the contemporaneous record of my attempt, complete with false starts and other brainos. (The code is in Common Lisp. Lispers may be shocked to see how little Lisp I knew then, a few months before I started work on PCL. And no guarantees that this code exemplifies good Lisp style or good anything else.)

The next problem I tackled with TDD was The “Impossible Book” problem, posed on the Test-driven Development Yahoo! group around that time. Again, I recorded my attempt as I went. I’m not sure these are great examples of TDD in action; I mention them merely as evidence that I have actually spent some time trying out TDD.

My Impossible Book attempt is also, perhaps, relevant to the current discussion since I was up against the same kind of difficultly as Ron Jeffries was while trying to write his Sudoku solver. Namely, I had no real idea how to solve the problem.

However—perhaps because there was no other infrastructure code to distract myself with—I was lucky enough to realize that no amount of testing was going to help me solve a problem I didn’t know how to solve. So instead I decided, as I say in my coding diary, that “TDD is about driving the implementation”. So I went off and read a bit about how to solve the problem and then used TDD to come up with an implementation. In this case I was lucky that there was already a readily accessible write up of a way to solve the problem. If there hadn’t been I would have had to do more research and probably would have had to learn some math in order to figure out how to apply it to the problem at hand. In other words, the thing that was going to speed me up was not trying harder with TDD but recognizing that I needed to step back and try something else.

Since those experiments with TDD, I’ve mostly been writing books so I haven’t been doing much work on production code. Working on my own coding projects, which tend to be exploratory, not very large, and special purpose (i.e. they only have to work for me) I have not found myself inclined to use TDD or even a lot of unit testing. I’m not claiming that that’s due to a principled appraisal of the benefits and drawbacks of TDD or unit tests; it just hasn’t felt like the problems I’ve had writing the software I’ve wanted to write would be fixed by having more unit tests nor that writing test-first would speed up my exploration.

And that, I guess, brings me back to how I was drawn into this conversation in the first place: I think testing, of all kinds, is an important part of serious software development. I think TDD is, at the very least, an interesting way to write software and I suspect that it might be a very good way to write some kinds of software. But I think the claim that TDD always speeds you up is just bunk. It may be that for the kinds of software Uncle Bob and Tim Bray write, in the kinds of organizations where they work, over the kinds of time scales they care about, it really does always speed things up. I’m happy to believe Uncle Bob when he says that he’s seen the benefits of TDD in his own and in others’ work.

But I also think that when Jamie Zawinski says that writing unit tests would have slowed down the first release of Netscape or when Donald Knuth says that he thinks he saved time by writing out TeX in a notebook without any testing at all until he had figured out how the whole program was going to work, those are data points that need to be accounted for, not dismissed with insults about “living in the stone age” and “being willfully ignorant”. Maybe Zawinski and Knuth are wrong about their own experience. Or maybe they were making different trade offs than Uncle Bob and Bray would chose to make. At any rate, I agree with Tim Bray when he says

If you read the comments around this debate, it’s increasingly obvious that we as a profession don’t have consensus around the value of TDD. Many of our loudmouths (like me for example) have become evangelists probably to the point of being obnoxious. But there remains a strong developer faction out there, mostly just muttering in their beards, who think TDD is another flavor of architecture astronautics that’s gonna slow them down and get in their way.

Maybe TDD’s detractors are, as Uncle Bob claims, analogous to the 19th century surgeons poo-pooing the benefits of washing their hands before surgery but I find Uncle Bob’s rhetorical stance of absolute certainty disconcerting and, ironically, anti-persuasive. That is, I thought better of TDD before I read his recent postings about it. But that’s a silly reason to accept or reject a practice that might do me some good. If I go back to writing production code, I’ll certainly resume my own contemplation on the best way to mix testing with development and wouldn’t be surprised if TDD found a place in my own practice.

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