Object Mentor Blog: Rich Hickey on Testing

Rich Hickey on Testing

Dean Wampler — Fri, 05 Jun 2009 12:40:00 -0500

It was an interesting week at JavaOne, with lots of talks and hallway discussions about new languages on the JVM. One of those languages is Clojure.

Rich Hickey, the creator of Clojure, gave a talk at the Bay Area Clojure User Group Wednesday evening. During the Q&A part, he said that he’s not big on writing tests, although he always runs the tests that other people have written before he commits changes.

Of course, there are many people, including us Object Mentors, who consider TDD to be an essential part of professional software development. Obviously not everyone agrees. James Coplien has been another critic of this view.

We should never accept any dogma. Why is TDD considered important? What does it purport to do? TDD provides two important benefits.

Driving the design.
Building a suite of automated regression tests.

So, if you can satisfy both requirements without TDD, then technically you don’t need it. In Rich’s case, he said he spends a lot of time thinking about what he’s going to do before he does it. In this way, he satisfies the first requirement, driving the design. I had a spirited discussion with some Thoughtworkers afterwards and Ola Bini said what a lot of us think, “I do that thinking by writing tests.” I’ll freely admit that when I am really experimenting with ideas, I might just write code, but once I know how to proceed, I return to the beginning and test drive the “production” code.

Rich also made an off-hand comment that if he screws something up, he’s got thousands of users who will let him know! That ex post facto testing, along with the Rich’s own devotion to doing high-quality work, does a good job of handling regressions.

But Rich mentioned something else that is also very important. In a functional language, where values are immutability and mutable state is handled in specific, principled ways, regressions don’t happen nearly as often. Clojure has one of the most deeply thought out approaches for handling state, which is the genius of Clojure.

I asked Rich how long he worked on Clojure before releasing it to the world. He spent about 2 1/2 years, much of that time working exclusively on Clojure (and eating through his savings). When he finally “announced” it, his “marketing” consisted of one email to some friends in the Common Lisp community. The rest was viral, a testament to the justified excitement Clojure has generated.

For me, I’ll probably always do my design thinking through tests, especially when I’m writing code in imperative languages, like Java and Ruby. I’ll continue to encourage my clients to use TDD, because I find that TDD is the most productive way to achieve high quality. I want the safety net of a good test suite. I’m also writing more and more of my code in a functional style, with minimal side effects and mutable data. You should, too.

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