Recently I became intrigued with something Seb Rose said on his blog about ‘recycling’ tests. He talks about first producing a test for a ‘low fidelity’ version of the solution, and refining it as you learn better what the solution should look like. In a follow-up post he deals with some criticisms that other posters had of the technique, but actually seems to agree with Alistair Cockburn, that it’s probably not important enough a technique to need a name. I disagree, it’s a technique I use a lot, although most often when using an approval testing approach. I prefer to call it simply iterative development. A low fidelity version of the output that is gradually improved until the customer/product owner says “that’s what I want” is iterative development. It’s a very natural fit with approval testing – once the output is good enough to be approved, you check it in as a regression test that checks it never changes. It’s also a very natural fit for a problem where the solution is fundamentally visual, like printing a diamond. I also find it very helpful when the customer hasn’t exactly decided what they want. In this kata, it’s not such an issue, but in general, quickly putting out a low-fidelity version of what you think they want and then having a discussion about how to proceed can save you a lot of trouble.

The other posters seemed to be advocating a TDD approach where you find ‘universal truths’ about the problem and encode them in tests, so you never have to go back and revisit tests that you made pass earlier. In order to take small steps, you have to break down the problem into small pieces. Once you have identified a piece of the problem and solved it, it should stay solved as you carry on to the next piece. That seems to be what I would call ‘incremental’ development.

There’s a classic explaination of the difference between iterative and incremental that Jeff Patton came up with a few years ago using the Mona Lisa painting. It’s a good explaination, but I find experiencing abstract concepts like this in an actual coding problem can make a world of difference to how well you can reason about and apply them. So I thought it would be interesting to look at these two approaches to TDD using the Diamond Kata.

I have a regular coding dojo with my team these days, so a few weeks ago, I explained my thinking about incremental and iterative, showed them Jeff Patton’s picture, and asked them to do the kata one way or the other so we could compare. I probably didn’t explain it very well, because the discussion afterwards was quite inconclusive, and looking at their code, I didn’t think anyone had really managed to exclusively work one way or the other. So I decided to try to force them into it, by preparing the test cases in advance.

I came up with some starting code for the exercise, available here. I have two sets of unit tests, the first with a standard incremental approach, where you never delete any test cases. The second gets you to ‘recycle’ tests, and work more iteratively towards the final solution. In both cases, you are led through the problem in small steps. The first and last tests are the same, the difference is the route you take in between.

When I tried this exercise with my team, it went a lot better. I randomly assigned half the pairs to use the ‘iterative’ tests, and the rest to use ‘incremental’ tests. Then after about 45-55 minutes, I had them start over using the other tests. After another 45 minutes or so I stopped them and we had a group discussion comparing the approaches. I asked the ‘suggested questions for the retrospective‘ I’d prepared, and it seemed to work. Having test-driven the solution both ways, people could intelligently discuss the pros and cons of each approach, and reason about which situations might suit one or the other.

As Seb said, ‘recycling tests’ is a tool in your developer toolbox, and doing this kata might help you understand how to best use that tool. I’d love to hear from you if you try this excercise in your coding dojo, do leave a comment.


  1. Johannes Brodwall says:

    I’m quite excited about the diamond kata problem myself ever since Jon Jagger introduced me to it a few years ago.

    But I keep worrying about the problem in regards to TDD.

    If you envision the “domain” as a coordinate system with the origin in the center of the diamond, this opens up the possibility of a brilliant solution (especially pleasing in Ruby).

    Problem is: I am still trying to find a sequence of tests that would take you there.

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