Exploration Through Example

Example-driven development, Agile testing, context-driven testing, Agile programming, Ruby, and other things of interest to Brian Marick

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Mon, 08 Aug 2005

Counterexamples

In my thinking about tests as examples, I've been thinking of them as good examples:

The right system behaves like this. And like this. And don't forget this.

But what about counterexamples?

A system that did this would be the wrong system. And so would a system that did this.

There's some evidence that differences are important in understanding.

• The linguist Ferdinand de Saussere taught that meaning of the word "boat" isn't "a small vessel for travel on water." Rather the meaning of "boat" is generated by contrast with other words like "ship", "raft", "yawl", "statue of a boat", etc. (Derrida would later go on to make perhaps too much of the fact that there's no limit to the recursion, since all those other words are also defined by difference.)

• In the early '70s, Patrick Winston wrote a program that learned the concept of "arch" from a series of examples and "near misses". My copy of his book has long since gone to the place paperclips, coathangers, and individual socks go, so I can't check if the near-miss counterexamples merely improved the program or were essential to its success.

• My kids are now of the age (nine and ten) where they ask for dictionary-like definitions of words. But when they were younger, they more obviously learned by difference: they'd point at something, give the wrong name, then accept the correction without further discussion. ("Duck." "No, that's a goose." "Dog." "Yes, a nice dog.") Presumably the counterexamples helped with that amazing burst of vocabulary young kids have.

So what about those times when the programmer proudly calls the product owner over to show the newest screen and watches her face fall just before she says, "That's not really what I had in mind"? Or those times when a small group is talking about a story and a programmer pops up with an idea or a supposed consequence that's wrong? That's an opportunity to - briefly! - attend to what's different about the way two people are thinking.

Does anyone make explicit use of counterexamples? How? What have you learned?

## Posted at 20:05 in category /ideas [permalink] [top]

Link: Crispin on test-first customer tests

Lisa crispin has a nice, short article on how her team uses business-facing tests to drive development. A couple of points I particularly like:

• Good examples of questions someone (often the tester) should ask about even a simple story.
• An emphasis on just-in-time test creation.
• How the need to support tests encouraged business logic to go in the right place.

## Posted at 11:07 in category /agile [permalink] [top]