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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Sun, 15 May 2005

Expert code

I've been working on the chapter in Scripting for Testers where readers start defining classes. As the story behind the following text begins, I have to figure out how to make a failing test pass.

Here's a thought about a solution: After pushing the new context line onto potential_context, I could check if it's too long. If so, I should shorten it. That would look like this:

def unusual_lines(line_array)
  return_value = []
  potential_context = []
  line_array.each { | one_line |
    if unusual?(one_line)
      return_value += potential_context
      potential_context = []
      potential_context.shift if potential_context.length > 5

It would pass the test, but am I happy with it?

No. I fear if statements. I especially fear if statements within other if statements. They're too hard to get right and too confusing to read. Fortunately, there are often ways to set things up so that the code would always make the same decision---and hence doesn't need an if. Here, there are two possibilities: make it so that the code always throws away the first element or such that it never has to throw away any element.

When I thought about it that way, I realized that if potential_context always had five elements, the code would always shift the first one away...

Then there's text on how to make that work. I'm teaching an old idea: replace explicit runtime decisions with appropriate setup.

Just after writing that, I flashed on something that Gary Klein writes in Sources of Power. His research group was preparing to interview expert firefighters about their decision-making.

We asked the commander to tell us about some difficult decisions he had made.

"I don't make decisions," he announced to his startled listeners. "I don't remember when I've ever made a decision." [...]

He agreed there were options, yet it was usually obvious what to do in any given situation. He insisted he never [compared options]. There just was no time. (pp. 11-12)

It was not that the commanders were refusing to compare options; rather, they did not have to compare options. [...] [They] could come up with a good course of action right from the start. [...] Even faced with a complex situation, the commanders would see it as familiar and know how to react.

The commanders' secret was that their experience let them see a situation, even a nonroutine one, as an example of a prototype, so they knew the typical course of action right away. (p. 17)

[T]here are times for deliberating about options. Usually these are times when experience is inadequate and logical thinking is a substitute for recognizing a situation as typical. [...] Deliberating about options makes a lot of sense for novices [...] (p. 23)

What makes someone an expert is the ability to act appropriately without resorting to conventional rational thought, except perhaps for after-the-fact explanations. (And as both Sources of Power and oodles of old work in expert systems show, experts often find it difficult or impossible to explain the reasons behind their actions, even to themselves.)

If the parallel isn't completely bogus, expert code will have few if statements and be hard to debug.

## Posted at 16:14 in category /coding [permalink] [top]

About Brian Marick
I consult mainly on Agile software development, with a special focus on how testing fits in.

Contact me here: marick@exampler.com.




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