Tuesday, October 1, 2019

TDD: Safety in Numbers - a Bowling Game Adventure

Last night, I decided to work through a bowling game exercise, but it didn't quite turn out as I had expected.

The goal was eliminate duplication; how much intention revealing code could I introduce before moving onto the second test?

As suggested by Uncle Bob, I started with the degenerate case:

It is, of course, trivial to get this test passing. We simply hard code the required answer into the score method.

The step that I expected to follow was to immediately start introducing domain concepts, like frames, into the production code while there was still but a single test constraint in place.

And it was a very uncomfortable experience - I realized fairly quickly that a simple pass signal wasn't enough to give me confidence that the match I was introducing was actually manipulating the figures correctly -- not enough to give me confidence that I wasn't introducing silly fence post errors.

After discarding the work and contemplating the ceiling for a time, I decided that I was having trouble because zero is the additive identity -- I couldn't look at my actual result and deduce how many numbers had been added together, because 10, 20, 100 zeros all sum to the same amount.

This evening, I tried a different initial test:

The results are much better - fence post mistakes change the observable behavior in this circumstance, and are therefore easy to catch. The deviations from the expected results give an immediate hint at the error. We know from taking small steps which edit introduced a fault, but the distinct behaviors mean that it is easy to recognize the precise nature of the fault.

In this evenings ending, I managed to get all the way to make the next change easy: using the gutter game as my second test produced a trivial pass, because the faults I introduced during refactoring had already been detected and mitigated.

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