TDDbE: Addition, Finally

Beck introduces the idea of rewriting the TODO list as both a warmup exercise, and as permission to clean up the little items first rather than copying them to the new list.

I'm not sure how to write the story of the whole addition, so we'll start with a simpler example

Again, the lesson of shifting to smaller steps as a counter measure to uncertainty; accompanied by my usual interjection that baby-footin' is optional, not mandatory.  Have the skill to work at multiple cadences and shift between them, then use the cadence that is appropriate at the time.

I hope you will see through this how TDD gives you control over the size of the steps.

Vindication!

TDD can't guarantee that we will have flashes of insight at the right moment.  However, confidence-giving tests and carefully factored code give us preparation for insight, and preparation for applying that insight when it comes.

That said, I think that testSimpleAddition really goes sideways.  I think there are two flaws here: first, that the metaphors aren't very good.  Second, the experimenting with the metaphor is something that should emerge during refactoring, not during the red task.

What we should be doing is taking the metaphor we understand (adding dollars to dollars), and making that work, then refactoring the underlying implementation to find the working metaphor (encoding into the design our improved understanding of the proper way to think), then lifting the new metaphor into the published API and finally (assuming it makes sense) sunsetting the dollars to dollars metaphor.

In other words:

for each desired change, make the change easy (warning: this may be hard), then make the easy change -- Kent Beck, 2012

This is not a fair fight: I'm writing 10+ years after Beck shared this approach, whereas the work I'm criticizing was written ten years before.  If he waited to write the book until he understood everything thoroughly, the book would not exist for me to critique.

Part of the point of this review exercise is to rediscover where we were, in the expectation that we will find past practices that should be improved.