Some have said that computers are just rocks we’ve taught to think. Others think computers run on magic blue smoke, and once you let the magic smoke out they’ll never work again. The truth, as usual, is somewhere between the two extremes. It’s not magic, and while arcing 120 VAC to ground across a chip will make a cloud of blue smoke and the chip will ever work again, it’s not magic. And no matter how many MFLOPS a chip can execute, it’s not really doing math. It just lets the electrons flow one way or another through a series of adjustable switches. From the outside though, it does seem like someone cast a spell on some tiny grains of sand (silicon) now the sand is doing math.
Whether it’s magic or good teaching, what does this have to do with Intent, let alone Test Driven Development? The connection is that intent is what drives both. The teaching was driven by the intent to build a machine that can do math quickly and reliably. Over and over again. And of course one of the primary rules of magic is that you have to keep the intent of the spell in mind when you cast it. Whether it’s Harry Potter’s “alohamora”, a djinn’s three wishes, or almost any other example of magic in the literature, it’s the intent behind the spell, not just the words, that defines what the spell operates on and how it works.
And it’s Intent that connects us to TDD. The intent of the tests in TDD is to express what should and should not happen. They’re an explicit expression of our intent for how the API should be used. They’re an explicit expression of what the limits and boundaries of the code are. They express what will work, what won’t, and how you know if it worked or not. And explicit is always better than implicit.
Leaving it implicitly expressed by the definition of the API and hoping users intuit your intent will only cause problems in the end. Hyrum’s Law tells us that, over time, anything users can do, they will do. That turns implicit requirements into explicit requirements as you work to avoid any breaking changes. Flight Simulator was like that. We needed to ensure all of the 3rd party tools and content worked, and with each new version it got a little more difficult to maintain compatibility with all those things that leaked through our interfaces.
Now you know how thinking rocks and the intent of magic are related to software development in general and TDD specifically. But magic has a lot more in common with development than that. After all, according to the literature, with magic, unless you follow the rules exactly things don’t always turn out the way you expected. At best nothing happens at all. At worst, something terrible happens. For more discussion of how the rules of magic also apply to software development, check out this thread from @bethcodes.
And beware the wily fae.