by godelski | 2026-03-11 18:50:33+00:00
> But maybe you're separating automated testing and TDD as two separate concepts?

I hope it's clear that I am given my content and how I stress I write tests. The existence of tests do not make development TDD.

The first D in TDD stands for "driven". While my sibling comment explains the traditional paradigm it can also be seen in an iterative sense. Like just developing a new feature or even a bug. You start with developing a test, treating it like spec, and then write code to that spec. Look at many of your sibling comments and you'll see that they follow this framing. Think carefully about it and adversarially. Can you figure out its failure mode? Everything has a failure mode, so it's important to know.

Having tests doesn't mean they drive the development. So there's many ways to develop software that aren't TDD but have tests. The important part is to not treat tests as proofs or spec. They are a measurement like any other; a hint. They can't prove correctness (that your code does what you intend it to do). They can't prove that it is bug free. But they hint at those things. Those things won't happen unless we formalize the code and not only is that costly in time to formalize but often will result in unacceptable computational overhead.

I'll give an example of why TDD is so bad. I taught a class a year ago (upper div Uni students) and gave them some skeleton code, a spec sheet, and some unit tests. I explicitly told them that the tests are similar to my private tests, which will be used to grade them, but that they should not rely on them for correctness and I encourage them to write their own. The next few months my office hours were filled with "but my code passes the tests" and me walking students through the tests and discussing their limitations along with the instructions. You'd be amazed at how often the same conversations happened with the same students over and over. A large portion of the class did this. Some just assumed tests had complete coverage and never questioned them while others read the tests and couldn't figure out their limits. But you know the students who never struggled in this way? The students who first approached the problem through design and even understood that even the spec sheet is a guide. That it tells requirements, not completeness. Since the homeworks built on one another those students had the easiest time. Some struggled at first, but many of them got the right levels of abstraction that I know I could throw new features at them and they could integrate without much hassle. They knew the spec wasn't complete. I mean of course it wasn't, we told them from the get go that their homeworks were increments to building a much larger program. And the only difference between that and real world programming is that that isn't always explicitly told to you and that the end goal is less clear. Which only makes this design style more important.

The only thing that should drive the software development is an unobtainable ideal (or literal correctness). A utopia. This prevents reduces metric hacking, as there is none to hack. It helps keep you flexible as you are unable to fool yourself into believing the code is bug free or "correct". Your code is either "good enough" or not. There's no "it's perfect" or "is correct", there's only triage. So I'll ask you even here, can you find the failure mode? Why is that question so important to this way of thinking?