> Tests do not ensure that your functions are correct; they ensure that you are alerted when their behavior changes.
I agree with that part and I am not against tests, just the idea of writing tests first.
> helps you design good interfaces
I am sure plenty of people will disagree but I think testability is overrated and leads to code that is too abstract and complicated. Writing tests first will help you write code that is testable, but everything is a compromise, and to make code more testable, you have to sacrifice something, usually in the form of adding complexity and layers of indirection. Testability is good of course, but it is a game of compromises, and for me, there are other priorities.
It makes sense at a high level though, like for public APIs. Ideally, I'd rather write both sides at the same time, as to have a real use case rather than just a test, and have both evolve together, but it is not always possible. In that case, writing the test first may be the next best thing.
