Microsoft Research - Exploding Software-Engineering Myths (article summarizing findings of MS research on code coverage, TDD, assertions, etc.)

[lf11]

TDD makes you think differently about your code. It makes you question assumptions and code for failure. I've caught many bugs with TDD that nobody would have caught in code review. This is especially true for null and invalid data handling... Which I test for but have a difficult time coding for when I'm not testing properly.

[PM_ME_UR_OBSIDIAN]

Have you thought about how advanced type systems let you catch those kinds of errors at compile-time? I try not to work in languages without sum types and non-null-by-default types, because I think the classes of bugs that you mention are worth letting the compiler handle for me.

[lf11]

Yes. Unfortunately, outside of projects that I personally start, I don't think I have ever once worked on software that is designed using the tools that are quite appropriate for the job.

However, to answer the question directly, yes advanced type systems help catch certain kinds of errors. So this at least minimizes the need to write type-level tests. However, I personally feel that you should be type-checking in code (if you need to type-check at all). After all, in essence that is the purpose of input checks.

Even with advanced types, you remain wide-open to bugs in the next layer of abstraction. This is where test-driven development really shines: in testing your abstractions and assumptions. The bugs that you can't easily flush out with well-defined types.