There's no way to prove that you proved the right thing, unfortunately. And, of course, your proof may not be correct. (Proof checkers can help, and I would hav

There's no way to prove that you proved the right thing, unfortunately. And, of course, your proof may not be correct. (Proof checkers can help, and I would have confidence in the correctness of a type-check as proof.) Only having tests is "fine" for most circumstances because, unfortunately, proofs are intractable for most systems. During a software engineering talk I recently attended, an interview candidate stated that the largest proved-correct code base is on the order of 10,000 lines of code. Most people don't have the resources to do even that. But, philosophically, I always want tests for the same reason that I run experiments: I want empirical evidence, not just reasoning. Empirical evidence increases my confidence that we have accounted for what we think we have.