I did not expect that someone actually does CDD ;)
Honestly, integration tests are great but much too expensive to test all code path. I actually love unit tests, they are very fast and give you immediate feedback in you development cycle. Hundreds of test run in a matter of seconds. You will never achieve that with integration tests.
The problem with unit tests is that even 100% test coverage do not mean you have cought most bugs. A decent type system cuts down the surface of error by a lot.
I tend to disagree about cost. An integration test is just a unit test that tests multiple parts together. For me this means testing a whole component at once. Property bases testing helps a lot, because you can have a lot more tests than what you would write otherwise. For example for a form, you would test if inputting random data in the form gets verified and saved correctly. Then you generate 100 tests that will check that for you. In CI you can even run more to be sure.
The really expensive tests are end-to-end tests, where you spin up a database fill it with data and just run the tests by scripting interactions with your UI.
There are two different types of bugs: unexpected behavior and wrong behavior (in a functional way).
The former may result in things like NPEs in languages like Java. That is definitely something that could be prevent in stronger typed languages like Haskell eg. however, the second type could still happen and then could happen in each and every line of code. Sure, you can write integration test but it’s much harder to cover all expenses edge cases than in unit tests. Using CI is obviously very important but mostly solves the problem of “works kn my machine”. It’s definitely way to slow to use it while implementing a feature. Then, you want feedback loops of 500ms-1s.
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