> later one new dev adds one more case and forgets to handle it in every call siteOkay, so you are dreaming of a situation where you allow a new developer who doesn't know anything about how to engineer software unfettered access to your codebase and he goes in and starts mucking about to where he ends up doing things that no sensible human would ever consider acceptable?
I accept you like to live dangerously and have no real care for the product you are building. But, still, you've only covered a limited subset of all the similar things this new developer who doesn't know the first thing about building software can make a mess of. Ocaml just isn't going to cut it.
To reach the full gamut you need, say, a proper dependent type system. But are you really going to throw all your Ocaml code away and start writing Rocq just to deal with this guy you should have never let touch your code in the first place? Probably not. So, what now? Your only practical option is to fire him and find someone who actually understands how to write software. And at that point your tests will catch such forgetfulness just fine.
> No test will catch this 100%
Testing cannot exhaust all cases where the search space is infinite as that requires infinite time to evaluate. In this case, you have exactly 4 states. That is evaluated on the order of nanoseconds. If your tests are not capturing every one of those four permutations, it seems clear that you need to stop letting any random Joe who has never done more than some quick vibe coding anywhere near your code. These are solved problems.
And, hell, even if you did switch to a language with a proper type system, like Rocq, so you could mathematically guarantee all aspects of your program, the shitty programmer will still fail to write theorems that are free of holes. So don't think that even lets you of the hook in hiring those who live by the vibes. There is simply no escaping the need for solid engineering.
When you have that, exhaustiveness checking still remains a useful tool, but you aren't going to forget something and ship broken code to production if you don't have it. This isn't a realistic scenario outside of the contrived. Again, The construct is useful enough that you don't need to oversell it on imagined hypotheticals.