This is why we invented type systems. No need to examine call chains, just examine input types. The types will not only tell you what assumptions you can make, but the compiler will even tell you if you make an invalid assumption!
If instead of validating that someone has sent you a phone number in one spot and then passing along a string, you can as easily have a function construct an UncheckedPhoneNumber. You can choose to only construct VerifiedPhoneNumbers if the user has gone through a code check. Both would allow you to pluck a PhoneNumber out of them for where you need to have generic calling code.
You can use this sort of pattern to encode anything into the type system. Takes a little more upfront typing than all of those being strings, but your program will be sure of what it actually has at every point. It's pretty nice.
Yep! I have seen so much pushed into a type system that in the end there was hardly any code needed to do validation or scaffolding… to the point where it felt magical
You can enforce them (statically) by other means if you’re determined enough, eg by using lint rules which enforce type-like semantics which the type system itself doesn’t express.
This does rely on the language having a sophisticated-enough type system to be able to extract enough type information for the rules to work in the first place.
True, but there are still documented interface contracts you can program against. The compiler won’t catch violations of the non-type parts, but the requirements are still well-defined with a proper interface contract. It is a trade-off, but so is repeating the same case distinction in multiple parts of the program, or having to pass around the context needed to make the case distinction.
