> Also, other than Swift, I don't know of any programming language that let's me explicitly mention if a function could throw an exception.

Checked exceptions in Java. I both like and hate them.

They force the function to be explicit about what exceptions they can raise. If you call a function that raises a checked exception, you must either explicitly handle the exception, or mark that it is propagated up by adding the same exception to the calling method signature.

However, they cause a lot of pain for working with higher order methods (map, filter, flatmap, etc). Because they change the signature of the method (and therefore the interface it can satisfy), you need to use a lot of generic variants of higher order methods to accept them, or like a lot of libraries, you end up writing wrapper functions that convert the checked exceptions into unchecked runtime exceptions, such that they don't modify the method signatures. This then leads to a lot of weird code, and uncaught exceptions at runtime, taking down the application.