slow_add is not simple. It reimplements the Python + operator, which is quite complex, specially if you look at it from a type system perspective. As noted, its

slow_add is not simple. It reimplements the Python + operator, which is quite complex, specially if you look at it from a type system perspective. As noted, its implementation is very core to the language and it has several edge cases. That's not usually what you do when you create a function. Rather, you have very specific types that you support. It's only "simple" because adding things together is a basic operation. Even if you write a function that encapsulates some funky arithmetic. You will probably be using more operations, to the point that anything but numbers will really make sense. That said, yes, adding types to an API that wasn't implemented with a typing system in mind can be challenging. But I feel like you get a better API at the end of that exercise.