We should be careful to appreciate the “types” in Julia for what they are at their essence: a way to direct dispatch of methods. Certainly the Julia compiler reasons about types in order to generate efficient code. But Julia types also affect the meaning of programs, not just the performance. In fact, the presence or absence of “type annotations” on arguments in a method definition don’t affect the code that that method generates. It simply affects whether that method gets dispatched to or not for a given function call.

I think it’s helpful to consider that this particular use of types is morally different from when type annotations are used to 1. Document intent, 2. Run programs faster, 3. Reason about correctness statically.