> it seems to run fine until you go over a bump, then it kills you That's half of duck-typing. You're missing the exception response where safety is enforced.

> it seems to run fine until you go over a bump, then it kills you That's half of duck-typing. You're missing the exception response where safety is enforced. A static type system fails with the wrong type. A duck-typed system responds with something as sensible as it can reason with what it was handed. Must often, that's a hard failure because most code is brittle and doesn't need the robustness (i.e. don't build what you aren't going to need).