This is an argument for type annotations to be integrated into every dynamically typed language, rather than tacked on via an external tool.
I have, and I wanted to like it. On its face it seems like it should be a lot better than Go's--after all, it supports generics and union types! But it falls over in trivial cases, like recursive types (i.e., there's no way to model tree structures such as JSON or linked lists). A few other hard/impossible/confusing things come to mind:
1. How do you declare a typevar for a certain scope. If I define a type parameter `T` for function `foo`, I only want `T` to be scoped to `foo`. I don't want the type checker getting confused with `T`s for other functions/classes/etc. 2. What is the signature for a function that takes args/kwargs? 3. It straight up doesn't work with popular libraries like SQLAlchemy (last I checked, these were simply not supported because the likes of SQLAlchemy are "too magical"--this is a fair take, but frustratingly limiting for users).
These are just a few because my memory is poor, but I run into these sorts of things by the dozens every time I try to use mypy. It's just not ready for prime time. Go's type system is limiting, but its limitations are much more predictable and even less limiting (it turns out recursive types and poor-man's union types are quite a bit better than first-class, non-recursive union types, for example).
