The announcement post says the actual type-checking happens in a persistent server that watches for filesystem changes. That sounds pretty close to continuously running a linter. It also says "without breaking things", so I'm a little fuzzy on whether badly-typed code will actually execute or not.

For that matter, can you call a typed function from an untyped one? Or, worse, a typed method? If the typing is purely static, there's no way to know the method you're calling is actually typed, so there's no actual guarantee that it receives the types it's declared unless your entire program is typed. It doesn't seem like a very strong guarantee if both the caller and the callee have to opt into the typing.

If you're looking for a tool that exists right now, you either have an existing codebase and can't port it to Hack if it's not already PHP anyway (for the same reason Facebook couldn't port away from PHP), or you're starting from nothing and could just use a statically-typed language in the first place.

I don't know if I'd even be excited about the prospect of optional static typing in Python. (It hasn't gotten me interested in Dart, for example.) I'd kinda rather see the effort poured into something that could do static duck-typed analysis/inference, e.g. balk if I pass an argument that could be a non-string into a function that tries to call `.startswith` on it. (Ah, but maybe it could theoretically be a string or None, and I only know it isn't None for reasons the type system can't see, and now I hate the type system.)

I didn't say it was a trivial problem. I just don't feel excited by the solution.