[hyp0]

I strongly agree with module typing (and being more relaxed within). Because of caller dependence on modules, the difficulty of changing static types becomes a benefit. Plus, they need to be documented anyway; and tooling support helps use they as if they were primitives.

As others have said, I love the approach of inference giving static type benefits, for free. If you get tooling support, at no extra work, why not adopt it?

But static types can be a hard sell for JS programmers. What sort of reception has it gotten inside facebook?

[avik]

Still early days but the reception has been strongly positive. We may be a biased bunch but we like our code mostly statically typed with the flexibility provided by dynamically typed languages. A large part of this culture is due to the immense internal success of Hack (http://hacklang.org/).