[kmavm]

You're asking about Hack, and the only response I see so far is about HHVM. So!

I've seen Hack used in a multi-tens-of-MLOC codebase to gradually insert types. It made huge differences in the kinds of changes that were possible; you can, e.g., rename a class or method, or change the order of its arguments, with confidence comparable to that in a C++ codebase. Most developers hack-ified everything they could get their hands on, and did all new work in Hack, without any external encouragement.

[biesnecker]

Yup, this. I work on (the original?) multi-tens-of-MLOC PHP-cum-Hack codebase, and writing new code is every bit as safe type-wise as C++, with none of the compile time. It's pretty great.

Dealing with the parts that aren't Hackified, though, is horrific. Complex codebases with no types are just ridiculous.