> type hints which are required (I think) Only in strict mode; in partial or decl mode, you can leave off the annotations. In fact, a large fraction of PHP fil

> type hints which are required (I think) Only in strict mode; in partial or decl mode, you can leave off the annotations. In fact, a large fraction of PHP files are also valid Hack files in partial mode. The article has a decent explanation of the modes, and see http://docs.hhvm.com/manual/en/hack.modes.php http://docs.hhvm.com/manual/en/hack.modes.php for our official documentation. > Technically HHVM can run projects that are a mix of PHP/Hack, but I wouldn't recommend that. Mixing your codebase is worse for portability, in my opinion, as implementors are more likely to implement just Hack than Hack + PHP. I'm not sure I understand your argument here. Hack was designed to be seamlessly used in a mixed PHP/Hack codebase. Facebook itself is still in such a mixed state, and will likely remain so for the foreseeable future; we've gotten a ton of benefit from converting most of our code, but there's a long tail that's just not worth pushing through all at once.