Does a partially statically typed language work in practice?
Are FBers happy with Hack? How much PHP is there running around vs Hack code?
I know Instagram is primarily Python - how about other products? Do they have different stacks?
I don't think there's anywhere in fb that uses vanilla php instead of hack, except for some core php stuff in bits of code, which is intentional.
That being said, I tried to use Hack for a personal non fb project and found it pretty annoying, mostly because a lot of the commonly used php functions are not typed, so you lose all the benefits. It's great with a codebase like fb though.