Hack doesn't get compiled, heh, it's interpreted so it's quite a bit different to what I'm used to. It's more like TypeScript in that regard, but TypeScript is still compiled... I've never used an interpreted typed language before, hence the question!
Well HHVM is a JIT compiler isn't it? Also it applies just as well to an intermediate representation (like bytecode, or some simpler language that gets evaluated).
HHVM enforces the checks purely dynamically as it actually executes code. There's a totally separate tool that does the full static type analysis. See my sibling comment for details: https://news.ycombinator.com/item?id=7808586
