(I work at Facebook but not on Hack.) There's a lot to be said about backward compatibility, and much of Hack's virtuosity stems in its smooth interoperability

(I work at Facebook but not on Hack.) There's a lot to be said about backward compatibility, and much of Hack's virtuosity stems in its smooth interoperability with PHP - many millions of lines of it. There's nothing like working on such a large codebase to convince one how difficult disruption of any kind is. The language definition and semantic checker are difficult, but are stereotypically tasks that cannot be distributed to many engineers; instead, a few senior engineers took that task to the benefit of all others.