Facebook's web frontend is implemented in PHP, a dynamically typed language
with more strange and error-prone features even than another well-known
dynamically typed language in widespread use. Hack is Facebook's answer to the
legacy problem of millions of lines of PHP: a new language, based on PHP, with
rich static types, async, typed XML literals, and other features. New code is
written in Hack; old PHP code is gradually becoming Hackified. My own interest
is in Hack's type system, and this will be the focus of my talk.