From Wired, Facebook Introduces ‘Hack,’ the Programming Language of the Future
You can think of Hack as a new version of PHP. It too runs on the Hip Hop Virtual Machine, but it lets coders use both dynamic typing and static typing. This is what’s called gradual typing, and until now, it has mostly been an academic exercise. Facebook, O’Sullivan says, is the first to bring gradual typing to a “real, industrial strength” language.
Hack is open source and is available at hacklang.org. It supports type annotation, generics, lambdas and host of other features on top of PHP.