Facebook’s main website, ads platform, and much of its internal tooling is implemented in PHP, a language not known for elegance or best practice in programming language design. Five years ago Facebook embarked on an ambitious project to migrate its code base to Hack, which takes the syntax of PHP, removes the worst features, and adds static typing and modern constructs for asynchronous programming. Its type system is an interesting mixture of ideas from Java, C#, Scala, and Caml, with flow-sensitive typing thrown in to capture typical PHP idioms. Type-driven development is now more than accepted: developers demand ever richer types, and evolution of the codebase goes hand-in-hand with evolution of the type system and programming language. Development-driven typing, anyone?