I'll leave the first part of your question for someone less close to the project. For the toolchain, when using it: it's currently mostly the PHP toolchain (e.

I'll leave the first part of your question for someone less close to the project. For the toolchain, when using it: it's currently mostly the PHP toolchain (e.g. composer) with some additions: - the typechecker: this is a fast (I usually get full results in well under a second) static analysis tool, fully integrated with Nuclide (our IDE), and with excellent support in VSCode via a third-party plugin. This is a massive improvement in developer experience, especially when refactoring. Usually the unit tests pass first try once I've fixed the problems that Hack finds. - an autoloader that can handle any definition (e.g. functions, typedefs, constants), not just classes - the built-in webserver isn't just for development - no need for fastcgi + apache etc, though that is supported As for building/working on HHVM itself, it's basically modern C++ using CMake, and the Hack typechecker is OCaml.