Facebook Explanation about Hack:
“Our principal addition is static typing. We have developed a system to annotate function signatures and class members with type information; our type checking algorithm (the type checker) infers the rest. Type checking is incremental, such that even in the same file, some code can be converted to Hack while the rest remains PHP. Technically speaking, Hack is a gradually typed language: dynamically typed code interoperates seamlessly with statically typed code.”
“The Hack adds additional features beyond static type checking, including collections, lambda expressions, and run time enforcement of return and additional parameter types….We are delighted to open-source both Hack and the tools that we used to convert our codebase hacklang.org.”

 hacklang.org