Technically speaking, Hack is a “gradually typed*”* language: dynamically typed code interoperates seamlessly with statically typed code. Type checking is incremental, such that even within a single file some code can be converted to Hack while the rest remains dynamically typed. 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. We have also added many new features that we believe will help make developers more productive. “variable variables” and the extract() function).

We made a conscious choice not to support a handful of deprecated functions and features that were incompatible with static typing (e.g. In fact, most PHP files are already valid Hack files. We believe that it offers the best of both dynamically typed and statically typed languages, and that it will be valuable to projects of all sizes. We believed there had to be a sweet spot. Conversely, statically typed languages provide more of a safety net, but often at the cost of quick iteration. Traditionally, dynamically typed languages allow for rapid development but sacrifice the ability to catch errors early and introspect code quickly, particularly on larger codebases. Before Hack, we had a simple language with a quick feedback loop - but how could we mitigate the sorts of problems described above? Could early error detection coexist with rapid iteration, all while preserving our investment in PHP? Could improved code analysis and introspection help make developers more productive with tools like auto-complete? Another example is a complex API, where developers may have a solid understanding of its semantics but still spend time looking up mundane method names in documentation.Īt Facebook scale - with thousands of engineers shipping new code twice a day - slowdowns like these are even more problematic. The code above is a great example of a common mistake where a method could unexpectedly be called on a null object, causing an error that wouldn’t be caught until runtime. MotivationĮvery PHP programmer is familiar with day-to-day tasks that can be tricky or cumbersome. We’re also proud to release an open source version of Hack to the public at as part of our HHVM runtime platform, which will now support both Hack and PHP. Over the last year, we have migrated nearly our entire PHP codebase to Hack, thanks to both organic adoption and a number of homegrown refactoring tools. We have deployed Hack at Facebook and it has been a great success. Hack reconciles the fast development cycle of PHP with the discipline provided by static typing, while adding many features commonly found in other modern programming languages. Today we’re releasing Hack, a programming language we developed for HHVM that interoperates seamlessly with PHP.

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