Where credit belongs for Hack

It’s been exciting to see the positive reception in the tech press and the open source community to Hack, the new programming language that my team at Facebook released last week.

At the same time, one part of that coverage has made me wince: a couple of articles give the impression that I created Hack, but I didn’t. What I do is manage the Hack team, and since I’m incredibly proud of the work they do, I’d like to shine a little light on the people who really deserve the credit.

A few years ago, Julien Verlaguet and Alok Menghrajani had a notion that Facebook could improve some aspects of how it ships software. They developed a proof-of-concept of a system that could detect certain kinds of logic error in a program, and showed it to a few colleagues. Buoyed by the positive feedback they received, they decided to invest more effort in the project, which would in due course become Hack.

This process underscores one of the aspects I find most appealing about engineering at Facebook: engineers are welcome to try out bold ideas, and to push on with them if they look like they’ve got a good prospect of paying off.

The work that Julien and Alok did to get the project off the ground was far from simple. They had to design not just a type system, but one where the type checking algorithms could scale to run efficiently and incrementally across hundreds of thousands of source files. Not just a static type system, but one that could interoperate seamlessly with PHP’s dynamic types. Even the notionally simple act of watching the filesystem to quickly find changed files is impressively difficult; just ask anyone who’s had to wrestle with race conditions involving Linux’s inotify subsystem.

When presented with the early prototype of Hack, Drew Paroski went beyond simply offering support: he immediately signed up to enrich Hack by building the Collections system, a set of clean, modern APIs for dealing with bulk data. While the face that Collections presents to the programmer is easy to understand, there’s once again a lot of subtle work going on behind the scenes (including a lot of runtime support in HHVM, to make them efficient). Collections interoperate seamlessly with PHP arrays, both when a program is being typechecked and while it’s running.

While designing a language and building a typechecker are substantial efforts in their own right, a big reason that we’re excited about Hack is that it is already a success within Facebook. This is in large part due to the work of two engineers, Gabe Levi and Josh Watzman, who converted a large part of our codebase to use Hack’s type annotations. Pulling this off is a surprisingly subtle task. In a large dynamically typed codebase, there can be both latent type errors and code that is more elegantly left dynamically typed. Introducing static types on a large scale involves a series of judgment calls: is what we’re looking at an error; a piece of code that we can refactor so that we can express its types statically; or something that’s just fine as it stands?

As the rest of the team proceeded with the language design, engineering, and conversion work, Joel Marcey built out the HHVM and Hack documentation, while Eugene Letuchy both added features to the language (e.g. richer support for traits) and made significant contributions to our PHP-to-Hack conversion work.

I’m proud to be associated with such skilled people who put so much passion and care into their work. When it comes to Hack, I’m simply the messenger, and these engineers have all along been the ones making it happen.