Hack Developer Day Recap

Last week, Facebook invited 150 developers to building 15 on their Menlo Park campus for Hack Developer Day 2014, the first of hopefully many full-day events for all things Hack and HHVM. The event, which was free for anyone that could get themselves over to Facebook’s office, sold out between the time I found out about it and was able to approve my travel. I’m very thankful to Bryan O’Sullivan for finding me a spot at the last minute, and Francesca Krihely, MongoDB’s community manager, for telling me about it in the first place.

Facebook had their own recap of the event posted before my plane landed back on the East Coast, but I had several pages of presentation notes and a recently renewed commitment to blogging, so here we are.

Introducing Hack with Julien Verlaguet

While HHVM has had its fair share of publicity over the past year, details about Hack have only recently come to surface. Julien started off the day with an introduction into the how the language integrates with HHVM and a tour of its type-checking features. Since being introduced to HHVM, Hack has been an opt-in feature unlocked by using <?hh in lieu of <?php. As we’d hear throughout the day, this design was vital to making Hack’s adoption within Facebook as painless as possible. Developers were free to pick up Hack on a file-by-file basis and fall back to PHP mode as needed. More importantly, there was never a need to migrate millions of lines of legacy code over to Hack in one shot.

Hack operates in one of three modes: strict, partial (the default), or decl. Strict mode appeals to the pedant in all of us, and activates a stringent level of type checking that rules out any interoperability with untyped code. Partial and decl modes are more forgiving and will actually allow the developer to call into PHP. Decl mode also serves a purpose when migrating legacy code to Hack, as typed function signatures will be sufficient to satisfy a call from strict mode.

For better or worse, PHP developers are accustomed to an instant feedback loop. We can save, refresh a browser tab, and immediately see the effects of our code changes. With this in mind, Facebook created a hh_client utility, separate from the HHVM server, which can quickly (e.g. 100ms) scan a project’s entire source tree for type errors. Aside from speed, flexibility was an important consideration. The tool is easily integrated into most editors as a build command.

One thing that stood out during Julien’s talk was his explanation of how Hack handles unresolved types. Hack’s type checking sits atop the dynamic nature of PHP; it doesn’t replace it. This means that it needs to gracefully handle ambiguous situations such as $x = $cond ? 1 : true. The variable $x is not declared with a type and may be an integer or boolean after the ternary assignment. Hack ends up deferring type resolution until it’s absolutely necessary (e.g. $x is being passed to a function). We see similar behavior when using generics, such as the built-in Collection classes:

The bottom line here is that until this generic collection is exposed to code that takes or returns a type that will cause incompatibility, the type checker will be relaxed as to what it allows to be added.1

Before leaving the stage, Julien gave us a brief preview of an in-browser IDE for Hack development. One immediate question was whether this was built atop an existing project, such as Ace or Atom. Facebook confirmed that it was an organic project that had been in development for at least a year. In the ten seconds it was on screen, we saw some code intel and type checking in action. In a later session, we’d learn that the editor sported an embedded JavaScript port of hh_client, SCM integration, and debugging facilities to step through an application request launched in a second browser tab.

Converting PHP to Hack with Josh Watzman

Josh may be a familiar to those on the PHP-FIG mailing list. Last year, he reached out to the list and invited members to preview Hack before the project was formally announced. Before joining the Language and Tools team, Josh used Hack (or a previous iteration of it) on the News Feed team for two years. In this session, he would share a bit about Facebook’s workflow for migrating legacy PHP code over to Hack syntax.

The first part of this process is the hackificator command, which does exactly three things:

  1. Replaces <?php with <?hh one file at a time. Strict, partial, and decl modes are attempted in order, and the file reverts back to <?php if none of the modes are valid.

  2. Add nullable annotations (e.g. ?string) to argument types where null was specified as a default value.

  3. Supply missing constructor arguments as need (e.g. new Foo; becomes new Foo();).

Manual intervention will still be required, but hackificator does a fine job of automating some tedious changes. Before running the tool, Josh began with a review of how Hack’s partial and decl modes interoperate with untyped PHP:

In the general principle of Hack, if there’s something that we can’t determine if it’s right or wrong, we assume that the programmer knows what they’re doing and that it’s right.

Using hack-conversion-demo as an example, Josh walked through conversion of a basic class inheritance hierarchy from PHP to Hack in two fashions: depth and breadth. Starting with depth mode, he converted class files to partial Hack where possible and deferred to decl mode as needed (e.g. when a child class method referenced an undeclared, untyped property). The alternative breadth conversion took advantage of “the general principle of Hack” and saw us convert child classes to partial Hack and leave the parent class as untyped PHP. Each approach has its pros and cons. Depth allowed for better coverage through the inheritance hierarchy, while breadth achieved greater coverage overall.

The second step of the conversion process entails running hh_server --convert, which examines functions and class properties and attempts to infer suitable supertypes that satisfy the type checker. These are referred to as soft types, as there is no guarantee that the inference will be valid at runtime. The resulting soft types will be prefixed with @, which instructs Hack to downgrade type violations from fatal errors to loggable warnings.

The final conversion step requires error logs from our application (either unit test runs or production logs were suggested), which should indicate if any type inferences added by hh_server --convert are invalid. With logs in hand, the hack_remove_soft_types command can be used in one of two modes:

  1. Parse HHVM’s error logs for soft type warnings, delete those log entries, and remove the corresponding type annotations.

  2. Harden type annotations within a file by removing @ error suppression.

This entire process is covered in more detail in the Hack conversion documentation.

HHVM on Heroku with Craig Kerstiens and Peter van Hardenberg

Before we adjourned for lunch, Craig and Peter took the stage to briefly talk about recent developments at Heroku for deploying to HHVM. A community-supported buildpack has existed since last year, but David Zuelke has been hard at work over the past month on a more robust solution. Peter’s demo, which is a fork of Facebook’s basic demo, is published on GitHub.

Hack Language Features with Drew Paroski and Eugene Letuchy

Post-lunch sessions are often a snoozefest, but Drew and Eugene kept everyone’s attention with one of the most interesting presentations of the day. The talk jumped through a host of new features to Hack, from small variations to PHP syntax to more significant additions, such as async functionality.

Lamba expressions were one of the first topics. Defined with a literal syntax via the ==> operator, these are an alternative to anonymous functions and should be accepted by any code that expects a PHP callable. One important distinction is that lamba’s inherit their scope implicitly. This is similar to how closures operate in JavaScript, and means that PHP’s use statement is no longer necessary.

Collections are an alternative to using PHP arrays for anything and everything. The Vector, Map, and Set types should need no introduction. All collection types can be defined with a literal syntax, are fully compatible with generics, and have a corresponding immutable class. Immutability can be useful for type checking and allows for some performance optimization in HHVM. Additionally, the collection classes have several functional methods (e.g. map(), filter()), which are reminiscent of Doctrine’s Collection interface. As you might expect, the collection methods place very nicely with Hack’s concise lambda syntax.

If I could pick one feature to pluck from Hack and drop into PHP core today, it would be collections hands down. A recurring issue while developing the MonogDB PHP driver and Doctrine ODM has been property converting arrays to and from BSON. Distinguishing associative arrays from sequential, numerically-indexed arrays, is a trivial algorithm, but satisfying the user’s expectations for behavior is a neverending challenge. PHP-1051 is one such example.

While most of the room was wiping drool from their chins after Drew’s collection demo, Eugene took the stage to walk us through Hack’s async support. If you had read most of the online discussions in the month following Hack’s release, you would conclude that (a) nearly everyone thought this feature was amazing and (b) hardly anyone understood how it actually worked. To be fair, there originally wasn’t much documentation outside of a few scripts in the HHVM test suite.

Eugene aptly described Hack’s async implementation as “cooperative multi-tasking within a request.” It does not attempt to mimic a full asynchronous framework that we find in Node.js or React. Instead, Hack allows functions and callers to opt-in to asynchronous execution via the async and await keywords, respectively. Hack’s compiler will then apply some transformation to the code under the hood, similar to what is done for generators. At runtime, async functions will return an Awaitable<T> instance immediately. Calling code is therefore able to invoke multiple async functions and finally call join() on the Awaitable when its time to block until the result(s) are available. As a rule of thumb, join() is best left to the top-most scope of the request (e.g. before a controller might need to return a response).

Hack also made some small improvements to traits. While traits can be a boon for reducing code duplication, they introduce a pain point when it comes to API dependence, particularly for Hack’s type checker. In practice, traits are often destined to be used within a class hierarchy, but PHP offers no way for a trait to restrict how it should be used. Hack allows traits to require that its using class be an instance of a particular class or interface.

User attributes were next on the list. In short, these are Hack’s alternative to PHP docblock annotations. The syntax was immediately recognizable to me, since I’ve been working with an Open Academy class this semester to prototype a MongoDB driver for HHVM. HHVM extensions have the option of being written in pure PHP (actually Hack syntax) or a blend of PHP and C++ using HNI. In the latter case, PHP functions can be declared without a body and prefixed <<__Native>> (a user attribute) to instruct HHVM to look for a corresponding C++ implementation. In our project, most of the driver can be written in PHP, but some functions resort to C++ in order to interact with libmongoc and libbson.

Before this talk, I wasn’t aware that user attributes were utilized for other language features outside of HNI. For instance, the <<Override>> attribute functions as a child class’ counterpart to abstract and requires the method to exist in a base class. This can be used to prevent leaving behind dead code after refactoring out methods from base classes.

One significant difference between user attributes and familiar PHP annotations is that the attributes are first-class citizens in Hack’s syntax. With Doctrine’s annotations library, we need to resort to parsing docblocks at runtime; however, Hack makes attributes easily accessible through getUserAttributes() method in the Reflection API. Beyond that, they sport similar features to existing PHP annotations and can be attached to classes, functions, and member properties. Similar to Doctrine’s annotations, user attributes can have parameters. At present, Hack supports array and scalar values; support for collection type literals (e.g. Map) may come later.

Last but not least, we had a quick blurb on trailing commas. One of PHP’s minor idiosyncrasies has been allowing trailing commas after the last element in an array, but not after the final argument in a function declaration. In the interest of reducing noise in diffs for multi-line function declarations, Hack fixed this. We heard it was a huge hit with Paul Tarjan,,,.

HHVM Status Update with Sara Golemon and Paul Tarjan

Paul and Sara came on to update the audience on HHVM’s development progress. This session likely had the broadest appeal of any, as HHVM has garnered the attention of many PHP developers who have no intention of abandoning ship for Hack’s new syntax but are interested in a faster runtime for their applications. For libaries and frameworks such as Doctrine and Symfony, both of which were well-represented in the audience, we’d like to ensure that our projects work as well with HHVM as they do with PHP proper. On a related note, the HHVM team also had a Open Academy class this semester assigned to moving a handful of open-source PHP projects closer to 100% test suite compatibility (all progress is being tracked here).

For those that have seen Sara present at HHVM over the past year at the various PHP conferences, this session had few surprises. She began with an overview of HHVM’s release schedule. Facebook aims to publish a new major release each year. Minor releases are expected every eight weeks, which syncs up with every fourth Facebook production release (those happen once a fortnight). As an homage to the “HH” in HHVM, each of the 26 releases this year are named after hip hop artists. There’s still time to submit your suggestions for next year’s category.

Now that HHVM has started to appear in Linux package managers, Facebook will commit to supporting older major versions of HHVM with bug and security fixes as needed. Sara also hinted that the package structures should mimic PHP, so we could expect to see hhvm-dbg and hhvm-dev packages for debug builds and development headers, respectively, in the near future. The hhvm-dev package cannot arrive soon enough, as extension development currently requires building HHVM from source, which makes Travis CI integration a pipe dream.

Support for platforms other than x86 Linux is also on the agenda. Facebook enlisted all five feet and two inches of Elizabeth Smith to bring Windows compatibility up to speed, and progress is also being made on OSX and ARM. Compatibility for HHVM’s JIT and interpreter modes are tracked separately, but the end goal is obviously full support for both on each platform.

Throughout the presentation, Paul and Sara gave frequent shoutouts to the HHVM community outside of Facebook. Nearly every slide had a couple of faces in the margin, highlighting some of the project’s most active contributors. Special mention went to Simon Welsh, who has single-handedly converted more than 20 HHVM extensions from IDL to HNI. This is all part of a team-wide effort being tracked here. Sara candidly admitted that the original IDL API for HHVM’s extensions was bad enough to make PHP’s own C API look appealing. HNI is a huge improvement and has really accelerated the development of new extensions, MongoDB’s driver project included.

One last topic to address was Facebook’s commitment to keeping HHVM’s development open. Based on questions asked throughout the day, it was obvious that Facebook has quite a bit of FUD to overcome when it comes to promoting adoption of Hack and, to a lesser extent, HHVM. Sara said that code reviews will happen on reviews.facebook.net, a public-facing Phabricator deployment. Currently, the GitHub repository is synced once daily, but there are plans to reduce that latency and implement a two-way sync. The HHVM team is also considering opening up their biweekly design meetings to the public via either live video or recordings.

HHVM Performance with Edwin Smith

We started the day with discussions about type checking, high-level Hack syntax, and PaaS computing platforms. Edwin was about to take us to the opposite end of the computing spectrum with a tour of processor dies and assembly code, but not before telling us a story.

Five years ago, Facebook switched over to HipHop, the infamous PHP-to-C++ compiler and predecessor of HHVM. The company had an internal goal to fully migrate over to HHVM’s JIT by the end of 2012; however, in early 2012 the HHVM team realized they would need to triple the project’s performance in order to meet that goal. This lead to their first lockdown, which would last six weeks. The team isolated themselves, grew out their beards (for what it’s worth, Sara did try), and focused on racking up as many performance wins as possible over the next 42 days. Three weeks in, they had hit their goal, but decided to press on.

Each development sprint now follows a rhythmic schedule: build things, measure, generate ideas, lockdown, and cleaning up technical debt. Last fall, the team blogged about their most recent lockdown, which focused on improving performance and compatibility with leading PHP projects. The results spoke for themselves.

From here, Edwin segued into explaining how Facebook uses PHP and the team’s approach to unlocking the performance improvements they needed out of HHVM. Facebook’s PHP workload makes heavy use of APC as a key/value store (on the order of 100s of MB per server). Memory heaps for requests range anywhere from 10-100MB, most of which is arrays and strings (News Feed is one example). Additionally, requests sometimes need to pull data from external data sources that aren’t in memory. Between network IO, local storage, and RAM, Facebook has to be concerned with the latency of everything. Ultimately, the goal is to keep the CPU’s cores as preoccupied as possible. Idle time is the enemy.

One illustration given was the progression from HipHop’s early ZendArray data structure, designed after its namesake in PHP core, to HphpArray, which aligned values closer together in memory to require less jumps from CPU cache to RAM. MixedArray, introduced in 2013, takes this a step pointer and optionally allows for the entire array to be pulled into cache without individual pointers for its elements (referred to as PackedArray). Similar improvements were made to the native string type.

Beyond data structures, HHVM also needs to optimize the code itself. This ranges from minimizing jumps, which require register stashing, by analyzing a running program for the most common execution path, to ensuring that functions within HHVM’s own binary are placed efficiently. Using a suite of performance tools, the team located the “hot” functions and took steps to ensure they were linked nearest each other in memory. Referred to as “Bert’s Heuristic”, this unlocked an unheard-of 4% performance gain in one shot.

Edwin is optimistic that the HHVM team has at least four to five years of performance wins ahead of them before they hit a wall. There are still gains to be made with HHVM’s garbage collector (JVM is far more advanced), and the team has yet to really leverage Hack’s type annotations with a region compiler to eliminate redundant guard statements.

Facebook IDE with Joel Pobar and Joel Beales

For our last session, “The Joels” came on to give us a closer look at the in-browser IDE that Julien revealed in the morning. Mr. Beales walked us through the IDE using JoelBook as an example. As a bit of background context, JoelBook has been described by leading venture capitalists as “Facebook for people named Joel.” Folks on Hacker News have already hailed it as “the killer app of 2014.” For reasons not entirely clear, the audience seemed more interested in the IDE.

The Facebook IDE is already being used and enjoyed within the company for Hack development. Beyond type checking and auto-completion, the built-in debugging functionality, which is currently integrated with Facebook’s dev environment, looked quite user-friendly (akin to debugging JavaScript with Chrome or Firebug). Facebook intends to make a version of the IDE publicly available in the summer of this year.


After the closing remarks, Facebook reconfigured the room for an evening hackathon. I have to confess that I spent most of this time working through a backlog of Doctrine issues while most other folks were trying out Hack. I did manage to compile HHVM from source over the ensuing three hours. We recently updated mongo-hhvm-driver from 2.4.x to 3.0.x and I realized I didn’t have the proper development headers available locally (this is why we need hhvm-dev!). Kudos to whomever it was that reminded me of the -j compiler option, as it made things much more bearable. I’ll say nothing of Victor Berchet’s efforts to assassinate my character:

During the hackathon, I had a moment to poke Fabien about his thoughts on the day’s events as Symfony edges closer to 100% test suite compatibility with HHVM. While there’s no indication that Symfony would ever drop PHP, Fabien was talking with several Hack developers about the feasibility of automating a PHP-to-Hack conversion based on docblock comments and some of the tools shown in Josh’s earlier presentation. Take this with a grain of salt, but if that idea comes to fruition, it could allow Symfony to be utilized in a strict Hack project. Additionally, Hack’s type checking could conceivably be integrated into Symfony’s CI process to catch deviations between documented types and runtime behavior.

For those already using Hack and looking for a web framework, there are a couple of options out there already. Simon Welsh and James Miller, who work together at PocketRent and both flew out from New Zealand to attend to event, are actively developing beatbox. Unfortunately, searching for “hack framework” is futile, and the results only get worse if you add “PHP” to the mix. Hopefully the HHVM team can compile a list of projects in a future blog post.


This was the first Facebook event I’ve attended, excluding last year’s field trip after ZendCon when 25 PHP developers all attempted to visit under Sara’s name (apparently there’s a limit for that sort of thing). The staff, everyone from speakers to catering to the AV crew, did an amazing job throughout the day. The event was also streamed live for those that couldn’t physically attend, so videos are available of each presentation and the Q&A sessions that followed.

Since the event was held on a Wednesday, I spent the remainder of the week working at MongoDB’s Palo Alto office with Hannes. Until he moves to New York (fingers crossed), the PHP team (Derick included) is spread out across eight hours of time zones, which makes any sort of in-person collaboration a rare treat. We have some exciting plans in store for the PHP driver as we hope to port it over to libmongoc and libbson. Combined with some learnings from the HHVM research I’ve been leading, I’m hopeful that we’ll be in a good place next year to efficiently support multiple platforms (PHP C, HHVM, and pure PHP, à la mongofill) with a set of high-level driver components (published as Composer packages) and a couple of platform-specific, no-frills core drivers.

It was a great week for learning, reconnecting with old friends, and meeting new ones. I have no idea when I’ll make it back to Palo Alto again, but you can bet the very first place I’ll visit (after the obligatory In-N-Out stop) will be Philz Coffee to order a Mint Mojito Iced Coffee.2

"Mint Mojito Iced Coffee from Philz Coffee"

  1. Generics and Type Inference 

  2. I gave up trying to find a natural way to work a mention of this into the post, but suffice it to say this beverage is utterly amazing.