Future Technology

Hakana: Taking Hack Significantly – Slack Engineering

6 min read

TL; DR: We’re saying a brand new open supply sort checker for Hack, known as Hakana.

Slack launched in 2014, constructed with plenty of love and in addition plenty of PHP code.

We began migrating to a distinct language known as Hack in 2016. Hack was created by Fb after that they had struggled to scale their operations with PHP. It provided extra type-safety than PHP, and it got here with an interpreter (known as HHVM) that might run PHP code sooner than PHP’s personal interpreter.

A lot has modified in PHP-land since we switched. PHP is quicker than it was, and it has borrowed various Hack options (similar to constructor property promotion). A substantial amount of the PHP neighborhood has additionally embraced sort checking — there are actually some nice third-party sort checkers to select from.

Sticking with Hack has given us entry to further runtime velocity boosts, performance-enhancing language constructs like `async`, and a typechecker that’s extra strict by default than PHP typecheckers. However we’ve missed out on options offered by PHP typecheckers, together with the power to customise sort inference guidelines to seek out points particular to our codebase and automatic safety vulnerability detection.

Slack has a whole bunch of builders writing Hack. We need to give them the absolute best expertise, so final 12 months we began constructing a kind checker that might fill these gaps.

We’ve dubbed that static evaluation software Hakana, and it’s now available on GitHub!

Hakana is predicated on Psalm, an open-source PHP static evaluation software I created, and it’s written in Rust. Hakana re-uses a Hack parser that’s bundled with the Hack interpreter.

A bonus of writing it in Rust: with a little bit of prodding, Hakana can run nearly anyplace. For instance, it runs in your web browser by way of WASM.

How we use Hakana

At Slack we run Hakana in CI to implement good code habits in a variety of areas. Right here’s an incomplete listing:

  • It prevents unused features and unused non-public strategies.
  • It prevents unused assignments inside closures.
  • It detects each unattainable and redundant type-checks.
  • It warns us about potential SQL-injection assaults and cross-site scripting vulnerabilities (extra on this beneath).
  • It prevents misuse of inside Slack APIs (by way of plugin hooks).

We additionally use Hakana to automate type-aware API migrations (once more by way of plugin hooks) and to delete unused features in bulk. Because of Rust, these whole-codebase migrations are comparatively fast.


PHP makes it very easy to make a dynamically-rendered web site. PHP additionally makes it very easy to create an completely insecure dynamically-rendered web site.

Hack improves on this barely, by supporting a system for producing HTML output known as XHP. XHP is secure-by-default towards cross-site scripting assaults, nevertheless it doesn’t cease you from leaking buyer information, and Hack doesn’t forestall you from capturing your self within the foot with a variety of different safety vulnerabilities.

For a bunch of causes (together with compliance obligations) Slack wanted a software that might uncover these vulnerabilities. Psalm, the sort checker that Hakana is predicated on, already does safety evaluation, so it was comparatively easy so as to add safety evaluation to Hakana as properly.

Hakana isn’t the primary safety evaluation software for Hack — for years, Fb has been utilizing an inside, closed-source software called Zoncolan — however Hakana is the primary that everybody can use.

Hakana works in a lot the identical method as Zoncolan. It examines how information can stream between completely different features in a codebase, and checks if attacker-controlled information can present up in locations it shouldn’t.

To this point, Hakana has discovered various exploitable vulnerabilities in manufacturing code at Slack (that had been instantly mounted, and we checked our logs to make sure that the vulnerabilities had not really ever been exploited).

Safety within the sort system

Hakana’s safety evaluation mode is a type of interprocedural evaluation — it appears to be like on the method information flows between features. Hakana additionally helps detecting one sort of vulnerability (SQL injection) by way of intraprocedural evaluation, simply by analyzing sorts at perform boundaries.

To do that, Hakana borrows the idea of literal string sorts from Psalm. In Hack code we will outline a kind alias:

sort db_query_string = string;

Although the official Hack typechecker simply treats this as a string, Hakana treats it as a particular sort `literal-string`, a subtype of string that may solely be concatenated or interpolated with different literal-strings. Passing a string right into a perform that expects a literal-string causes Hakana to emit an error:

perform get_id_query(string $id): db_query_string 
    return "choose * from customers the place id = '$id'";
    // Error: The sort `string` is extra basic
    // than the declared return sort `literal-string`

Extending Hakana

PHP static evaluation instruments are typically highly-customisable. Customisable static evaluation is absolutely helpful for PHP, as a result of its interpreter permits a number of tips (e.g. magic methods) that may confound one-size-fits-all static evaluation instruments.

Most of these tips don’t work in Hack code, so there’s far much less of a necessity for customisable static evaluation. Even so, we’ve discovered plenty of worth in extending Hakana with plugins.

For instance, we use a customized plugin to inform Hakana {that a} technique name on our inside End result object, $some_result->is_ok(), is equal to the extra verbose $some_result is ResultSuccess<_> typecheck.

We additionally use customized plugins to carry out type-aware migrations at velocity throughout the complete codebase.

Constructing a plugin system for Rust isn’t easy — our customized model of Hakana wraps the open-source core as a library, utilizing its plugin hooks the place obligatory — nevertheless it’s an essential characteristic for us.


Hakana was tailored from Psalm, a kind checker written in PHP. Whereas PHP is quick for an interpreted language, it will possibly’t compete with a compiled software. To investigate a codebase of Slack’s dimension — many hundreds of thousands of strains of code — we wanted a software that’s as quick as potential.

Hakana, written in Rust, runs about 5x sooner than the PHP-based software on which it’s modeled.

We haven’t spent a lot time tuning efficiency, however when analyzing the complete Slack codebase (about 5 million strains of code) efficiency is on par with the official Hack typechecker. That’s adequate for us, for now.

Why open-source Hakana?

Hack type of appears to be like like PHP however with extra sorts. In that respect it’s been in comparison with TypeScript. However in contrast to TypeScript, which compiles right down to JavaScript, Hack requires its customers to alter plenty of their server infrastructure too.

Resulting from that top switching value, at the moment Hack is barely used at just a few firms. It’s potential that no person else apart from Slack can have a cause to make use of Hakana.

Even so, there are just a few the explanation why we predict it’s price open-sourcing:

  • 🔍 The broader programming language neighborhood could have helpful enter — particularly relating to safety evaluation.
  • 🤝 Psalm, one other open-source software, was the premise for Hakana. By open-sourcing our personal software we’re returning the favor.
  • 🏢 Although it wouldn’t be simple, firms with extraordinarily massive PHP codebases may take into account forking Hakana and altering it to research PHP code.


Slack is an indispensable software for hundreds of thousands world wide. Hack is now an integral a part of Slack, and so we’ve created a software that we hope will develop into indispensable to our Hack builders. We’re open-sourcing it today with the hope that others will discover it helpful and attention-grabbing.

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