What is hakana?

Hakana is a code quality tool built by Slack for projects written in Hack, a programming language used heavily at Slack and Facebook. It runs alongside the standard Hack typechecker to catch deeper problems that the built-in tool misses, things like security vulnerabilities, logic bugs, and dead code that serves no purpose. At its core, the tool focuses on figuring out precise types for everything in your codebase. "Types" here mean understanding what kind of data each variable, function, and class works with. By reasoning carefully about these types, the tool can spot mismatches and risky patterns that might lead to runtime errors or security holes. It also enables type-aware migrations, meaning it can help you safely restructure large amounts of code while understanding how different pieces connect. Who benefits from this? Teams with substantial Hack codebases, think of a company like Slack itself, where engineers need automated help keeping millions of lines of code healthy. For example, if someone introduces a function that handles user input insecurely, this tool can flag it before it ships. If there are entire files or methods no longer called anywhere, the tool identifies them so they can be removed. This keeps large codebases lean and maintainable over time. One notable tradeoff: the tool is not designed for instant, real-time feedback as you type. Instead, it's meant for periodic analysis or on-save checks via its language server integration. This slower, more thorough approach allows for deeper analysis than a tool optimized for speed alone. It also borrows HHVM's parser, the core engine that reads Hack code, rather than building one from scratch, which keeps it closely aligned with how Hack actually works.