How Would You Type-Check 30 Million Lines of PHP? (with Julien Verlaguet)

How would you manage 30 million lines of PHP? You can't rewrite it - you'd be fired long before you delivered. That was the problem facing Facebook around 2011, and their answer was to add a type system, giving birth to the Hack language. But adding types to PHP turned out to be the easy part. The hard part was building a type checker that developers used to "edit, save, refresh" would actually tolerate. So how do you type-check tens of millions of lines of code fast enough that nobody notices?

Julien Verlaguet co-designed Hack at Facebook, and has spent the fifteen years since chasing a better way to build incremental systems. We start with the internals of Hack's language server - forked OCaml workers, a shared immutable heap, lock-free dependency tracking, and the months spent hunting non-deterministic bugs. From there we follow the thread to Skip, a language designed around tracking mutability in the type system, with stops at why Haskell's monads and "reactive Java" didn't fit, functors vs type classes, and Skip's bang operator for working with immutable objects. Then it's on to the Skip reactive framework: eager vs lazy collections, a garbage collector without long pauses, a reactive SQL database that didn't find a market, TypeScript bindings, and Skipper, an AI coding agent built on a sound version of TypeScript.

If you're interested in compilers, language design, or building real-time systems that stay fast as they grow, Julien has plenty of hard-won lessons to share.

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SkipLabs: https://skiplabs.io/

Skip Framework Docs: https://skiplabs.io/docs

Skip on GitHub: https://github.com/SkipLabs/skip

Skiplang: https://skiplang.com/

Skipper Launch Announcement: https://skiplabs.io/blog/press_release

Hack: https://hacklang.org/

HHVM: https://hhvm.com/

OCaml: https://ocaml.org/

OCaml Functors: https://ocaml.org/docs/functors

Haskell: https://www.haskell.org/

Clojure: https://clojure.org/

Scala Implicit Parameters: https://docs.scala-lang.org/tour/implicit-parameters.html

Salsa (Incremental Computation for Rust): https://github.com/salsa-rs/salsa

ReactiveX: https://reactivex.io/

SQLite: https://www.sqlite.org/

TPC-H Benchmark: https://www.tpc.org/tpch/

PostgreSQL: https://www.postgresql.org/

PostgreSQL NOTIFY: https://www.postgresql.org/docs/current/sql-notify.html

Server-Sent Events: https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events

JavaScript Proxy Objects: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy

Doxygen: https://www.doxygen.nl/

SKDB and Reactive Databases (Materialized View): https://materializedview.io/p/skdb-reactive-databases-julien-verlaguet

Julien on LinkedIn: https://www.linkedin.com/in/julien-verlaguet-b5710a20/

Julien on X: https://x.com/JulienVerlaguet

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