Parallelism without panic: a user’s guide to multicore safety in OCaml

OCaml has a reputation for keeping its promises: strong types, solid abstractions, and very few runtime surprises. Multicore parallelism changes the rules a bit: suddenly, we can encounter race conditions, the kind of bugs that only show up at 3 AM, disappear when you add a print statement, and return months later just to spite you. If we want to stay in OCaml’s comfort zone, we need good tools.

In this tutorial, participants start from a buggy lock-free queue and work their way toward a correct implementation using the tools available today for multicore safety in OCaml: a race detector (ThreadSanitizer), property-based concurrency testing (QCheck-Lin), and a model checker (dscheck). Step by step, they will discover why standard testing falls short for concurrent code, learn to apply each tool to find specific classes of bugs, and fix them until the queue is fully correct. These tools are similar, albeit less mature, to those found in other languages, but when combined, they already provide a surprisingly robust workflow for writing reliable multicore code.

At the end of the tutorial, we will also have a look at a more radical option: OxCaml, a promising but experimental mode-system extension inspired by Rust’s ownership model. The idea is simple: if you really dislike runtime bugs, or if you’re writing critical software, you shouldn’t just detect data races, you should make them statically impossible to write.

In brief, OCaml already provides the tools you need for everyday multicore safety, and OxCaml points toward an even more reassuring future for those who require the strongest guarantees.

Preparation

This tutorial includes hands-on exercises that require a working OCaml environment. Please follow the setup instructions available in the repository before the tutorial, as some options can take up to some times to complete: lyrm/bobkonf2026#setup

The fastest option is GitHub Codespaces, which runs entirely in your browser and takes only a few minutes. A local devcontainer setup and a manual local setup are also available.

Carine Morel

lyrm.bsky.social

I started out as a physicist, then got happily lost in functional programming with OCaml and resurfaced as a software engineer. When OCaml gained shared-memory parallelism, I discovered a new passion, and I’ve been diving deeper ever since.

Sudha Parimala

Sudha has been programming with OCaml for 6+ years. These days she’s mostly into build tooling, and multicore OCaml. She’s based in the Netherlands these days. Outside of programming, she likes reading, making art, and visiting museums.