package riot

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An actor-model multi-core scheduler for OCaml 5


Dune Dependency






An actor-model multi-core scheduler for OCaml 5.

Quick Start | Tutorial | Reference   

Riot is an actor-model multi-core scheduler for OCaml 5. It brings Erlang-style concurrency to the language, where lightweight processes communicate via message-passing.

open Riot
type Message.t += Hello_world

let () = @@ fun () ->
  let pid =
    spawn (fun () ->
        match receive () with
        | Hello_world ->
   (fun f -> f "hello world from %a!" Pid.pp (self ())))
  send pid Hello_world

At its core Riot aims to offer:

  • Automatic multi-core scheduling – when you spawn a new Riot process, it will automatically get allocated on a random scheduler.

  • Lightweight processes – spawn 10 or 10,000 processes as you see fit.

  • Fast, type-safe message passing

  • Selective receive expressions – when receiving messages, you can skim through a process mailbox to consume them in arbitrary order.

  • Process links and monitors to keep track of the lifecycle of processes

Riot also includes:

  • Supervisors to build process hierarchies

  • Logging and Telemetry designed to be multicore friendly

  • an Application interface to orchestrate startup/shutdown of systems

  • Generic Servers for designing encapsulated services like with Elixir's GenServer


At the same time, there's a few things that Riot is not, and does not aim to be.

Primarily, Riot is not a full port of the Erlang VM and it won't support several of its use-cases, like:

  • supporting Erlang or Elixir bytecode

  • hot-code reloading in live applications

  • function-call level tracing in live applications

  • ad-hoc distribution

Quick Start

opam install riot

After that, you can use any of the examples as a base for your app, and run them:

dune exec ./my_app.exe


Riot is the continuation of the work I started with Caramel, an Erlang-backend for the OCaml compiler.

It was heavily inspired by eio by the OCaml Multicore team and miou by Calascibetta Romain and the Robur team, as I learned more about Algebraic Effects. In particular the Proc_state is based on the State module in Miou.

And a thousand thanks to Calascibetta Romain and Antonio Monteiro for the discussions and feedback.


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