Introducing Butler Virtual Operating System

This post introduces haskell-butler, a virtual operating system that runs multi-players applications on the web. In two parts, I present:

  • The project’s overview, and,
  • A motivating use-case; a remote desktop that looks like this:

Hobby Project

Before I begin, let me set the expectations. Butler is a hobby project inspired by the SerenityOS project. I am working on it on my free time as a challenge and learning experience. Moreover I am considering alternative runtimes and protocols. Therefore the implementation is subject to change.

That being said, the project reached a point where it is now enjoyable, and I’m happy to share its current state with you.

Modular Operating System

Butler’s goal is to implement a virtual operating system with a strong focus on concurrency. Butler is not a computer hardware operating system. Instead, Butler is an application environment that runs on top of an existing system.

Thus I created the following modules:


Starting with the Core layer, I designed low level interfaces for the host system:

  • Process for thread management, powered by the ki library.
  • Pipe for inter-process communication using stm.
  • File for directory and file objects.
  • Network to serve Web Application Interface wai.

Then in the Main layer, I implemented higher level interfaces on top of the Core primitive:

  • Scheduler for the process hierarchy and supervision.
  • Display for graphical user interfaces.
  • Session for users.
  • Frame for the a data exchange format.

Finally, the App layer provides the application environment:

  • Window for app instance.
  • Cursor for user pointer.
  • Sound to produce and receive audio.
  • Agent to forward local agent.

I find it valuable to model these APIs after regular operating system constructs. They are available through a single top level Butler module. The next section introduces the application environment.

Application Actor

Butler applications are implemented as standalone processes that operate a AppEvent pipe. Checkout the App definition. Applications start with a AppContext provided by the display.

In order to run multiple application concurrently, Butler features a data exchange format to share a single socket per client. Applications output:

  • HTMX payload, by suffixing the application id. For example the UI are mounted on: <div id="w-$pid">.
  • Binary payload, by encoding the application id at the beginning of the buffer: | $pid | server-data |

Applications receive:

  • HTMX trigger, by suffixing the trigger name with the application id: {"HEADERS": {"HX-Trigger": "$trigger-$pid"}}
  • Binary payload, using the same application id encoding: | $pid | client-data |

The key design is that clients are dumb terminals and the state is controlled server side. A good example is the text editor app named Noter. This app leverages the whole application environment to implement an etherpad like text editor.

Haskell RunTime System

In this section I explain the choice of using Haskell. Beyond the language’s common features, here are two reasons for using Haskell:

  • The language lets you break down any problem into smaller chunks, using let or where bindings. I find this syntax and way of thinking very helpful for simplifying complex software.
  • The REPL evaluates modification almost instantaneously. I run development code through ghcid, and thanks to a simple client’s reconnect handler, the services are hot reloaded in milliseconds when the source changes.

However, even though the Haskell ecosystem is quickly improving, it is not a mainstream language, and it can be rough around the edges. Here are a couple of alternative languages that I am also considering:

  • Rust, because it is very similar to Haskell. Though it can be more complicated: for example, some expressions don’t seem to be decomposable into sub expressions without triggering the borrow checker. And the lack of REPL to evaluate the code interactively is frustrating.
  • Gleam, because it leverages the BEAM virtual machine, which is a great fit for application actors.

That being said, Haskell works great for Butler. So far I have only used its most basic features, and the project source is only 5k lines of code according to polyglot.

HTML5 Graphical User Interface

In this section I explain the choice of using HTML5 for the GUI. I choose HTML5 because it is an ubiquitous standard and the Web API offers a fully featured application environment. Thanks to the hypermedia paradigm enabled by htmx, it is a great fit for Butler.

Though it would be interesting to investigate morphdom and rich client alternatives, such as Web GPU or Flutter.

Application Demos

Here is a list of demos I am using to validate the core implementation:

I am mostly focusing on the desktop use-case, but I also helped my colleague fboucher use Butler to create a game named HazardHunter.


A terminal and VNC client are already provided to access non Butler applications, and perhaps it is possible to re-implemented these legacy system as Butler native applications. For example, I am looking forward adding:

  • Programming REPL.
  • Media player named “Ampler”.
  • IRC and Matrix client.
  • Agenda and calendar.
  • More video games.

Moreover, I would like to develop the Butler APIs to include:

  • Application testing framework, to simulate actor’s inputs and validate the outputs.
  • User’s mailbox and notification system.
  • Application debugging strace.
  • External identity providers.
  • Authorization system.

Finally, it would be interesting to investigate if and how an app registry could be implemented. For example as a list of packages that could be dynamically loaded/unloaded into a running instance.


The actor framework looks like a great solution for web applications, and I am quite happy with the Butler’s implementation. Moreover, I am excited about the community discussion for moving Towards an actor framework for Haskell. My implementation is rather limited, and I would be happy to use a common library similar to the erlang/otp.

Butler may sound like a crazy project, but I believe it is important to pursue fun goals that appear impossible. I would like to quote the author of SerenityOS Andreas Kling: “Never underestimate the power of a bunch of nerds building something just for fun.”

Finally I would like to thank elmiko for his early feedback and encouragement.


Links to this page
  • ZuriHac 2023 trip report

    I made a few demos of butler, my multiplayer virtual operating system, as well as animation-fractal, an engine to create live visuals. There I got great feedback and new improvement ideas. In particular, someone generously showed me with a pen and paper how I could implement a filter for my fractal inputs. I am now looking forward to implementing this technique in a new library.

  • Beautiful Haskell

    Some days there are streams of negative posts about Haskell, with which I disagree. There are many reasons why I enjoy using Haskell, many of which I have already mentioned in previous posts. See for example the Haskell RunTime System or Why Haxl?. In this post, I present:

#blog #haskell #butler