OCaml Weekly News

The event will be held at Jane Street's London office, located at 2 & A Half Devonshire Square, Premier Place, London, EC2M 4UJ. Check in will take place in the building lobby - please bring a photo ID with you on the day. ⌛ The event will begin at 17:30 with talks starting around 18:30. 🍕Food and beverages will be served.

• Add a visual indicator for the state of the previewer (disconnected, refreshing, …) (#220, #222)
• Allow frontmatter in included files (#228)
• (#229) Add an LSP server, available though the slipshow lsp command, to interact with your editor. The server supports:
  • Diagnostics
  • Documentation for actions on hover
  • Go to definition (from IDs in action, to the associated element)
  • Occurrences of ID
  • Completion for IDs in actions
• Add preview server capabilities to the LSP server (#233)
• Add more syntax to include raw html, either in the file or as external file (#236)

• In frontmatter, distinguish between attributes (the attributes for the whole file) and toplevel-attributes (the attributes for the whole presentation). Change toplevel-attributes to attributes in your presentation to fix the breaking change. (#229)

• Fixed hot-reload not resuming after a disconnect (#220)

Solo5 is both a C framework and a set of tools for creating and running programmes within a sandbox that, initially, allows only 5 interactions with the outside world:

1. reading and writing Ethernet frames to a net device
2. reading and writing pages to a block device
3. exit

All the functions available in the sandboxed environment are described in this header.

What is notable about Solo5 is that this API works with VirtIO, Xen (and QubesOS) and our minimalist hypervisor hvt (which runs on KVM, BHyve and VMM). This means that a Solo5 application can be deployed across a wide range of platforms. On Ubuntu, you can install Solo5 via:

$ curl -fsSL https://apt.robur.coop/gpg.pub | \
  gpg --dearmor > /etc/apt/trusted.gpg.d/apt.robur.coop.gpg
$ echo "deb [signed-by=/etc/apt/trusted.gpg.d/apt.robur.coop.gpg] https://apt.robur.coop ubuntu-24.04 main" > /etc/apt/sources.list.d/robur.list
$ apt update
$ apt install solo5

At Robur, we prefer to build hvt unikernels. These unikernels only require the solo5-hvt "tender" (~200 KB) to run, which is available on Linux, FreeBSD and OpenBSD (WSL supports what is known as nested virtualisation, so it is also possible to run a unikernel on WSL!).

The advantage of hvt is its minimalism (a hallmark of Solo5 in general). This latest release completes the sandboxing on Linux by using libseccomp to retain privileges only for what is necessary to run the unikernel (which amounts to 18 system calls in total). This sandboxing was already available for FreeBSD with Capsicum and OpenBSD with pledge(2).

This release also marks an improvement in throughput when writing Ethernet frames to a tap interface on Linux, achieved through the use of eventfd(2) and a parallel thread that writes directly to the tap interface. This allows us to use fewer costly VM exits.

To this end, we ran a benchmark using iperf3 (and a unikernel implementing this protocol with mnet) to demonstrate the improvement in throughput.

At Robur, we aim to make deploying unikernels as straightforward as possible. That is why we have developed a suite of software and unikernels designed to deploy a range of services:

1. Albatross is a project (also installable via apt) that allows you to run a daemon capable of launching and monitoring hvt unikernels. The advantage is that this service was specifically designed for hvt unikernels (although, more recently, we have been able to deploy systems).
2. dnsvizor is one of our unikernels that resolves the issue of IP addressing and DNS resolution. The idea is to launch such a unikernel and have subsequent ones configure their networks in relation to it.
3. Finally, we are actively developing mollymawk, a unikernel that provides a web interface for Albatross to enable the deployment of new unikernels (in the simplest way possible).

We have also developed aussi (still very much in the experimental stage), which is an OCI runtime for our unikernels. Among other things, this allows us to use Docker to build and deploy our unikernels. An example using our unikernels annuaire is available here.

Finally, there is the other aspect of unikernels and their development. At Robur, we are therefore trying to develop libraries (particularly protocols and formats) to help potential new users develop new unikernels themselves (so that there is a sense of ownership).

So please do not hesitate to ask us any questions on this subject!

In this regard, for several years now we have been developing a number of libraries that enable the development of standard unikernels:

• we are continuing to develop Miou as our scheduler for our unikernels (as well as our services)
• we are developing mnet and utcp, which is our new TCP/IP stack for our unikernels
• mfat, a FAT32 file system for unikernels, is also available
• and finally vifu, a web framework for developing your website as a unikernel, is also available
• and many other projects that we have been maintaining for several years now!

This release gives us the opportunity to provide a comprehensive (and accessible) overview of our cooperative’s work. This work essentially involves creating the necessary components for the development and deployment of unikernels in OCaml, whilst embracing the idea of reclaiming control over the means of communication and production.

Happy hacking!

• Location: Paris (co-located with FPW 2026)
• Date: August 27th (Subject to change! Possible alternative: August 31st)
• Submission deadline: July 1st, AoE
• Speaker notification: July 27th
• Submission website: https://types-hotcrp.paris.inria.fr/ocaml26/

The OCaml Workshop welcomes a broad audience of OCaml users ranging from enthusiasts who are discovering the magic of OCaml to wizards well-proficient in the cast of unsafe spells. Their common denominator is their passion for OCaml and the desire to learn more, connect with fellow OCamlers, and collectively find ways to improve the language.

We invite talk proposals just as broad: anything OCaml related is welcome!

The topics are not limited to the following, but, to give an idea, examples from previous years include: OCaml editing tools, verified OCaml artefacts, interoperability between OCaml and other languages, the OCaml code of conduct, compiler optimisations, OS portability, OCaml testing frameworks, packages for concurrency in OCaml, etc.

The full catalogue from previous editions can be accessed through the links below:

• 2025: https://ocaml.org/conferences/ocaml-workshop-2025
• 2024: https://ocaml.org/conferences/ocaml-workshop-2024
• 2023: https://ocaml.org/conferences/ocaml-workshop-2023
• 2022: https://ocaml.org/conferences/ocaml-workshop-2022
• 2021: https://ocaml.org/conferences/ocaml-workshop-2021

In addition to the Standard Talk format of 20 minutes, we allow the following formats:

• Demo. 30 minutes tutorial-style demonstration of a tool.
• Informed Position. 20 minutes presentation on topics in the design space of OCaml (such as, but not limited to, the inclusion or removal of a feature).
• Experience Report. 20 minutes report on the use of OCaml or a tool.

The submission website is available at: https://types-hotcrp.paris.inria.fr/ocaml26/

Please submit a description of the talk (typically two to three pages long; it could also be less or more): the problems that are addressed and the solutions or methods that are proposed. If you believe the delivery itself is a unique feature of the talk, please feel free to also include a description of how you plan to deliver the talk.

LaTeX-produced PDFs are common but not required.

Last year's accepted presentations are available online: https://icfp25.sigplan.org/home/ocaml-2025#event-overview

Proposals largely written by LLMs are not acceptable and will be desk-rejected. The use of LLMs to correct grammar and enhance style is perfectly fine (especially if English is not your first language), but their use to produce material directly is dangerous and unprofessional, and undermines both authorship and reviewer effort.

This edition of the Caml Workshop will be located in Paris as part of Functional Programming Workshops (FPW) 2026, alongside other workshops and events taking place at the end of August. This is a departure from the usual habit of having the OCaml Workshop co-located with ICFP. Because some FPW'26 events are bi-located with ICFP, the events will take place at roughly the same dates.

The ML Family Workshop is also co-located with FW '26 this year and may be of interest to attendees. While the OCaml Workshop focuses on developments in the OCaml ecosystem, the ML Workshop is language-agnostic and more research oriented. Authors whose submissions fit both venues are welcome to indicate this at submission time or reach out to the program chairs.

• Clément Allain
• Pedro Carrott
• Simon Cruanes
• Marko Doko
• Patrick Ferris
• Jean-Christophe Filliâtre
• Sadiq Jaffer
• Robbert Krebbers
• Vincent Laviron
• Tim McGilchrist
• António Monteiro
• Carine Morel
• Andreas Rossberg
• Vimala Soundarapandian
• Jérôme Vouillon
• John Whitington
• Paulo de Vilhena (Chair)
• Sudha Parimala (Chair)