Hello Here is the latest OCaml Weekly News, for the week of November 25 to December 02, 2025. Table of Contents ───────────────── avro-simple, an OCaml implementation of Apache Avro Sunsetting of DkML distribution opam 2.5.0 is out! Transient emacs command for dune Slipshow! Two open roles at Tarides: Compiler Engineer and VP of Software Engineering RFSM version 2.2 Alice v0.2.0, now with LSP Gendarme, a modular marshalling library FUN OCaml is live on YouTube and Twitch Other OCaml News Old CWN avro-simple, an OCaml implementation of Apache Avro ═══════════════════════════════════════════════════ Archive: Tim McGilchrist announced ───────────────────────── I’ve been building a library for Apache Avro called [avro-simple]. It is an OCaml implementation of [Apache Avro] with codec-based design, schema evolution support, and container file format. The key principles for this library are: • *Value-centric design*: Manual codec construction using combinators (no code generation required) • *Pure OCaml*: No external C dependencies for core functionality • *Schema evolution*: Built-in support for reading data with different schemas • *Container files*: Full support for Avro Object Container File format • *Compression*: Multiple compression codecs (null, deflate, snappy, zstandard) with compression plugins • *Streaming*: Memory-efficient block-level streaming for large files • *Type-safe*: Codec-enforced types with composable combinators This compares to [ocaml-avro] which uses code generation based on JSON schemas and is missing some of the schema evolution features. I'm mainly using this for reading and writing Avro container files, however it should be possible to integrate with [ocaml-kafka] if you use Kafka. The performance is reasonable so far, 1.4 times slower than the fastest Rust based library I could find and I haven't really tried to optimise it yet. HEAPs faster than the official Apache Avro libraries in Python. There are a few other places I think I can improve the performance and memory usage, but it should be quite usable for small to medium sized files. Enjoy :slight_smile: [avro-simple] [Apache Avro] [ocaml-avro] [ocaml-kafka] Sunsetting of DkML distribution ═══════════════════════════════ Archive: jbeckford announced ─────────────────── DkML, the Windows-friendly distribution of OCaml, is being sunset. DkML was launched to get Windows + OCaml off life-support when an earlier Windows distribution was retired. Mission accomplished. And thanks to OCSF for the support in the early years! Existing users: Official end-of-life for the distribution will be *12/31/2026*. 1 year and 1 month from this posting the distribution binaries, C libraries and overlay repositories will no longer be hosted. Please use the native Windows support from opam! To be clear, the Windows *distribution* is the following two products which will disappear: • *dkml-installer*: This is the Windows installer available through `winget' (). *I’ll withdraw it from winget soon; that won’t affect existing users.* • *setup-dkml*: This is the GitHub/GitLab CI for MSVC. In contrast, the following products are much broader than Windows, are actively maintained, and will *_not be sunset_*: • /dkml-compiler/ and /dkml-base-compiler/. These have patches for [Android] and [bytecode]. • /MlFront/ and /dk/. This is the build and scripting system; more on that in upcoming announcements. [Android] [bytecode] opam 2.5.0 is out! ══════════════════ Archive: Kate announced ────────────── Hi everyone, We're happy to announce the release of opam 2.5.0 and encourage all users to upgrade. *Note*: the following section will recap the various major changes in opam `2.5.0' for anyone who haven't already read the previous pre-release announcements. For those who did, note that nothing changed between `2.5.0~beta1' and the final `2.5.0'. What’s new? Some highlights: ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ • :high_speed_train: Speedup opam update up to 70%. Thanks to [@arozovyk], `opam update' now load opam file incrementally, only parsing the files that have changed since the last time you called `opam update'. Before that, opam files in opam repositories were all loaded after an update if there was any change. The performance improvement of this change thus depends on how often you call `opam update' and what type of repository and OS you are using. ([#5824]) • :spiral_shell: Improved shell integration. A number of users have been hitting issues with opam's shell integration where parts of a previous environment was kept in the current environment, causing a number of issues. These can be triggered by, for example, nuking your opam root directory (by default `~/.opam' or `%LocalAppData%\opam'). For this particular case we are still working on a fix, but many other users have reported similar issues without nuking their root directory and in that case we believe to have fixed the majority of issues. ([dbuenzli/topkg#142], [#4649], [#5761]) • :spiral_shell:² We've also changed the default file to which `opam init' writes the opam shell integration to be `.bashrc' instead of the previous `.profile' or `.bash_profile' when `bash' is detected. Doing it this way prevents some issues with existing `.profile' files that source the `.bashrc' file and causing an infinity loop when opam asks users to make sure to source their `.bashrc' file into their `.profile' file. ([#5819], [#4201], [#3990]) • :shield: The opam install script now installs an appropriate `apparmor' profile on systems configured with `apparmor' (this is enabled by default on Ubuntu). This change is not strictly speaking related to this release as it is deployed for every versions. ([#5968]). • :ocean: Many more UI additions and improvements, bug fixes, … :open_book: You can read our [blog post] for more information about these changes and more, and for even more details you can take a look at the [release note] or the [changelog]. [@arozovyk] [#5824] [dbuenzli/topkg#142] [#4649] [#5761] [#5819] [#4201] [#3990] [#5968] [blog post] [release note] [changelog] Try it! ╌╌╌╌╌╌╌ The upgrade instructions are unchanged: For Unix systems ┌──── │ bash -c "sh <(curl -fsSL https://opam.ocaml.org/install.sh) --version 2.5.0" └──── or from PowerShell for Windows systems ┌──── │ Invoke-Expression "& { $(Invoke-RestMethod https://opam.ocaml.org/install.ps1) } -Version 2.5.0" └──── Please report any issues to the [bug-tracker]. Happy hacking, <> <> The opam team <> <> :camel: [bug-tracker] Transient emacs command for dune ════════════════════════════════ Archive: Paul-Elliot announced ───────────────────── Hello! :tada: On the behalf of everyone who has contributed to a training set for an LLM by writing something on the internet, I'm happy to announce the release of [`dune-transient'], an emacs transient command to drive dune! :tada: When invoked with `M-x dune-transient', it opens a first panel: Some keys provide direct access to common workflows. Pressing B will open a subsequent panel allowing you to build your very own command in the transient way: Now that you know the gist of the tool, let me quote an important part of the README^[This text was written by me so it's acceptable to publish it here!]: It was built entirely by an AI. I was only in charge of copy-pasting various bits of inscrutable text from one place to the other. As a consequence, I'm afraid this project cannot accept contributions made by humans. We need to wait a bit before humans are reliable enough. You are however welcome to open PR, provided both the PR text and the code in it has been written by an AI, without human review. If you find it useful, you are welcome to use it, even if you are not 100% an AI. You've been warned! [`dune-transient'] Slipshow! ═════════ Archive: Paul-Elliot announced ───────────────────── I'm back! Slipshow 0.7.0: The Slipshow of Dorian Gray ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ It's with guilty pleasure that I announce the new [release] of Slipshow 0.7.0: The Slipshow of Dorian Gray, on [opam]. ┌──── │ $ opam update │ $ opam upgrade slipshow └──── This release contains a handful of bugfixes. But the highlight of the changelog is an experimental support for recording and replaying annotations! [Peek 2025-11-26 13-25] The gif above was made using the new feature. You can also [view it live]. If you want an example of a more interesting use of the feature than a picture of me grow old, you can also see [this presentation]. You can also find the doc [here]. The exclusive feature, that might not be obvious above, is that once you've recorded a series of strokes, you can edit it. For instance, here is the timeline for this [presentation]: You can see it live and edit it by pressing `Shift + R' in the presentation, and selecting one of the recording in the dropdown on the bottom left. However experimental this is, I can't wait to see what kind of good and bad uses it can have for your presentations. Please post them here! Thanks a lot to [NLNet] for their invaluable [support]. And before the changelog, an exclusive information relevant to OCaml users! The drawing edition UI, and actually the whole drawing mechanism, is made using Functional Reactive Programming, in particular with @let-def's [brr-lwd] library! As always, I'd like to say thank you to the developers of open source libraries, you are so great. Here are the release notes: [release] [opam] [Peek 2025-11-26 13-25] [view it live] [this presentation] [here] [presentation] [NLNet] [support] [brr-lwd] BREAKING CHANGES! ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ • I removed support for the "setext" headings: Underlining a title with dashes will no longer make a title. Sorry for breaking a standard, but that messes too much with the `---' separator. Replace it with atx headings: `# This is a title'. • A bug in the carousel was fixed, breaking the workaround. If you used `+2' to go to the next meaningful page, you can now change that, either to `+1' or to `all'. • The semantics of `focus' and `unfocus' was changed. Before, you had to unfocus as many times as you focused. Now, you have to unfocus only once. • If you find any migration problem, please open an issue and I'll help for the migration! Compiler ╌╌╌╌╌╌╌╌ • Embed Liberation Sans fonts (and use them) (#150) • Fix missing favicon (which was missing since speaker view) (#150) • Fix changing step number from speaker note does not update serve mode state (#154) • Fix blank lines considered as elements in carousel (#170) • Allow to specify port in `slipshow serve' with `-p' or `--port' (#176) • Fix link with no content in block raising a syntax error (#180) • Remove support for Setext headings (#178) Engine ╌╌╌╌╌╌ • Allow to record and replay strokes (#187) • Fix speaker note scrolling (#150) • Fix script undos recovery when script execution fails (#150) • Hide paused/unrevealed elems also for mouse (#150) • Don't execute scripts when computing toc (#150) • Mute medias in speaker view (#152) • Use the [perfect-freehand] library to generate strokes. (#151) • Fix order of execution of actions (`center' after `enter') (#171) • Fix pauses not being scoped in slides (#179) • Fix exiting not where it should (#179) • Fix `unfocus' behavior to match the docs (#179) • Fix wrong position bug on custom dimensions (#182) • Fix infinitely jiggling autoresizing (#187) • Fix not being able to draw outside of inner presentation (#187) • Fix permanent fast-moving bug (#187) [perfect-freehand] Two open roles at Tarides: Compiler Engineer and VP of Software Engineering ═══════════════════════════════════════════════════════════════════════════ Archive: Thomas Gazagnaire announced ─────────────────────────── Hi all, Tarides’ mission is to help make OCaml mainstream. We work across the stack: contributing upstream to the compiler, maintaining ecosystem tooling, and supporting organisations that rely on OCaml in production. Our work ranges from training and team extensions to full product development. For instance, Tarides recently spun off [Parsimoni] which builds software-defined satellite systems using a platform based on MirageOS (in OCaml) and Unikraft. We also collaborate with companies that already operate large OCaml codebases, such as Jane Street and Semgrep, supporting them on targeted projects and long-term initiatives. To sustain this breadth of activity, we are opening two roles on . [Parsimoni] *Compiler Engineer* ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ The compiler team at Tarides has been central to major changes in OCaml over the past years, including the design and integration of the multicore runtime and effect handlers in OCaml 5. Beyond that milestone, the team continues to shape the evolution of the language and its runtime: improving the OCaml/WASM toolchain, refining the behavior of OCaml 5 programs under load, and exploring new directions for the type system and tooling. The role suits someone who enjoys language implementation, runtime internals, and the practical constraints of deploying OCaml in production environments (especially performance). Details: *VP of Software Engineering* ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ As our projects and teams have grown, we are looking for a VP of Software Engineering to guide the group of team leads across Tarides. The role involves structuring delivery, keeping teams aligned with the technical roadmap, and maintaining a clear interface between engineering and the rest of the company. It is a VP-level position, but experienced team leads or engineering managers who are ready to step into a wider leadership role should consider applying. Details: If you would like to discuss either post or want more context about current projects, feel free to reach out. Best, Thomas Thomas Gazagnaire later added ───────────────────────────── And now a third one, to improve the OCaml runtime's performance and create a successor to venerable [sandmark]. [sandmark] Runtime Systems Engineer ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ The multicore work merged in OCaml 5.0 introduced native support for scalable concurrency and shared-memory parallelism. Its design combines effect handlers for expressing structured concurrent workflows, a concurrent garbage collector aimed at responsive networked applications, and a modular memory model that supports local reasoning without sacrificing performance. With these foundations now in the mainline compiler, Tarides is helping partners migrate their systems to OCaml 5. This third role focuses on the practical side of that transition: developing tooling to understand runtime behaviour, maintaining benchmarks that guide runtime improvements, and collaborating on future directions for the scheduler and GC. It is well-suited to someone who wants to work on the boundary between research-grade runtime design and the realities of large OCaml deployments. Details: RFSM version 2.2 ════════════════ Archive: jserot announced ──────────────── It’s my pleasure to announce the availability of version `2.2' of the `RFSM' language and compiler. `RFSM' is a domain specific language for describing, simulating and generating code from *reactive finite state machines*. From a description of a system composed of a set of reactive FSMs, the `RFSM' compiler can generate • graphical description of the system in the `.dot' format • execution traces in the `.vcd' file format • code in `C', `SystemC' and `VHDL' for simulation or implementation on a target platform The most significant change since version 2.0 is the introduction of a *server mode* allowing the compiler to be called on fragments of code. This feature is used by the [Grasp] application, a graphical front-end to `RFSM' (superseding the previous `Rfsm-Light' application). `RFSM' is available from this [GH page] or as an [OPAM package]. Comments, feedbacks and bug reports welcome ! [Grasp] [GH page] [OPAM package] Alice v0.2.0, now with LSP ══════════════════════════ Archive: Steve Sherratt announced ──────────────────────── I'm happy to announce the release of [Alice v0.2.0]. Alice is a radical, experimental OCaml build system, package manager, and toolchain manager for Windows and Unix-based OSes. Its goal is to allow anyone to program in OCaml with as little friction as possible. The main new addition is support for LSP. Alice now generates some additional files that can be interpreted by OCaml-LSP/Merlin. To use OCaml-LSP in an Alice project, make sure to configure your editor to start the LSP server with the command `ocamllsp --fallback-read-dot-merlin' and install `dot-merlin-reader' (either by running `alice tools install' or from the `dot-merlin-reader' opam package). See [this page] for more info on setting up LSP for Alice projects. Alice has gotten slightly easier to install. There's now a package in the [WinGet] repository so Windows users can install Alice by running: ┌──── │ winget install OCaml.Alice └──── There's also a Homebrew tap for Alice, so macOS users can install Alice by running: ┌──── │ brew install alicecaml/homebrew-tap/alice └──── The [Install page] has an interactive OS picker that displays install instructions for the current OS by default. [Alice v0.2.0] [this page] [WinGet] [Install page] Gendarme, a modular marshalling library ═══════════════════════════════════════ Archive: Benjamin Somers announced ───────────────────────── Hi everyone! This is my first post on this forum, and I’m pleased to present [Gendarme], a generic-but-opinionated library to marshal and unmarshal OCaml data types in a variety of formats. [Gendarme] Why a new library? ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ OCaml has a few libraries in the `ppx_deriving' family, like the famous [`ppx_deriving_yojson'], allowing to very conveniently generate marshallers and unmarshallers for OCaml types. However, two elements didn’t suit me in this approach: • These libraries pollute the namespace quite a bit when we start combining them (*e.g.* when developing user-facing apps that allow ingesting several serialization formats); • Adding support for a new format is hard and requires some PPX expertise most OCaml users don’t have. [`ppx_deriving_yojson'] How the project is born ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ When discovering the Go language, I was pleasantly surprised by how easy it was to marshal and unmarshal structs with simple annotations, and wanted a similar hassle-free mechanism in OCaml. I also wanted to learn about GADTs, as I never had found any use for them in my projects before. This project was originally named `Marshal', but a module of the same name already exists in the standard library. “Gendarme” is one way to translate “Marshal” in French. What’s particular about Gendarme? ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ Gendarme is a modular, extendable, PPX-heavy marshaller and unmarshaller based on type witnesses, supporting various data formats (CSV, JSON, TOML, YAML). The curious reader may find **way more** information on [the project’s repository], but here’s the gist: ┌──── │ [%%marshal.load Yojson] │ │ type t = { t_foo: int list [@json "foo"]; │ t_bar: t list [@json "bar"] } [@@marshal] │ type u = t * int [@@marshal] │ │ let v = ({ t_foo = [1; 2]; t_bar = [{ t_foo = [3; 4]; t_bar = [] }] }, 3) │ │ let json = [%encode.Json] ~v u │ (* │ val json : string = "[{\"foo\":[1,2],\"bar\":[{\"foo\":[3,4],\"bar\":[]}]},3]" │ *) └──── Annotating a type `my_type' with `[@@marshal]' and providing the required additional data (such as field names in the case of records) builds a witness value `my_type' of type `my_type Gendarme.ty' that tells Gendarme how to marshal and unmarshal values. Currently, [only a subset of OCaml core types are supported], but generic support gets improved as the need appears in my personal and professional projects. Gendarme was written both with users (application and library developers) and developers (people developing new Gendarme encoders) in mind. If your target format is able to encode objects/records (optional but nice to have), lists/arrays, and supports arbitrarily nesting them, then writing a new encoder requires writing **at most** 100 lines of what is essentially pattern-matching cases to tell Gendarme what to do with your data (see for example [the code for `gendarme-ezjsonm']). Each encoder is heavily tested, and most of the encoders that we ship have more lines of code for tests than for their actual logic. This is a very quick introduction, but if you are interested in this project, head over to [its README] to learn more! I’m obviously happy to answer any questions you may have. [the project’s repository] [only a subset of OCaml core types are supported] [the code for `gendarme-ezjsonm'] [its README] FUN OCaml is live on YouTube and Twitch ═══════════════════════════════════════ Archive: Continuing this thread, Sabine Schmaltz announced ───────────────────────────────────────────────── FUN OCaml talks are now uploaded to [FUN OCaml's watch.ocaml.org channel]: ETA: still transcoding :sweat_smile: but due to appear any moment [FUN OCaml's watch.ocaml.org channel] Other OCaml News ════════════════ From the ocaml.org blog ─────────────────────── Here are links from many OCaml blogs aggregated at [the ocaml.org blog]. • [Keeping your branch up-to-date] • [The AI French Connection to the Practice of Science] • [OCaml 5.4 native Arm32 branch] • [Bringing Emacs Support to OCaml's LSP Server with `ocaml-eglot'] • [Advent of FPGA — A Jane Street Challenge] • [Four Ps for Building Massive Collective Knowledge Systems] [the ocaml.org blog] [Keeping your branch up-to-date] [The AI French Connection to the Practice of Science] [OCaml 5.4 native Arm32 branch] [Bringing Emacs Support to OCaml's LSP Server with `ocaml-eglot'] [Advent of FPGA — A Jane Street Challenge] [Four Ps for Building Massive Collective Knowledge Systems] Old CWN ═══════ If you happen to miss a CWN, you can [send me a message] and I'll mail it to you, or go take a look at [the archive] or the [RSS feed of the archives]. If you also wish to receive it every week by mail, you may subscribe to the [caml-list]. [Alan Schmitt] [send me a message] [the archive] [RSS feed of the archives] [caml-list] [Alan Schmitt]