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Here is the latest OCaml Weekly News, for the week of September 16 to 23, 2025.

Table of Contents

Introduction to the Dune build system, by OCamlPro

Archive: https://discuss.ocaml.org/t/blog-introduction-to-the-dune-build-system-by-ocamlpro/17279/1

OCamlPro announced

Greetings Cameleers,

We’ve just released a new blog post: OCaml Onboarding: Introduction to the Dune build system

This is a practical, compact guide for those starting their OCaml and Dune journey — or helping others do so. Instead of diving into internals, we focus on what you need to know to get up and running with Dune confidently.

We walk through:

  • How Dune thinks about builds and directories
  • The role of dune files and static declarations
  • The tooling one interacts with day-to-day

The post keeps things hands-on and beginner-focused, with just enough theory to make the practical bits stick. We end by circling back to dune init — showing how understanding the basics makes the scaffolding tool far more intuitive.

📝 Read it on the OCamlPro Blog

Perfect for sharing with colleagues or newcomers dipping their toes into OCaml and Dune.

As always, feedback welcome! Until next time, — The OCamlPro Team

memprof-limits (first official release): Memory limits, allocation limits, and thread cancellation, with interrupt-safe resources

Archive: https://discuss.ocaml.org/t/ann-memprof-limits-first-official-release-memory-limits-allocation-limits-and-thread-cancellation-with-interrupt-safe-resources/8135/3

Guillaume Munch-Maccagnoni announced

I am pleased to announce the version 0.3.0 of the package memprof-limits. The main contribution of this release is the support for OCaml 5 with multiple parallel domains.

Memprof-limits can henceforth be used to conveniently interrupt (CPU-bound) domains in your parallel computations, using cancellation tokens, with the support of features to ensure interrupt-safety and resource-safety.

As example of uses, plans are underway to use memprof-limits to ensure the resource-safety of user interrupts in the Rocq prover (eliminating a class of bugs) and to add tactics for portable timeouts (that count allocations rather than elapsed time).

mlfront-shell - reference implementation of a build system

Archive: https://discuss.ocaml.org/t/ann-mlfront-shell-reference-implementation-of-a-build-system/17186/2

jbeckford announced

There have been many improvements since the last update. Here are some of them:

  1. The trace store (needed to not rebuild everything from scratch) has been implemented.
  2. Build file ASTs are parsed, marshalled and put into a key-value directory (which can be shared or cached in CI).
  3. Build keys are generated and used to protect sharing of the parsed AST.
  4. A full walkthrough of “building” the 7zip executable from 7zip binary assets (it is more complicated than it sounds) is on the documentation page
  5. An –autofix option to update build files with checksums, and allowing SHA1 checksum for local files (both are for eventual support of Meta’s watchman tool)
  6. A new format for the JSON files (many functions versus one). The old format is deprecated.
  7. Many many bug fixes, including specification updates.

Zanuda – OCaml linter experiment

Archive: https://discuss.ocaml.org/t/ann-zanuda-ocaml-linter-experiment/11784/13

Continuing this thread, Kakadu announced

Zanuda 2.0.0 with OCaml 5.3 support have hit opam.

YOCaml, a framework for static site generator

Archive: https://discuss.ocaml.org/t/ann-yocaml-a-framework-for-static-site-generator/15393/9

Xavier Van de Woestyne announced

Release 2.5.0

We are delighted to present the new release of YOCaml: 2.5.0! As you can see, there have been a few intermediate versions since our last announcement. However, we have reached a new milestone: YOCaml can now be used seamlessly with an Applicative API (instead of the Arrow one), making many tasks much easier to express!

We have also finally taken the time to write a tutorial that explains how to use YOCaml to create a complete blog, step by step, and our goal is to expand it over time to add more and more guides! (The documentation/guide generator (https://github.com/yocaml/yocaml-www is also written in YOCaml and gives an idea of what can be done fairly quickly.)

We look forward to receiving your feedback! YOCaml is a free and collaborative project, so any contributions (including the guide) are more than welcome! We would also be DELIGHTED to see your creations with YOCaml!

Happy Hacking!

Lwt.6.0.0~beta (direct-style, multi-domain parallelism)

Archive: https://discuss.ocaml.org/t/ann-lwt-6-0-0-beta-direct-style-multi-domain-parallelism/17283/1

Raphaël Proust announced

After some feedback and some work, I'm happy to announce the release of lwt.6.0.0~beta00 and lwt_direct.6.0.0~beta00! (https://github.com/ocaml/opam-repository/pull/28558)

major CHANGES are:

  • direct style mode (see alpha release announce): use await : 'a Lwt.t -> 'a to transform any promises into a simple value. This allows you to break out of the monad which makes it possible to use libraries previously incompatible with Lwt.
  • multi-domain support: run separate schedulers in separate domains (some of the more advanced functions on Lwt are now domain-dependent, e.g., run_in_main becomes run_in_domain and takes one additional parameter)

Feedback is very welcome. Happy beta-testing and good luck with the parallel-programming!

schm-ocaml

Archive: https://discuss.ocaml.org/t/ann-schm-ocaml/17293/1

Florent Monnier announced

schm-ocaml provides a scheme-like syntax.

schm3 integrates nicely with the rescript-version that provides the ocaml syntax.

schm4 integrates with one of the later ocaml 4 versions, and tries to be accessible from .c

http://decapode314.free.fr/ocaml2/schm/

There is a tutorial:

http://decapode314.free.fr/ocaml2/schm/schm-tut.html

http://decapode314.free.fr/ocaml2/schm/dl/schm4-0.03.zip

The oo got lost in a computer crash.

So there is no oo.

detri cmd

Archive: https://discuss.ocaml.org/t/ann-detri-cmd/17301/1

Florent Monnier announced

There is now a fifth variant for detri:

http://decapode314.free.fr/ocaml/detris.html

This fifth one outputs .html, in-stead of console.

If you took a previous version for the console, it was already something you could do by-yourself easily rewriting the console esc-chars to html, but now there is a ready-made one.

(There is also a small tutorial, but it should be re-written,

http://decapode314.free.fr/ocaml/detri/tut/detri-tut.html

)

seven ocaml tutorials

Archive: https://discuss.ocaml.org/t/ann-s-tut-seven-ocaml-tutorials/17302/1

Florent Monnier announced

You will find seven new ocaml-tutorials on this page that I wrote recently:

http://decapode314.free.fr/ocaml2/blog/

It was not writen with a chat-bot, but you will maybe notice a difference in the style of writing, since I’m using a chat-bot.

Ortac/Wrapper: a new plugin for specification driven unit testing

Archive: https://discuss.ocaml.org/t/ann-ortac-wrapper-a-new-plugin-for-specification-driven-unit-testing/17304/1

Charlène_Gros announced

Hello everyone,

We, at Tarides, are excited to announce the release of a new plugin for ortac: Ortac/Wrapper!

This plugin is part of the Gospel project, a contract-based behavioural specification language for OCaml. ortac is a tool that converts an OCaml module interface with Gospel specifications into code to check those specifications. There are various ways to check specifications, all provided by plugins, and this post announces the new plugin: Ortac/Wrapper!

This plugin is designed to generate a wrapped module that exposes the same interface as the original module but instruments all function calls with assertions corresponding to the Gospel specifications. The main objective is to assist with unit testing. You provide the Gospel specification for your file, and Ortac will instrument it. When you run the unit tests on the wrapped version, if a specification is violated, Ortac will crash with an explicit error, telling you which portion of your code is incorrect and which specifications were violated.

This work has been started by Clément Pascutto during his PhD at LMF and Tarides https://theses.hal.science/tel-04696708v1. I continued his work to support some Gospel features such as the old operator and models.

Installation

To install the Wrapper plugin, use the following command: opam install ortac-wrapper

This will install the following OPAM packages:

  • ortac-core.opam which provides the ortac command-line tool and the core functionalities used by all plugins,
  • ortac-runtime.opam which provides the support library for the code generated by the Wrapper Ortac plugin,
  • ortac-wrapper.opam which provides the Wrapper plugin for the ortac command-line tool.
  • To automatically generate dune files

    If you need dune rules to integrate Ortac into your project, you can install the Dune plugin: opam install ortac-dune This will install the following OPAM packages:

    • ortac-core.opam which provides the ortac command-line tool and the core functionalities used by all plugins,
    • ortac-dune.opam which provides the Dune plugin for the ortac command-line tool.

Try it!

Let’s dive into a mini tutorial to see how the Ortac/Wrapper plugin can be used to enhance your unit testing with Gospel specifications. We’ll walk through creating a simple polymorphic container type with limited capacity and see how to specify and test its behavior using Gospel.

  • 1- Define the type and models

    First, we define a polymorphic container type 'a t with Gospel specifications. This type will have a fixed capacity and a mutable list of contents. 

    type 'a t
(*@ model capacity: int
        mutable model contents: 'a list
        with t
        invariant t.capacity > 0
        invariant List.length t.contents <= t.capacity *)

    Here, we define two models:

    • capacity: Represents the fixed size of the container.
    • contents: Represents the mutable list of elements currently stored in the container.

    The invariants ensure that the capacity is always positive and that the contents list never exceeds the declared capacity.

  • 2- Specify function behavior

    Next, we specify the behavior of functions that manipulate the type 'a t. We’ll define a create function to initialize the container and an add function to insert elements into the container. 

    val create: int -> 'a t
(*@ t = create c
        requires c > 0
        ensures t.capacity = c
        ensures t.contents = [] *)
val add: 'a t -> 'a -> unit
(*@ add t x
        modifies t.contents
        ensures t.contents = x :: (old t.contents) *)

    Here the functions create and add are specified in Gospel.

    • The create function requires the capacity c to be strictly positive and ensures that the model of the new container has the specified capacity and an empty list of contents.
    • The add function modifies the contents of the container and ensures that the new element x is added to the list of contents.
  • 3- Define projection functions

    To validate these specifications at runtime, you need to provide projection functions that link OCaml values to their Gospel models. Projection functions can be defined in two ways.

    • Using the same name as the model.
    • Using a different name, annotated with the attribute @@projection_for and the name of its Gospel model.

    For our example, we define the projection functions as follows. 

    val capacity : 'a t -> int
val to_list : 'a t -> 'a list [@@projection_for contents]

    Where we encounter the two types of naming.

    • The capacity function directly corresponds to the capacity model.
    • The to_seq function is explicitly declared as the projection for the contents model using the @@projection_for attribute.

    These projection functions are mandatory for the Wrapper plugin to instrument the specifications. If any projection function is missing, nothing will be generated and an error will be printed.

  • 4- Generate the wrapped version

    Once you have both of the interface file annotated with Gospel and your implementation, you can start the generation. If you have installed the ortac-dune package (which is recommended), you need to add the following in the dune file where you want to put the tests. 

    (rule
 (alias runtest)
 (mode promote)
 (action
  (with-stdout-to
   dune.wrapper.inc
   (setenv
    ORTAC_ONLY_PLUGIN
    dune-rules
    (run ortac dune wrapper <path to lib/lib.mli>)))))

    This will generate an additional Dune file called dune.wrapper.inc that you will need to include once created: (include dune.wrapper.inc). Also, add the name of the wrapped module to the libraries stanza of the test folder.

  • 5- Add unit tests

    In order to test the Lib module, we can now simply write a program using the wrapped version. No need to specify the expected behaviour as the instrumentation will take care of that.

    For example, if you run the following program: 

    open Lib_wrapped
let () =
  let q = create 3 in
  add q 1;
  let q2 = create (-1) in
  add q2 1;
  ()

    You will obtain the following result: 

    File "lib.mli", line 8, characters 0-175:
Runtime error in function ~create'
  - the pre-condition
      `c > 0'
    was violated.
Fatal error: exception Ortac_runtime.Error(_)

    This process helps ensure that your code adheres to the specified behavior, making your unit tests more robust and informative.

Feel free to report

For more information here is the link of the dedicated README. If you encounter any bugs or misunderstandings, please feel free to report them as an issue on GitHub.

We hope this plugin will be useful to you and look forward to your feedback!

Acknowledgments

This work is partly founded by the ANR grant ANR-22-CE48-0013.

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