OCaml Weekly News

Previous Week Up Next Week


Here is the latest OCaml Weekly News, for the week of March 07 to 14, 2023.

Table of Contents

My experience contributing to the LLVM bindings

alan announced

Over the past few months, I’ve been contributing patches to the LLVM bindings. In particular, I ported the bindings to OCaml 5, which involved removing naked pointers. I wrote about what I learned here.

Which binding should i use for sqlite , for mariadb and for postgresql

Alain De Vos asked and Nicolas Ojeda Bar replied

Which binding should i use for sqlite , for mariadb & for postgresql ?

At LexiFi we have used https://github.com/mmottl/postgresql-ocaml for PostgreSQL and https://github.com/mmottl/sqlite3-ocaml for SQLite for a long time and found them solid.

Kim Nguyễn also replied

If you are looking for a unified API, there is Caqti. It uses the lower level bindings mentioned by @nojb (and also supports MariaDB using mariadb-ocaml. It feels like the “jdbc for OCaml” but with much more!

It provides three high-level API, either a standard blocking interface, or asynchronous with Lwt or asynchronous with Janestreet’s Async).

Frédéric Loyer then added

Caqti is nice and can be completed nicely with ppx_rapper which transforms a commented SQL string into a typed wrapper.

See https://github.com/roddyyaga/ppx_rapper

Monday, 6th March - Outreachy Intern Presentation (Dec ’22 Round)

Patrick Ferris announced

The presentation is now online: https://watch.ocaml.org/w/pQSAfZ9kDSsSnr8Bxzocn3

Thank you @Prisca for the talk and everybody for joining live. We took this opportunity to reupload everything to the OCaml Outreachy channel under the bactrian user: https://watch.ocaml.org/c/outreachy_ocaml/videos

Anil Madhavapeddy asked and Prisca replied

Your slides look great, with lots of nice OCaml/TopoJSON code fragments; might you consider making them available online somewhere? Making yourself a homepage, or using something like Speakerdeck are both good options.

@avsm Thank you very much for the kind words. I just did. This is the link https://speakerdeck.com/priceless/presentation

I noticed you published a release of the library to opam – you should add yourself to the authors list in dune-project, so that you are credited in the documentation page for TopoJSON .

Thank you for pointing that out as well. @patrickferris and I will correct it.

First release of ppx_subliner, a ppx deriver and rewriter for Cmldliner sub-command

Boning announced

I am very pleased to announce the first release of ppx_subliner! The package is now available through OPAM.

I am always a big fan of ppx_deriving_cmdliner. It helps you construct Cmdliner.Term.t from record types and makes writing cli parsing intuitive and painless. But it lacks the ability to generate values for sub-command groups and the final Cmdliner evaluations. Therefore, ppx_subliner comes to life.

ppx_subliner can work with ppx_deriving_cmdliner to generate sub-command groups. You can do so by simply tagging the extension to a variant type.

type foo = { my_arg : string } [@@deriving cmdliner]

type params =
  | Foo of foo
  | Bar
[@@deriving_inline subliner]
    [@@@ocaml.warning "-32"]
    val params_cmdliner_group_cmds : (params -> 'a) -> 'a Cmdliner.Cmd.t list
  end[@@ocaml.doc "@inline"]

It will generate a function which takes in a handle function and return the sub-command list. Here is a simple handle function.

let handle = function
  | Foo { my_arg } -> print_endline ("Foo " ^ my_arg)
  | Bar -> print_endline "Bar"

From here, you either construct the final evaluation manually:

let cmd =
  let open Cmdliner in
  let doc = "Some docs" in
  let info = Cmd.info ~doc "foobar" in
  Cmd.group info (params_cmdliner_group_cmds handle)

let () = exit (Cmdliner.eval cmd)

or use the [%%subliner.cmds] rewriter, which reuses the setfield syntax:

(* {eval function}.{type name} <- {function expression> *)
[%%subliner.cmds eval.params <- handle]
[@@name "foobar"] [@@version "3.14"]
(** Some docs *)
$ foobar.exe foo --my-arg 123
Foo 123

Both the deriver and rewriter will respect the OCaml docstring. You can also use [@name], [@man], [@envs] etc to configure all aspects of the underlying Cmdliner.Cmd.info value.

You can also use different evaluation function and set optional arguments:

[%%subliner.cmds (eval_result ~catch:false).params <-
    | Foo { my_arg } -> print_endline ("Foo " ^ my_arg) |> Result.ok
    | Bar -> print_endline "Bar" |> Result.ok)]

Please see more details in the documentation.

What’s next

I want to support inline record and enum as arg in the future, and maybe replicate some of ppx_deriving_cmdliner’s functionality, but better support for deriving_inline, lists (ie. -I liba -I libb instead of -I liba,libb), positional arguments and more compile time validation. We shall see.

Hope this is helpful. Happy hacking!


Cross-compiling for Embedded arm32 Target (RP2040 / Cortex-M0+)

Deep in this thread, Calascibetta Romain said

Note that you probably should take a look on gilbraltar which is a full OCaml runtime for Rasperry Pi 4 (arm64). We already did several works on this ocaml toolchain, a “guirlande OS” or a music player OS via Jack.

VOCaml: add and remove type annotations in VS Code

Lukasz Stafiniak announced

I’m happy to announce a VS Code extension VOCaml (GitHub release) where I’ll be putting my OCaml coding helpers. Currently, it has three commands:

  • VOCaml: Add Type Annotations inserts type annotations around let bindings.
  • VOCaml: Remove Type Annotations removes type annotations from let bindings.
  • VOCaml: Visit File from Current Line triggers a Quick Open dialog populated with a file and line number retrieved from the line at cursor position.

VOCaml is a companion to ppx_minidebug, but it can be used for its own merits. It has some limitations, report issues if you try it!

Sunsetting opam-repository-mingw

David Allsopp announced

As previously announced, “opam-repository-mingw” is no longer receiving updates.

We’re actively working on getting the Windows compiler packages into ocaml/opam-repository. There are two issues which are taking (me) a little while to finish solving, but more on that further below.

In the gap - of hopefully only a month or so - for this being ready, there’s is an issue that new releases are of course not available when opam-repository-mingw is being used with ocaml/setup-ocaml@v2 GitHub actions workflows. I’m hoping here to set out what’s happening, and what steps you may need to take to keep your GitHub Actions Windows workflows running smoothly over the next few months.

What’s happening right now?

We’ve updated setup-ocaml to use ocaml-opam/opam-repository-mingw instead of fdopen/opam-repository-mingw (see ocaml/setup-ocaml#651). This clone has been augmented with:

  • OCaml 4.14.1 packages, in the same style as the 4.14.0 forked packages (the “pre-compiled” package variants exist, but they’re not pre-compiled)
  • Changes to the constraints for existing packages only

If you’re using setup-ocaml in its default configuration, you should notice no change except that 4.14.x builds should now use 4.14.1 and the initial build will be a little slower as it builds from sources (GitHub Actions caching will then take over for subsequent runs).

For new releases of packages, it’s necessary to add opam-repository to the repositories selections for the switches. It’s important that opam-repository is at a lower priority than opam-repository-mingw for existing packages, so it’s better to use these lines in your ocaml/setup-ocaml@v2 step than to issue opam repo add --rank=1000 later:

uses: ocaml/setup-ocaml@v2
  opam-repositories: |
    opam-repository-mingw: https://github.com/ocaml-opam/opam-repository-mingw.git#sunset
    default: https://github.com/ocaml/opam-repository.git

What do I do when things are broken?

There’s an issue tracker on ocaml-opam/opam-repository-mingw, and this is a very good place to start.

If a version of a package isn’t building, there are three possible remedies:

  • Previous versions of the package may have carried non-upstreamed patches in opam-repository-mingw.

opam-repository’s policy is not to carry such patches. In this case, the package actually doesn’t work on Windows.

  • opam-repository should be updated to have os != "win32" added to the available field for the package
  • An issue on the package’s upstream repo should be opened highlighting the need to upstream patches (or even a pull request with them!)
  • The patches in opam-repository-mingw make changes which may not necessarily be accepted/acceptable upstream in their current form, so the issue may be a better starting point than simply taking a patch and opening a pull request for it (for example, the utop package contains patches which may require further work and review)
  • The package relies on environment changes in “OCaml for Windows”. For example, the Zarith package works in “OCaml for Windows” because the compiler packages unconditionally set the CC environment variable. This change is both not particularly desirable change to upstream (it is very confusing, for example, when working on the compiler itself) and also extremely difficult to upstream, so the fix here is instead to change the package’s availability with (os != "win32" | os-distribution = "cygwinports") and constrain away OCaml 5 on Windows ("ocaml" {< "5.0" | os != "win32"})
  • Package constraints on existing packages need updating in ocaml-opam/opam-repository-mingw. For example, the release of ppxlib 0.29 required some existing packages to have upperbounds added.

What about OCaml 5.0.0?

OCaml 5.0.0 was released with support for the mingw-w64 port only, however, there’s a quite major bug which wasn’t caught by OCaml’s testsuite, but is relatively easily triggered by opam packages. I’ve previously announced how to add OCaml 5 to a workflow. For the time being, the packages for OCaml 5 aren’t automatically made available.

What’s next?

The ultimate goal is to be using an upstream build of opam.exe with ocaml/opam-repository, just as on Unix. Once opam 2.2 is generally available (we’re aiming for an alpha release at the end of March) and the compiler packages in opam-repository support the Windows builds, we will recommend stopping use of opam-repository-mingw completely. The default in setup-ocaml won’t change straight away, since that risks breaking existing workflows.

With upstream compiler support, we’ll be able to extend some of the existing bulk build support already being done for Linux to Windows and start to close the gap of patches in opam-repository-mingw.

Windows compiler packages

I mentioned earlier the problems with moving the compiler packages into opam-repository, and just for general interest this elaborates on them.

The first issue affects the use of the Visual Studio port (“MSVC”) and is a consequence of the somewhat strange way that the C compiler is added to the environment when using the Visual Studio C compiler. “OCaml for Windows” (as well as Diskuv) use a wrapper command (it’s ocaml-env in “OCaml for Windows” and with-dkml in Diskuv). Those commands are Windows-specific, which is an issue for upstream opam. There’s an alternate way which sets the environment variables in a more opam-like way. Doing it that way, though, requires an improvement to opam’s environment handling which is in opam 2.2, otherwise there’s an easy risk of “blowing” the environment.

The second issue is selecting the C compiler. On Unix, this is easy with ocaml-base-compiler because there is only one “system” C compiler. Windows has two ports of OCaml, and the configuration requires it to be explicitly selected. That requires input from the user on switch creation for a Windows switch.

“OCaml for Windows” solves this by packaging the Windows compilers with the variant name appended, just as opam-repository used to, so ocaml-variants.4.14.1+mingw64 selects the the mingw-w64 port and ocaml-variants.4.14.1+msvc64 selects the MSVC64 port. The problem, as we already had in opam-repository, is that this adds 4 packages for each release of OCaml in ocaml-variants, and leads to a combinatorial explosion when we start considering flambda and other relevant compiler options.

opam-repository switched to using the ocaml-option- packages to solve the combinatorial explosion which was already present in opam-repository. The demonstration repo for OCaml 5 on Windows is already using an adapted version of this so that ocaml-option-mingw selects the mingw-w64 port (by default 64-bit, with ocaml-option-32bit then selecting the 32-bit port).

This work is all in progress and being tested alongside changes in opam 2.2 to support the depext experience on Windows. The only reason that’s not being upstreamed piecemeal is that changes to the compiler packages in opam-repository trigger switch rebuilds all over the world, so we don’t want to that until we’re sure that the packages are correct. The intention is to do this around the time of the alpha release of opam 2.2, once the work in opam itself has settled down.

Thanks for getting to the end, and happy Windows CI testing!

Outreachy Summer 2023

Paul-Elliot announced

Just wanted to say that I also submitted an OCaml Outreachy project!

The goal of the project is to improve the error reporting in the PPXs ecosystem. This includes both modifying ppxlib’s behavior when catching an exception raised by a rewriter; and modify existing PPXs to better report error (by embedding them inside the AST when relevant, allowing for both multiple error reporting, and creation of a partial AST that contains all information that could be created).

VS Code extension Alcotest Test Explorer 0.4.0

Roland Csaszar announced

Thanks to @Muqiu-Han a severe bug has been fixed: an exception raised by failwith wasn’t recognized as failure and the test result had been set to ’passed’.


Version 0.4.0 (2023-03-10)

  • Mark a test case as failed if a [FAIL] tag is present, even if the actual error message can’t be parsed.


  • Make the error parser recognize the exception generated by failwith. (Fix #1)

Internal Changes

  • Add test cases to check that the changes actually work.
  • Fix typo in issue template.
  • Correct the job name in the release GitHub workflow.


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 online.