OCaml Weekly News

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Here is the latest OCaml Weekly News, for the week of October 25 to November 01, 2022.

Table of Contents

What are pros and cons of Eliom web framework?

Deep in the thread, ZAN DoYe said

I’ve made a cjkv info app with eliom, the old demo version is here(most of its functions are turned off and hidden): https://cjkv.smaji.org/ It collects variation, source info, input method code of characters among China, Japan, Korea, Vietnam, Hongkong, Taiwan, ethnic minorities and ancient books. e.g.



Overall, eliom framework is fantastic. All the links are eliom_service, its parameters and itself are typed. Session manage is powerful and flexible. Components are reactive with each other, seamlessly between server and client. An app written in eliom is intrinsically an SSR SPA, an integral whole.

Writing an eliom app, all I want to say is: damn! it’s too good!


  • dune support
  • merlin, ocaml-language-server

    Your project is built by a manually managed Makefile and merln/lsp support is broken when you mix client/server code together in the same folder.

    You only get these supports when you write standalone server and client separately.

    But these flaws can be overcome. I wrote a custom code translator, which generates server code and client code from an eliom file into separate folder and make them still work together seamlessly. And in this case, I don’t need a Makefile, dune and ocaml-lsp works as normal projects.

  • ocsigenserver

    It’s good. But it updates infrequently and doesn’t keep pace with other libraries closely. Because eliom depends on it and its dependence constraint sometimes requires old libraries, better create a separate opam switch when working with an eliom project.


In fact, it’s not a wart of eliom. It’s lwt https://github.com/mirage/ocaml-cohttp/issues/545 That’s why I stop working on the old demo and now rewriting it again with another tec stack.

If only eliom depended on lwt/async optionally and came with an official code translator to overcome the dune/lsp integration issue.

Domain-shims: an implementation of `Domain` for OCaml 4 (beta version)

gasche announced

I just wrote a small package, domain-shims, that provides a non-multicore implementation of the new Domain module of the OCaml 5 standard library:

repository: https://gitlab.com/gasche/domain-shims

the one source file: https://gitlab.com/gasche/domain-shims/-/blob/trunk/lib/domain.ml

This is a fun hack, but it may also be useful for people who write some code using Domain to test it under OCaml 5, but still want to be able to build and run their project using their normal OCaml 4 setup.

I haven’t put a version number and made a formal release yet.

The main limitation of the library is that it does not implement the Effect module providing effect handlers, so it is not true that all OCaml 5 code will be compatible. I discuss this in more details in the README # no effect support, quoted below:

OCaml 5 also provides effect handlers as a new Effect module of the standard library. We do not provide an implementation of Effect, which is much harder to emulate using regular OCaml 4.

Someone could experiment with an implementation using the delimcc library for delimited control. (Please do this and report! It is probably fun.) We expect that the result would be much slower than the OCaml 5 implementation, probably by an order of magnitude. This would be okay for toy experiments. But it would not be good enough to seriously run libraries relying on effect handlers for performance-sensitive operations, such as Domainslib or Eio.

The other option would be to implement shim versions of Effect-using libraries. We are considering providing a shim for Domainslib in particular, which is very easy to do without emulating effects at all. This general approach sounds scary (no limit to the number of programs you need to provide shims for), but remember that OCaml 5 users are not supposed to use effect handlers too much yet, it was only made available for lightweight concurrent schedulers.

Happy hacking :-)

OCaml Platform Installer alpha release

Deep in this thread, Hannes Mehnert asked and Paul-Elliot replied

Hello Hannes and thanks for your interest!

do you have machine-readable data what went into the binaries that are being downloaded (i.e. which version of what) [nowadays the buzzword is “Software Bill of Materials] – NB: and opam switch export –full –freeze is a good start for the opam packages

In fact, currently only the ocaml-platform binary is being downloaded. The tools are then compiled in a sandbox switch on the local machine, and cached in a local opam repository. Only the version of OCaml and of the tool is kept as info in the local cache.

do you have security in mind? the curl | sudo bash is not very nice from a security perspective, since that may do any harm.

We do have security in mind! We wanted a one line copy-paste installation for new users. So, we went for an install script, although we will consider your advice on setting up a package repository!

is there an update path? when I run your installer, how to upgrade to the latest version of “platform installer” (can I check whether an update is available and what will change?)?

Currently, the update path is simply to re-run the install script:

bash < <(curl -sL https://github.com/tarides/ocaml-platform-installer/releases/latest/download/installer.sh)

But there is no way to know if there is a new release, apart from checking the github releases page. Thanks a lot for the inputs! I’ll reach out to you soon by private message, as your work and experience on reproducible builds might help for the ongoing remote repo!

geojson and geojsone

Patrick Ferris announced

On behalf of the geocaml org, I’m happy to announce the first release of geojson and geojsone.

What is GeoJSON?

GeoJSON is defined in RFC7946 as

…a geospatial data interchange format based on JavaScript Object Notation (JSON).

It is a schema for writing geospatial information in JSON often as some combination of “geometries” (e.g. a Point or a Polygon). These are all using the World Geodetic System 1984 (WGS84) coordinate reference system.

An example

A collections of features where the first feature contains a single point.

  "type": "FeatureCollection",
  "features": [
      "type": "Feature",
      "properties": {
        "name": "Place du Capitole"
      "geometry": {
        "coordinates": [
        "type": "Point"

Geojson Library

The geojson library provides functions for reading and writing GeoJSON objects. You must, however, provide a simple interface for the JSON library of your choosing. This was a design decision made to offer better support for using the library in the browser. You can read more about it in the documentation.

Geojsone Library

The geojsone library is inspired by the jsonm library. It vendors jsonm and ports it to a non-blocking implementation provided using effects. Internally it then reconstructs ezjsonm using eio dubbed ezjsone. The purpose of this library is to deal with the fact that GeoJSON documents can be quite big. It should be considered quite experimental. I did a little bit of benchmarking and it seemed to suggest that effects helps lower the memory usage (sometimes…).


Thanks to all of the Outreachy applicants that helped with this library and in particular @IIITM-Jay who did some work at the start of his Outreachy internship on the library.

Other OCaml News

From the ocaml.org blog

Here are links from many OCaml blogs aggregated at the ocaml.org blog.


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