Weeknotes 2026-w38 [weekly-2026-w38]

September 18, 2026
Patrick Ferris

Msh log 2026-38 [msh-log-2026-w38]

September 18, 2026
Patrick Ferris

After running some Dockerfile analysis last week, I have focused on formalising some operational semantics for the core POSIX-shell inspired language for a "mergeable shell".

This work borrows from Smoosh, but extends the semantics to include reasoning about namespaces, filesystems and effects.

For instance, here are two inference rules for field splitting and parameter substitution. Here, $\sigma$ tracks the shell state, $\phi$ is the filesystem and $\tau$ tracks the side effects.

There are lots of interesting examples of this, like assignment during parameter substitution (e.g. ${FOO:=BAR}) or the side effects that take place during program name resolution.

OCaml Maintenance [ocaml-maintenance-2026-w38]

September 18, 2026
Patrick Ferris

A mixture of work in ppxlib and geocaml.

Ppxlib migration woes

September 18, 2026
Patrick Ferris

Ppxlib continues to prove itself as a tricky library to maintain. This week, a report came in from David about package_type attributes being dropped. These are new attributes that are attached to package expressions which simply did not have them before.

The solution here was to store these attributes on parent nodes (e.g. the core_type a Ptyp_pack might be a part of) when migrating from 5.4 to 5.3 alongside a ppxlib.migration attribute to distinguish them from the core_type's attributes. When migrating back up we then re-split the attributes.

David also landed a new syntax feature arriving in OCaml 5.6 that adds upstream support for expression "holes", something Merlin supports. It is now possible to replace any OCaml expression with _, for example, let x = 1 + _.

This needed explicit ppxlib support which, thankfully, was relatively straightforward as hole expressions do not have a lot of associated metadata or arguments.

Our benchmarking infrastructure is also back on its feet.

Thanks to Nathan for all the reviewing.

`rtree.0.3.0` release [rtree-0-3-0]

September 18, 2026
Patrick Ferris

I cut a release of ocaml-rtree that removes the repr dependency. See the relevant discuss post.

This was a feature that Michael had asked for when building laserver. I took the time to open a PR to laserver to use rtree.0.3.0 which does simplify the code.

I wanted to ensure the library was still working, so I grabbed some of the LAS data for Lough Neagh, provided by OpenDataNI for 2017. After converting it to cloud-optimised point cloud (COPC) data via pdal (yes, two different "cloud" meanings) I managed to view the south-east portion of Lough Neagh.

This is overlooking the Lough Neagh Discovery Centre and Croaghan Island.

Outreachy

September 18, 2026
Patrick Ferris

After securing funding for this Outreachy round, I submitted an ocaml-tiff project. A year ago, we had an intern successfully add write support to the library.

This time, I hope to continue building out the library with an intern who is interested in learning OCaml, functional programming and geospatial data formats! One subgoal of the internship will be to create a tiffinfo-like command line tool that is distributed as part of the library. I also hope to add more compression algorithm support and iron out some API kinks.

References

Reference. Executable formal semantics for the POSIX shell [greenberg2020smoosh]

2019
Michael Greenberg, Austin J. Blatt
10.1145/3371111
https://doi.org/10.1145/3371111

Abstract

2019
Michael Greenberg, Austin J. Blatt

The POSIX shell is a widely deployed, powerful tool for managing computer systems. The shell is the expert’s control panel, a necessary tool for configuring, compiling, installing, maintaining, and deploying systems. Even though it is powerful, critical infrastructure, the POSIX shell is maligned and misunderstood. Its power and its subtlety are a dangerous combination. We define a formal, mechanized, executable small-step semantics for the POSIX shell, which we call Smoosh. We compared Smoosh against seven other shells that aim for some measure of POSIX compliance (bash, dash, zsh, OSH, mksh, ksh93, and yash). Using three test suites—the POSIX test suite, the Modernish test suite and shell diagnostic, and a test suite of our own device—we found Smoosh’s semantics to be the most conformant to the POSIX standard. Modernish judges Smoosh to have the fewest bugs (just one, from using dash’s parser) and no quirks. To show that our semantics is useful beyond yielding a conformant, executable shell, we also implemented a symbolic stepper to illuminate the subtle behavior of the shell. Smoosh will serve as a foundation for formal study of the POSIX shell, supporting research on and development of new shells, new tooling for shells, and new shell designs.

Context

OCaml Roundup: September 2026 [ocaml-roundup-september-2026]

October 7, 2026
Patrick Ferris

Plenty of OCaml work in September, including a few releases of libraries I maintain:

rtree got a new release.
ppx_deriving_yamlx saw its first release.
I managed to upstream some Forester work that was living inside Graft.
Some work on ppxlib that is on its way into the world.

Weeklies

October 7, 2026
Patrick Ferris

Weeknotes 2026-w38 [weekly-2026-w38]

September 18, 2026
Patrick Ferris

Msh log 2026-38 [msh-log-2026-w38]

September 18, 2026
Patrick Ferris

After running some Dockerfile analysis last week, I have focused on formalising some operational semantics for the core POSIX-shell inspired language for a "mergeable shell".

This work borrows from Smoosh, but extends the semantics to include reasoning about namespaces, filesystems and effects.

For instance, here are two inference rules for field splitting and parameter substitution. Here, $\sigma$ tracks the shell state, $\phi$ is the filesystem and $\tau$ tracks the side effects.

There are lots of interesting examples of this, like assignment during parameter substitution (e.g. ${FOO:=BAR}) or the side effects that take place during program name resolution.

OCaml Maintenance [ocaml-maintenance-2026-w38]

September 18, 2026
Patrick Ferris

A mixture of work in ppxlib and geocaml.

Ppxlib migration woes

September 18, 2026
Patrick Ferris

This needed explicit ppxlib support which, thankfully, was relatively straightforward as hole expressions do not have a lot of associated metadata or arguments.

Our benchmarking infrastructure is also back on its feet.

Thanks to Nathan for all the reviewing.

`rtree.0.3.0` release [rtree-0-3-0]

September 18, 2026
Patrick Ferris

I cut a release of ocaml-rtree that removes the repr dependency. See the relevant discuss post.

This was a feature that Michael had asked for when building laserver. I took the time to open a PR to laserver to use rtree.0.3.0 which does simplify the code.

This is overlooking the Lough Neagh Discovery Centre and Croaghan Island.

Outreachy

September 18, 2026
Patrick Ferris

After securing funding for this Outreachy round, I submitted an ocaml-tiff project. A year ago, we had an intern successfully add write support to the library.

Weekly 2026 w39 [weekly-2026-w39]

September 30, 2026
Patrick Ferris

Shells

September 30, 2026
Patrick Ferris

Work on the msh paper continued this week. One interesting result (for an appendix) was seeing what were the most commonly used commands in the Dockerfile dataset:

Which again, leads to some notion of overkill in terms of using a shell as the means to create container images when all we do is move some files around and install packages! I'm wondering if we're missing a real opportunity to streamline this process via something like the package calculus -- a package manager-agnostic means for declaring your dependencies.

Embed that as a programmable notion in your container image language alongside a container image-aware HTTP GET tool (to replace a vast majority of curl calls) and a handful of busybox commands, and you are left with (nearly) the raw shell only being used for the long tail of domain-specific things (e.g. invoking a build system).

Joy in old documents

September 30, 2026
Patrick Ferris

I'll talk about the joy I found in reading some historical bits and bobs tracking down information about shells, containers and operating systems more generally.

Part of this archaeological effort was trying to find when CHROOT was introduced into UNIX. Various online tools, including AI summaries, point to UNIX v7 but the system call is not obviously defined there. A bit more digging reveals it is listed under CHDIR(2) and not on its own.

Anyway, flicking through the manual was somewhat of a joy. The preface reads:

This new form of the Seventh Edition manual attests to the gratifying popularity of the UNIX Operating System. Delivered with no support and with an amateurishly published manual, this elegant and productive system has nevertheless earned worldwide respect, sometimes bordering on adulation...
Thousands of users today work from faint reproductions of reproductions of the original manual, whose ragged pages dangle and slide out of beat-up ring binders...

This bordering-on-poetic prose reminds me of some passages written for the MULTICS manual too (which I have talked about before).

All to say, I'm often thinking about the importance of writing these days and injecting some personality into it.

Part II Discussions

September 30, 2026
Patrick Ferris

I am hopefully going to supervise a part II project this year either on a fun ergonomic frontend to eBPF programs or a new spin on the geospatial DSL. More to follow once things are finalised.

Upstreaming Forester Pretty-printer

September 30, 2026
Patrick Ferris

I finally got around to upstreaming the Forester pretty-printing API I have been using in Graft. This allows you to print, hopefully, syntactically correct Forester code directly from the AST (a Code.t in forester parlance).

Happily, this all took place on tangled. It will be nice to keep this updated alongside Forester and point Graft to it. It feels like many people are moving away from a community-oriented model of software development, instead building a hybrid bazaarian cathedral monorepo powered by AI. I'm not here to really say what is right and what is wrong, but it seems to me that the sense of community is fragmenting and undue stress is being placed on many maintainers who no longer enjoy their job.

New release of ppx_deriving_ [ppx_deriving_yamlx-release]

September 30, 2026
Patrick Ferris

Raphael has proposed a new yamlx backend for ppx_deriving_yaml. It seems Ahrefs have been using his patch. I rejigged that PR to parameterise ppx_deriving_yaml over a small set of constructors and patterns so that the core codec logic could be reused.

I am fairly pleased with this abstraction. It allows any data format that supports objects and list to reuse the codec. For example, it would be trivial now to add a ppx_deriving_cbor.

Patrick Ferris' Weeklies [weeklies]

A collection of personal weekly(ish) updates about my work. If you want a more specific feed of OCaml-related work, please see OCaml weeklies. My personal weeklies will include the OCaml specific ones too.

An RSS feed is available.

Below are weeklies from 2026.

Weekly 2026 W43 [weekly-2026-w43]

October 26, 2026
Patrick Ferris

This week was a bit of a flop as I ended up coming down with something and was unable to work for about half of it. A shorter note this week due to this.

Paper Writing

October 26, 2026
Patrick Ferris

I had a productive conversation with Anil about the direction of the paper I am currently writing. Plenty of work to do with the deadline looming, but I am starting to think the framework we're building could have wider use than the applications I first thought of and that is somewhat exciting.

More on this during the week as I'm back to operational strength!

Forester 6.0

October 26, 2026
Patrick Ferris

A new release of Forester is imminent and with that comes upgrades and fixes to Graft. If you are reading this post then you are enjoying the delights of Forester 6.0 compiling artefacts from an in-the-works version of Graft.

The upgrade was mostly smooth, though some of my CSS was a bit broken, and it was easier to do a merge of the most important bits with the new Forester stylesheets.

Depending on the internals of Forester is a little tricky as it is first and foremost a CLI tool. The library APIs are likely to be less stable and are a bit of a moving target. The changes, however, were pleasantly small as far as I can tell, limited mostly to some changes in the reporting infrastructure and the strictness of all Forester codes needing a location (somewhat tricky as there is no one-to-one mapping from markdown/yaml/bibtex to these codes). For the latter case, we do our best and where possible try to map the generated codes back to the fragment of the source language Graft has transpiled.

Now that I am dog-fooding Graft and Forester, it will be easier to keep things working ahead of the next release.

Thanks to Kento and Jon for their work on improving Forester.

Outreachy

October 26, 2026
Patrick Ferris

I have lots of thoughts about Outreachy this year. The volume and quality of contributions is quite unlike anything I have seen before, in large part due to the current ease of accessibility and quality of LLMs. This does fly in the face of somewhat strong policies set out by Outreachy to help maintainers handle AI contributions.

Going forward, I imagine it will be difficult to balance pedagogical values with financial rewards given wide-spread AI usage (regardless of project-specific or organisational AI policies). Not something that quite exists, in the same way, say, for university undergraduate students.

I am still trying to find my feet in the middle of this. I have never felt quite this alone in the open-source software world.

Misc.

October 26, 2026
Patrick Ferris

Claudius Playground

October 26, 2026
Patrick Ferris

Michael spoke to me about some accessible ways to play around with his fantasy console library Claudius. At this point, I fancy myself a bit of an expert at shoving OCaml code into browsers, so I pulled together a rough and ready Claudius Playground (code is freely available on tangled). Happy coding!

Music

October 26, 2026
Patrick Ferris

I was able to go see the Mountain Goats before the start of last week, with Craig Finn supporting. I enjoyed Isaiah 45:23.

The Mountain Goats at Vicar St. Dublin, 2026-10-18.

Shells, SOOCon26 and Part IIs [weekly-2026-w41]

October 9, 2026
Patrick Ferris

Last week I gave a precise meaning to the "mergeability" of container images. The further I go down this rabbit hole, the less convinced I am of the exact benefits of such an approach. However, there are lots of off-shoots to explore as well, which all seem very worthwhile and might have better semantics than merging (e.g. incremental computation and provenance-tracking).

Uniform Representations [uniform-repr]

October 12, 2026
Patrick Ferris

Whilst working on things related to Merry and Shelter, I couldn't help but think how much easier parts of this problem would be if we were using Plan9 for one simple reason: the uniform access to OS resources. Half the battle on Linux is trying to trace or sandbox a process with respect to some set of OS resources (in an ideal world, all of them). Syscalls are perhaps the closest thing we have, but there isn't a one-to-one mapping between user actions and syscalls (e.g. opening a file using io_uring does not call open(2) or openat(2)).

I think there is often a trade-off here in terms of performance and simplicity. Plan9's 9P protocol was not exactly known for its performance. But, I still find myself, to use a phrase my colleague Ryan once used when explaining his use of emacs, comfy with uniform representations. Another example, a tree in Forester.

It is pleasant when these representations have a defined structure, even if the contents are more malleable. For instance, I don't want everything to be schema-less JSON. Forester trees have tags and that allows me to impose a schema in a structured way that doesn't lead to parsing errors. Similarly, 9P offers a simple protocol on which all things are built. As I understand it, the semantics are given by the interpretation of the file server that is resolved, but the protocol is uniform. Anyway, just some thoughts.

State of Open Conference 2026 -- Belfast [soocon2026-belfast]

October 9, 2026
Patrick Ferris
https://stateofopencon.com/belfast-soocon26/

This week I was fortunate enough to attend (as a volunteer) SOOCon26 in Belfast in the Ulster Bank building, just opposite the City Hall. Thanks to Anil Madhavapeddy for linking me up with Amanda Brock to make it happen at the last hour.

If I can boil down the conference into a general topic it would be: how are large language models and coding agents impacting commercial, business-oriented open-source software. I found the conference's near-total focus on AI a little surprising and a little disappointing, but I suppose it reflects the current state of open source. For example, most, if not all, the working developers and business owners at this conference, for one reason or another, felt the need to relate their work to AI, both in their talks and in hallway conversations. Something I continue to struggle with, though, is that almost all references to "AI" pointed to large, expensive, closed frontier models (such as Claude, ChatGPT, Copilot and Gemini). That being said, there were talks on data (and AI) sovereignty as well as mentions of the increasingly capabale open models (e.g. Qwen).

I would like to add that the conference was essentially offline: no recordings or streaming. With that in mind, I want to respect the fact that most attendees and presenters could speak freely and candidly, so I will only focus on some of the higher level threads of discussion without attributing too much to any individual.

AI: Productivity

October 9, 2026
Patrick Ferris

The morning focused mainly on the question: what is a software developer's job in 2026? This culminated in a panel discussion on "building trust in AI". Already, I would say the conference's mind was made up, AI is here to stay; it is just a tool; we should learn how to hold it properly. I think, in part, this was mainly due to the group of speakers who were all, more or less, executives within for-profit businesses that incorporate open source in some way: Github, BIMP.ai and Budibase. The only exception was a speaker from the Institute of Directors. I think this gave the morning session a commercial flavour which suggested the tech industry's large-scale adoption of AI tooling, a position I'm not sure is as common among, or available to, smaller open-source maintainers.

One stream of thought that was popular was arguing that languages like C, where it is easy to introduce memory bugs and vulnerabilities, are on the way out. Copilot's runtime rewrite in Rust was championed as a success story for the kinds of productivity that can be unlocked for highly-skilled developers. A pinch of salt is needed though, as Microsoft did sponsor the event. My own thoughts, however, are that there are quite a few success stories we can learn from where software was translated from one programming language to another using an LLM. Migration seems well-defined and well-scoped, often with large, pre-existing test suites that can easily guide a tool towards some form of correctness. In the same breath, it is not necessarily that straightforward, for example, see some lobste.rs commentary on the Bun rewrite in Rust.

Another undercurrent throughout the morning talks was how LLMs could free up developers to actively engage with users. Some quotes from the talks included "experiment with empathy" and "AI doesn't write the story". On the surface, this seems like a reasonable argument. But, shouldn't these large teams of people have already been thinking about their users, with or without AI? I'm also not convinced this free time will appear. When the metric of success is at least proportional to the rate of new features, it would seem that a tool, like an LLM, would only crank out more widgets rather than better widgets.

A final worrying conversation from the panel, discussed the fact that companies are not hiring graduates. This was self-evident from a few other people I met in and around the conference: computer science graduates still looking for jobs.

AI: Community, Data Centres and Vulnerabilities

October 9, 2026
Patrick Ferris

The post-lunch session included a talk about the future of community open source, a talk about AI data centres and finally a panel about the "vulnpocalypse".

The first talk focused on the realities of doing community open source and how, in effect, AI and LLMs are not (yet) a part of our shared open culture, citing large, anti-AI open-source projects, some which have banned AI contributions (e.g. NLNet Labs, Codemirror) and pro-AI projects which have closed contributions (e.g. Ladybird). Of course, open source never meant the right to contribute, but it still signals a, perhaps, worrying shift in open-source, cultural norms. An arXiv paper collected some AI policies on popular repositories which is worth a skim.

Another topic in the talk was the "time to open". The speaker argued that it was now possible to completely reverse engineer closed-source products and tools in as little as 48 hours. This could lead to more open source business models, as closed models are no longer viable. The speaker suggested that the open-source community around a project is now more important than the code itself. I think I fall somewhere closer to believing that they are both important. I worry about the proliferation of agentically-managed software stacks which companies and individuals use with little incentive to upstream patches back to the community (or release their new code for the community). This kind of isolationism seems more likely given the ability to create your own implementation of any piece of software.

Data Centres

October 9, 2026
Patrick Ferris

The talk on hyperscalar data centres presented some great work from the folks at Action to Protect Rural Scotland (APRS). With some serious energy projections:

This gives an estimated around 7,306 MW currently in the planning system or in pre-application consultation. Peak winter electricity demand in Scotland is just over 4,000 MW.

It seemed like the feeling in the room was that these data centres are inevitable, in spite of these jarring numbers; and that somewhere down the line, the ends will be just so good, that the means will be justified.

It seems, once again, I must reach for Pete Seeger.

Perhaps the biggest change will come when we don't have to change much at all When maniacs holler "grow, grow, grow, grow!"; we can choose to be small.

Vulnerabilities

October 9, 2026
Patrick Ferris

I don't think this needs too much explaining, but visit Daniel Stenberg's mastodon feed and that will catch you up. Edit: I wanted to also add a more recent post from Daniel highlighting the other work of the core curl team.

Conclusion of sorts

October 9, 2026
Patrick Ferris

Unfortunately, I missed a good portion of the final session as I was helping clear up. The talks were: "The cost of Sovereignty", "Knowledge Graphs for Digital Transformation" and "You can't download trust".

I managed to have a good chat with Phil Weir, director of Flax and Teal, about some of their work including Alizarin, a tool for working with knowledge graphs and Arches data. It was refreshing to hear about the details of an actual project someone was building.

Ultimately, I would say the day left me disheartened for the immediate future of (open-source) software. What I found most unsettling about SOOCon is what I find unsettling about the tech industry at large. It would seem that the industry is leaving new graduates in the lurch. Additionally, environmental concerns are falling somewhere (far?) below commerical concerns.

I'm also anxious that open source's ethos is being eclipsed by the need for companies (and individual employees) to demonstrate their relevance in an AI-focused economy, and that that performance of relevance is really about integrating, into whatever you're doing, the products coming out of a few powerful companies, which are not invested in the values which make open source make sense.

I think conferences like State of Open are important, and we need more conversations from all kinds of open-source community members. I'd like to see more talks from students, sole maintainers of highly depended-on software, folks from other non-profit-driven open-source initiatives like Outreachy or the Processing Foundation (these are just examples I know). I recognise that, perhaps, SOOCon26 Belfast was suffering from being in Belfast, hardly the most connected metropolis in the world... but it is the home of The Big Fish. I'm excited to hopefully see it return in the years to come.

My friends, Maddie and Emma, enjoying the splendour of Belfast!

Thanks to Meredith, my partner, for helping me edit this and form some much more coherent ideas.

Part II Projects

October 9, 2026
Patrick Ferris

After some super-human administration by Anil Madhavapeddy, I have closed this week with three part II projects to supervise.

An Embedded DSL for Querying OpenStreetMap [part-ii-edsl-osm]

Matthew Occleshaw

This project, originally conceived by Anil Madhavapeddy, hopes to make it easier to answer questions like "What is the safest route to Pembroke College from Cambridge train station?". Here, "safest", can have lots of meanings: the best lit road, a route with tactile paving or accessible pedestrian signals etc.

In addition to this, we may wish to provide suggestions for how to improve the current folksonomy of information OSM stores.

From OCaml to Graphics Shader Languages [part-ii-shaders]

Rohan Wadhawan

This project wants to reuse OCaml's typechecker to provide a safe way to create graphics shaders. As an extension, a ppx rewriter will provide a translation layer from OCaml to the embedded DSL making it possible to write shaders in a subset of the OCaml language.

When interacting with the GPU for graphics programming, parts of the rendering pipeline are written as standalone shader programs. The standard method of using these is to store the shader source code as a string, which is then compiled at runtime by the GPU driver. This is not a performance issue, as shader programs are usually small enough that compilation time is negligible. However, it does mean simple syntax errors or incorrect types, which would normally be caught at compilation, won’t manifest until the entire program is run.
I propose a new library, Shaders_of_ocaml, allowing for shader programs to be written inline as native OCaml, which is then compiled to shader code strings to be passed to the existing graphics libraries. This would allow the OCaml compiler to provide type checking for every shader written.
-- From Rohan's Proposal

Ripple: An OCaml Library for Incremental Computation [part-ii-incr]

Boris Hall

Following the work of Adaptive Functional Programming and Implicit self-adjusting computation, this project will build an incremental computation engine in OCaml alongside an expressive combinator library for build such computations. As an extension, a small DSL will be built that makes use of the engine to build an implicitly incremental programming language.

The core of this project is to implement an eDSL and a runtime in OCaml for self-adjusting computation. I also propose a new TDT which allows for generic monoidal folds over modifiable sequences: the library will include a native foldMap primitive that allows performant reduction over a modifiable sequence of modifiable values for any general monoid, giving logarithmic updates with a small trace. I will then compare foldMap to the generic fold implemented in plain SAC.
-- From Boris' Proposal

Ppxlib

October 9, 2026
Patrick Ferris

Just a quick footnote about [Ppxlib]: I opened a PR tomerge multiple instances of a deriver. I threw together this OCaml script for checking implementation and interface files. Running it on Forester, it picked up two instances which can be avoided.

open Ppxlib

let attr_compare (a1 : attribute) (a2 : attribute) =
  String.compare a1.attr_name.txt a2.attr_name.txt

let has_derivings_recursively =
  object
    inherit [bool] Ast_traverse.fold as super

    method! structure_item s acc =
      match s.pstr_desc with
      | Pstr_type (Recursive, (_ :: _ as tds)) ->
        let attributes =
          List.fold_left (fun acc td -> td.ptype_attributes @ acc) [] tds
        in
        let uniq = List.sort_uniq attr_compare attributes in
        List.length attributes <> List.length uniq || super#structure_item s acc
      | _ -> acc

    method! signature_item s acc =
      match s.psig_desc with
      | Psig_type (Recursive, (_ :: _ as tds)) ->
        let attributes =
          List.fold_left (fun acc td -> td.ptype_attributes @ acc) [] tds
        in
        let uniq = List.sort_uniq attr_compare attributes in
        List.length attributes <> List.length uniq || super#signature_item s acc
      | _ -> acc
  end

let main f =
  let is_ml = Filename.extension f = ".ml" in
  let is_mli = Filename.extension f = ".mli" in
  let kind =
    if is_ml then Some Ppxlib_private.Utils.Kind.Impl
    else if is_mli then Some Intf
    else None
  in
  match kind with
  | None -> Format.eprintf "Skipping %s@,%!" f
  | Some kind -> (
    match Driver.load_input ~kind ~relocate:false ~input_name:f f with
    | Error (e, _) ->
      Format.eprintf "Input err %a: %s" Location.print (Location.Error.get_location e) (Location.Error.message e);
      exit 127
    | Ok (_, _, Impl s) ->
      if has_derivings_recursively#structure s false then
        Format.printf ">>> Maybe fix: %s@,%!" f
      else ()
    | Ok (_, _, Intf s) ->
      if has_derivings_recursively#signature s false then
        Format.printf ">>> Maybe fix: %s@,%!" f
      else ())

Note, that given the implementation of attr_compare, this will be an overapproximation.

Weekly 2026 w39 [weekly-2026-w39]

September 30, 2026
Patrick Ferris

Shells

September 30, 2026
Patrick Ferris

Work on the msh paper continued this week. One interesting result (for an appendix) was seeing what were the most commonly used commands in the Dockerfile dataset:

Joy in old documents

September 30, 2026
Patrick Ferris

I'll talk about the joy I found in reading some historical bits and bobs tracking down information about shells, containers and operating systems more generally.

Anyway, flicking through the manual was somewhat of a joy. The preface reads:

This new form of the Seventh Edition manual attests to the gratifying popularity of the UNIX Operating System. Delivered with no support and with an amateurishly published manual, this elegant and productive system has nevertheless earned worldwide respect, sometimes bordering on adulation...
Thousands of users today work from faint reproductions of reproductions of the original manual, whose ragged pages dangle and slide out of beat-up ring binders...

This bordering-on-poetic prose reminds me of some passages written for the MULTICS manual too (which I have talked about before).

All to say, I'm often thinking about the importance of writing these days and injecting some personality into it.

Part II Discussions

September 30, 2026
Patrick Ferris

I am hopefully going to supervise a part II project this year either on a fun ergonomic frontend to eBPF programs or a new spin on the geospatial DSL. More to follow once things are finalised.

Upstreaming Forester Pretty-printer

September 30, 2026
Patrick Ferris

New release of ppx_deriving_ [ppx_deriving_yamlx-release]

September 30, 2026
Patrick Ferris

I am fairly pleased with this abstraction. It allows any data format that supports objects and list to reuse the codec. For example, it would be trivial now to add a ppx_deriving_cbor.

Weeknotes 2026-w38 [weekly-2026-w38]

September 18, 2026
Patrick Ferris

Msh log 2026-38 [msh-log-2026-w38]

September 18, 2026
Patrick Ferris

After running some Dockerfile analysis last week, I have focused on formalising some operational semantics for the core POSIX-shell inspired language for a "mergeable shell".

This work borrows from Smoosh, but extends the semantics to include reasoning about namespaces, filesystems and effects.

For instance, here are two inference rules for field splitting and parameter substitution. Here, $\sigma$ tracks the shell state, $\phi$ is the filesystem and $\tau$ tracks the side effects.

There are lots of interesting examples of this, like assignment during parameter substitution (e.g. ${FOO:=BAR}) or the side effects that take place during program name resolution.

OCaml Maintenance [ocaml-maintenance-2026-w38]

September 18, 2026
Patrick Ferris

A mixture of work in ppxlib and geocaml.

Ppxlib migration woes

September 18, 2026
Patrick Ferris

This needed explicit ppxlib support which, thankfully, was relatively straightforward as hole expressions do not have a lot of associated metadata or arguments.

Our benchmarking infrastructure is also back on its feet.

Thanks to Nathan for all the reviewing.

`rtree.0.3.0` release [rtree-0-3-0]

September 18, 2026
Patrick Ferris

I cut a release of ocaml-rtree that removes the repr dependency. See the relevant discuss post.

This was a feature that Michael had asked for when building laserver. I took the time to open a PR to laserver to use rtree.0.3.0 which does simplify the code.

This is overlooking the Lough Neagh Discovery Centre and Croaghan Island.

Outreachy

September 18, 2026
Patrick Ferris

After securing funding for this Outreachy round, I submitted an ocaml-tiff project. A year ago, we had an intern successfully add write support to the library.

Analysing some Dockerfiles [weekly-2026-w37]

September 12, 2026
Patrick Ferris

After spending a few weeks thinking about the semantics of POSIX shells, this week saw me analysing lots of Dockerfiles looking for some evidence for how they are being used. It also saw environment variables landing in Eio thanks to Thomas and Anil. We also had some issues in ppxlib migrations.

Dockerfile Analysis [msh-dockerfile-analysis]

September 12, 2026
Patrick Ferris

Part of my research at the moment is thinking about how shells are used. What commands are run? What constructs are used (e.g. redirections, compound commands...)? Dockerfiles are a defacto standard means for building container images and for the most part they are a shell with weird semantics and some meta-commands (sounds familiar!).

I had some assumptions about the kinds of shell commands people use (myself included) when writing Dockerfiles. For example, making use of AND-lists for grouping commands in RUN instructions. But I had no evidence to back this up. So I ran some analysis over the dataset from this paper from Henkel et al..

Total Dockerfiles: 218_253 (errors: 808)
Total Docker Instructions: 2_729_118
Total RUN instructions: 657_909 (errors: 4_847)
========

Count (Windows Dockerfiles): 841 / 218_253
Count (ENV commands): 268_898 / 2_729_118
Count (COPY commands): 165_968 / 2_729_118
Count (ADD commands): 86_552 / 2_729_118
Count (WORKDIR commands): 104_112 / 2_729_118
Count (using at least one &&): 267_113 / 657_909
Count (uses of exec): 25 / 657_909
Count (system pkg manager: [apt, apt-get, dnf, apk, pacman, nix, brew, yum]): 192_092 / 657_909
Count (internet retrievers: [curl, wget]): 95_433 / 657_909
Count (might use python): 9_082 / 657_909
Counts:
[(rm, 169_386), (echo, 113_768), (mkdir, 100_924), (cd, 74_904),
 (chmod, 55_998), (set, 25_738), (mv, 24_034), (sed, 21_232),
 (chown, 18_610), (grep, 16_604), (cp, 15_637), (export, 13_154),
 ([, 10_411), (touch, 3_152), (exit, 2_288), (printf, 2_275), (awk, 2_010),
 (., 1_077), (test, 1_004), (source, 904), (pwd, 199), (unset, 105),
 (diff, 40), (trap, 2)]

I think there's some evidence here to support some assumptions I already had:

A significant proportion of RUN instructions in Dockerfiles invoke package managers.
A significant proportion of RUN instructions invoke curl or wget.
There is something rather Zipfian about how shell built-in utilities are used.
There is limited used of exec (and therefore limited use of opening file descriptors directly via the shell, as opposed to a command running in a subprocess).
A lot of time is spent changing directories, either temporally (via cd in a RUN instruction, or by using WORKDIR).
Lots of ENV setting.

Parsing Dockerfiles

September 12, 2026
Patrick Ferris

For reasons that are probably quite clear, whenever I end up doing a small task like this, I always reach for OCaml to do it. The idea is that I'm invested in that community and hope to see it thrive. And if that means contributing some fixes, libraries or advice along the way, it seems to me that that is time well spent.

When I started, I assumed ocaml-dockerfile had support for parsing files; it turns out it doesn't, but I found Thomas G.'s port that does. So I took it for a spin and the results were less than I had hoped for. Of the 219,061 Dockerfiles, 97,123 were not parseable! After a series of commits I managed to get that number down to 808.

Eio process environment

September 12, 2026
Patrick Ferris

What started out seemingly straightforward (add a capability-based API to Eio for accessing environment variables) ended up being a more nuanced discussion about what the best API would be. In the end, Thomas added an Eio.Process.Env API to manipulate soon-to-be opaque environments. This paved the way for the portable environment support.

The API Eio exposes is readonly so we do not have to worry too much about race conditions (even though users are free to Unix.putenv).

One quirk of this work was reaffirming something I had already stumbled across in Merry. Accessing and searching large environments has a cost! For my rather large Nix environment, we can see the slowdown pretty clearly.

                  name,   major-allocated,   minor-allocated,   monotonic-clock
        get_env/all 10,        346.619865,      38577.227215,      29248.828738
        get_env/all 50,       1677.003302,     181788.755923,     138307.102538
       get_env/all 100,       3323.770915,     361218.802888,     274768.577259
   get_env/all_once 10,        121.955078,      14694.900546,      11652.096583
   get_env/all_once 50,        563.133065,      61951.683658,      48161.605588
  get_env/all_once 100,       1113.238600,     121431.915770,      93960.646400
     get_env/direct 10,          0.000000,          0.000000,        974.649587
     get_env/direct 50,          0.000000,          0.000000,       4369.486360
    get_env/direct 100,          0.000000,          0.000000,       8753.393503

The all benchmark calls environment every time and then finds a variable using Eio.Process.Env.get_opt, all_once calls environment once and then Eio.Process.Env.get_opt every time and direct uses the Eio.Process.get_opt directly every time. The differences are a lot less stark when the environment is pretty small.

Ppxlib Migrations

September 12, 2026
Patrick Ferris

Thankfully, ppxlib has been mostly stable for a number of months. In part, thanks to the diligent efforts of Nathan. However, David uncovered some unpleasant bugs in the migration code for some new 5.4 AST features. Unfortunately, it is only for attributes which are a source of pain for ppxlib. I spent some time reviewing the fixes, and it looks like we still need some more work to ensure the attributes attached to package types roundtrip cleanly.

Misc.

September 12, 2026
Patrick Ferris

Some other work I recorded separately and include here for the eager reader.

OCaml Roundup: August 2026 [ocaml-roundup-august-2026]

July 12, 2026
Patrick Ferris

Much of August was spent thinking about POSIX shell semantics, and in particular extending notions from smoosh to include side effects (and maybe concurrency). This is still nascent work, but some ideas and flavours are described below.

Merging Shells

July 12, 2026
Patrick Ferris

The work came from wanting to merge two shell sessions into one. Setting aside the practicalities of an implementation, it makes sense to start first theoretically. When can I merge two shell executions, and when can I not?

It would be nice to think of unset DOWNLOAD_DIR as a function. This function takes the shell state and the operating state as arguments and returns a modified shell state and an "unmodified" operating system state (at least for our purposes). Meanwhile, branch-b may have a lot more side effects, but when focusing on the environment only we can see in a sense the two branches are conflict free. Well, sorta. We also need to ensure we do not also read DOWNLOAD_DIR in branch-b if we intend to remove it in their merge.

This work, in part, is borrowing ideas from MRDTs and can reuse void processes to help reason about when side effects can occur, as well as open-trace (which is now becoming os-trace).

I hope to continue this work, see my latest weekly for more on this.

Eio in the browser

July 12, 2026
Patrick Ferris

I spent some time carving out Eio_browser into a standalone library, something I depend on for Merry to work in the browser.

Eio_browser [eio_browser]

September 12, 2026
Patrick Ferris
https://git.sirref.org/eio_browser

Eio_browser is a backend for Eio that works in the browser. It reuses work I started at Tarides that picked up some momentum but has since been a little dormant.

Crucially, it provides a functional standard environment like Eio_main or any of the OS-specific backends.

module Stdenv : sig
  type base =
    < stdout : Eio_unix.sink_ty r
    ; stderr : Eio_unix.sink_ty r
    ; clock : float Eio.Time.clock_ty r
    ; mono_clock : Eio.Time.Mono.ty r
    ; secure_random : Eio.Flow.source_ty r
    ; debug : Eio.Debug.t
    ; backend_id : string >
end

val run : (Stdenv.base -> unit) -> unit
(** [run fn] runs [fn] passing it the {! Stdenv.base} environment.
    See {! Backend.run} for more information about how effects are handled.*)

Browsher and Namespaces [weekly-2026-w33]

August 17, 2026
Patrick Ferris

The work presented constitutes a good bit more than a weeks worth of work. That being said, let's dive in.

Browsher [browsher]

August 17, 2026
Patrick Ferris

After a week or two of abstracting the internals of Merry, I finally got a shell running in the browser. You can have a play with it here:

https://patrick.sirref.org/msh

The final hurdle was providing an abstract interface to file handles. Eio, the library that so far has enabled much of the OS abstraction, doesn't have a particularly good cross-platform file descriptor API. Eio_unix does extend sources and sinks with the Unix_fd tag, but there is no Eio.Fd.dup2 that provides a platform-agnostic API for file handle manipulation. Even if it did, a POSIX shell does a fair amount of assuming that your file handles are integers which might prove tricky on Windows.

The shell in the browser completely reuses the logic of Merry. It also links in a browser-aware subpackage of bruit (the OCaml port of linenoise I wrote) and a few, half-baked POSIX utilities I wrote (like ls and cat). But you can at least run something like:

$ echo "hello" > hello.txt
$ ls -la .
$ cat < hello.txt

Being able to, in some sense, run a shell anywhere is actually quite an attractive proposition. Shells, with all of their wonderful footguns, are still a reliable and familiar tool for interacting with a running system. I'm interested in the idea of running shells in more exotic deployments (e.g., unikernels) where it becomes an interface for dynamical reconfiguration or a tool for debugging.

Updates on Eio_mem

August 17, 2026
Patrick Ferris

At the heart of browsher is Eio_mem (an in-memory filesystem for Eio). The Eio_mem API now comes with (and exposes) a number of file descriptor operations:

val pipe : Eio.Switch.t -> Fd.t * Fd.t
val dup : sw:Eio.Switch.t -> Fd.t -> Fd.t
val dup2 : sw:Eio.Switch.t -> src:Fd.t -> tgt:Fd.t -> unit

I was happy to see Eio_mem getting some use in Thomas G.'s fork eio-fs where he was able to use it for chaos engineering.

Namespaces [namespace-musings]

August 17, 2026
Patrick Ferris

I spent a few days recently trying to come up with some formalism for the notion of names, namespaces and name resolution.

On the surface, it seems bigraphs might be a fruitful approach. The orthogonal treatment of locality and connectivity maps fairly well onto name structure (e.g, Module.X.foo, /foo/bar/hello.txt or foo.example.org) and redirection (e.g., symlinks or CNAMEs). Bigraph reaction rules could then model name resolution. A resolution process can move through the hierarchical name structure and across links at which point you might have some interesting properties to prove or check about a particular namespace system. Additionally, namespaces and resolution mechanism might compose in different ways and this might be worth formalising and comparing.

I didn't get much further than modelling some bits of a filesystem in BigraphER.

# filesystem
ctrl Dir = 0;
atomic ctrl File = 1;
atomic ctrl SymLink = 2;

# Query -- the thing that gets resolved.
atomic ctrl Q = 1;

# Reaction Rules
react resolve_symlink =
  Q{x} | SymLink{x, y} --> Q{y} | SymLink{x, y};

# Initial State
big s0 =
  Dir.
    (SymLink{sym_a, file_a}
     |
     Q{sym_a}
     |
     File{file_a}
    );

# Bigraphical Reactive System
begin brs 
  init s0;
  rules = [ {resolve_symlink} ];
end

Resulting in these two transition states.

Bear in mind this was just a quick experiment. The first bigraph shows our query at sym_a and the second bigraph (using the resolve_symlink reduction rule) shows the query at file_a. All of this takes places within Dir. It seems that BigraphER represents the tree structure graphically with black arrows (rather than spatial containment).

Papers

August 17, 2026
Patrick Ferris

I'm working on a paper right now for Merry and Shelter. It is mostly coming together okay; and I am enjoying the creative aspects of this paper, those things with no right answer just "hey, I think this is a good/useful/fun idea".

My TikZ-abilities have also had a refresher.

TODOs

August 17, 2026
Patrick Ferris

Over the next week, I hope to finish up a first draft of the aforementioned paper. I also want to publish a backlog of packages I have for OCaml: bruit, ocaml-bibtex, a fresh cut of Graft etc.

Ramblings

August 17, 2026
Patrick Ferris

I had some half-formed thoughts lying around in my forest, so I thought I might include those here:

Boilerplate [on-boilerplate]

August 11, 2026
Patrick Ferris

The word boilerplate gained popularity as a colloquialism used by newspaper and magazine editors for pre-prepared editorials and stories that were syndicated across the US. In effect, boilerplates were news stamps that could produce identical content.

I have been thinking about the nature of boilerplate code. How is it similar or different to boilerplate in its original meaning.

Distinguishing true boilerplate code from code that feels repetitive or boring to write is, I think, important. Argument parsing falls into this category quite well. Consider the following flag I added to Merry's set built-in.

let all_export =
  let doc = "Export all variables, like -o allexport." in
  Arg.(value & flag & info [ "a" ] ~docv:"ALLEXPORT" ~doc)

It uses cmdliner to define a new -a flag that will, in effect, make all variable definitions act like the export built-in. LLMs have been praised for their ability to do this specific task in OCaml.

What exactly is boilerplate about this code snippet? It seems to me that there is some sort of ceremony to defining a flag. There is boilerplate in the structure with more meaningful content threaded through. Below, is the example reduced to just the boilerplate replacing the for more meaningful parts with Merlin's typed holes: _.

let all_export =
  let doc = _ in
  Arg.(value & flag & info [ _ ] ~docv:_ ~doc)

Arguably the use of flag is quite important too as that is where the magic happens if you want an optional argument, or a positional one etc. As a thought experiment, having agents deliver fill-in-the-blanks style code might be quite useful. In a type system that can support holes (e.g. like in OCaml or Hazel) it becomes a more pleasant exercise, but helps keep you in the loop. I, myself, do not use any AI in the code I write; so I admit that I am pretty far from the problem.

The problem I have with this approach is that it is fairly easy to automate in a predictable and easy way! I just added a snippet subcommand to octx, but you could easily wrap this up in whatever your editor of choice is.

$ octx snippet cmdliner-flag
let cmdliner_flag =
  let doc = _ in
  Arg.(value & flag & info [ _ ] ~docv:_ ~doc)

The more I think on this problem the harder I find good uses for LLMs besides churning out vasts amount of end-to-end code; something I am personally not that interested in doing. It seems to me, to reap the benefits of LLMs, one must relinquish a certain amount of agency; anything less, and you could easily replace its usage with a cheaper, more predictable tool.

Shell Benchmarks, Back to Writing [weekly-2026-w26]

July 4, 2026
Patrick Ferris

A little late due to travel, but I did a little benchmarking of Merry.

Signal Handling in Merry

July 4, 2026
Patrick Ferris

In order to even run shellbench, I first had to fix some bugs in the shell implementation. Shellbench aims to warm up the shell before starting any benchmarks; it includes this prelude before getting started:

__ready=
trap __ready=1 HUP
kill -HUP "$MAIN_PID"
until ["$__ready" ]; do __dummy=; done

Whilst writing Merry, I hadn't really considered that the return context from a trap needed to be taken into consideration! I had assumed they would be silently ignored, how else would we know where to thread that context back into the shell's execution.

Even after reading through the specification for trap I am still none the wiser. I've opted to place some sensible signal polling points during the execution of the commands to run any signals that have come in. The single handlers in Merry simply push thunks onto a run queue. Looking at the Dash source code, it seems to be something that happens just before evaluation.

Benchmarking Merry [merry-benchmarks]

July 4, 2026
Patrick Ferris

Medium-to-small potatoes

July 4, 2026
Patrick Ferris

I used shellbench to understand where some of the performance pitfalls were coming from.

$ ./shellbench -s bash,zsh,msh sample/func.sh
-----------------------------------------------------------------------------------
name                                                     bash        zsh        msh
-----------------------------------------------------------------------------------
func.sh: no func                                      775,539  1,124,485      5,229
func.sh: func                                         398,861    197,169     93,424
-----------------------------------------------------------------------------------
* count: number of executions per second

This particular benchmark first runs nothing (i.e. :) and then a function lookup and execution. Surprisingly msh is slower at doing nothing! I perf-ed a run of msh test.sh with just a single : in the file:

Test built-in

July 4, 2026
Patrick Ferris

After making various improvements to Merry that almost halved the memory usage (e.g., removing extraneous string manipulations, adding a fast-path to the PATH variable for resolving programs). I was still pretty far off the performance of these other shells. I decided to have a look at dash which has a simple implementation.

This flame graph is for the following shell script:

f () { :; }
while __count=$(($__count+1)) && ["$__count" -le 10000 ]; do
	f
done
echo "count is $__count"

And here are some rudimentary benchmarks:

$ time -p dash test.sh
count is 10001
real 0.01
user 0.01
sys 0.00
$ time -p msh test.sh
count is 10001
real 15.53
user 4.28
sys 12.28

It turns out the massive slowdown is very simple to explain. In the dash flamegraph there is a oexpr stack; which comes from the bltin/test.c file. dash, along with the other shells, does not spawn a process for the [-test command! Forcing dash to do use the binary [ gives:

$ time -p dash test.sh
count is 10001
real 7.55
user 3.95
sys 3.44

Which is much more respectable in terms of performance compared to msh. Plenty of work to do though!

Parsing arithmetic expressions

July 4, 2026
Patrick Ferris

There was some obvious low-hanging fruit to help improve some of the performance differences. The first was pre-parsing arithmetic expressions. Morbig, the static parser, does not implement arithmetic expression parsing; so Merry does it for us. Before, it was on-demand, as we encountered an expression of the form $((<e>)). Now, it happens up front. This should improve tight-loops.

Writing

July 4, 2026
Patrick Ferris

More on this soon, but I have started writing two separate papers bringing all of this work together.

Three weeks in the Shell [weekly-2026-w25]

June 16, 2026
Patrick Ferris

The last three weeks have been focused work on building Merry. In practice, I wanted to run the following two commands through Merry's shell: msh:

apt-get install debootstrap -y
debootstrap stable /stable-chroot http://deb.debian.org/debian/

Seemingly innocuous. But both commands run thousands of lines of shell script. Whether that's Debian package maintainer scripts or state-machines for processing package release files.

What follows is a handful of the bugs and the shell scripts that caused them.

Field Splitting

June 16, 2026
Patrick Ferris

Edge-cases abound in the POSIX shell specification; and field splitting is one of the most serious perpetrators. Field splitting happens after word expansion. It is a known footgun amongst shell script authors.

$ count () { echo "Count: $#"; }; FOO="hello world"; count $FOO
Count: 2

Quoting variables that need expanding will prevent field splitting from taking place, but it is an easy thing to forget.

Field splitting uses the IFS (internal field separator) environment variable to determine how to do the splitting, and it comes with some extra rules beyond "split on these characters".

$ IFS=" "; FOO=" hello    world"; echo $FOO
hello world
$ IFS=" "; FOO=" hello    world"; echo "$FOO"
 hello    world
$ IFS=":"; FOO="a:b:c:"; echo $FOO
a b c
$ IFS=":"; FOO="a:b::c:d"; echo $FOO
a b  c d
$ IFS=":"; FOO=":a:b:c"; echo $FOO
 a b c
$ IFS=; FOO="unset   IFS"; echo $FOO
unset   IFS

Variable assignment

June 16, 2026
Patrick Ferris

How a shell stores variables also impacts field splitting. Naively, Merry stored variables as strings, but really they should be stored as their expanded form.

Consider this example of saving a function output in a variable called base.

iter () {
 echo "a"
 echo "b"
 echo "c"
}

base=$(iter)

for n in $base; do
  echo "Got $n"
done

for n in "$base"; do
  echo "Got $n"
done

Later, when we come to iterate across the fields of $base we wouldn't be able to reconstruct the intial fields that $(iter) produced if we simply saved the result as a string. Too much information would have been lost.

Redirections

June 16, 2026
Patrick Ferris

Redirections control what file descriptors point to. Each redirection has a lifetime associated with it. For example: cat < README.md really says "open README.md and execute the cat program mapping the process's FD 0 to the FD of README.md". When cat returns that redirection is no longer valid.

Other redirections are not so simple. Consider:

$ echo "!dlrow olleh" > input.txt
$ (read line; echo $line) < input.txt | rev
hello world!

There are lots of redirections here too. Firstly the standard input for everything inside the braced command (read line2; echo $line) should be the open file descriptor for input.txt. The standard output of the braced command should be the writing end of a pipe whose reading end is passed to rev as that process's standard input. The final standard output for rev should be unchanged from what it was originally (the shell's standard output).

Getting redirections right is hard, consider this snippet from the debootstrap scripts (all in the context of stdout and stderr pointing to a debootstrap.log file and file descriptor 4 pointing at the true stdout).

for m in $MIRRORS; do
    pkgdest="$TARGET/$($DLDEST pkg "$s" "$c" "$a" "$m" "$path")"
    if [! -e "$pkgdest" ]; then continue; fi
    pkgs_to_get="$(download_debs "$m" "$pkgdest" $pkgs_to_get 5>&1 1>&6)"
    if [ -z"$pkgs_to_get" ]; then break; fi
done 6>&1

More niche built-ins

June 16, 2026
Patrick Ferris

As the dust begins to settle on the nuts and bolts of implementing a POSIX-ish shell, new challenges are allowed to emerge from deep within scripts.

One such challenge is the seemingly endless (I know, they're finite) shell built-ins! Whilst trying to run debootstrap using its fakechroot variant, I bumped into getopts... a whole command-line parsing shell built-in.

An In-memory Filesystem for Eio [eio_mem]

June 16, 2026
Patrick Ferris

Towards the end of last week and the beginning of this week, I threw together a prototype for an in-memory filesystem for Eio: Eio_mem. Concretely, this provides a means for constructing an Eio.Fs.dir_ty Eio.Path.t value that is completely in-memory.

Thanks to Eio's capability-first approach, any existing code implemented against the _ Eio.Path.t API should just work with the in-memory filesystem.

There's a convenient Eio_mem.Fs.load function that can take a path from one filesystem (most likely a normal one from Eio_linux or Eio_posix) and replicate it into the in-memory filesystem. For example, you might have a static web-server pointing to a directory like "/var/www". The following might be the only change you need to make it completely in-memory (example from ocaml-cohttp):

 let () =   let port = ref 8080 in
   Arg.parse
     [ ("-p", Arg.Set_int port, " Listening port number(8080 by default)") ]     ignore "An HTTP/1.1 server";
   Eio_main.run @@ fun env ->
+  Eio_mem.run @@ fun menv ->
   Eio.Switch.run @@ fun sw ->
-  (* Restrict to current directory: *)
-  let htdocs = Eio.Stdenv.cwd env in
+  (* Restrict to in-memory current directory: *)
+  Eio_mem.Fs.load ~filter (Eio.Stdenv.cwd env);
+  let htdocs = Eio.Stdenv.cwd menv in
   let socket =
     Eio.Net.listen env#net ~sw ~backlog:128 ~reuse_addr:true
       (`Tcp (Eio.Net.Ipaddr.V4.loopback, !port))

The keen-eyed reader might have noticed the filter parameter. This can be used to ignore certain paths (here I ignored .git and _build which were adding substantial time to loading the filesystem).

Using meio to debug Merry [weekly-2026-w23]

June 4, 2026
Patrick Ferris

During a large rewrite of Merry, I introduced a regression causing certain shell programs to hang indefinitely. I thought it might be a good opportunity to try out Meio.

First, let's look at a trace of Meio from some real-world programs. For example: meio -- msh -c "sleep 100".

One problem that comes up immediately from this example is that the tree hierarchy of tasks and cancellation contexts makes it tricky to reason about the logging. Here, we can see the pipeline's cancellation context clearly labelled pipeline-id1. This is an Eio.Switch.t for the pipeline sleep 100. The actual execution log for sleep 100 bubbles back up to the main fiber (that has no name, so it is called <task>). The daemon fiber, for ensuring the spawned process is reaped, is attached to the switch and is displayed below it.

Meio has a long way to go. Most programs are too fast to be monitored in this way. Using something like tokio-console's model might be a better idea here focusing less on the tree strucuture of the program, and more on the performance and monitoring of individual tasks. Without this, it turns out Meio was not all that useful for debugging this specific issue (which ended up being some bad pipe management on my part).

Catching up in May [weekly-2026-w22]

June 1, 2026
Patrick Ferris

I never quite found the time in May to write weeknotes. I think that's, in some ways, quite a good thing. My philosophy on weeknotes is not that they are a public punchcard of my progress. They are a tool for organising thoughts and keeping my colleagues abreast of what I am up to. Only the latter suggests any necessity for timeliness and given I frequently message, call and zulip them, if a few weeks fall between the cracks then that's okay!

All that being said, I have tried to provide some thoughts and things I have been up to over the course of May and not just week 22 of the year.

Tools for Shells

June 1, 2026
Patrick Ferris

Some tools and libraries related to Merry.

Meio

June 1, 2026
Patrick Ferris

The shell work continues, and as the complexity internally increases I have found myself wishing for some more tools for debugging concurrent Eio programs. Given enough pipes and compound commands, it is fairly easy to introduce a fair deal of concurrency to your shell programs. To this end I hope I have improved meio somewhat in this fork.

In particular, there were several things I wanted to improve.

Simple UI improvements -- the set of tools for building TUI UIs from nottui are, on closer inspection, rather good and simple. For example, turning a particular UI component into a scrollable pane is as simple as wrapping it in a call to this function.
Logging more runtime events -- Eio programs emit information via OCaml's runtime events. This is how meio knows which fiber is currently active for example. It also emits information like "the current fiber created a new promise" and "this fiber is blocked waiting for a promise". I have not figured out an intuitive way for representing this in meio's tree-view, but I did turn these into "system logs" that appear in the per-fiber logs which is already an improvement. Thanks to Thomas for helping understand the runtime event sequence for logging this information.
Timestamping where possible -- all runtime events come with a timestamp, these were mostly discarded by meio.

Some aspects of meio made it clear that labelling Eio.Switches and Eio.Promises might be a good practice. After reading Michael's Eio forays, I can't help but wonder if a more opinionated Eio for Systems Programmers guide might be useful. I think the Eio README does a wonderful job of introducing most of the low-level concepts, but not quite how they are used by the working systems programmer.

Glob

June 1, 2026
Patrick Ferris

A little dismayed at the lack of progress I was making with Merry, I switched to a specific problem I knew I wanted to fix -- removing the dependency on glob(3), replacing it with a pure OCaml implementation.

let get_txts fs =
  Glob_eio.glob fs Glob.(of_component [ dir_sep; string "home"; dir_sep; star; string ".txt" ])

Anil provided some issues with fixes too which was very appreciated.

Beyond factoring out fork+exec, Merry should now be portable to the browser. Of course there are lots of other refactorings needed to (e.g. a fake-filesystem) -- but I'm interested in how I can use Eio's filesystem capabilities to do this pretty seemlessly. Having an emeddable, pure OCaml shell may also unlock some cool debugging capabilities for unikernels too!

Geocaml

June 1, 2026
Patrick Ferris

Michael has been valiantly trying some of the Geocaml libraries in his work on rendering LiDAR point clouds. Where possible, I have been reviewing Michael's PRs and helping implement features he might find useful. I also took his Laserver for a spin, and suggested a few changes.

Rtree

June 1, 2026
Patrick Ferris

After a comical series of attempts, I (or rather Anil) released rtree.0.2.0 which contained:

A bug fix where not all the tree values were being returned.
A new bounds function for getting an Rtree's maximum bound.
A set of simple removal functions.

PROJ

June 1, 2026
Patrick Ferris

Michael added some new functions to ocaml-proj. Previously, I had worked on making PROJ more portable by using virtual libraries. This was a little restrictive, as it forced users to always implement a JS version of any functionality that exists in the native version of PROJ.

To make it easier to contribute (and allow the C bindings to contain more specific bindings) I carved out the two virtual library implementations into standalone libraries, and reimplemented the virtual library support using these libraries. Now projects can depend directly on proj_c if they wish to have more features in exchange for being less portable.

Ppxlib

June 1, 2026
Patrick Ferris

Thankfully, work on ppxlib has been relatively quiet. Nathan also opened a PR for initial OCaml 5.6 support. Keeping on top of 5.6 features is probably the right way to go about this (rather than one giant PR during the 5.6 release cycle).

I'm particularly happy to have some better migration error handling. The previous errors, though uncommon, were confusing ppx users and hopefully this will help ppx authors and ppxlib maintainers triaging important encoding bugs or feature wishes.

Self-hosting

June 1, 2026
Patrick Ferris

I now have a fairly workable setup for self-hosting source code with git.sirref.org and tests.sirref.org. One piece that is missing, is a publishing workflow. I spent a little time building a YOCaml static-site generator for packaging OCaml libraries. Unfortunately, this was full of rabbit holes and busy work which was frustrating.

Tar-eio

June 1, 2026
Patrick Ferris

One such rabbit hole was tar-eio. It was in a bit of a sorry state, so I took the opportunity to improve the API for new users. Tarring a directory is now as simple as:

let foo = Eio.Path.(cwd / "foo") in
let foo_tar = Eio.Path.(cwd / "foo.tar") in
Eio.Path.with_open_out ~create:(`Or_truncate 0o755) foo_tar @@ fun foo_tar ->
Eio.Switch.run @@ fun sw ->
let t = Tar_eio.create ~sw foo in
Tar_eio.run t (Tar_eio.flow_of_file tar)

There was also a series of security issues that the tar library had that were fixed too, you can read about those in Thomas's weeknotes.

Shell forks revisited [weekly-2026-w17]

April 28, 2026
Patrick Ferris

Whilst I did get up to some other work last week, I decided to dedicate this weekly to fork-ing in the shell.

Must we fork?

April 28, 2026
Patrick Ferris

I have spent some time trying to figure out: can you implement a shell that never has to fork and run its own code (as opposed to fork+exec-ing, which is necessary)? There are, at least, two bits of POSIX shell semantics that make this hard: concurrency and redirects, particularly in the presence of shell built-ins.

Individual commands in a pipeline, according to the POSIX specification, will have their standard input and output connected up before other redirections.

The standard input, standard output, or both of a command shall be considered to be assigned by the pipeline before any redirection specified by redirection operators that are part of the command.

Additionally:

...each command of a multi-command pipeline is in a subshell environment; as an extension, however, any or all commands in a pipeline may be executed in the current environment.

Where a subshell is defined as:

Subshell: A shell execution environment, distinguished from the main or current shell execution environment.

We will note that subshell $\neq$ child process. In practice, however, the two are used fairly interchangeably given the semantics of spawning a child process. Perhaps the two most crucial aspects of a given command in a subshell are that:

It is run in parallel with the other commands.
It should be reentrant.

In what follows we will assume that we are discussing a multi-command pipeline $p$ .

$\texttt {p} = \texttt {cmd}_1 | ... | \texttt {cmd}_n \text { where } n \geq 2$

Pipeline parallelism

April 28, 2026
Patrick Ferris

The POSIX specification does not mandate parallelism explicitly for commands in a pipeline. However, in practice the semantics of a pipeline necessitate concurrency at the very least.

Suppose we have $\texttt {cmd}_1 | \texttt {cmd}_2$ where $\texttt {cmd}_1$ produces an infinite amount of output (e.g., cat /dev/urandom) and $\texttt {cmd}_2$ consumes a finite amount of input (e.g., hxd-xxd -l100). What is the result of running this pipeline with the default POSIX shell options?

$ dash -c "cat /dev/urandom | hxd.xxd -l100"
00000000: 5033 7981 ae44 d475 074c 5361 0a9e 3828  P3y..D.u.LSa..8(
00000010: f88c 1f9e 3b0e bcde 528a f78c b1e4 7195  ....;...R.....q.
00000020: 16f2 8325 c90b 9310 539c af04 8289 c3a0  ...%....S.......
00000030: 57c5 680a 2c81 a999 71b6 708e 95df e720  W.h.,...q.p....
00000040: 004c 407a 8365 9448 65c6 b435 bed2 4084  .L@z.e.He..5..@.
00000050: 62ea 96ab 2b23 0385 f9d2 fc30 d090 1719  b...+#.....0....
00000060: 68d1 c514                                h...
cat: write error: Broken pipe

There are three factors that influence the pipeline's execution behaviour (i.e. what is done about the commands that are run and the exit status we return).

pipefail: With pipefail set to false (the default), the exit status of the pipeline is entirely decided upon by the exit status of the rightmost command (above $\texttt {cmd}_2$ ). With pipefail, the pipe will exit with 0 if all commands exit with 0; otherwise the non-zero exit status of the rightmost command is taken.
!: If the pipeline begins with ! then this inverts the exit code in the way you might expect.
All commands must terminate.

Whilst in theory $\texttt {cmd}_1$ does not terminate, the termination of $\texttt {cmd}_2$ closes the reading end of the pipe resulting in the Broken pipe system error.

Let's replace $\texttt {cmd}_1$ with the following compound command that also produces, left to its own devices, infinite output on stdout.

while echo "some bytes"; do
  :
done | hxd.xxd -l100

In this example, we no longer have the luxury of being able to fork and exec. The compound command (the while loop) forces our hand -- we must rely on the internal execution logic of the shell to keep it going. How then do we prevent $\texttt {cmd}_1$ from blocking our shell from spawning $\texttt {cmd}_2$ ?

For most shells, this is where they might fork themselves and allow the while loop to execute in the child. The parent continuing on. Alternatively, the work could continue on in a separate thread (in OCaml parlance, domain). Fibers (green threads, coroutines etc.) are only possible if there are a sufficient number of co-operative yields (which can be hard to track: any control flow or shell built-in would need to ensure they periodically yield to the other commands in the pipeline).

With that being said, it would seem we have workarounds to ensure the parallel aspects of pipelines are possible to build without forking the shell state itself.

Command Reentrancy

April 28, 2026
Patrick Ferris

Command reentrancy deals with the fact that commands in a pipeline run in subshells. In practice, this means any $\texttt {cmd}_i$ should not impact the execution environment of any other $\texttt {cmd}_j$ . Commands may certainly interfere with one another: a rogue rm might remove a file that some other command was about to read or was in the middle of reading!

$\S 2.13$ of the POSIX shell specification describes the execution environment. In Merry we lift this into the ctx type making it completely immutable; the only way to update the context is to destructively declare new fields, e.g., { ctx with cwd = "/home/bactrian" }. All shell built-ins work directly from the context as opposed to any state that might also be kept by the process itself (e.g., we do not call getcwd(3) to expand PWD, but instead read it from the context).

Most state is handled nicely by this functional implementation of the shell; except open (or not) file descriptors. The exec built-in is particularly thorny in this regard:

If exec is specified with no operands, any redirections associated with the exec command shall be made in the current shell execution environment.

So, exec 3>&1 says "in the current execution environment map file descriptor 3 to stdout" and exec > /dev/null says "in the current execution environment map stdout to /dev/null. This is often used to silence anything writing to stdout.

info () {
  echo "INFO: $1" >&3
}

exec 3>&1 > /dev/null
info "Starting up server..."
echo "Some noisy program spamming STDOUT"
info "All is quiet..."

If we are not running shell built-ins inside new child processes, then we must lift the file descriptor table into the context as well and disallow any two built-ins to run in parallel. Consider this example:

exec < ./hello.txt

while read line; do
  echo "Got $line" >&2
done | (exec < ./world.txt; cat)

If we allow exec to change the set of open file descriptors of the shell process directly, then individual commands in the pipeline are no longer re-entrant. The pipeline here is not being used to pipe bytes from one command to another, but instead to expose concurrency and the subshell behaviour.

The second exec < ./world.txt has no impact on the currently opened file on stdin, the while loop continues to read ./hello.txt. This is straightforward to implement with fork as you can dup2 any files you need. The bookkeeping and isolation is all provided by the POSIX process semantics. Without fork, the file descriptors would have to be saved and restored on every suspension and resumption to provide an illusion of isolation.

We could relax our rule to "disallow any two built-ins to run in parallel" by taking a lock on the file descriptor table, but considering the lock is held from the point of resumption until the next suspension it might not achieve very much in practice.

Some of the intricacies of scheduling are also covered in Michael Greenberg's An executable and formal semantics of the POSIX shell.

To motivate the question, consider the following two pipelines:
(1) while true; do echo 5; done | true
(2) while true; do echo 5; done | { read x; echo $((x+42)); }
Both pipelines spawn two processes, both of which use shell builtins exclusively: neither of these pipelines needs to make an execve system call (though some systems may implement true or echo as executables, Smoosh and most shells build them in).

Scheduling is an opaque part of the POSIX specification. Indeed, as we have argued here, there is no semantics for scheduling. Here is a similar pipeline run through a script that straces for clone and fork calls on various shells:

% ./test/check_fork.sh "while echo 5; do : ; done | true"
bash  3
dash  3
yash  1 
zsh   1
ksh   1
ash   3

It might make an interesting use case for effects in OCaml -- whilst we can certainly piggyback on Eio's existing suspension mechanisms, this is slightly different in that we need to ensure some preconditions are true before resuming certain computations. Fiber-local state is insufficient as programs implicitly depend on global state of the process (in particular the FD table). In this sense, we need some form of suspension-resumption hooks.

Landlocked Eio [weekly-2026-w16]

April 18, 2026
Patrick Ferris

I spent the start of last week wrapping up the "local" CI I mentioned the previous week. Unfortunately it was mostly a packaging nightmare, followed by missing features in ocaml-ci-local. It would be great to have a richer CLI tool for running tests on a directory of git repositories, but unfortunately most of the existing tooling has prioritised the Github/Gitlab etc. APIs.

After coming across minimal during the week, I took it as an opportunity to learn a little more about Linux's Landlock mechanism.

Landlocked Eio [landlocked-eio]

April 18, 2026
Patrick Ferris

Landlock is a sandboxing mechanism that has existed in the Linux kernel since 5.13. It allows unprivileged processes a means to restrict their ambient rights (e.g. creating a file or opening a network connection). These rights are ambient in the ambient authority sense, i.e., any process can attempt to perform an action without needing access to any particular capability to do so.

Ambient systems are often contrasted with capability-based ones. In terms of operating systems, this is most commonly illustrated with access-control-lists-as-columns and capabilities-as-rows in an access matrix.

Eio is a capability-based library in OCaml -- resources are provided at the start of the application and can be passed around wherever they are needed. The design is covered in Thomas Leonard's lambda capabilities blog post. Unfortunately, any application (willingly or via a third-party library) may ambiently access OS resources. In OCaml, this may be by using the standard library functions (e.g. Sys.readdir), things from the Unix module, a C FFI function or by spawning a subprocess.

I had a go at "landlocking" Eio applications last week after stumbling across minimal's hakoniwa library for process isolation (which is very reminiscent of void processes).

module Landlock = Eio_linux.Landlock
let ( / ) = Eio.Path.( / )

let run_eio file =
  Eio.traceln "Writing to file";
  Eio.Path.save ~create:(`Exclusive 0o644) file "hello, world!";
  Eio.traceln "Reading file: %s" (Eio.Path.load file);
  Eio.Path.unlink file

let bad_program () =
  In_channel.with_open_bin "/etc/passwd" In_channel.input_all
  |> Eio.traceln "/etc/passwd: %s"

let () =
  Eio_linux.run @@ fun env ->
  Eio.Path.with_open_dir (Eio.Stdenv.cwd env / ".") @@ fun dir ->
  let parent_fd = 
    Eio_unix.Resource.fd_opt (fst dir) |> Option.get
  in
  let file = dir / "hello.txt" in
  let rules =
    Landlock.Rule.
      [
        path_beneath ~parent_fd
          ~flags:Flags.Fs.(
            write_file + read_file + make_reg + remove_file
           );
      ]
  in
  match Landlock.enter rules with
  | Ok () ->
      run_eio file;
      bad_program ()
  | Error `Not_supported -> Eio.traceln "Landlock not supported!"

Running this program (with the necessary privileges) prints the following.

+Writing to file
+Reading file: hello, world!
Fatal error: exception Sys_error("/etc/passwd: Permission denied")

Eio already has a similar function for the BSDs using capsicum. In capsicum, processes enter a capability mode:

cap_enter() places the current process into capability mode, a mode of execution in which processes may only issue system calls operating on file descriptors or reading limited global system state.

This plays nicely with Eio's capabilities. Within the scope of Eio.Path.with_open_dir, the directory will remain usable after entering "cap" mode. This is not the case with landlock unless you add a specific path_beneath rule to the ruleset (like in the example above). However, you could imagine plumbing the necessary information throughout the Eio_linux backend for it to build a set of rules that can be applied at any given moment in order to mimic the capsicum semantics.

Self-hosting Music [self-host-music]

April 18, 2026
Patrick Ferris

After my friend, Greta, sent me some articles about music streaming services and their exploitation of artists, I decided enough was enough and setup navidrome. It is compatible with Subsonic and so far I'm very pleased to be streaming my own music to my devices. I am using substreamer on iOS. On my laptop, I tried out Andreas Rossberg's Camp

What you got here is an old-school music player heavily inspired by good old Winamp [1], with a focus on decent music library and playlist handling.

I took Anil's OCaml Installer tool for a spin with great success... after a few mishaps mostly related to system dependencies.

$ nix-shell -p pkg-config gcc pulseaudio miniaudio libxi libxrandr libxinerama libffi mesa libxcursor libGL 
$ oi-linux-x86_64 run --with=https://github.com/patricoferris/camp#nix -- camp

The fixes in my fork of Camp are just to install the assets using dune and also statically provide the -lpulse flag (which should probably be dynamically picked up). Otherwise, raylib or miniaudio selects the Null playback device. Roll-on cross-ecosystem package management.

Moving away from Github [weekly-2026-w15]

April 13, 2026
Patrick Ferris

A short week for me as I was hiking with family in and around Porthmadog, Wales.

Reliance on Github

April 13, 2026
Patrick Ferris

I joined Github in 2016, but only began using it in earnest (according to my Github activity) in July 2018: right after Microsoft announced its intent to acquire Github; and just before they closed the $7.5bn deal. I originally joined to boost my résumé and I hoped to receive the odd open-source contribution to projects I was working on (like ppx_deriving_yaml). Since then, I have used Github extensively both at work (Tarides) and for more personal projects.

There have been many reasons to jump ship from Github ever since the Microsoft acquisition (e.g. their planned (and quickly unplanned) announcement to charge self-hosted Github runners). Ultimately, I was convinced when I noticed the new AI tool on the homepage with options to select your "versatile and highly intelligent" model. There is no need for me to stay on the platform for my personal projects. AI integration into Github has nullified those earlier motivations for hosting my projects on the site. The number of contributions has increased, but many of them fix superficial elements of an underlying bug. In my (maybe too uncharitable) imagination, it is all too easy for contributors to prompt their built-in LLM to "fix this broken unit test" and for the fix to layer a single-use workaround on top, instead of identifying and correcting the relevant code. This is, at least, what it looks like to me when I receive a new review request on my open-source projects.

At the end of last week I set up my own Git infrastructure following Ryan's nixos setup. In the future, the source of truth for my projects will be git.sirref.org (I will likely continue to mirror to tangled.org or Github).

Reduce, Reuse, Recycle

April 13, 2026
Patrick Ferris

Part of self-hosting your own source code is managing some form of automatic testing. I hesitate to describe it as "continuous integration" as the term is confusing and overloaded. Luckily, parts of the OCaml community have been building test infrastructure for a while.

I have a proclivity for rebuilding things from scratch be it to learn or to build something the way I want. It is a proclivity I am trying to reign in as I find less and less time to hack on projects. I think this is a good thing. Reusing and recycling existing technologies, in earnest, feels like a pushback on the not-invented-here syndrome that seems to be snowballing thanks to LLMs and code generation tools.

To that end, I dusted off my OCurrent knowledge, and repurposed the "local" mode of OCaml-CI to work with my git repositories. After battling through nested layers of git submodules, I managed to Nix-ify this and get it running at tests.sirref.org. I needed to add multi-repository support as well as Current.Monitors for the local git refs. By no means perfect, but it is nice to own the testing infrastructure (and the cost of downloading OCaml images and the compute to run the tests).

Forking in Shells & Library Maintenance [weekly-2026-w14]

April 3, 2026
Patrick Ferris

This week, Merry got close to executing apt-get install ca-certificates and running Debian's debootstrap scripts correctly. Nathan and I released Ppxlib 0.38.0, I added support to ocaml-proj for compiling to the browser and I fixed a long-standing bug in hilite.

Forking in Merry

April 3, 2026
Patrick Ferris

So far, Merry has managed to support many features of the POSIX shell specification without needing to do a fork(2) without an exec. Or, to put it another way, all the forking that Merry does, will only be followed by running C code. This is actually a feature of Eio's Process API. It is required to make process execution safe in the context of multiple domains in OCaml (mostly due to ensuring consistency within the garbage collector). However, it is perfectly okay to fork and run OCaml code provided there is only a single domain.

In a shell, many features of the shell language require that commands (or built-ins, function applications etc.) execute within a subshell. The specification does not go into detail about how shells should implement this, but many choose to fork in order to preserve some state in the parent (for example, the file descriptor table). Some shells try to minimise the number of forks as an optimisation. The korn shell (ksh) is one such shell.

Using the notation $(command) will cause command to execute in a subshell of the current ksh. In many instances, ksh will not actually fork/exec a subshell when command is a built-in or a shell function.

By trying to adhere to Multicore OCaml's "thou shalt not fork" commandment, Merry is much more similar to ksh in this regard. However, whenever there is an interaction with a shell built-in and semantics that need a child process, things get tricky very quickly. Consider the following:

while echo "hello"; do
  :
done | head -n 3

We have a mix of shell built-ins, normal commands, a compound command (the while loop) and a pipeline. Each individual command in the pipeline requires you to run them in a subshell and set them up before executing them. Without a fork for that first command, you may end up looping forever which is not the intended behaviour here. I had a good conversation with Michael about the implications of this for reproducibility.

For now, I have started implementing a fork for some of the shell features whilst still trying to maintain the functional core of the implementation.

An OCaml 5.5 compatible Ppxlib

April 3, 2026
Patrick Ferris

Nathan and I released an OCaml 5.5 compatible version of ppxlib.

Ppxlib Releases › 0.38.0 [ppxlib-release-0-38-0]

March 30, 2026
Patrick Ferris

Nathan and I released ppxlib.0.38.0 last week. Its main feature is full migration support for the upcoming OCaml 5.5 compiler (currently in its alpha3 release). This means supporting the handful of new features landing in OCaml 5.5: modular explicits, external type declarations and arbitrary "local" structure items.

As Nathan and I continue to find a plausible maintenance story for ppxlib, Nathan has opened an issue on the OCaml compiler to discuss the idea of adding additional extension points to the language.

This comes from the desire to be able to store encoded versions of new OCaml features inside older abstract syntax trees.

There are also some nice bug fixes in there too:

A potential OOM loop has now been removed.
Locations have been restored to long identifiers!

Browser support for ocaml-proj [ocaml-proj-virt]

April 3, 2026
Patrick Ferris

A while ago I built some "modern" bindings to PROJ4 in OCaml. After reading Jon Ludlam's weeknotes (and speaking with him and Anil, I thought it might be nice to add a Javascript backend to those bindings). This was relatively straight-forward using Dune's virtual libraries and is available on Github.

With virtual libraries, your own data analysis could (if you wished) depend solely on the proj library and later choose to either link it with proj.c or proj.js depending on where the analysis is being deployed.

I briefly looked at WASM support, but quickly realised there was not much appetite for it and trying to compile around the C FFI was going to be hard.

Hilite updates

April 3, 2026
Patrick Ferris

Since I helped relaunch the ocaml.org website a few years ago, I have maintained hilite, a tool for doing build-time syntax highligting.

It is being used on the ocaml.org website, in yocaml_markdown and also in Michael's webplats (when it works...).

And it was Michael who led me to finally fixing and adding syntax highlighting support for Python and Go. Once I was on this roll, I finally took a look at fixing the long-standing bug of trying to highlight ocaml-mdx code, like the following:

# 50.123;;
# 1 "file.ml";;

In fact, this code now highlights fine, but was the source of the bug. The syntax highlighting grammer confuses the ocaml-mdx # as a toplevel directive and trouble ensues.

Ppxlib release and Merry updates [weekly-2026-w13]

March 30, 2026
Patrick Ferris

Ppxlib Releases › 0.38.0 [ppxlib-release-0-38-0]

March 30, 2026
Patrick Ferris

As Nathan and I continue to find a plausible maintenance story for ppxlib, Nathan has opened an issue on the OCaml compiler to discuss the idea of adding additional extension points to the language.

This comes from the desire to be able to store encoded versions of new OCaml features inside older abstract syntax trees.

There are also some nice bug fixes in there too:

A potential OOM loop has now been removed.
Locations have been restored to long identifiers!

Merry updates

March 30, 2026
Patrick Ferris

It turns out writing a POSIX(ish) shell is hard; at least, there is a vast number of slightly obscure features that one needs to support. I think this is surprising because most people writing shell scripts write simple shell scripts; scripts that use a much smaller subset of features. In the same breath, those same developers are likely to use something like apt-get install bash which runs a plethora of more advanced (and non-POSIX) shell scripts!

Exec redirects

March 30, 2026
Patrick Ferris

In a shell script, you can use the built-in command exec to replace the current process with a new one (e.g. exec vim). However, there is a different mode of operation for exec:

If exec is specified with no operands, any redirections associated with the exec command shall be made in the current shell execution environment.

One of my litmus tests for Merry is the Debian debootstrap scripts (h/t Anil). One thing that it does is the following:

err () {
  printf "err $1" >&4
}

exec 4>&1

A lot of detail has been elided for clarity. exec 4>&1 sets up a redirection for the shell's execution environment in which file descriptor 4 is now an alias for standard output. So, writing to 4 (by redirecting a command's standard output to 4 i.e. >&4) will output to where standard output is going (most likely the terminal).

For this to work, shell's must dup2 the relevant file descriptors which have the following condition:

If the file descriptor newfd was previously open, it is closed before being reused; the close is performed silently (i.e., any errors during the close are not reported by dup2()).

This is all very well, unless your program has an important file already open that happens to have file descriptor 4. The Eio Linux and POSIX backends suffer from this problem. Both make use of a file-based synchronisation mechanism for waking up the eventloop should another domain push a completion to the scheduler's run queue. On Linux this is via an eventfd and in POSIX, a pipe.

This dup2 will close the eventfd and will likely grind Eio to a halt (or an assert false). For now, I have resorted to vendoring Eio and moving the eventfd file descriptors to higher values, though I have opened a PR to make this more configurable. This bug is quite easy to write up... it was not so easy to find!

As a random example, consider the debconf/confmodule script which offers very little room for a buggy implementation!

Outreachy

March 30, 2026
Patrick Ferris

The presentations from the demonstrations for this round of Outreachy are now online!

Write support in OCaml TIFF library [outreachy-ocaml-tiff]

February 9, 2026
Patrick Ferris

I am mentoring Tambe Salome during the December 2025 Outreachy round to add support for writing TIFF files in the ocaml-tiff library.

You can now see the video of the demonstration day presentation:

A POSIX Shell in OCaml [weekly-2026-w12]

March 19, 2026
Patrick Ferris

Long time, no weekly. Since the start of this year I have been building a POSIX shell in OCaml called msh (with the underlying library called Merry). Merry is available online now.

A POSIX(ish) shell in OCaml

March 19, 2026
Patrick Ferris

Merry is a POSIX(ish) in OCaml. It uses Eio alongside Morbig (a static parser for POSIX shell).

Why another (POSIX) shell?

March 19, 2026
Patrick Ferris

Shells have been around for a long time. In my research on the notion of metashell I wrote about Louis Pouzin originally coining the term:

Metashells › Louis Pouzin's "SHELL" [pouzin-shell]

May 19, 2025
Patrick Ferris

I spent some time reading part of the multics design documentation this week. Louis Pouzin coined the term "SHELL" in this document, and I was reminded yet again just how important it is to be a good writer even as a "computer science researcher". For example, this excerpt from the requirements section of the document

The previous definitions imply that a command MUST be designed while keeping in mind the user, sitting at his console, wondering about what might be going on, mistyping or forgetting arguments, even if fully aware of the conventions, and possibly interfering with the command by hasty quits, carriage returns, and other temperamental reactions.

And then later, when defining the "SHELL".

We may envision a common procedure called automatically by the supervisor whenever a user types in some message at his console, at a time when he has no other process in active execution under console control (presently called command level). This procedure acts as an interface between console messages and subroutine. The purpose of such a procedure is to create a medium of exchange into which one could activate any procedure, if it were called inside of another program. Hereafter, for simplification, we shall refer to that procedure as the "SHELL".

It still surprises how little the undergraduate degree in computer science at Cambridge focuses on writing skills.

I built Shelter exploring the idea of a shell-like interface that allowed users to undo their shell actions (amongst other cool tricks). Unfortunately shell-like is not enough. Shelter cut many corners to masquerade as a shell (e.g. appending env to understand how a command may have altered the execution environment). I felt it was necessary to make Shelter a "SHELL"! To do that, I needed a solid foundation to build on.

msh, the POSIX shell that comes with Merry, is by no means POSIX-complete in terms of features. But it is my daily driver at this point. It includes a pure OCaml rewrite of linenoise (a small, self-contained alternative to the venerable readline) called bruit.

If you have docker installed on your machine, you can take it for a spin today:

docker run -it --rm patrickferris/msh

The patrickferris/msh docker image is just for trying it out. It is based on the OCaml 5.3 alpine image.

Alternatively, you can build msh from source and have it available in your opam switch.

opam pin git+https://tangled.org/patrick.sirref.org/merry

There are many small paper cuts left to patch over, but most of it is porcelain (e.g. ctrl+left-arrow for moving in bruit). Unfortunately, these are the kinds of things that you will immediately stumble upon.

What makes Merry different?

March 19, 2026
Patrick Ferris

Nothing.

Merry is supposed to be a solid, POSIX-ish base to build on. Unfortunaltely, as it turns out, the subset of features from the POSIX specification that people actually use... is pretty much all of it. Every possible redirection, variable expansion, shell built-in and compound command make some appearance. Not to mention the non-POSIX bits of shell we all take for granted (e.g. &>-redirection).

Merry and msh are useable. You will most likely find bugs if you use them. If, when using msh, you find something obscure happening you can enable debug mode either my setting the variable MSH_DEBUG or by invoking msh with -v -v.

What's next?

March 19, 2026
Patrick Ferris

It is soon time to combine Shelter and Merry into the time-travelling, POSIX-ish shell that I have been trying to build since I first started working on Shelter.

For this to be successful, I need Merry to be able to pass plenty of tests and right now that involves trying to install plenty of packages using tools like apk and apt.

Shell MRDT

March 19, 2026
Patrick Ferris

What I am particularly interested in reasoning about, is the execution context in Merry. This value, alongside the file-system, constitutes a fairly deep understanding of the state that changes in each step of a shell's evaluation loop.

This was, in terms of Shelter, the missing piece for truly building some kind of MRDT across shell sessions.

TIFF in OCaml

March 19, 2026
Patrick Ferris

I picked up from the excellent Outreachy work of Tambe Salome in getting write-support in ocaml-tiff. We are getting closer to the kind of API I envisaged in this latest round of refinement and review.

let ( / ) = Eio.Path.( / )

let checkerboard ~size =
  let v = Nx.zeros Nx.uint8 [| size; size |] in
  for row = 0 to size - 1 do
    for col = 0 to size - 1 do
      if (row + col) mod 2 = 0 then Nx.set_item [ row; col ] 254 v
    done
  done;
  Nx.to_bigarray v

let () =
  Eio_posix.run @@ fun env ->
  Tiff_eio.with_open_out (env#cwd / "example.tiff") @@ fun w ->
  let tif = Tiff.make (checkerboard ~size:256) in
  Tiff.to_file tif w

I have also been extremely pleased to see further external collaborators appearing:

Nicolas helping out with metadata maintainence.
Virgile adding support for reading multi-image TIFF files.

Below are weeklies from 2025.

Data Provenance in Shelter [weekly-2025-w49]

November 24, 2025
Patrick

A late update that stretches more than a week...

failed to render transclusion https://shelter.sirref.org/shelter-log-002/

OCaml Roundup: November 2025 [ocaml-roundup-november-2025]

December 9, 2025
Patrick Ferris

A roundup of some of my OCaml activity this month!

Ppxlib Updates

December 9, 2025
Patrick Ferris

I managed to get a substantial amount of ppxlib work completed in the last month. Some of those bits are described in detail below.

Initial OCaml 5.5 Support [ppxlib-5.5-support]

December 9, 2025
Patrick Ferris

We recently merged 5.5 support into our main branch (thanks to Nathan for the review). It cast a light on some new features coming to OCaml including the ability to define any structure locally. Most OCaml programmers will be familiar with things like local opens (e.g. let open M in). This has now been extended to include any structure item (more or less, you cannot nest value bindings).

External types have also landed, bringing with them a new "type kind" in the AST. You can read about them in the manual.

Bug Fixes for Attribute Rewriters

December 9, 2025
Patrick Ferris

There was a bug in the error handling for attribute rewriters -- a relatively new feature to have been added to ppxlib.

In some places, ppxlib makes use of a With_error.t monad. Unlike the Result.t monad, With_error.t allows you to collect errors rather than find the first error and thread it through your program. This can make sense if your program can meaningfully do work after encountering an error. Unfortunately, in the logic for replacing attributes, the code could not recover from an error where there were duplicate attributes.

Value binding constraints in ppxlib

December 9, 2025
Patrick Ferris

I spent a good few hours understanding why the CI tests in my PR to mirage/repr were still failing. It turns out, the bug, is fairly simple to reproduce. Consider the following OCaml code:

let g : int = 1

We have a let-binding that contains a type constraint. Where, in the abstract syntax tree sense, do we attach the constraint? Since OCaml 5.1, we have quite a few options for how this can be represented.

The first is using the Ppat_constraint AST node which allows you to attach a type constraint to any pattern.

    | Ppat_constraint of pattern * core_type
    (* (P : T)*)

This tends to be used for things like function arguments (e.g. fun (x : int) -> x + 1). So it doesn't quite capture what we want for this binding constraint.

On older OCaml compilers, let g : int = 1 also includes a Pexp_constraint on the body of the value binding. This makes sense if we consider a more general case of the syntax where we allow functions.

let g x : int = x + 1

In this example, we no longer parse a Ppat_constraint and instead this is treated as syntactic sugar for:

let g x = (x + 1 : int)

In OCaml 5.1 a new field for value bindings was added allowing type constraints to be tracked directly in the bindings themselves.

and value_binding =
  {
    pvb_pat: pattern;
    pvb_expr: expression;
    pvb_constraint: poly_constraint option;
    pvb_attributes: attributes;
    pvb_loc: Location.t;
  }(** [let pat : type_constraint = exp]*)

Our type constraint (: int) can be recorded in the pvb_constraint directly. This is a nice change, bringing the concrete syntax of the language closer to the parsetree. For ppxlib, however, it does create a bit of a headache as we now have to encode the different versions of expressing type constraints into the AST of the compiler before 5.1!

So what was the bug? The author's of repr had added some value bindings to circumvent some "unused value warnings" in OCaml. They choose to write these bindings like:

let (_ : int) = 1

Here, I have simplified the types and the body of the binding. The significant thing to notice is the parentheses surrounding the value binding's pattern. This forces the AST representation to be a single Ppat_constraint node regardless of the current compiler version. No Pexp_constraint and no pvb_constraint.

However, using ppxlib with a compiler less than 5.1 meant that this did not roundtrip cleanly. Instead, the value after passing through ppxlib's migrations was:

let _ : int = 1

After bottoming out at the issue, I discovered we already had a test case for this in ppxlib! Whatsomore, ocamlformat.0.20.0 (which repr was still using) actually transforms instances of let x : t = ... into let (x : t) = ...! This limitation does not seem inherent to the parsetree, the printer or the migrations. But I have asked the (very kind and friendly) previous maintainers about these changes.

Outreachy

December 9, 2025
Patrick Ferris

December 8th 2025 marks the start of the next round of Outreachy. I have been preparing some issues for geocaml/ocaml-tiff and I am excited to get started with Tambe Salome!

Graft and Bib [graft-and-bib-update]

December 9, 2025
Patrick Ferris

I spent some time at the start of the month updating and fixing Graft and Bib.

I added named subtrees to the markdown syntax. This allows users to create subtrees with headings that are also externally linkable.

With some more bug fixing, I hope to release both libraries before the new year!

Shelter imports [weekly-2025-w46]

November 15, 2025
Patrick Ferris

Most of this week was spent fixing bugs in Shelter in an attempt to get it ready to run the LIFE pipeline.

To do so, Shelter requires a mechanism by which to pull in files from the outside world. That could be configuration files, source trees, data etc. For now, this has taken the form of a crude @ import statement. In true Forester fashion, I have transcluded the foreign tree from my Shelter logs.

failed to render transclusion https://shelter.sirref.org/shelter-log-001/

Eio Debugging [eio-cap-debugging]

November 15, 2025
Patrick Ferris

One of the promises of Eio's capability model is an improved debugging experience. Your program must be designed in such a way as to make resources explicit. If you have a load_config function, more than likely it will need access to the file-system.

val load_config : _ Eio.Path.t -> Config.t
(** [load_config path] loads a configuration from [path].*)

Once all of your resources are explicit and threaded through your program, it becomes much easier to swap out one resource for another (provided they have the same interface).

This week I was trying to get the LIFE pipeline running in shelter. Part of that involves pulling the appropriate geospatial container image, but my code was terrifically broken, failing with what looked to me with a Docker error.

With only this clue, I knew that it was probably in the execution of some command in a child process, but which one? To trace this quickly, I could plug in a slightly different instance of the _ Eio.Process.mgr capability. Essentially unchanged from the one provided by the unix backend except that it would also print to stderr the command it was trying to run!

let debug_process_mgr (mgr : 'a Eio_unix.Process.mgr) : 'a Eio_unix.Process.mgr
    =
  let module D = struct
    type t = unit

    let spawn_unix () ~sw ?cwd ?pgid ?uid ?gid ~env ~fds ~executable args =
      Eio.traceln "Spawning subprocess... %a" Fmt.(list string) args;
      Eio_unix.Process.spawn_unix ~sw ?cwd ?pgid ?uid ?gid mgr ~env ~fds
        ~executable args
  end in
  let module V = Eio_unix.Process.Make_mgr (D) in
  Eio.Resource.T ((), Eio_unix.Process.Pi.mgr_unix (module V))

Whist there is a fair amount of jumping through first-class-module-shaped hoops here, all I am doing is rewrapping an existing "method" to a spawn a unix child process. Then using this helper function, I can push that into my standard environment.

let env =
  Eio_unix.Stdenv.with_env
  ~process_mgr:(debug_process_mgr env#process_mgr)
  env
in
Shelter.main config (env :> _ Shelter.env) dir cmd_file

And sure enough I was querying for the Docker user before pulling the image! I hadn't run into this, as for the last while I have been using alpine and debian images that already exists locally.

Taking stock & Graft updates [weekly-2025-w45]

November 10, 2025
Patrick Ferris

I ended up working on a real mix of things last week. I managed to get two large posts uploaded to this site: an ICFP recap and a deeper-dive into the random bits of OCaml hacking. This was pretty useful in terms of taking stock of where I was at, I always find post-conference, I'm a little lost. The conversations and ideas from ICFP are simultaneously exciting but also pretty inactionable.

Graft Updates [graft-updates]

November 10, 2025
Patrick Ferris

It would seem that there is a potential second user of Graft! A great source of motivation for some tidying and feature implementing. At the end of last week I fixed some little bugs in Graft and (more importantly) allowed users to name their subtrees. Upstream Graft also now pins and uses my Bibtex implementation.

OCaml Geotessera

November 10, 2025
Patrick Ferris

As I discussed in my OCaml roundup, I spent a bit of time polishing the OCaml geotessera code. In particular this meant starting to handle low-level geospatial transformations allowing users to project latitudes and longitudes into the rows and columns of their raster data. I feel like I'm stumbling onto a path Michael has carved out with Yirgacheffe. I'm following along with his work excitedly and we had a good catch-up last week discussing how libraries should handle things like automatic, numeric precision conversion.

As a fully paid-up member of the static types fan club, I'm often evangelising the benefits of types. However, I hit a little quirk of the Nx library in OCaml (a numpy equivalent). Take a look at this sum function:

let sum nx = Nx.sum nx |> Nx.item [];;
val sum : ('a, 'b) Nx.t -> 'a
let arr = Nx.create Nx.int8 [| 2 |] [| 64; 64 |];;
val arr : (int, Bigarray.int8_signed_elt) Nx.t
sum arr;;
- : int = -128

Even though sum takes an ('a, 'b) Nx.t array that stores elements with representation 'b and allows you to read/write to the elements with OCaml values of type 'a, the sum function performs the calculation in the world of the 'b representation type (here signed 8-bit integers). So we get an overflow! Perhaps not too surprising but easy to overlook! The moral of the story is that we still need to be careful in the OCaml world too.

Shelter

November 10, 2025
Patrick Ferris

I did a little cleaning up of the Shelter code too. I noticed a few bugs from the demos I did at ICFP. Most were pretty straight-forward to fix, and the test-suite is now checking parallel for-loops and conditional execution are working.

This week

November 10, 2025
Patrick Ferris

I hope to get a few things done this week:

Run some part of the LIFE pipeline through Shelter.
Deploy the Shelter daemon allowing other people to connect and take it for a spin.
Spend some time writing and reading, pulling ideas together rather than always writing code!
I have a slightly harebrained scheme that combines Shelter, atproto, Forester, capn(proto/web) and webcomponents... I want to see if I can pull that together.

Back to shelter [weekly-2025-07-18]

July 18, 2025

Capnproto Interface to Shelter

July 18, 2025

I spent some time this week thinking about decoupling Shelter's builds from the CLI tool. This would allow users to connect to a remote daemon to build and store their sessions, opening up the possibility of using Shelter on Windows and macOS.

The design so far keeps the metadata local, in an Irmin database. I will probably then augment the stored information with some kind of unique daemon identifier to know where the actual data is.

This is all still quite fresh, and I will have more by the end of next week.

OCaml-related Things

July 18, 2025

Something I decided to do last week, was to make a clearer delineation between my OCaml work and research. That line is not always very distinct, but I have split of a separate set of weeklies for OCaml related things.

Fear not, the OCaml weeklies are a subset of these weeklies and so I will, when I remember, transclude them for you.

OCaml Weekly 2025 w29 [ocaml-weekly-2025-w29]

July 15, 2025
Patrick Ferris

Ppxlib

July 15, 2025
Patrick Ferris

I met with Nathan this week to discuss future plans for ppxlib. The current state of affairs is that ppxlib is becoming unmaintainable. This is primarily a knock-on effect from changes being made to OCaml's parsetree (e.g. labelled tuples being added in 5.4). Nathan has a plan that will provide two key properties.

Migrations, which allow old compilers to be used with new ppxlib releases, will be more compatible. For example, we will be able to migrate new features downwards and back up without raising an error.
Ppx authors will be able to use new features in an opt-in workflow, rather than ppxlib bumping the internal AST (like we did in ppxlib.0.36.0). This will reduce the maintenance burden significantly whilst still allowing users to write ppxes for new OCaml features.

I also started looking into some older issues in ppxlib related to performance. This is work-in-progress, but I am trying to improve the performance of some passes done by ppxlib. To better understand what was making ppxlib slow, I wanted to use runtime_events_tools but I was dismayed to see it wanting to install over 100 packages! I opened a PR to reduce the number of packages. I think this kind of work goes a little unrecognised as it is not very glamorous. However, I think it really benefits the OCaml community in the long run.

Outreachy

July 15, 2025
Patrick Ferris

In Outreachy news, we had a wonderful mid-internship video call with all the interns and mentors to catch-up on how everyone is getting along. Seeing the progress everyone has made was great! I am very grateful for the work that Michael and Steve have put in so far to make this a very successful Outreachy round for OCaml.

In sadder news, an email was shared with all Outreachy mentors detailing the increasingly critical financial situation the project finds itself in. There are ongoing discussions about how costs can be cut including potentially only running a single round a year.

Graft

July 15, 2025
Patrick Ferris

With the release of Forester.5.0, I made a plan to make a release of Graft.0.1. Unfortunately this is blocked by a new release of hilite, a tool I built for doing build-time syntax highlighting for OCaml code. This powers the syntax highlighting on ocaml.org.

PhD Work and Scientific Programming [rest-of-monthly-2025-06]

June 30, 2025
Patrick Ferris

I have been away on holidays. Some of that was spent hiking in the Massif de Calanques in Marseille, unfortunately this week they were on fire. Anyway, this "weekly" fills in the blanks for the rest of June 2025.

A Calanque just outside of Marseille, a canyon-like structure of calcified stone on the edge of the Mediterranean.

Revisiting hazel_of_ocaml

June 30, 2025
Patrick Ferris

I spent some time thinking and writing about hazel_of_ocaml and the benefits of OCaml as source language and Hazel as a target language. The stability of OCaml with a powerful type-system makes it very expressive.

Hazel, on the other hand, is a hotbed of PL theory research. The typed holes, which are fully supported within the language, are particular powerful targets for a transpiler. They allow transpiler developers to incrementally support the language whilst still getting full feedback of their generated source code. Whilst it is a niche use-case, I thought it was an interesting point to note.

A second year report

June 30, 2025
Patrick Ferris

I wrote my second year report at the end of June. The process was a bit chaotic, but I think ultimately it was useful for clarifying some ideas I had rattling about in my head.

Part of that was working on my publications page and, through a conversation with Anil, realising that my work on Graft was in many ways a part of my own research.

If you are interested, feel free to peruse both my first year and second year reports. Take them with a pinch of salt! I think it is useful to be fairly open with this sort of thing when you can be. Maybe someone will find them useful.

What is scientific programming?

June 30, 2025
Patrick Ferris

This question arose whilst writing my second year report. It got me thinking about how we distinguish scientific programming from other kinds of programming. I came across a paper by Tim Storer: Bridiging the chasm: A survey of Software Engineering Practice in Scientific Programming. I enjoyed a few aspects of this paper.

Relating scientific programming to the scientific method so directly was pretty useful insight for me. Falsifable hypotheses, repeatable experiments and reproducible results when applied as characteristics of the computer stack (e.g. OS, programming language, filesystem etc.) might be a useful lens to argue for better tools to enable this.
The breadth of Storer's case-study analysis on how scientific programming has "gone wrong" is very good, if anyone needs a reference to draw on to help support their tools or research, this seems like a good index to use.

Ppxlib

June 30, 2025
Patrick Ferris

I was reminded again of the pain of helping maintain ppxlib. We are coming to the conclusion, that in its current form, it is becoming unmaintainable. I took a stab at updating bisect_ppx to the latest ppxlib. With OCaml entering a potentially turbulent parsetree era, it might be time to take stock of this and propose some fillers to help. Nathan Rebours and I are meeting next week to discuss ideas he has to make parsetree migrations smooth sailing for all!

Geocaml

June 30, 2025
Patrick Ferris

Work on ocaml-tiff has stalled, which is totally fine. Michael and I independently are working on other things and have obligations to fulfill. Maybe some day we'll carve out enough time together to push forward on this project, but not this week.

Forester as a Target Syntax [weekly-2025-06-02]

June 2, 2025
Patrick Ferris

This week included some time finishing opentrace and subsequently folding it into shelter. I have been writing up some more of the draft paper for shelter which I am excited to share in the near future.

I revisited the upgrading vpnkit PR and pushed some more fixes. I have been thinking, again, about the promise of a direct-style world for OCaml that just hasn't quite landed yet. C'est la vie.

Forester and Graft

June 2, 2025
Patrick Ferris

I spent a bit of time finally pulling out my changes to Forester to add markdown and bibtex support into a standalone tool: graft.

Graft [graft]

June 2, 2025
Patrick Ferris
https://github.com/patricoferris/graft

Graft is a Forester preprocessor.

It takes a forester (a directory of trees) written in a mixture of Markdown, Bibtex and Forester syntax and produces a new forest completely written in Forester syntax.

Usage

June 2, 2025
Patrick Ferris

graft simply preprocesses a forest generating Forester trees from .md, .bib and .tree files. It will copy the structure of the input directory in the output directory.

$ graft preprocess --output=grafted-trees trees
$ forester build

This assumes that you have updated your Forester toml file to put the grafted-trees directory as your source of trees.

[forest]
trees = [ "grafted-trees" ]

Example

June 2, 2025
Patrick Ferris

A typical "tree" might look something like

---
title: Opentrace and Supervisions
date: 2025-05-26
author: Patrick Ferris
---

Over the past two weeks I have mainly split my time (amongst many things) developing [opentrace](open-trace)
and doing revision supervisions.

```forester
\put\transclude/numbered{false}
\transclude{open-trace}
```

A few things to note:

The yaml frontmatter allows you to add some of the metadata fields from Forester.
At any point in your markdown there is an escape hatch to Forester using a forester codeblock.

It is very satisfying to find the separation of concerns works quite well. For a while I had been rebasing my development branch on Forester. I was also worried about trying to get the code upstream as it pulled in many dependencies. It seems that I have a very workable solution. I welcome contributions to graft including extra input formats. I am also considering extending the Forester configuration to contain some graft configuration for how it should generate new trees (e.g. at the moment every entry in a bibtex file is given a new tree).

Maths Support

June 2, 2025
Patrick Ferris

As part of that process, graft now supports Markdown KaTeX. For example:

Definition. Mergeable Replicated Data Type Implementation [mrdts]

Patrick Ferris

A mergeable replicated data type (MRDT) implementation for a data type $\tau$ is a tuple $D_{\tau } = (\Sigma , \sigma _{0}, do, merge)$ where:

$\Sigma$ is the set of all possible states at a branch,
$\sigma _{0} \in \Sigma$ is the initial state,
$do : Op_{\tau } \times \Sigma \times Timestamp \rightarrow \Sigma \times Val_{\tau }$ implements every data type operation,
$merge : \Sigma \times \Sigma \times \Sigma \rightarrow \Sigma$ implements the three-way merge strategy.

Definition 2.1 from "Certified Mergeable Replicated Data Types".

Revision Supervisions

June 2, 2025
Patrick Ferris

I have been doing revision supervisions for Discrete Maths and Foundations of Computer Science. I have also been marking Operating Systems past paper questions for the same group of first years.

Opentrace and Supervisions [weekly-2025-05-26]

May 26, 2025
Patrick Ferris

Over the past two weeks I have mainly split my time (amongst many things) developing opentrace and doing revision supervisions.

Opentrace [open-trace]

May 19, 2025
Patrick Ferris
https://tangled.sh/@patrick.sirref.org/opentrace

Thanks to Koonwen's excellent libbpf bindings in OCaml, I have been building a little tool called opentrace to make it easier to track an executable's inputs and outputs.

This work was inspired my Michael's self-proclaimed "gross hack": pyshark. Whilst pyshark achieves its goals by injecting code into commonly used python objects and methods, opentrace uses eBPF. By using a lower-level API (hooks in the kernel), opentrace can remain programming language agnostic. However, less information is none about the user's intent compared to something like pyshark.

Monitoring the System

May 19, 2025
Patrick Ferris

opentrace has an all command that will trace the entire system.

$ sudo opentrace all --flags=O_WRONLY
pid,cgid,comm,kind,flags,mode,filename,return
16324,4417,".nheko-wrapped",openat,577,438,"/home/patrick/.cache/nheko/nheko/curl_alt_svc_cache.txt",47
16324,4417,".nheko-wrapped",openat,193,33188,"/home/patrick/.cache/nheko/nheko/cr3ZUHqBErIOe3PlwJ1SuU8zFKKxL12VzrRoHYMH.tmp",47
16324,4417,".nheko-wrapped",openat,577,438,"/home/patrick/.cache/nheko/nheko/curl_alt_svc_cache.txt",47
16324,4417,".nheko-wrapped",openat,193,33188,"/home/patrick/.cache/nheko/nheko/QfZTdZSuC56NdcDQ3aMxJc3BhMhAj8PmtYW1zFDP.tmp",47
2530,4235,"systemd",openat,524865,438,"/sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/app.slice/gammastep.service/cgroup.subtree_control",41
2530,4235,"systemd",openat,524545,0,"/sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/app.slice/gammastep.service/memory.min",41
2530,4235,"systemd",openat,524545,0,"/sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/app.slice/gammastep.service/memory.low",41
2530,4235,"systemd",openat,524545,0,"/sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/app.slice/gammastep.service/memory.high",41
2530,4235,"systemd",openat,524545,0,"/sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/app.slice/gammastep.service/memory.max",41

The --flags=O_WRONLY argument filters the events where the O_WRONLY flag was set in the call to open.

We also get the name of the current executable linked to the task (comm). The -wrapped is an artefact of using Nix.

Tracing an Executable

May 19, 2025
Patrick Ferris

The primary use case for this tool is to inspect what files your program might be reading and writing.

$ sudo opentrace exec --format=json --flags=O_CREAT -- opam list
$ cat trace.json | jq ".[] | .fname"
"/home/patrick/.opam/cshell/.opam-switch/packages/cache"
"/home/patrick/.opam/log/log-118747-29da3d.out"
"/home/patrick/.opam/log/log-118747-29da3d.err"
"/home/patrick/.opam/log/log-118747-29da3d.env"
"/home/patrick/.opam/log/log-118747-29da3d.info"

The "flags" argument can specify a small boolean formula for checking the open flags of a particular event with | (or), & (and), and ~ (not). Parentheses can be used for precedence.

$ sudo opentrace exec --flags="O_WRONLY|O_RDONLY" -- ocaml --version

Spawning Subprocesses

May 19, 2025
Patrick Ferris

One feature opentrace needs (in this proof-of-concept phase) is the ability to also trace subprocesses.

opentrace is primarily an eBPF program that is loaded into the kernel and communicates with an OCaml program. Events are communicated via a ring buffer and most of the post-processing happens in OCaml. To capture subprocesses, opentrace creates a new control group (cgroup) and places the new process into that group. This gives opentrace a new identifier to track, namely the cgroup.

So consider the following program.

let () =
  Eio_posix.run @@ fun env ->
  Eio.Path.(save ~create:(`Or_truncate 0o664) (env#fs / "hello.txt") "hello");
  Eio.Process.run env#process_mgr
    [ "/bin/bash"; "-c"; "echo 'heya' > heya.txt" ]

It first creates a file using direct calls to functions like openat. Then it spawns a process which creates a new file called heya.txt. This happens in a separate process. However, with the --cgroups flag we can capture both interactions with the operating system.

$ sudo opentrace exec --cgroups --flags="O_CREAT|O_TRUNC" ./main.exe
pid,cgid,comm,kind,flags,mode,filename,return
153187,530807,"main.exe",openat,526914,436,"hello.txt",5
153192,530807,"bash",openat,577,438,"heya.txt",3

Eio's Process API

May 19, 2025
Patrick Ferris

I have used the Eio_unix fork action process API to be able to extend what happens in the child process. Loading most eBPF programs into the kernel requires special privileges hence the need for sudo. When a user requests for a particular program to be executed and traced, opentrace spawns a process via the Eio Process API. Opentrace defines a few new so-called "fork actions", little fragments of C code that are run after the call to fork (clone). Most likely this ends with a call to execve, but other calls are possible for example setuid allowing opentrace to change the user of the child process so it does not run as root. Similarly, this is where (if used) we create the cgroup and place the process into that group.

Limitations: Io_uring

May 19, 2025
Patrick Ferris

Whilst testing opentrace against some of the tools I use nearly daily, I noticed some events were being missed. I tried tracing forester, and only the initial read of forest.toml was logged. It dawned on me that the reason for this was that forester (via eio) was using io_uring to perform most of the IO. Most attempts to open files were bypassing the open system calls, and instead they were being performed by the kernel after reading a submission request for an openat2-style call!

This is not news to seasoned, Linux systems programmers. Io_uring bypasses SECCOMP filters for exactly the same reasons.

$ sudo opentrace exec -- forester build
$ cat trace.csv
pid,cgid,comm,kind,flags,mode,filename,return
155007,535570,"forester",openat,524288,0,"forest.toml",5
155007,535570,"forester",Uring,2621440,0,"",0
155021,535570,"cp",openat,131072,0,"/home/patrick/documents/forest/theme/favicon-32x32.png",4
155007,535570,"forester",Uring,2686976,0,"",0
155007,535570,"iou-wrk-155007",Uring,557634,420,"",0
155007,535570,"iou-wrk-155007",Uring,557634,420,"",0
155007,535570,"iou-wrk-155007",Uring,557634,420,"",0

It is interesting to note two things here:

We can tell that forester reads the configuration file probably using something like In_channel in OCaml (it does).
It appears that Uring is performing IO in both worker threads and directly.

The file paths are empty at the moment as I cannot find a clean way to trace openat submission requests into Uring without it sometimes going very wrong. I have tried quite a few methods (e.g. tracing do_filp_open) and at the moment I am tracing io_openat2, but this seems quite brittle, and often the filename is completely garbled, so I do not set it. If anyone has any ideas to trace Io_uring more reliably, I am all ears!

Supervisions

May 26, 2025
Patrick Ferris

The first year students I teach will be doing their first year exams soon. I have been helping them revise for foundations of computer science and discrete mathematics. Whilst doing so, I have been thinking a good deal about Jon Sterling's post about assessment bureaucracy. In particular:

At moments like this, it is a good idea to pause and reflect on whether it is better for our students that each faculty member spend a cumulative two months doing literally nothing but assessment and higher-order practices related to assessment, vs. other activities that could benefit our students more (including actual teaching, of which we do astonishingly little at Cambridge).

Metashells [weekly-2025-05-12]

May 19, 2025
Patrick Ferris

This week, I feel I have been stuck fighting the OCaml ecosystem trying to keep my OxCaml work afloat. Aside from that, Ryan and I made some really nice progress with Shelter, culminating in Ryan describing it as a metashell.

Shelter the metashell

May 19, 2025
Patrick Ferris

The main progress this week with Shelter was composing runc's terminal mode with entering raw terminal input mode on the Shelter side. This is inspired by Ryan's own work on capability interfaces.

Shelter remains mostly intact, acting as an interactive shell. However, just before executing a command we switch to receiving and sending raw terminal inputs and outputs. This means tools like vim now work in Shelter! Not only that, but users can now activate an inner shell (e.g. zsh) and enjoy all the usual features of a fully-fledged shell (tab complete, fuzzy history search etc.) and upon exiting that shell, Shelter will snapshot the session. This lets you alter the granularity of snapshots from the command-line.

Louis Pouzin's "SHELL" [pouzin-shell]

May 19, 2025
Patrick Ferris

The previous definitions imply that a command MUST be designed while keeping in mind the user, sitting at his console, wondering about what might be going on, mistyping or forgetting arguments, even if fully aware of the conventions, and possibly interfering with the command by hasty quits, carriage returns, and other temperamental reactions.

And then later, when defining the "SHELL".

We may envision a common procedure called automatically by the supervisor whenever a user types in some message at his console, at a time when he has no other process in active execution under console control (presently called command level). This procedure acts as an interface between console messages and subroutine. The purpose of such a procedure is to create a medium of exchange into which one could activate any procedure, if it were called inside of another program. Hereafter, for simplification, we shall refer to that procedure as the "SHELL".

It still surprises how little the undergraduate degree in computer science at Cambridge focuses on writing skills.

OxCaml

May 19, 2025
Patrick Ferris

Last week, I got a toplevel with OxCaml working. This required a serious amount of work to understand the changes Janestreet have made to obscure parts of the OCaml compiler and then working those into tools like js_of_ocaml.

This week, Janestreet pushed their latest rounds of changes and of course everything broke! I spent some more time fixing it all back up. I'm not entirely sure how maintainable this is. The problem is that, whilst things compile, the programs do not work together! Only when someone uses the program do the bugs surface.

Other OCaml Work

May 19, 2025
Patrick Ferris

I worked on some other parts of the ecosystem this week.

Ppxlib

May 19, 2025
Patrick Ferris

I helped review some changes to enable Janestreet to have ppx rewriters via attributes (usually they are via extension points). It is a bit of a controversial change to ppxlib as we try to keep the API predictable for users:

Extension points are rewritten: this means the part that is rewritten is nicely delimited by the extension points start and end.
Attributes extend: attributes do not rewrite the code they are attached to but rather extend the code with new AST nodes.

We will see what we decided to do (most likely provide the functionality behind some kind of "expert" interface).

Tiff

May 19, 2025
Patrick Ferris

See Michael's notes.

I spent some time trying to speedup the LZW decompression of TIFF files in the pure OCaml tiff library this week(end). The two big changes to help with this are pretty common when speeding up these parts OCaml programs:

Allocate less
Does less work

In terms of allocating less, the original implementation was using a char list to represent LZW strings. Manipulating these becomes quite costly, particularly since the most common operation is appending a single character to the end of a list. Converting this to use OCaml's immutable string saved a ton of allocations.

In terms of doing less work, I opted to bypass Cstructs sane (but slow) bounds checks in some of the "hotter" parts of the code. In particular, LZW ends up reading potentially huge arrays full of bytes one-by-one. So reading each byte needs to be quite snappy. This is a bit of a trade-off in terms of "safety" but we are in control of this code so I'm not too worried about that.

Here are some results decompressing a fairly large array of some elevation data.

before: | tiff/lzw/cea │ 523851.2259 mjw/run │ 3289761.5414 mnw/run │ 9806796.7121 ns/run│
after:  │ tiff/lzw/cea │  27846.2408 mjw/run │  587928.7527 mnw/run │ 8457161.3761 ns/run│

Hazel and OxCaml [weekly-2025-05-04]

May 10, 2025
Patrick Ferris

I missed a week of posting last week, mainly because I spent more time writing posts.

Hazel of OCaml

May 10, 2025
Patrick Ferris

I mentioned previously that I was building a tool to transpile OCaml code to Hazel. This work is now in a good enough state that, along with one of my students, we have transpiled a good number of OCaml programs to help them write their evaluation for their third-year project.

I wrote up a little summary of that work, which I've transcluded below.

A Transpiler from OCaml to Hazel [hazel-of-ocaml]

May 2, 2025
Patrick Ferris

Over the past few months, I have been piecing together a transpiler from Hazel to OCaml. This is, in part, to help one of my third-year undergraduate students who is working on type error debugging in Hazel.

Typed Holes

May 2, 2025
Patrick Ferris

Hazel is a functional programming language with typed holes. Holes are pieces of your program that have not yet been filled in. Holes can appear anywhere in your program both as expression or types. Hazel can still evaluate your program in the presence of holes.

To get a flavour of Hazel, take a regular map function for lists.

let map = fun f -> fun xs -> case xs
  | [] => []
  | x :: xs => f (x) :: map(f)(xs) 
end in
map(fun x -> ?)([1, 2, 3])

The question mark (?) is a hole. The program evaluates to the following expression of type [?] (for people more familiar with OCaml types ? list).

[ ?, ?, ? ]

Hazel supports local type inference but nothing involving unification variables. For example, a simple add_one function in Hazel (fun x -> x + 1) has type ? -> Int.

From OCaml to Hazel

May 2, 2025
Patrick Ferris

The ability to transpile OCaml programs to Hazel programs is motivated by one simple thought: there are more OCaml programs than there are Hazel programs. This could help bootstrap projects by alleviating the need to rewrite boilerplate code (e.g. URI parsing or standard library functions for strings).

A Transformation of Syntax

May 2, 2025
Patrick Ferris

Hazel markets itself as an "Elm/ML-like functional programming language". From the previous example of map, it should be apparent just how close to OCaml the language is.

It turns out that a majority of the transpiler is a transformation of syntax. Take a simple ADT for an arithmetic programming language.

type expr =
  | Float of float
  | Add of expr * expr
  | Sub of expr * expr
  | Mul of expr * expr
  | Div of expr * expr

And when we run hazel_of_ocaml over this OCaml type declaration.

type expr =
  + Float(Float)
  + Add((expr, expr))
  + Sub((expr, expr))
  + Mul((expr, expr))
  + Div((expr, expr))
 in ?

Not much has changed expect some syntax. Hazel does not have a notion of top-level expression so hazel_of_ocaml wraps the program into one set of value bindings. For the most part, Hazel acts as a subset of the pure, functional part of OCaml. At the time of writing, this subset is fairly limited with no support for modules or labelled records out of the box (there are plenty of development branches with these features).

If we try out the same map function but written in OCaml and transpiled to Hazel we get.

let rec map f = function
  | [] -> []
  | x :: xs -> f x :: map f xs

Which becomes the following hazel program.

let map = fun f -> fun x1 -> case x1
  | [] => []
  | x :: xs => f(x) :: map(f)(xs)
end in ?

We could have a field day discussing the syntax of OCaml and Hazel (parentheses for function arguments, well-scoped cases for pattern-matching, a different arrow for pattern-matching etc.). What would be more interesting is taking a look at how to handle polymorphism in Hazel.

Explicit Polymorphism

May 2, 2025
Patrick Ferris

Hazel has explicit polymorphism. So far, we have not seen it as we have let the types have holes in them. The map function in OCaml has the following type.

val map : 
  ('a -> 'b) -> 'a list -> 'b list

We must remind ourselves (by reading Craig's excellent blogpost on the matter) that in OCaml

... type variables in signatures are implicitly universally-quantified

So in reality, we have that map has the following type.

val map : ∀ a b. (a ->  b) -> a list -> b list

In Hazel, we have to explicitly type our map function to be polymorphic. Not only does this mean the type annotation requires universally quantified type variables, but we must also perform type application wherever we choose to apply the map function (whether that be recursively or somewhere later in our program).

let map : forall a -> forall b -> (a -> b) -> [a] -> [b] =
  typfun a -> typfun b -> fun f -> fun xs -> case xs
    | [] => []
    | x :: xs => f (x) :: map@<a>@<b>(f)(xs) 
end in
map@<Int>@<Int>(fun x -> ?)([1, 2, 3])

forall introduces a universally quantified type variable into our type annotation, and typfun introduces it into the function itself (à la System F). Type application requires @<T> where T is some type. This allows hazel to quite easily support higher rank polymorphism, but we will not worry too much about that.

Propagating OCaml Types into Hazel

May 2, 2025
Patrick Ferris

Most often, OCaml users interact with prenex polymorphism (rank-1) where the universal quantifiers are at the front of the type. OCaml does support quantifiers inside certain types like records.

What this means for the transpiler is that we can reuse OCaml's type inference to safely instantiate the correct type annotations and type applications in Hazel! To do this, hazel_of_ocaml uses Merlin to inspect the type of the function in either a value binding or at the point of a function application.

Take a simple, polymorphic length function.

let rec length = function
 | [] -> 0
 | _ :: xs -> 1 + length xs

let int_len = length [ 1; 2; 3 ]
let str_len = length [ "only"; "two" ]

When we run this through hazel_of_ocaml with the -type flag we get.

let length : forall a -> [a] -> Int = typfun a -> fun x1 -> case x1
  | [] => 0
  | _ :: xs => 1 + length@<a>(xs)
end in
let int_len : Int = length@<Int>(1 :: 2 :: [3]) in
let str_len : Int = length@<String>("only" :: ["two"])
in ?

hazel_of_ocaml has correctly instantiated the type for length inside the recursive function and then in each case with the integer list and the string list.

A Corpus of Hazel Programs

May 2, 2025
Patrick Ferris

The impetus for this work was to derive a corpus of ill-typed Hazel programs. Luckily, such a corpus exists for OCaml! Seidel et al. created a corpus of OCaml programs from their undergraduate students at UC San Diego. Some of these programs have been transpiled to Hazel.

Future Work

May 2, 2025
Patrick Ferris

Hazel is a fun, research programming language. Potential third-year students may find it interesting to take this work further. For example, how would this look in terms of a module system? From a purely engineering perspective, plenty of work would be needed to convert a multi-library OCaml project to Hazel (e.g. handling the cmi files).

Another line of research would be to have Hazel target one of the intermediate representations in OCaml which would give Hazel a fully functioning compiler to "native" code?

OxCaml

May 10, 2025
Patrick Ferris

I spent some time this week getting more familiar with Oxidized OCaml. I have a habit of wrapping new OCaml tools and libraries into toplevel, browser applications. For example, try-irmin and try-eio.

Naturally, I tried to wrap OxCaml into a toplevel so people could play around with the new modes that are part of the OxCaml type system. This turned out to be a lengthy debugging session (where type declarations did not align so the raw Obj.repr js_of_ocaml representation was broken for some parts of the toplevel). I would say that I do question the time-spent/value trade-off, but a mostly working toplevel with OxCaml is available at: https://patrick.sirref.org/oxcaml.

Shelter in the Forest [weekly-2025-04-21]

April 21, 2025
Patrick Ferris

I spent much of this week working on Shelter and things related to it. Some of that time was also spent on Forester.

Forester

April 21, 2025
Patrick Ferris

I added two important quality-of-life features to my frontend to Forester this week.

Bibtex Support

April 21, 2025
Patrick Ferris

I had previously mentioned adding support to Forester for Markdown. This week I added support for Bibtex too. From any *.bib file in your forest, Forester will now dutifully recognise it as a Bibtex file and convert, as best it can, all the entries into Reference trees.

I'm becoming quite convinced of this model at the moment. I'm using Forester's Code.t as a target representation. In fact, to ease the process, I really shouldn't spend all my time on my website, I have reused my Yaml.t -> Code.t and Markdown.t -> Code.t functions in the Bibtex parser.

To see it in action, you could have a look at the Build Systems à la Carte paper which is generated completely from Bibtex.

Full Heading Support

April 21, 2025
Patrick Ferris

The eagled-eyed viewer may have noticed that the table of contents for this page has more than one level. I finally caved and spent an evening rejigging my Cmarkit.Doc.t -> Tree code which was hacky and broken and is now less hacky and less broken.

In addition, headings support links and emphasis etc.

Lunch with Jon Sterling

April 21, 2025
Patrick Ferris

I had a delightful lunch with Jon Sterling discussing the future of Forester, the nature of the Web (old and new) and the success posting weekly updates for our colleagues. Thanks Jon.

Shelter Fixes

April 21, 2025
Patrick Ferris

I spent a good chunk of my week fixing bugs in Shelter with the aim to perhaps setup a VM somewhere and let people kick the tyres of what we've got so far.

The first bug is pretty annoying. At the moment, our filsystem backend is ZFS and we make heavy use of snapshots and cloning in order to provide time-travelling capabilities. Unfortunately, ZFS will take a snapshot before data has fully made it to disk (or whatever is the equivalent point it should reach in ZFS). Commands that generated lots of disk activity would be snapshotted in a half finished state and this would cause all sorts of problems. Thanks to Mark Elvers for the pointer to how OBuilder uses ZFS for the OCaml macOS builders which unmount datasets immediately therefore inducing a flush of sorts. Shelter now follows a similar model with all of the slow downs that create. Anil and I discussed some amalgamation of overlayfs, tmpfs and ZFS to alleviate some of this but for now that's a premature optimisation.

A small eDSL for Shelter

April 21, 2025
Patrick Ferris

Whilst testing Shelter, I ended up wanting a way to programmatically invoke the different run commands. This is similar to say a Dockerfile, but maybe with a little more expressivity.

This lead me to revisit the Build systems à la Carte paper and rediscover selective applicative functors.

I started playing around with a selective applicative interface to Shelter, this would allow you to express your dependencies statically but select them dynamically (as the paper says).

module D = Shl (Identity)

let shelterfile =
  let open D in
  let base_image = from "alpine" in
  let is_node_lst img = String.equal "v22.15.0" (stdout img) in
  let cmds base =
    let node_version = run "node --version" base in
    Select.if'
      (Select.map ~f:is_node_lst node_version)
      (run "node -e 'console.log('success!')")
      (run "node -e 'console.log('failure!')")
      base
  in
  with_session "node" (cmds base_image)

From this, we get a slightly more expressive way to describe images.

Geocaml TIFF Library

April 21, 2025
Patrick Ferris

I was pleasantly surprised to receive a pull request from Michael adding support to ocaml-tiff for reading TIFF files compressed using LZW. I was also surprised to hear the TIFF LZW is a little different to others.

In trying to get this PR merged, I moved the initialisation of the Eio eventloop to outside each individual test case. This one change then completely broke the entire test suite. After a period of debugging and help from Thomas Leonard the root cause was OCaml's OUnit2 library using process-level parallelism (via Unix.fork), sharing the ring between the parent and the child lead to the issues.

Part II Students

April 21, 2025
Patrick Ferris

As the new term begins, it signals that there are only just over two weeks for the final year undegrads at Cambridge to submit their dissertations.

The four students that I help supervise have been sending me drafts of their work (and reminder you can read about their projects) and I'm very impressed. I'm sure the next two weeks will be stressful, but I'm proud of what they have accomplished over the past academic year.

Catching Up! [weekly-2025-04-14]

April 21, 2025
Patrick Ferris

This is a bit of a catch-up post. A week or so in Belfast has thrown me a little off course with the weeklies.

Outreachy December 2024 and Beyond

April 21, 2025
Patrick Ferris

In the last week or so, we have come to the end of the December 2024 round of Outreachy. For those who do not know about Outreachy, it facilitates and organises internships in open-source. Internships for those who are historically underrepresented and impacted by systematic bias in tech.

We, the OCaml community, held our biannual Demo Day where our current cohort of interns can show off what they have been building for three months. This round, we only had one intern, Abdulaziz, who worked on the OCaml API diffing tool.

The next round has just completed the contribution phase and our next cohort of interns is being selected.

Ppxlib Supports OCaml 5.4 (mostly)

April 21, 2025
Patrick Ferris

This week, I added support for OCaml 5.4 to ppxlib. A rather non-trivial change to the codebase due to changes in the representation in Longidents (which now have location information for all segments of the Longident). OCaml 5.4 has labelled tuples, a light-weight record-like syntax similar to labelled arguments in functions.

let x = ~x:1, 10, ~y:1 
val x : x:int * int * y:int

let add (~x, ~y) = x + y
val add : (x:int * y:int) -> int

As of writing this post, we are waiting for the full OCaml 5.4 feature freeze and magic number bump. Note this is support for OCaml 5.4 (in ppxlib terms, we know have migration functions for 5.3 <-> 5.4) not a bump to use the 5.4 parsetree in ppxlib.

The Future of Ppxlib

I have now been helping maintain Ppxlib for a while and I am beginning to wonder about the long-term vision for the project. Ppxlib is a central component to any modern OCaml library or tool, Sherlocode reckons there's 18.3k instances of [@@deriving. Yet, every compiler release is a painful, two-phase process for the maintainers. First, we have to support the new release of the compiler. This is mostly okay but does involve adding lots of code to Ppxlib (we need a whole copy of the Parsetree!). Later on down the line, we have to bump the AST and this often breaks many ppxes.

The biggest issue is still the tension between abstract types and pattern-matching. By exposing the Parsetree directly to user's, they can pattern-match against the AST and build their ppxes in a rather straight forward way. However, this comes with the risk that the type may change thus breaking their code.

I have mentioned before about plans to use "views" to help with this, but I do not see that happening any time soon. It feels to me, we should be able to allow a user to select their AST to work with. How this interacts with modules like Ast_builder is unclear to me, but it would mean user's can remain on older ASTs even when ppxlib bumps the internal, main AST.

Sherlorocq

April 21, 2025
Patrick Ferris

Rocq is a theorem prover (originally called coq) from many of the same folks who brought you OCaml. I have been interested in theorem provers for a while, more from an engineering perspective and less from a theoretical one. My good friend, Derek Sorenson, taught me about Rocq when we encoded mergeable replicated datatypes. Later I had a go at encoding decentralised information flow control (also in Fstar).

During this process, I found it quite challenging to find Rocq code that I could learn from. Compared to using say OCaml's Stdlib, using Rocq's standard library of datatypes and proofs did not come naturally (not helped by the fact that at the time I could not get the LSP to play ball).

This lead me to port Arthur Wendling's excellent Sherlocode to do the same thing only for Rocq code. Luckily, it uses regular expression matching with a trigram index so it is not tied to any particular programming language. Additionally, it is somewhat opam-centric (not actually that much) which is perfect for Rocq!

Anyway, I've had some interest from the people at Inria to bring the server back to life. Their wish is my command!

Sherlorocq is back online!

Shelter, Hazel and More! [weekly-2025-03-31]

March 31, 2025
Patrick Ferris

Last week I focused on Shelter -- our idea that shells should have the same ability as reproducible build tools like Nix or Docker. To this end I now have a fairly fleshed out prototype.

Shelter Prototype

March 31, 2025
Patrick Ferris

Shelter is a spin-off from the work Michael and I started with Shark. It takes the same ideas but applies them directly to a shell-like interface.

We're still in the middle of working all of this, but you can read more about it at Shelter.

Forester Hacking

March 31, 2025
Patrick Ferris

As you can probably tell, my website is still using Forester. I rebased my Markdown branch to include the new Atom syndication feature.

Alongside that I added support for arbitrary HTML injection into Forester via codeblocks in Markdown. This was actually very straightforward thanks to Markup and being able to re-parse Forester syntax in the middle of converting a Markdown document. The HTML for the shell in Shelter uses this feature.

If you are interested in taking this custom Forester for a spin, there's a branch on Github. In fact, nearly the only change beyond letting the core engine know about markdown files, is adding a new parser frontend.

Hazel

March 31, 2025
Patrick Ferris

For one of my Part II students, I've been prototyping a transpiler from OCaml to Hazel. This has gone pretty well and now supports type annotations as well as straight-forward implementation translation.

Consider the following OCaml map function.

let rec map f = function
  | [] -> []
  | x :: xs -> (f x) :: map f xs

The tool, hazel_of_ocaml can translate this to Hazel code, including making the polymorphism explicit.

let map : forall a -> forall b -> (a -> b) -> [a] -> [b] 
  = typfun a -> typfun b -> fun f -> fun x1 -> case x1
  | [] => []
  | x :: xs => f(x) :: map(f)(xs)
end in ?

You can copy and paste that codeblock into the hazel playground. But do note that it still needs some manual editing to add the type applications in directly (map@<a>@<b>). With the right amount of type-inferencing and scoping I actually think that you could place those type applications in yourself. This could make a nice Part II project I think.

Ppxlib

March 31, 2025
Patrick Ferris

I recently wrote about the painful experience of miragrating ppxlib to the 5.2 OCaml AST. This week, Nathan Rebours and I merged a PR to add the 5.3 AST! Ppxlib has been playing catch-up with the compiler and we decided it was best to try to catch up quickly and deal with the ecosystem fallout all at once rather than incrementally. With this new AST merged, ppx authors can now use the new Pexp_effect parsetree node. I'll write a little more about this in a separate post soon.

Vpnkit, Void Processes, LSP Servers [weekly-2025-02-17]

February 17, 2025

Previous weeklies used strong emphasis to distinguish sections. This comes from Forester's philosophy about atomicity of the content in your forest.

However, subtrees are supported! I quickly hacked together the ability to use subheadings to indicate subtrees. This is strictly less expressive than the \subtree{} of Forester's default syntax as we cannot close heading sections in Markdown.

This weekly uses subtrees.

Vpnkit

February 17, 2025

I spent some time this week trying to upgrade vpnkit to OCaml 5. I was originally working on a paper idea which might need benchmarks, but Anil and I decided we could simply point to the port I did and show how it has simplified much of the code.

Void Processes [void-processes]

February 17, 2025

Work continued on implementing (and fully exploring) void processes. A lot of the groundwork already exists in Jake Hillion's master's thesis.

This week I added a feature that I needed to help build the processes we need for the shell I'm building: mount points with modes!

In addition to the root mount (taken care of with pivot_root), we need to be able to add additional mounts into the process' environment. These can now be added. All mount points can be mounted readonly or readwrite.

Here is the "Hello, World!" example (the /say/hey program has been statically compiled using musl-gcc).

let status =
  let void =
    empty 
    |> mount ~mode:R ~src:hey_dir ~tgt:"say"
    |> exec [ "/say/hey" ]
  in
  let t = Void.spawn ~sw void in
  Promise.await (Void.exit_status t)

There really is nothing else in there. Without specifying a root mount, the void process is started with an empty tmpfs root. Next on the list is networking!

LSP Servers

February 17, 2025

I got a little side-tracked building a library for writing LSP servers in OCaml: mlsp. This may seem a little unrelated, but it isn't. The LSP has become the de facto standard for communicating between an editor and a programming language environment. If you have used VSCode to write a program in Python, chances are you are using the official extension which gives you linting, formatting, code navigation etc. All of these features are communicating using the LSP.

It seems Code Mirror can already proxy over a websocket for LSP support too (we might not even need that as we can compile OCaml directly to JavaScript/Webassembly and have the whole thing running locally!).

Open-Source & Community

February 17, 2025

Anil and I had a great conversation this week about building community especially as it pertains to open-source and OCaml.

I've been going back over Governing the Commons, but have already discovered The Real World of Technology!

More thoughts on all of this soon... maybe

Teaching, teaching, teaching... [weekly-2025-02-10]

February 10, 2025

On paper, I don't have that many students. I teach four undergraduates (first year students at Pembroke College) Discrete Maths. I supervise three third year students for their final year project and another one I co-supervise with Michael Dales. However, I do end up spending at least two full days a week on teaching. Something I really enjoy and take seriously. The time it takes is also quite unpredictable; last week for instance all the third year students had their mid-project demonstrations (a five-minute presentation in front of their peers and a few professor-types). My first year students also found two slides particularly challenging to understand from their lectures and asked if I could help explain what was going on, so I produced some materials for that.

OCaml

Within the OCaml universe I spent a good bit of time trying to maintain a couple of different packages:

Mirage Crypto: this library provides cryptographic primitives for OCaml programs. Unfortunately, the maintainers removed direct support for Eio replacing it with a "Unix" alternative. This is not a fair swap as now Eio programs must make a dependency to Unix! Speaking to Anil, the best approach might be to make Eio programs handle this directly. I think this highlighted how fragmented open-source maintenance can be as no user of Eio seems to have bumped into this yet and the upstream maintainers did not communicate a rather large breaking change.
Ppxlib: In addition to the 5.2 AST bump (which is nearly ready to be merged), I queued up a 5.3 AST bump right behind it. I plan to write up a more detailed post about the challenges of maintaining this part of ppxlib.

Paris

I spent some time in Paris at the end of the week. I enjoyed visiting the Musée d'Orsay and in particular their collection of impressionist paintings. Here's my favourite from that visit by Camille Pissarro.

Woman in an Orchard (Spring Sunshine in the Meadow at Eragny). — Woman in an Orchard (Spring Sunshine in the Meadow at Eragny)

AT Protocol and an IR for Wikis [weekly-2025-01-27]

January 27, 2025

AT Protocol

This week I've been diving into the AT Protocol.

The Authenticated Transfer Protocol, aka atproto, is a decentralized protocol for large-scale social web applications.

The protocol could be a candidate for the glue that holds together a distributed, computational wiki network. The protocol, it seems, is very similar to IPFS. Thankfully, a few years ago, I was working on building out a suite of OCaml libraries for working with IPFS. For example, ocaml-cid, self-describing content-addressed identifiers.

# let s = "zb2rhe5P4gXftAwvA4eXQ5HJwsER2owDyS9sKaQRRVQPn93bA";;
val s : string = "zb2rhe5P4gXftAwvA4eXQ5HJwsER2owDyS9sKaQRRVQPn93bA"
# let cid = Cid.of_string s |> Result.get_ok;;
val cid : Cid.t = <abstr>
# Cid.pp_human Format.std_formatter cid;;
cidv1 - base58btc - raw - ident(sha2-256) length(32) digest(6e 6f f7 95 0a 36 18 7a  80 16 13 42 6e 85 8d ce
                                                            68 6c d7 d7 e3 c0 fc 42  ee 03 30 07 2d 24 5c 95

To this end I have built out some more OCaml libraries for working with atproto, including:

atproto-data: the atproto data model, similar to JSON-LD.
ocaml-did: an OCaml library for working with decentralized identifiers.
atproto-lexicon: atproto's schema format, I've been building a quick tool for doing an OCaml translation from these schemas.

I managed to get a post published from the OCaml library after fixing it up and porting it to Eio.

An IR for Wikis

I started working on a proof-of-concept intermediate representation for Wikis -- I imagine it a bit like malfunction but for computational wikis i.e. a target for Wiki building tools that allows different front-ends and servers to communicate in a common IR for exposing key functionalities of a wiki:

Links: External links, cross-wiki backlinks
Versioned, temporal feeds
Etc.

Other PhD Work

I met with most of my Part II students this week, and I'm excited about their work. Progress reports are due this week and next they have a presentation to give.

In discrete maths this week we did induction. Next up is a big section on sets, functions, bijections etc.

Misc.

I was happy to find the Information Controls Fellowship Program.

The Information Controls Fellowship Program (ICFP) cultivates research, outputs, and creative collaboration on topics related to repressive internet censorship and surveillance.

Forester, ICFP, Wikis [weekly-2025-01-20]

January 20, 2025

This week was my first full week back from the break and I found it challenging trying to get back into what I had been working on previously.

ICFP Papers

In conversation with Anil, we looked at options for submitting a paper to ICFP. I wrote up some notes on some options we discussed.

Forester

I spent some time this week converting this website to using Forester. I'm not a huge fan of the syntax, especially as a lot of my site's content was already in markdown. So I wrote a markdown frontend to forester which is available on Github.

The markdown frontend integrates very nicely and only a few changes were needed in the core logic of Forester itself. Additionally, for any features not directly supported in markdown there is an escape hatch using code blocks such as:

```forester
\put\transclude/numbered{false}

\transclude{pxf-1000}
```

Personally, I'm still getting to grips with the bottom-up approach to building this site, atomically creating notes and reference cards that then are linked in many places.

I'm excited to see how I can integrate some of the Forester concepts into "Shark".

OCaml

In the OCaml world, I spent time on ppxlib, reviewing PRs to update the lower bounds of the library, fixing effect syntax problems and bumping internal AST to 5.2. I've also spent some time looking into de-objecting Eio, making the OCaml types more friendly to new users. I need to revive my port of Vpnkit to Eio for the thoughts on ICFP 2025 too.

I also want to modernise and make more public my OCaml code for creating little shells in OCaml too -- I think the ideas here really have legs and would like to find a conference to submit them too.

I also met with the single, OCaml Outreachy intern working on ocaml-api-watch.

Misc.

I met with most of my Part II students this week which was nice to catch up and see how their projects are going. I also started marking work for my first year students who are at the induction part of their Discrete Maths course.

Write support in OCaml TIFF library [outreachy-ocaml-tiff]

February 9, 2026
Patrick Ferris

I am mentoring Tambe Salome during the December 2025 Outreachy round to add support for writing TIFF files in the ocaml-tiff library.

You can now see the video of the demonstration day presentation:

Project. Outreachy [outreachy]

https://www.outreachy.org/

Outreachy provides internships in open source and open science. Outreachy provides internships to people subject to systemic bias and impacted by underrepresentation in the technical industry where they are living.

I am one of the coordinators for the OCaml community for Outreachy. I have also mentored multiple three-month internships for the community.

Write support in OCaml TIFF library [outreachy-ocaml-tiff]

February 9, 2026
Patrick Ferris

I am mentoring Tambe Salome during the December 2025 Outreachy round to add support for writing TIFF files in the ocaml-tiff library.

You can now see the video of the demonstration day presentation:

Implement a non-blocking, streaming codec for TopoJSON [topojson-0001]

December 1, 2022
https://github.com/geocaml/ocaml-geojson

GeoJSON is a JSON standardised format for handling simple geographical data. One common use of GeoJSON is to add map overlays such as with the popular JavaScript library leaflet. One issue with GeoJSON is redundant data, for example when two polygons share boundaries ideally only one would contain the full data about the boundary, however, with GeoJSON the data is duplicated.

TopoJSON is an extension to GeoJSON to encode topology. This allows for redundant data to be removed and file sizes to be greatly reduced. This is often very desirable especially when working with data in the browser. In a previous Outreachy internship a new OCaml library was implemented to provide an OCaml library for TopoJSON, this project will build on this adding more functionality to the library and providing a non-blocking, streaming codec version similar to the geojsone library.

Extend OCaml's GeoJSON library to support TopoJSON [topojson-0002]

June 1, 2022
https://github.com/geocaml/ocaml-geojson

Improve the ocaml.org Website [ocamlorg-0001]

June 1, 2021
https://ocaml.org

OCaml.org is the main website for OCaml, a functional, typed, high-level programming language. This project revolves around improving the website on multiple different fronts including: layout, accessibility and content.

Because a lot of the content of the website is about OCaml and the website itself is written in OCaml, this would be a great opportunity for interns to learn the language and functional programming concepts.

ocaml-tiff [ocaml-tiff]

https://github.com/geocaml/ocaml-tiff

Part of the geocaml suite of geospatial libraries for OCaml.