Patrick Ferris 2026 2 9 https://patrick.sirref.org/merry/ merry /merry/ Merry https://tangled.org/patrick.sirref.org/merry Merry is a library for building POSIX-ish shells in OCaml. It comes with a Eio called msh. Supporting the full POSIX specification is no easy feat, however, msh already supports plenty of the more useful bits of the spec . References Context Backlinks Patrick Ferris 2026 4 3 https://patrick.sirref.org/weekly-2026-w14/ weekly-2026-w14 /weekly-2026-w14/ Forking in Shells & Library Maintenance 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 . Patrick Ferris 2026 4 3 Forking in Merry So far, Merry has managed to support many features of the POSIX shell specification without needing to do a 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 However, it is perfectly okay to fork and run OCaml code provided there is only a single domain. subshell. The specification does not go into detail about how shells should implement this, but many choose to fork forks as an optimisation. The is one such shell. Using the notation 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 while 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. Patrick Ferris 2026 4 3 An OCaml 5.5 compatible Ppxlib Nathan and I released an OCaml 5.5 compatible version of ppxlib . Patrick Ferris 2026 3 30 https://patrick.sirref.org/ppxlib-release-0-38-0/ ppxlib-release-0-38-0 /ppxlib-release-0-38-0/ Ppxlib Releases › 0.38.0 Nathan and I released ppxlib.0.38.0 last week. Its main feature is full migration support for the upcoming OCaml 5.5 alpha3 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! Patrick Ferris 2026 4 3 Browser support for ocaml-proj A while ago I built some "modern" bindings to PROJ4 in OCaml . After reading Jon Ludlam 's weeknotes 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 . 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. Patrick Ferris 2026 4 3 Hilite updates 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 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. Patrick Ferris 2026 3 30 https://patrick.sirref.org/weekly-2026-w13/ weekly-2026-w13 /weekly-2026-w13/ Ppxlib release and Merry updates Patrick Ferris 2026 3 30 https://patrick.sirref.org/ppxlib-release-0-38-0/ ppxlib-release-0-38-0 /ppxlib-release-0-38-0/ Ppxlib Releases › 0.38.0 Nathan and I released ppxlib.0.38.0 last week. Its main feature is full migration support for the upcoming OCaml 5.5 alpha3 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! Patrick Ferris 2026 3 30 Merry updates 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 Patrick Ferris 2026 3 30 Exec redirects In a shell script, you can use the built-in command exec to replace the current exec vimdifferent 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 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 standard output to 4 i.e. >&4 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 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 assert falseEio 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! Patrick Ferris 2026 3 30 Outreachy The presentations from the demonstrations for this round of Outreachy are now online! Patrick Ferris 2026 2 9 https://patrick.sirref.org/outreachy-ocaml-tiff/ outreachy-ocaml-tiff /outreachy-ocaml-tiff/ Write support in OCaml TIFF library 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: Patrick Ferris 2026 3 19 https://patrick.sirref.org/weekly-2026-w12/ weekly-2026-w12 /weekly-2026-w12/ A POSIX Shell in OCaml Long time, no weekly. Since the start of this year I have been building a POSIX shell in OCaml called mshMerry Merry is available online now. Patrick Ferris 2026 3 19 Merry Eio alongside Morbig Patrick Ferris 2026 3 19 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: Patrick Ferris 2025 5 19 https://patrick.sirref.org/pouzin-shell/ pouzin-shell /pouzin-shell/ Metashells › Louis Pouzin's "SHELL" 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 and subroutine. The purpose of such a procedure is to create a medium of exchange into which one could activate any procedure, inside of another program if it were called . 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 shell-like is not enough. Shelter cut many corners to masquerade as a env to understand how a command may have altered the 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 readline bruit . If you have docker installed on your machine, you can take it for a spin today: 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. There are many small paper cuts left to patch over, but most of it is porcelain ctrl+left-arrow for moving in bruit of things that you will immediately stumble upon. Patrick Ferris 2026 3 19 What makes Merry different? 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 &> 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. Patrick Ferris 2026 3 19 What's next? 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. Patrick Ferris 2026 3 19 Shell MRDT 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. Patrick Ferris 2026 3 19 TIFF in OCaml 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. Patrick Ferris 2026 2 9 https://patrick.sirref.org/fellowship-roundup/ fellowship-roundup /fellowship-roundup/ Fellowship Roundup Patrick Ferris 2026 2 9 Overview February is the final month of my current six-month fellowship work with Tarides . A lot of the work can be summarised by reading the roundups October , November , December and January However, they do not quite capture all the work I have been doing. Below, I pick individual projects and expand on them more holistically rather than pointing at individual PRs or issues. Patrick Ferris 2026 2 9 Ppxlib Most of my time over the past six months has been devoted to ppxlib maintenance. A large proportion of this time has been solo development work. Ppxlib Moreover, it is completely invaluable to Jane Street too. Like other AST-based ppxlib must follow a regular release schedule to support newer compilers. During the years of multicore OCaml development, ppxlib maintenance was a little easier as the Parsetree rarely changed. However, since then, the same cannot be said. Since OCaml 5.2, here are just some of the Parsetree changes: Functions are represented according to their arity. Local module opens on types. Effect syntax. Modular explicits. External types. Locations for all parts of long identifiers. Labeled tuples. In a perfect world, each of these require a codec for serialising the feature into ASTs that do not support the feature, plenty of tests and new Ast_builder/ Ast_pattern functions for using the feature. Nathan and I have been managing pretty well I would say, though we are not getting any technical debt work done. Patrick Ferris 2026 2 9 Outreachy As the OCaml coordinator for Outreachy I have been running the latest December 2025 round. We are lucky to have four projects on the go! Thibaut Mattio is mentoring two projects: an ML dashboard for Raven and an OxCaml backend for Raven . Xvw is mentoring a Yocaml project . I am mentoring a Geocaml project. Patrick Ferris 2026 2 9 Eio During my work on tools like Shelter I have made a few improvements to Eio including: A draft PR for incremental reading of directories . New fork actions for processes: setgid, setuid and process groups . Utility functions for overriding standard environments . Fixing documentation . Patrick Ferris 2026 2 9 OxCaml I have enjoyed my work on OxCaml went it has happend. I think a few tools have been useful including the try-oxcaml work. David and I also had fun at ICFP discussing some quick experiments related to io_uring and OxCaml . One road-blocker here is the comparative amount of time spent trying to keep the OxCaml ecosystem working. Given my limited time working on fellowship most of the time was spent making it just work, not exploring or experimenting with the features themselves. I think this has got better in recent months and perhaps jumping back in I would be surprised at the progress that has been made there. Patrick Ferris 2026 2 9 Forester Tarides are funding some work on forester . This site uses forester by way of graft and I have spent some time working on these tools during the fellowship. This includes a bytesrw-based bibtex library in OCaml that could be released soon. Patrick Ferris 2026 2 9 Papers I had the great fortune of attending ICFP where I presented two talks and was co-author on a few others too. Anil Madhavapeddy David J. Scott Patrick Ferris Ryan T. Gibb Thomas Gazagnaire 2025 https://patrick.sirref.org/madhavapeddy2025docker/ madhavapeddy2025docker /madhavapeddy2025docker/ Functional Networking for Millions of Docker Desktops (Experience Report) Reference https://dl.acm.org/doi/10.1145/3747525 Proc. ACM Program. Lang. 10.1145/3747525 Anil Madhavapeddy David J. Scott Patrick Ferris Ryan T. Gibb Thomas Gazagnaire 2025 Abstract Docker is a developer tool used by millions of developers to build, share and run software stacks. The Docker Desktop clients for Mac and Windows have long used a novel combination of virtualisation and OCaml unikernels to seamlessly run Linux containers on these non-Linux hosts. We reflect on a decade of shipping this functional OCaml code into production across hundreds of millions of developer desktops, and discuss the lessons learnt from our experiences in integrating OCaml deeply into the container architecture that now drives much of the global cloud. We conclude by observing just how good a fit for systems programming that the unikernel approach has been, particularly when combined with the OCaml module and type system. Patrick Ferris 2025 https://patrick.sirref.org/ferris2025scientific_programming/ ferris2025scientific_programming /ferris2025scientific_programming/ What we talk about when we talk about scientific programming Reference https://watch.eeg.cl.cam.ac.uk/w/aYXqXLtgQawYMjVXjSQtjx Patrick Ferris 2025 Abstract Patrick Ferris Anil Madhavapeddy 2025 https://patrick.sirref.org/ferris2025hazel_of_ocaml/ ferris2025hazel_of_ocaml /ferris2025hazel_of_ocaml/ Generating a corpus of Hazel programs from ill-typed OCaml programs Reference https://patrick.sirref.org/var/tyde2025.pdf Patrick Ferris Anil Madhavapeddy 2025 Abstract When developing a new programming language, having a large corpus of both correct and incorrect programs allows language designers to test and explore the capabilities of their new language. However, bootstrapping such a corpus of incorrect programs is time-consuming and arduous. We therefore explore how to reuse code from more mature languages to generate a corpus of ill-typed code for newer ones. We have developed a compiler to Hazel, an emerging language with typed holes, from the more mature OCaml ecosystem. We find it practical to generate a comprehensive corpus of ill-typed programs for Hazel development, and discuss future larger scale efforts towards bridging ecosystems. See also the online hazel of ocaml compiler Max Carroll Anil Madhavapeddy Patrick Ferris 2025 https://patrick.sirref.org/carroll2025decomposable_type_highlighting/ carroll2025decomposable_type_highlighting /carroll2025decomposable_type_highlighting/ Decomposable Type Highlighting for Bidirectional Type and Cast Systems Reference https://maxcarroll0.github.io/papers/workshops/HATRA-decomposable-type-highlighting/ Max Carroll Anil Madhavapeddy Patrick Ferris 2025 Abstract We explore how to provide programmers with an interactive interface for explaining the process by which static types and dynamic casts are derived, with the goal of improving the debugging of static and dynamic type errors. To this end, we define mathematical foundations for a decomposable highlighting system within a bidirectional system, and show how these can be propagated through dynamic types in a cast system. Our prototype implementation in the gradually typed Hazel language includes a web-based user interface, through which we highlight the importance of type level debugging. Michael Winston Dales Alison Eyres Patrick Ferris Francesca A. Ridley Simon Tarr Anil Madhavapeddy 2025 https://patrick.sirref.org/dales2025yirgacheffe/ dales2025yirgacheffe /dales2025yirgacheffe/ Yirgacheffe: A Declarative Approach to Geospatial Data Reference https://dl.acm.org/doi/10.1145/3759536.3763806 10.1145/3759536.3763806 Michael Winston Dales Alison Eyres Patrick Ferris Francesca A. Ridley Simon Tarr Anil Madhavapeddy 2025 Abstract I had some great conversations with KC too with respect to my current research. Patrick Ferris 2026 2 9 Future Work As I enter my final year of the PhD, my goal would be to try and align more of my fellowship work with my own research wherever it makes sense. This research focuses on the following hypothesis: Embedding deep provenance tracking, reversible execution and mergeable reproducible, publishable results; all whilst leaving existing workflows intact and being programming language agnostic. I want to focus on building these tools using Irmin , Eio , OxCaml etc. There are other projects I wish to continue regardless, like Outreachy Patrick Ferris 2026 2 9 Overlapping PhD Work There are multiple projects that overlap with my work. For example, Irmin , which I wrote a retrospective for. This is at the core of Shelter and suggests an extremely valid use case for a brancheable, mergeable database. In addition to this, continuing to work on Eio makes sense to me too. Eio currently has no maintainer and no active development, but the issues are piling up . In particular, I am making heavy use of Eio in Merry OCaml. Aside from research angles on brancheable and mergeable shells, I am also quite interested in how this might look in terms of OxCaml. I met with Anil and Thomas recently to discuss the future of Eio and there was some agreement that we are all heavy users of it and hope to keep up its maintenance and set of features. One large piece of work that is sorely needed here is moving Eio to bytes and not bigarray-backed Cstructs. Any libraries using bytesrw jsont Anil has been working on some of the pieces recently to make this better! Patrick Ferris 2026 2 9 Maintaining and Releasing my own libraries I have published quite a few OCaml libraries in my time, below is just a few Some of these are used in the community including hilite, ppx_deriving_yaml and the multi-codecs. I would like to do some general maintenance of some of these tools and release a few iterations of graft which now makes use of bib . Patrick Ferris 2026 2 9 Ppxlib Maintenance By far, maintenance of ppxlib takes up a majority of my time. I think, given the other work I need to focus on, it would be great to try to minimise the amount of time working on ppxlib . In particular, perhaps only maintaining ppxlib as far as OCaml release-readiness is a possibility and the OOM bug reported by Jane Street Patrick Ferris 2026 2 9 Forester, Graft and Writing With all of this work, I am eager to communicate what I am up to and my thoughts on open source, OCaml, OxCaml etc. I am glad to be using Forester my own work on Eio I hope to continue working on Graft and related tools like bib . But I would also like to start writing some long-form content similar to the Irmin retrospective . Related https://patrick.sirref.org/eio/ eio /eio/ Eio https://github.com/ocaml-multicore/eio Eio provides an effects-based direct-style IO stack for OCaml 5 Contributions