Patrick Ferris 2025 6 2 https://patrick.sirref.org/graft/ graft /graft/ Graft 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. Patrick Ferris 2025 6 2 Usage 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. Patrick Ferris 2025 6 2 Example A typical "tree" might look something like 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. References Context Patrick Ferris 2025 6 2 https://patrick.sirref.org/weekly-2025-06-02/ weekly-2025-06-02 /weekly-2025-06-02/ Forester as a Target Syntax 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. Patrick Ferris 2025 6 2 Forester and Graft I spent a bit of time finally pulling out my changes to Forester to add markdown and bibtex support into a standalone tool: graft. Patrick Ferris 2025 6 2 https://patrick.sirref.org/graft/ graft /graft/ Graft 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. Patrick Ferris 2025 6 2 Usage 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. Patrick Ferris 2025 6 2 Example A typical "tree" might look something like 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). Patrick Ferris 2025 6 2 Maths Support As part of that process, graft now supports Markdown KaTeX. For example: Patrick Ferris https://patrick.sirref.org/mrdts/ mrdts /mrdts/ Mergeable Replicated Data Type Implementation Definition A mergeable replicated data type (MRDT) implementation for a data type is a tuple where: is the set of all possible states at a branch, is the initial state, implements every data type operation, implements the three-way merge strategy. Definition 2.1 from "Certified Mergeable Replicated Data Types". Patrick Ferris 2025 6 2 Revision Supervisions 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. Backlinks 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 (see October, November, December and January). I have been writing throughout the fellowship. 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 is a cornerstone library of the OCaml ecosystem (whether people like it or not). Moreover, it is completely invaluable to Jane Street too. Like other AST-based tools (e.g. ocamlformat, merlin), 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 projects, the rate of change of OxCaml (including the opam ecosystem) meant 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 Programming for the planet undoubtedly involves programming scientifically, but what kind of programming are we talking about and what makes it scientific? In what ways does it differ from other forms of programming, if at all? Is scientific programming, data science or machine learning fundamentally different to constructing a compiler or building a high-throughput web server? By considering how the scientific method (with its falsifiable hypotheses and repeatable and reproducible experiments) relates to scientific programming, I hope to explore how computer science and traditional programming techniques are coming up short in meeting the requirements of scientific programmers. 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 We present Yirgacheffe, a declarative geospatial library that allows spatial algorithms to be implemented concisely, supports parallel execution, and avoids common errors by automatically handling data (large geospatial rasters) and resources (cores, memory, GPUs). Our primary user domain comprises ecologists, where a typical problem involves cleaning messy occurrence data, overlaying it over tiled rasters, combining layers, and deriving actionable insights from the results. We describe the successes of this approach towards driving key pipelines related to global biodiversity and describe the capability gaps that remain, hoping to motivate more research into geospatial domain-specific languages. 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 histories directly into an interactive programming environment (a POSIX-like shell) greatly decreases the gap between exploratory scientific work and 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, though just as a coordinator (not a mentor). 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, a POSIX(ish) shell written in 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 (e.g. jsont) incur an extra copy of data into and out of the bytes. 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 libraries (filtered to not include OCurrent projects). 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 important bug fixes (like 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 to do this, particularly as it is written in OCaml (and could make great use of 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. Patrick Ferris 2025 11 5 https://patrick.sirref.org/ocaml-roundup-october-2025/ ocaml-roundup-october-2025 /ocaml-roundup-october-2025/ OCaml Roundup: October 2025 Welcome to a monthly roundup of OCaml-related open-source work I have been involved with. If you haven't already, a quick look at my ICFP roundup might be a good preface to what follows. Patrick Ferris 2025 11 5 Outreachy December 2025 The contribution period for this year's Outreachy round took place for most of the month of October. This year I am excited to be going back to a Geocaml project, working primarily to add writing capabilities to ocaml-tiff. As per usual, I have been extremely pleased with the level of interest in the various OCaml projects being offered. Whilst we are still deliberating on our choices for interns this year, I won't say much more on the contributions, but if you want to get involved please do reach out! OCaml is a great language for geospatial work, striking a balance between reasonable performance for numerical code and also a rich type system for expressing complicated geospatial data-structures. Patrick Ferris 2025 11 5 Ppxlib and the Future of Compiler Support Nathan has put a tremendous amount of effort into laying a roadmap this month for how ppxlib, going forward, will support future compilers. This is in response to the calamity that was bumping the internal AST to 5.2, and whilst I think we are adding a good deal more complexity, it will hopefully keep the ppx universe on an even keel for longer periods of time. The first substantial PR for this modern world of ppxlib is in review! Aside from that I also debugged an issue that lead to ppxlib OOM-ing machines... of course, any recursive fold-like loop where the accumulator keeps growing is likely to do that! It was nice to be able to crack out memtrace to be able to hunt it down: Patrick Ferris 2025 11 5 Incremental Directory Reading for Eio A common tasks for many a CLI tool is to scan a directory of files, filter out the ones you are looking for and apply some transformation to said files. In fact, that is what Graft and Forester both do! A standard approach to this involves a recursive dance of readdir and stat, the former lists the entries in a directory whilst the latter lets you know what kind of file is at a particular path (e.g. symbolic link, directory etc.). This has a few downsides: We're making two systems calls (pretty much one of the most expensive things your program can do). If a directory contains thousands of files, we end up allocating a big list of strings after returning from readdir. On Linux, we can get around most of that with getdents64(2) which allows us to incrementally read a directory. Whatsomore, this system call also returns the file type so there's no need to do an extra stat! I prototyped an implementation of this called Eio.Path.walk which looks like: val walk : _ t -> File . Stat . kind * string Seq . t -> 'a -> 'a We effectively halve the number of system calls. In a benchmark that traverse deeply-nested directories with plenty of files, a full traversal whent from 1.46s to 110ms. I have since rejigged the implementation to automatically recurse for the user into subdirectories (not that the signature is unchanged) which does make it tricky to control the number of open file descriptors compared to if the user were in control. Patrick Ferris 2025 11 5 https://patrick.sirref.org/icfp-oxcaml-uring/ icfp-oxcaml-uring /icfp-oxcaml-uring/ OxCaml Experiments I have included here a transclusion from my ICFP post about an experiment using unboxed 32-bit integers in OCaml's Uring library. Patrick Ferris https://patrick.sirref.org/oxcaml-and-uring/ oxcaml-and-uring /oxcaml-and-uring/ OxCaml and Uring Jane Street were a big presence at ICFP 2025, carting along with them a shiny new OCaml compiler: OxCaml. If have been playing around with OxCaml recently but nothing outside toplevels in Javascript. Until now! After talking to David, I spent some time converting a small corner of the ocaml-uring library to use a part of OxCaml. In particular making the following change: module Heap = struct type ptr = int32 # ... end type cqe = user_data_id : Heap . ptr ; res : int32 # ; The idea being that a completion queue entry (a notification that some operation has completed) could be fully represented using 64 bits (two 32-bit, unboxed values). You can see how this impacted the library! I'm not certain about this change (and I'm sure I did it wrong) but it was nice to realise OxCaml gives you this kind of control. However, I am worried about the ergonomics of manipulating values like int32# and the temptation to case it into an int (presumably losing a good portion of the value of having an unboxed value in the first place). Patrick Ferris 2025 11 5 x-ocaml with OxCaml I sent a few PRs to the OxCaml repository to get the toplevel working again and lucky the underlying culprit for why patches were disappearing was found. Watch this space for a new toplevel in the browser soon. Patrick Ferris 2025 11 5 Geospatial OCaml In addition to the Outreachy work I mentioned above, I also found the time to keep the Geocaml project ticking along. A few projects saw some much-needed love including: Patrick Ferris 2025 11 5 PROJ in OCaml A modernisation of the bindings to PROJ. PROJ is probably the most used and tested code for working with geospatial projections. Any time you get a sateillite image or a blob of GeoJSON, the geospatial data will normally have some coordinate reference system (CRS) with it detailing exactly what the numbers that tell you where something is mean. One of the most common being WGS84 (used by GPS and GeoJSON for example). Each CRS has its benefits and drawbacks, and often you will need to convert between them. That is what PROJ does. Patrick Ferris 2025 11 5 WKT in OCaml Well-known Text (WKT) is a very simple encoding of geospatial objects and CRSs. For example, a polygon might look like: I used bytesrw to build a simple codec for the WKT format. It is probably about time to write the basic geospatial object library too, something similar to Rust's geo library. Patrick Ferris 2025 11 5 Geotessera A lot of people in my group are excited about Tessera. They have made it easy to used the Tessera embeddings via a python library called geotessera. In true OCaml-at-heart spirit, I tried my hand at porting this library. I had a good deal of success, converting the data into Nx array, extracting the CRS and transform out of the GeoTIFF landmasks using ocaml-tiff, reading datasets using ocaml-geojson etc. This made me excited but also keen to improve the APIs of most of these libraries too. I feel geocaml is at a critical mass where we can start to have this useful feedback loop for the libraries because there is enough functionality to actually use them! Patrick Ferris 2025 10 24 https://patrick.sirref.org/icfp-2025/ icfp-2025 /icfp-2025/ ICFP 2025 Two weeks ago I was fortunate enough to attend the International Conference on Functional Programming in Singapore. My first time in Asia and my second time at the conference, what follows are some thoughts and presentations I enjoyed whilst I was there. I must thank my office mate (and friend!) Ryan Gibb for his camaraderie and sage advice: "the best track is hallway track". Not to mention all of his photos too. Thanks Ryan! From right to left, top to bottom: Anil Madhavapeddy, Sadiq Jaffer, Patrick Ferris, Ryan Gibb and Roy Ang. Patrick Ferris 2025 10 24 Random Hacking Below are just some of the pieces of random hacking I got up to whilst in Singapore. Patrick Ferris https://patrick.sirref.org/oxcaml-and-uring/ oxcaml-and-uring /oxcaml-and-uring/ OxCaml and Uring Jane Street were a big presence at ICFP 2025, carting along with them a shiny new OCaml compiler: OxCaml. If have been playing around with OxCaml recently but nothing outside toplevels in Javascript. Until now! After talking to David, I spent some time converting a small corner of the ocaml-uring library to use a part of OxCaml. In particular making the following change: module Heap = struct type ptr = int32 # ... end type cqe = user_data_id : Heap . ptr ; res : int32 # ; The idea being that a completion queue entry (a notification that some operation has completed) could be fully represented using 64 bits (two 32-bit, unboxed values). You can see how this impacted the library! I'm not certain about this change (and I'm sure I did it wrong) but it was nice to realise OxCaml gives you this kind of control. However, I am worried about the ergonomics of manipulating values like int32# and the temptation to case it into an int (presumably losing a good portion of the value of having an unboxed value in the first place). Patrick Ferris 2025 10 24 Bibtex Parser On the flight over to Singapore, I started to write a post for this website. Some readers may remember that I use forester after a preprocessing step via graft to build this site. Part of that preprocessing consumes bibtex files and outputs forester trees. For the initial graft release I used the excellent Talaria Bibtex library. Unfortunately, it presented a pretty confusing API that, whilst nicely typed, felt a little cumbersome to use. Additionally, it was too specific in what it would accept as valid Bibtex. I wanted something that was a little more straight-forward to use (if a little fast-and-loose when it comes to types...). And so I built bib. This was my first real attempt at using the Bytesrw library. I was happy with the results, although I did have to copy a substantial amount of code from other bytesrw-compatible libraries. I'd be happy to shift to this bytes-first world if it wasn't for the ever-present-in-my-mind issue of relocatable values in the OCaml heap! Patrick Ferris 2025 10 24 Geocaml Libraries There has been some excellent outreachy work happening on the geocaml/ocaml-tiff library which is a potential Outreachy internship project for December 2025. More on this in a future post. On the way back (and a little during the conference), I spent some time building an OCaml counterpart to geotessera, a library for working with Tessera. The library is not ready for people to use directly, but I was working on various constituent parts including: Modern OCaml bindings to PROJ (a coordinate reference system projection library) Further work on a WKT library for OCaml Coming soon... ocaml-geotessera Patrick Ferris 2025 10 24 Talks Patrick Ferris 2025 10 24 Hazel of OCaml at TyDe The type-driven development (TyDe) workshop was held on the first day of ICFP at the NUS School of Computing. I find the workshop name to be a little misleading, but perhaps this comes from my engineering background. The talks focused a lot on type theory, presenting the formal mathematics of interesting type systems with less focus on why it might be useful to have such an exotic type system... but then again, that was the nature of the entire conference. I presented a talk on hazel_of_ocaml. The slides are here, along with the extended abstract. My talk ended up focusing a lot on the pedagogical benefits of having a type system that helped explain the most important thing a type system does... catch type errors! This work represents a small piece of work inside Max Carroll's excellent part II project, which he presented at the HATRA workshop: Decomposable Type Highlighting for Bidirectional Type and Cast Systems. Patrick Ferris 2025 10 24 Scientific Programming at PROPL I think it is important to be honest whenever you get a talk accepted that you otherwise feel should not have been. My talk "about scientific programming what we talk about when we talk" is one such talk. The slides are here. The talk itself was nothing to write home about, but I think there were a few nice ideas that came out of it and the conversations I had with fellow PROPL attenders. Patrick Ferris 2025 10 24 TMF Data Discrepancies Redux On the eve of my talk, I decided to re-run some analysis I did a few years ago on the tropical moist forest dataset, knowing that since then a few more years had become available. After fighting with some bit-rot in the Google Earth Engine script I had used, the new results were not very uplifting. Land Use Class Percent Change Undisturbed  -2.9 Degraded  -5.5 Deforested  8.6 Regrowth  0.5 Water  0.0 Other -0.6 The data in the table corresponds to the land use class values in Indonesia in the year 2008. The analysis computed the difference across each of the classes between the dataset released in 2021 and the dataset released in 2024. Most notably, the amount of deforested land has gone up by 8.6 percent! As a somewhat tangential, though related, aside, I had the pleasure of talking to Sam Westrick, Seong-Heon Jung and Sundara Vishnu Satish of the ParCour research group. They were telling me about MaPLe, a programming language from the MLs built for parallelism. We discussed some ideas of using real-world, geospatial algorithms as benchmarks for MaPLe. Michael, if you got this far remind me to chat to you about this! Patrick Ferris 2025 10 24 Shelter Reactions At the end of my talk, I briefly mentioned shelter. By far, this was the topic I got the most engagement on post-talk. During the conference, this was also true when people asked what I was working on. The tag line "a shell with undo" seemed to capture the attention of most people. I'm feeling a renewed sense of interest in my own work thanks to that, even if the "research" contributions are not very apparent. However, I also had a good discussion with KC about the underlying MRDT in Shelter and how we might think about collaborating in the future on MRDT-related work. Related to that, I managed to spend the evening in Chinatown with Roly Perera. Roly's work has focused on Fluid, a transparency-based programming language that allows authors to reveal their computation to their user. Excitingly, Roly presented a new text-based part of the language at PROPL. Our work is in the same universe, loosely orbiting some notion of reproducibility and explainability, but quite different in engineering work and how users might actually interact with our systems. I thoroughly enjoyed talking about solutions or research ideas that preserve existing workflows. Roly was great at pulling that out of the current work I have been doing. Patrick Ferris 2025 10 24 Other Talks and Highlights I was a part of, often in a very small way, some other talks that were presented at ICFP including Functional Networking for Docker, I encourage you to read Anil's notes on the topic. It is also worth mentioning Anil's post-posix talk (not unrelated to my Uring musings above). Michael gave an excellent, very live, talk about Yirgacheffe at PROPL which I thoroughly enjoyed. I'm also excited about his recent developments which I'm sure you can clue into over on his blog. Below is a non-exhaustive list of talks I enjoyed whilst at the conference. Some relevant, most of them not! By far, one of the best talks was Polynomial-time Program Equivalence for Machine Knitting. I think the talk really nailed the "here's the idea, the details are in the paper"-presenting style as they described an "...algorithm that canonicalizes the algebraic representations of the topological semantics of machine knitting programs". I attended most of the OxCaml tutorial presented by the incredibly friendly Gavin Gray. Ryan's talk Spatial Programming for Environmental Monitoring at PROPL was excellent. There's plenty more to talk about, and I'm sure my future posts will reference other parts of my experience at ICFP 2025. Thanks for reading, and if anybody is interested in any aspects of this do reach out! Patrick Ferris 2025 7 31 https://patrick.sirref.org/ocaml-weekly-2025-w30-w31/ ocaml-weekly-2025-w30-w31 /ocaml-weekly-2025-w30-w31/ OCaml Weekly 2025 w30 and w31 A bumper edition today, a fortnightly. Patrick Ferris 2025 7 31 Opam Releases I did a little personal spring-cleaning of packages I maintain and pushed a few releases to opam. This included a 0.1.0 release of graft! There is some light documentation online at https://graft.sirref.org. Elsewhere, Hilite got a 0.5.0 release. This release makes the core syntax highlighting separate from the markdown part of the library. You can now generate pairs of tokens and identifiers to plug into any "OCaml sourcecode to format" tool you might be building (e.g. graft!). Patrick Ferris 2025 7 31 Interactive OCaml Presentations I had fun trying to fuse slipshow and x-ocaml! 2025 7 23 https://patrick.sirref.org/slipshow-x-xocaml/ slipshow-x-xocaml /slipshow-x-xocaml/ Slipshow x x-ocaml A short, explanatory post about combining two very fun pieces of work in OCaml. Paul-Elliot has been building Slipshow for some time now where slides are slips and your presentations run vertically. More recently, Arthur has built x-ocaml, a web component library for executable OCaml cells embedded into OCaml. Using xocmd, a small tool I built for translating markdown codeblocks to x-ocaml components, your Slipshow's can now be executable! learn-effects.html]]> Take a look at an example! (or the source markdown). I really like this light-weight approach to building interactive presentations for explaining things in OCaml (e.g. over running a jupyter notebook server). I tried using x-ocaml with Irmin and something is not quite right with some of the runtime JS code being generated (see the issue in case you can help). This was intended to be used alongside a longer-form retrospective I am writing on my use of Irmin over the years. Patrick Ferris 2025 7 31 Outreachy I have been helping Michael's intern whilst he has been away on their project Claudius. It has reminded me, again, how inpenetretable some of OCaml's tooling is. In this case the generation of opam files from dune-project files, and in particular, the required manual steps when you wish to remove a dependency (1. remove it from the dune-project file, 2. run dune build to update the opam file, 3. remove all occurences of the library from dune files across your project). Patrick Ferris 2025 7 31 Ppxlib Work on Ppxlib has been fairly varied this past two weeks. I mentioned before about an exiting plan Nathan has planned to help ease the burden on ppxlib maintainers and ppx authors whenever there is an OCaml parsetree bump. That, however, is currently shelved as we are dealing with the fall out of the 5.2 bump. Repr got some fixes pushed to it, but this unconvered more issues. Patrick Ferris 2025 7 31 Misc. I spent some time cleaning up container-image, a tool primarily written by Thomas G. to fetch OCI images from repositories. The tidy up went surprisingly well, except for some spurious HTTP errors with AWS. Maybe I'll spend some proper time trying to help the state of HTTP clients in OCaml (of which there are many, but few that work very well out of the box in my experience). Patrick Ferris 2025 7 18 https://patrick.sirref.org/ocaml-quarterly-q2/ ocaml-quarterly-q2 /ocaml-quarterly-q2/ Quarterly OCaml Q2 Thanks to Tarides sponsorship, I get to work on open-source OCaml. This quarterly is a companion to my weeklies, summarising the last three months of development, peppered with ideas and thoughts about OCaml, its community and its future. Patrick Ferris 2025 7 18 What I wanted to work on? There were two main things I hoped to continue working on: ppxlib and outreachy. These are projects that I was previously working on, and in the case of Outreachy I have now been involved for many years. In addition to this, all of my Part II projects this year used OCaml in some regard. In general, I want to see more adoption of OCaml. Over the years this has taken many forms including my initial work on developing workflows for OCaml that just turned five years old. This directly fed into the rebranding and rethinking of ocaml.org itself. Patrick Ferris 2025 7 18 What I worked on? Patrick Ferris 2025 7 18 Ppxlib Ppxlib is the de facto standard library for building OCaml preprocessors. At the time of writing, opam list --depends-on=ppxlib informs me that there are 267 reverse dependencies. Janestreet is a heavy user of ppxes and has authored many. One of the main accomplishments this quarter was bumping the internal AST to 5.2. This allows ppx authors to use new OCaml language features in their ppxes. In bumping the AST, we knowingly broke compatability for pretty much every single reverse dependency. As best we can, we have been sending patches to ppx libraries and helping users migrate to the latest ppxlib. There is an interesting dicussion to had about the nature of open-source, and its interaction with industrial monorepos and community tended package repositories. Package ecosystems thrive whenever there is a dedicated community creating, maintaining and publishing packages. The idea is that the published world should be healthy. The publishing medium can act as natural limiting factor in the churn of breaking changes (in the case of OCaml this is via PRs to the opam-repository). This, I have come to notice, reacts poorly to changes coming from internally consistent monorepos where introducing breaking changes is easily fixed by applying patches there and then. Whatsmore, OCaml is often stated as an incredibly safe language to perform large refactorings thanks to its type system. Ppxlib sits awkwardly in the space of possible breaking changes. Tied to OCaml's parsetree, impacts of changes there ripple down to ppxlib. The compiler itself can remain internally consistent, and is protected as it need only parse source code. Ppxlib, on the other hand, exposes the parsetree to users and thus any changes to the parsetree will likely be felt by ppx authors. Since I started working on ppxlib, it feels as though the number of syntax changes has gone up (primarily from Janestreet work). Unless we make changes to how we provide support for these, maintainers of ppxlib will forever be stuck doing busy work! Thankfully, Nathan has thoughts on how to improve this. There are a slew of other features I have added to ppxlib including: Support for deriving from classes. Support for deriving from module bindings and signatures. Fixing compiler and ppxlib dummy locations. Bumping to 5.3. Migrations for 5.4. Patrick Ferris 2025 7 18 Outreachy Our two projects this year, one on claudius and one on dune, are going extremely well. At the time of writing, we just had a mid-internship call to catch up, and I was blown away by the progress each intern has made. Unfortunately, Outreachy is struggling with funding and the next round is perhaps not going to happen. This is a real shame and I am hoping that it will not be the case. Outreachy has been a wonderful source of new, committed OCaml developers. If you haven't already, do peruse the webpage on OCaml.org to see past internships or watch the demo day presentations. For the mentors involved, I believe it has also been a rewarding experience (though at times a challenging one). We are always looking for new mentors and project ideas, please do reach out to me if you are interested. Patrick Ferris 2025 7 18 Hazel OCaml's feature set allows it to shine at writing programming languages (and things of that ilk: compilers, interpretters, static analysis tools). Hazel is a research programming language with typed holes written completely in OCaml (via the reason dialect). Relating this back to the original intent of my work, to improve OCaml adoption, I believe this also means keeping existing users happy. In the last quarter I developed a compiler from OCaml to Hazel. Whilst new features are still being added to Hazel, I hope this could serve as a tool to help develop test-suites and standard library functions using existing OCaml solutions. In a student's work (Typed Debugging for Hazel), we used this tool to build a corpus of ill-typed Hazel programs to great effect. OCaml should continue to be a world-class programming language for building other programming languages. I hope to upstream some of this work to Hazel and provide some low effort maintenance to help keep their compiler in good shape. Patrick Ferris 2025 7 18 Systems Programming in OCaml In a cross-over with my own research, I have been developing many tools related to systems programming in OCaml including: An eBPF-based open syscall tracing tool. A library in OCaml for spawning void processes. A shell session manager that uses Irmin to manage sessions. It is nice to see a new push to finally land the direct-style Irmin PR! This work is means to develop the underlying libraries that support it. For example, I have opened a few PRs to Eio to add new "fork actions" to the spawn API. I also investigated the feasibility of changing the underlying mechanisms in Eio to use Picos. In the future, I think this could be important avoid further splitting the OCaml ecosystem. Patrick Ferris 2025 7 18 OxCaml I dabbled a little with OxCaml and build try-oxcaml to let people take it for a spin without having to perform opam repository gymnastics. It turned into a lot of work to track down some pretty inane bugs (type definitions differeing between js_of_ocaml and the OxCaml compiler, resulting in different Javascript runtime representations...). This unblocked a few of my colleagues to get OxCaml working on tools like odoc_notebooks and x-ocaml. Patrick Ferris 2025 7 18 Forester A good proportion of my work this quarter has been focused on how to present the very work that I am doing. Jon Sterling has been developing a tool for scientific thought called Forester which seemed like a possible candidate for writing and sharing my work. Porting my existing blog posts and website content from markdown to Forester's LaTeX-inspired syntax didn't seem like an option. In particular, many of my posts made use of additional markdown-based tools (like ocaml-mdx). This lead to the development of Graft: a preprocessor for Forester forests, converting markdown and bibtex to trees. Patrick Ferris 2025 7 18 What's next? So, I worked on most of what I wanted to work on and then some! Going forward I hope to keep maintaining ppxlib in some capacity and coordinating the OCaml community's Outreachy efforts. I hope to continue my small experiments (e.g. converting Eio to Picos). My own research makes heavy use of Irmin, and I would be interested to help with that too. More recently, a new library has been released for working with numerical data in OCaml called Raven: I am interested to use this library in Geocaml, a suite of geospatial tools written in OCaml that I maintain. I feel conflicted about the OxCaml efforts. I admit, I am not fully aware of the full benefits of the features, but I worry about some proliferation of modes that make the type system in OCaml unbearable to use. On top of this, with my ppxlib hat on, I worry about the impact of changing the compiler so frequently, placing strain on an already small community. That being said, by releasing OxCaml separately I do believe running it as an experimental set of packages will help understand the tool better. But I would not be building anything I intend to maintain or research with right now as that ecosystem is far too volatile. Finally, over the past two years I have done a lot of teaching. From Part II projects to supervisions. Going into next (academic) year, I intend to reduce my in-person teaching to focus on my research. However, I am interested in producing more materials for learning, maybe some of this in OCaml, perhaps something similar to Beautiful Racket. I have tried in the past to do these sorts of things, for example The Irmin Book. Patrick Ferris 2025 7 18 Thank you Thank you for reading this wrap up! And thank you again to Tarides for letting me work so freely on things that I think are good for the OCaml community. Patrick Ferris 2025 6 30 https://patrick.sirref.org/rest-of-monthly-2025-06/ rest-of-monthly-2025-06 /rest-of-monthly-2025-06/ PhD Work and Scientific Programming 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. Patrick Ferris 2025 6 30 Revisiting hazel_of_ocaml 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. Patrick Ferris 2025 6 30 A second year report 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. Patrick Ferris 2025 6 30 What is scientific programming? 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. Patrick Ferris 2025 6 30 Ppxlib 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! Patrick Ferris 2025 6 30 Geocaml 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. Patrick Ferris 2025 6 2 https://patrick.sirref.org/weekly-2025-06-02/ weekly-2025-06-02 /weekly-2025-06-02/ Forester as a Target Syntax 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. Patrick Ferris 2025 6 2 Forester and Graft I spent a bit of time finally pulling out my changes to Forester to add markdown and bibtex support into a standalone tool: graft. Patrick Ferris 2025 6 2 https://patrick.sirref.org/graft/ graft /graft/ Graft 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. Patrick Ferris 2025 6 2 Usage 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. Patrick Ferris 2025 6 2 Example A typical "tree" might look something like 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). Patrick Ferris 2025 6 2 Maths Support As part of that process, graft now supports Markdown KaTeX. For example: Patrick Ferris https://patrick.sirref.org/mrdts/ mrdts /mrdts/ Mergeable Replicated Data Type Implementation Definition A mergeable replicated data type (MRDT) implementation for a data type is a tuple where: is the set of all possible states at a branch, is the initial state, implements every data type operation, implements the three-way merge strategy. Definition 2.1 from "Certified Mergeable Replicated Data Types". Patrick Ferris 2025 6 2 Revision Supervisions 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. Patrick Ferris 2025 12 9 https://patrick.sirref.org/graft-and-bib-update/ graft-and-bib-update /graft-and-bib-update/ OCaml Roundup: November 2025 › Graft and Bib 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! Patrick Ferris 2025 11 10 https://patrick.sirref.org/graft-updates/ graft-updates /graft-updates/ Taking stock & Graft updates › Graft Updates 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. Patrick Ferris 2025 11 10 https://patrick.sirref.org/self-host-love/ self-host-love /self-host-love/ Thoughts on KC's "Foundations for Hacking on OCaml" › Self-host, self-host, self-host Self-hosting is another great way to build those skills. My website has had at least five different incarnations at this point. Some highlights include when it was completely made with GatsbyJS (when that was the new hottness), when it was a mishmash of OCurrent and Omd, to now, when it is one part Graft and one part Forester. Before I had a website, I was also blogging on Medium (I won't pretend they were the best blogs ever, but I learnt a lot by writing them). I have since moved past Medium as their values do not align with my own, but it was an important stepping stone. Thanks to Ryan, who is very generous with his time, I now self-host a variety of services using NixOS. Related https://patrick.sirref.org/forester/ forester /forester/ Forester Contributions