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 . References Context Patrick Ferris 2026 3 10 https://patrick.sirref.org/ocaml-roundup-february-2026/ ocaml-roundup-february-2026 /ocaml-roundup-february-2026/ OCaml Roundup: February 2026 Much like last month, this month has been busy with lots of work on ocaml-tiff thanks to Tambe Salome and ppxlib . Patrick Ferris 2026 3 10 Outreachy Tambe Salome , at the time of writing, has completed her internship! We will be hosting the biannual Demo Day celebration for this round's interns, so please do come along. Since the last update Tambe Salome and I have been slowly making progress on making the write-functionality more feature-complete. As I eluded last month , TIFF files are hard to write. Their internal structure is all over the place: directories, immediate and non-immediate values, 32-bit or 64-bit, little or big endian etc. We're tackling this bit by bit, evolving the library to be able to support writing to files. In the early days, when we only read TIFF files there was no need to something like in memory before writing them to the file, so we will need some kind of API change internally to support this. This month, I also bumped up against well-meaning contributions to ocaml-tiff that made use of AI. This left me with some thoughts that I am adding below. Patrick Ferris 2026 3 9 https://patrick.sirref.org/vibecoding-etiquette/ vibecoding-etiquette /vibecoding-etiquette/ Vibecoding Etiquette I am yet to jump head-first into using LLM-based tools like Claude Code or even ChatGPT to help with my programming in any serious way. After using some "free"-tier tools to try to better understand some tricky eBPF problems and to make sense of the semantics of POSIX shells, I was left unimpressed and ultimately felt my time was wasted. I feel unwilling to send my money to the corporations behind these tools. This is for a variety of reasons including the deeply worrying relationship with the "opertational planning" vs. "operational decision-making" given the leaders of the US DoW environmental concerns, exploitation of human labour and a fast-and-loose approach to copyright laws. All of which leaves the entire industry very unattractive to me. Everybody can draw their line where they want, and I am, in good faith, trying to find where mine should be. Unfortunately, my own decision to not use these tools is not completely in my control. What do I do about pull requests to open-source libraries I maintain that include agentic code? I am, after all, participating in the idea of open-source and inclusive collaboration. Agentic code also comes in many flavours, from blatant LLM-generated pull requests to heavily edited, human-in-the-loop contributions. An immediate first step for me is to develop some policy for contributions to projects I maintain regarding code where LLMs have had a hand. This can help set expectations for contributors. Eio's "AI-generated Code" subsection is succinct and prohibits contributions that solely use AI. It obfuscates how you think. Purely AI-generated code tells us little about how you think and the problems you might be having. This makes it harder to provide good feedback on PRs and issues. It is often more work to review. Particularly for the OCaml ecosystem and libraries like Eio, it seems that these tools are not very good and generate a lot of believable code that is in actual fact completely wrong. PR comments and the code submitted with them can say completely different things. It is a grey area for licensing. Models like ChatGPT have been trained on lots of code with different licenses and has been known to simply copy code as an answer to a prompt. We would like to avoid this headache as best we can. Additionally, I plead to other developers to own their use of these tools. Please, make Claude a co-author of the commits where Claude has generated any code at the very least. This is the bare minimum. I have strong feelings about the etiquette of sending AI summarised information or LLM-generated code to other people, but before any of that, the decent thing is to remove any uncertainty in the receiving party's mind of how the information they are receiving came to be. In this way, I appreciate the OCaml compiler's AI notice which includes: If a significant portion of your code, PR description, review comments or messages has been AI-generated, this must be disclosed, stating which tool was used and for what. Reviewing AI-produced code can require a different approach from human-written code. You should have an acknowledgement anyway even if another human contributor did some of the work. Patrick Ferris 2026 3 10 Ppxlib We are still in the throes of supporting OCaml 5.5 in ppxlib . Some of the features have proved more complicated to encode into migrations that "roundtrip". Roundtripping, in the context of ppxlib , is when a modern feature of OCaml must be preserved in older abstract syntax trees which may have no specific node to encode them. Patrick Ferris 2026 3 10 Bugs in Ptyp_open Unfortunately, Jon Sterling was bitten by this recently . module M = struct type t = int end type t = M . t list Here, the is an example of a new AST node called Ptyp_open. I had hastily not migrated Ptyp_open to older compilers. So what happened? Jon had used this new feature in his project with a 5.3 compiler with ppxlib 0.35. This is before our internal bump to the 5.2 AST so the code migrated all the way down to OCaml 4.14! Whilst passing from 5.2 to 5.1 we raised an error saying: Error: migration error: module open in types is not supported before OCaml 5.02 patched this issue and have planned to release a patched 0.35 in the not too distant future. If you can bump to ppxlib>=0.36 then you will not face this issue. Patrick Ferris 2026 3 10 Modular Explicits We ran into a fairly interesting corner of the OCaml language whilst trying to migrate modular explicits in ppxlib . As a recap, in OCaml 5.5 you can now right function types that depend on modules. For example: module type Add = sig type t val add : t -> t -> t end let add : module A : Add -> A . t -> A . t -> A . t = fun module A : Add a b -> A . add a b We have to tell the compiler the module type of A by adding : Add. However, there is another very similar syntax for these kinds of constraints, . And in fact, the former syntax was syntactic sugar for the latter. In OCaml 5.5, these two were distinguished from one another and is the required syntax for modular explicits. As you might imagine, this is a bit of a headache in terms of ppxlib migrations. In the end, we try to preserve the original intent of the author where possible, and you can read more about it on the PR for distinguishing the two. Patrick Ferris 2026 3 10 Six Month Retrospective This month marks the end of six months of work with Tarides . I wrote a small retrospective and plan for my next six months below. 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 . Backlinks Related 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 . 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 2 5 https://patrick.sirref.org/ocaml-roundup-january-2026/ ocaml-roundup-january-2026 /ocaml-roundup-january-2026/ OCaml Roundup: January 2026 A new year, another roundup in my open-source, OCaml activities. This month has been busy with lots of work on ocaml-tiff thanks to Tambe Salome and ppxlib . Patrick Ferris 2026 2 5 Outreachy Tambe Salome has been working to add write support to ocaml-tiff ; and we are well on the way to having good support. Writing TIFF files is no easy business. Unlike many formats, TIFF files have an internal metadata-data split, with values sometimes being place nowhere near the metadata describing the values in the file. In practice, it means if you do not wish to read the whole file into memory, then your IO layer must support seeking to file offsets. This means when you come to write the file, you might need to write at different offsets too. This becomes particularly complicated when you do not know ahead of time how copying a TIFF file we have access to but pushing all the copying via the OCaml values we read from our source TIFF. And Salome has made excellent progress: input.exif $ dune exec -- example/copy.exe test/data/uniform.tiff example.tiff $ exiftool example.tiff > output.exif $ patdiff input.exif output.exif ------ input.exif ++++++ output.exif @|-1,23 +1,23 ============================================================ |ExifTool Version Number : 13.39 -|File Size : 66 kB +|File Size : 118 bytes |File Permissions : -rw-r--r-- |File Type : TIFF |File Type Extension : tif |MIME Type : image/tiff |Exif Byte Order : Little-endian (Intel, II) |Image Width : 256 |Image Height : 256 |Bits Per Sample : 8 |Compression : Uncompressed |Photometric Interpretation : BlackIsZero |Strip Offsets : 122 |Rows Per Strip : 256 |Strip Byte Counts : 65536 |Planar Configuration : Chunky |Image Size : 256x256 |Megapixels : 0.066]]> From this slightly edited output, we can see that the OCaml libary is doing a good job at preserving the metadata. The file size discrepancy is because we have not tackled writing the raw data to the file yet! This is next on our TODOs, write data to a file given a strip-offset plan. Once we do that we can then start to think how might we come up with such a plan if we did not have one. Patrick Ferris 2026 2 5 Ppxlib The first cut of OCaml 5.5 is not too far away, at which point the ppxlib of new features sneaked into this release including: modular explicits, external type declarations and the previously mentioned local structure items. All of these features require migrations to support newer compilers. In this migration process, ppxlib lowers modern ASTs to the AST of OCaml 5.2. New migration process. We have a few PRs to support 5.5 fully: The initial PR to support 5.5 . Supporting all the new local structure items . Full support for modular explicits and external type declarations . Additionally, Nathan has been adding better feature encoding support for the 5.3 and 5.4. Unlike the migration code above, these also include Ast_builder and Ast_pattern functions for building and matching on the new AST nodes. This allows ppx authors to use those nodes in their ppxes. OCaml 5.3 effect syntax . OCaml 5.4 labeled tuples . Finally, we have an important bug fix where long identifiers were given a Location.none. Unfortunately, it is not possible to fully recover a long Read more about that fix here . Patrick Ferris 2026 1 11 https://patrick.sirref.org/ocaml-roundup-december-2025/ ocaml-roundup-december-2025 /ocaml-roundup-december-2025/ OCaml Roundup: December 2025 December was busy, both personally and in my OCaml-related work. If you haven't been following along have a read of my October and November roundups. Patrick Ferris 2026 1 11 https://patrick.sirref.org/outreachy-december-round/ outreachy-december-round /outreachy-december-round/ Outreachy December Round December marks the beginning of a new Outreachy round, and I am very excited to be working with Tambe Salome on improving the ocaml-tiff library. In particular, we will be adding the ability to write TIFF files from raw data. I imagine an API something like: val write : 'repr , 'kind Tiff . Data . t -> Tiff . File . wo -> unit So far Salome has been making excellent progress churning through some issues that are blocking us from starting the main project, including: Adding a sublibrary for Eio . Using the new Nx library . Uncovering bugs in our decoding of the number of bits per pixel Preliminary work on write support ! As a community, the mentors and mentees also had a community call to introduce ourselves to eachother. I am very excited about the Yocaml project and the two Raven projects. Patrick Ferris 2026 1 11 Ppxlib If you recall, I discussed the new representation for local constructs in the AST. Patrick Ferris 2025 12 9 https://patrick.sirref.org/ppxlib-5.5-support ppxlib-5.5-support /ppxlib-5.5-support Initial OCaml 5.5 Support We recently merged 5.5 support into our main some new features coming to OCaml including the ability to define any structure locally . Most OCaml programmers will be familiar with things like let open M in External types have also landed, bringing with them a new "type kind" in the AST. You can read about them in the manual . It transpires that since then, changes have landed in the compiler that mean just about any OCaml structure item can now be declared locally. I have documented an issue on ppxlib about the problems this will cause for us: Currently, we only migrate the previously supported structure items that let open allows anything to appear in a local let-binding except additional let let ...let include change had a 1:1 mapping from a 5.4 AST node to a 5.5 AST node, now we do not and will need to think of a suitable encoding. Patrick Ferris 2026 1 11 Migration Woes gasche for the report Longidents are used in OCaml to identify values like Foo.bar or . In OCaml 5.4, each segment of the identifier was given a precise location. This means that in the migration from 5.3 to 5.4 we have to reconstruct those locations. However, our code didn't do this, instead it simply defaulted to Location.none causing some imprecise error codes in preprocessed OCaml files. For example: let = NonExistingModule . run Has the following error: I have proposed a fix which partially brings back locations during the migration. Part of the problem, I think, is that the exact location of long identifiers containing applications Patrick Ferris 2025 12 9 https://patrick.sirref.org/ocaml-roundup-november-2025/ ocaml-roundup-november-2025 /ocaml-roundup-november-2025/ OCaml Roundup: November 2025 A roundup of some of my OCaml activity this month! Patrick Ferris 2025 12 9 Ppxlib Updates I managed to get a substantial amount of ppxlib work completed in the last month. Some of those bits are described in detail below. Patrick Ferris 2025 12 9 https://patrick.sirref.org/ppxlib-5.5-support ppxlib-5.5-support /ppxlib-5.5-support Initial OCaml 5.5 Support We recently merged 5.5 support into our main some new features coming to OCaml including the ability to define any structure locally . Most OCaml programmers will be familiar with things like let open M in External types have also landed, bringing with them a new "type kind" in the AST. You can read about them in the manual . Patrick Ferris 2025 12 9 Bug Fixes for Attribute Rewriters 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. Patrick Ferris 2025 12 9 Value binding constraints in ppxlib 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 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 ; : intpvb_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.0.20.0repr transforms instances of let x : t = ... into ! This limitation does not seem inherent to the parsetree, the printer or the migrations. But I have Patrick Ferris 2025 12 9 Outreachy 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 ! Patrick Ferris 2025 12 9 https://patrick.sirref.org/graft-and-bib-update/ graft-and-bib-update /graft-and-bib-update/ 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 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 We're making two 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 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 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 # ; unboxed values You can see how this impacted the library ! 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 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 it detailing exactly what the numbers that tell you where something is mean. One of the most common being WGS84 will need to convert between them. That is what PROJ does. Patrick Ferris 2025 11 5 WKT in OCaml 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. 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 # ; unboxed values You can see how this impacted the library ! 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 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 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. 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 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 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 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 8 7 https://patrick.sirref.org/irmin-retro/ irmin-retro /irmin-retro/ Irmin Retrospective Irmin is an OCaml library for building branchable and mergeable data stores. The data is mergeable in the sense of mergeable replicated data types . I have been using Irmin for over five years to build different kinds of interesting data stores including: A simple markdown-based note-taking web application . A content-addressed database . Mentoring an intern who worked on Irmin in the browser . Most recently, an Irmin-backed shell session manager . I was asked to provide some feedback recently on Irmin , so I thought writing a little retrospective here would be a good way to do that. The remit for the retrospective was about improving Irmin , so the content is focussed on pain points and areas of improvement. An in-memory Irmin store module Store = Irmin_mem . KV . Make Irmin . Contents . String Patrick Ferris 2025 8 7 What is Irmin? Irmin , at its simplest, is a key-value database. Users associate keys with values and can query and update these bindings. Additionally, this database supports versioning. This means independent snapshots of the database can coexist and users can switch between them and update them without fear of interfering with other versions. Different versions of the database can be combined by merging . When you set up your instance of an Irmin database, you also provide it with a merge function . # # show_type Irmin . Merge . f ;; type nonrec 'a f = old : 'a Irmin . Merge . promise -> 'a -> 'a -> 'a , Irmin . Merge . conflict result Lwt . t Patrick Ferris 2025 8 7 Design and API Irmin 's API is very git-inspired. There is a large overlap of shared vocabulary and concepts: repositories , branches , commits , heads etc. Probably the most confusing aspect of this is the notion of a Store. When I was describing Irmin above, I used the term database to help distinguish between some of these concepts. In Irmin's documentation, it is used for multiple Creating a Store " stores refer to the entire database, whereas in the Irmin API docs we have that: There are two kinds of store in Irmin: the ones based on persistent named branches and the ones based temporary detached heads. For Irmin library users, a Store.t can be thought of as a checkout of the database at a particular revision. This may be from a branch or a specific commit. # Store . of_branch ; ; - : Store . repo -> string -> Store . t Lwt . t = < fun > # Store . of_commit ; ; - : Store . commit -> Store . t Lwt . t = < fun > The overloading of the term is confusing. I think it leaves users unsure about what other people might mean when they say "store". Being careful with these terms, and the contexts in which they are used, would help avoid this confusion. Patrick Ferris 2025 8 7 Module and Functor Soup Undoubtedly for a majority of use-cases and users, Irmin is over-functorised. Nearly every module requires that you must apply some functor to access any useful code. In general, a user will have to interact with the Schema module when describing the types they want to instantiate their store with. To counteract this, Irmin has plenty of KV modules that provide a Make functor that only requires a user to provide a suitable content module # # show_module Irmin_mem . KV ;; module KV : sig type endpoint = unit type metadata = unit type hash type info = Store.info type contents_key = 'h type 'h node_key = 'h type 'h commit_key = 'h module ]]> sig ... end end I think it is fair to say the documentation is hard to follow as it is a module and functor soup. For example, looking at irmin.3.11.0, starting at the toplevel documentation page our path to finding this paricular module and functor is as follows: We jump into Irmin_mem from the nicely written landing page. We scroll down to find the KV module . We now make sense of the module's signature by following the KV_maker link . We see a at the end of the module, and we navigate through that . Finally we have come to our journey's end, and find the schema module with type constraints like type Branch.t = string. There is a big assumption there that you know what the Schema module is telling you and how it relates to your "store". There is a counter-argument here, in that Irmin is incredibly flexible in terms of what you can use as types for your keys, branches, hashes etc. I was quite easily able to produce for example. I think there is a middle ground. For Irmin to be usable it needs to reduce the complexity of the API. The complexity is due to, in large part, over-functorisation. If the functors cannot be removed then perhaps better documentation or more introductory libraries like irmin-containers would be helpful. Perhaps a standalone library that acts as an interface to Irmin stores would be helpful. I find myself time and again implementing something like that . Patrick Ferris 2025 8 7 Backends Galore Irmin has plenty of backends including a git-compatible one , a MirageOS backend , an in-browser IndexedDB backend and even a FoundationDB backend . Irmin needs to have fewer, better tested backends. Of course, some of these are likely driven by funding and use-case specific details. However, I think by focusing on a few key backends and striving for solid performance but also strong consitency guarantees, Irmin would become a more viable candidate for potential users. I also believe the browser backend plays a key part to making this work in order to facilate local-first applications . Patrick Ferris 2025 8 7 Syncing Remote Stores The synchronisation mechanisms in Irmin are powerful, but often the API is very confusing. For example, with the git-compatible Unix backend, nested deep in the documentation is the function required to create a remote endpoint . Once they have found this function, using it is not easy as it requires them to learn about the Mimic.ctx, which is an abstraction of the networking stack! Patrick Ferris 2025 8 7 Do Fewer Things and Do Them Well Irmin is simultaneously a database that supports: content-addressing, merging, key-value lookup, branches, transactions, complete OS portability etc. I think, in short, it tries to do too many things and comes up short on some of them in ways that really matter. When building the content-addressed database I needed strong guarantees about some atomic actions to perform on the underlying store. For example, setting the value and accessing the commit associated with it which was not possible with the API at that time in an atomic way. This kind of missing functionality is probably only understood after users stumble across it, but it also makes the library feel less focused and ready for production use-cases. Patrick Ferris 2025 8 7 Documentation It will probably come as no surprise that one of the main limitations of using Irmin is the lack of documentation and tutorials. This is a particular pain point as the API is not very straightforward. A while back, I started an Irmin book intending to help document the kinds of things real-world users would need in order to use Irmin in earnest. For example: ways to deal with type migrations or primers on runtime types . I still believe this sort of work would be invaluable, but there is a large cost to completing it, and it is not clear if there is enough interest in the project to warrant such an effort. The tutorials on the irmin website are still good starting points for most new users. But they quickly lack the depth for real-world scenarios. Patrick Ferris 2025 8 7 Feature Wishlist What follows are additional ideas for improving Irmin . Patrick Ferris 2025 8 7 Heterogeneous Stores missing feature for lots of first-time users of the library: you can only store values of a single type. This leads to a few common workarounds: Storing a serealised version of your values, essentially escaping the type-system and implementing a form of dynamic typing. Growing your value to hold lots of different types via some large variant type. Both of these options are feasible and have been used in practice. However, they are workarounds. A long time ago CraigFe created an RFC for heterogeneous stores , which I think about a lot. The main idea is to augment Irmin to be GADTs that carry type information about the kinds of values they access heterogeneuous variants of other data structures API would make Irmin more appealing as a library for persistent data storage. Patrick Ferris 2025 8 7 Real-world Retrospectives This echos some thoughts from the Documentation section. There are some very real-world Irmin Tezos blockchain were using Irmin which uses the irmin-pack backend. Does this make irmin-pack the best tested backend and perhaps should be the default backend for new users? Or is it hyper-specific to the Tezos use case? There are blogs on tarides.com about the irmin-pack backend , but they might be outdated and should not be the first place to find advice on which Irmin backend to use. Patrick Ferris 2025 8 7 Active Development and Engagement As far as I know, Irmin was at the forefront of technologies that have come to be described as local-first . There is a growing interest Ink & Switch's essay on the matter . And it would be great to see more research via Irmin at things like the lofi unconference and the PLF workshop at SPLASH 2023 ! Thank you for reading! And thank you to all the Irmin contributors. I hope this might be useful in the future for building the next-generation Irmin ! Patrick Ferris 2025 6 2 https://patrick.sirref.org/graft/ graft /graft/ Graft https://github.com/patricoferris/graft Graft is a Forester preprocessor. 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. Patrick Ferris 2025 5 9 https://patrick.sirref.org/try-oxcaml/ try-oxcaml /try-oxcaml/ Try OxCaml https://patrick.sirref.org/oxcaml This week, I have been trying out Janestreet's Oxidised OCaml ICFP paper This adds a system of modes to OCaml for expressing things like locality. ...we’re introducing a system of modes, which track properties like the locality and uniqueness of OCaml values. Modes allow the compiler to emit better, lower-allocation code, empower users to write safer APIs, and with the advent of multicore, statically guarantee data race freedom—all in a lightweight way that only affects those in need. For example, you might say: let is_empty s : string @@ local = String . length s = 0 To get a feel for how this changes the language, you can follow their instructions on their custom opam-repository . Alternatively, you can give it a go in your browser ! This is a full toplevel running with Base Happy OxCamling! https://patrick.sirref.org/anilmadhavapeddy/ anilmadhavapeddy /anilmadhavapeddy/ Anil Madhavapeddy person https://anil.recoil.org Professor of Planetary Computing at University of Cambridge . Patrick Ferris https://patrick.sirref.org/bib/ bib /bib/ Bib https://github.com/patricoferris/ocaml-bibtex Bib is a simple Bibtex codec for OCaml. https://patrick.sirref.org/dra27/ dra27 /dra27/ David Allsopp person https://www.dra27.uk/blog/ 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 https://patrick.sirref.org/forester/ forester /forester/ Forester https://patrick.sirref.org/geocaml/ geocaml /geocaml/ Geocaml Github Organisation https://github.com/geocaml Geocaml is an attempt to provide the OCaml community with a suite of geospatial libraries, for example: A Geojson library in pure OCaml. A tool for querying carbon intensity APIs . ISO3166 country codes . https://patrick.sirref.org/irmin/ irmin /irmin/ Irmin reference https://irmin.org Irmin is an OCaml library for building mergeable, branchable distributed data stores. https://patrick.sirref.org/kc/ kc /kc/ KC Sivaramakrishnan person Assistant Prof. IIT Madras https://kcsrk.info https://patrick.sirref.org/nathanreb/ nathanreb /nathanreb/ Nathan Rebours person https://github.com/nathanreb Maintainer of Ppxlib . https://patrick.sirref.org/outreachy/ outreachy /outreachy/ Outreachy Project 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. 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: 2022 12 1 https://patrick.sirref.org/topojson-0001/ topojson-0001 /topojson-0001/ Implement a non-blocking, streaming codec for TopoJSON 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. 2022 6 1 https://patrick.sirref.org/topojson-0002/ topojson-0002 /topojson-0002/ Extend OCaml's GeoJSON library to support TopoJSON 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. 2021 6 1 https://patrick.sirref.org/ocamlorg-0001/ ocamlorg-0001 /ocamlorg-0001/ Improve the ocaml.org Website 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. Patrick Ferris https://patrick.sirref.org/ocaml-blog/ ocaml-blog /ocaml-blog/ Patrick's OCaml Blog Welcome to the OCaml corner of my website. Here you will find long-form posts and shorter, more sporadic updates about my work in the OCaml community. This might include things about: Outreachy where I am the current OCaml community co-ordinator . Ppxlib where I help Nathan maintain the core library to build OCaml preprocessors. Any of the open-source tools I help maintain . An RSS feed is available . Posts Patrick Ferris https://patrick.sirref.org/modular-explicits/ modular-explicits /modular-explicits/ Modular Explicits in OCaml Abstraction, in OCaml, often refers to hiding a particular type's implementation using a signature . For example: module S : sig type t val add : t -> t -> t end = struct type t = int let add = Int . add end In this post, however, I use abstraction to refer to the notion of code-reuse. Using features of the OCaml language to allow one piece of code to be used across different implementations or instantiations of a particular kind of value. The goal of this post is to eventually discuss OCaml 5.5's modular explicits which is yet another feature that can allow a single piece of code to be used in various situations. Patrick Ferris https://patrick.sirref.org/abs-polymorphism/ abs-polymorphism /abs-polymorphism/ Polymorphism The most prevalent kind of abstraction is polymorphism. For example, List.length will work for all lists regardless of what kind of elements the list contains. However, polymorphism requires you to keep the polymorphic part of the type at arm's length. This can lead to suboptimal performance and/or limited usefulness. List.mem is a good example; in order to be polymorphic List.mem relies on , the polymorphic equality operator. In turn, this operator achieves its polymorphism by performing a structural comparison of the runtime representation of its arguments. # let rec mem ? eq = Stdlib . = v = function | -> false | x : : xs -> eq v x || mem ~ eq v xs val mem : ? eq : 'a -> 'a -> bool -> 'a -> 'a list -> bool = < fun > # let mem_int_list = mem ~ eq : Int . equal val mem_int_list : int -> int list -> bool = < fun > List.map, Hashtbl.iter it doesn't scale well from a user's perspective if the body of the function compare, length, pp same time, we want to avoid the precarious duck-typing of something like Python too. Patrick Ferris https://patrick.sirref.org/abs-functors/ abs-functors /abs-functors/ Functors Functors are perhaps the second most prevalent feature of OCaml when it comes to code reuse. In the OCaml sense, a functor is a module that is parameterised by other modules. Set.Make is a functor that requires a module adhereing to the Set.OrderedType interface. module IntSet = Set . Make Int module StringSet = Set . Make String Functors make it a lot easier to write implementations that depend on a large variety of functionality. Set.OrderedType is relatively tame in this sense, only needing a type t and a compare : t -> t -> int function. Irmin , on the other hand, has functors that require users to provide Schemas . I have more thoughts about this API design in my Irmin retrospective . It is important to consider the runtime costs of functors too. Whilst most of the time functors are instantiated at the toplevel, it does not come for free. You can have a play around with js_of_ocamlopt and see the generated output for how functors are applied. Patrick Ferris https://patrick.sirref.org/abs-fcm-exist/ abs-fcm-exist /abs-fcm-exist/ First-class Modules and Existentials Sitting somewhere inbetween polymorphism and functors are first-class modules with existentials. This is a rather advanced use of OCaml's type system to be able to pack together a value of a particular type alongside a module that works with those kinds of values. type set = Set : module Set . S with type t = 'a * 'a -> set let size Set module S , s = S . cardinal s Packaging up integer sets is fairly straight-forward. # let int_empty_set = Set module IntSet , IntSet . empty val int_empty_set : set = Set < module > , < poly > From a user's perspective we are now closer to the realm of dynamic typing. Indeed, existentials are the way to do that in OCaml. # type any = Any : 'a -> any type any = Any : 'a -> any # let any v = Any v val any : 'a -> any = < fun > Getting back into a typed world often involves some type witness gymnastics or runtime exceptions. Used sparingly, however, it can be very effective particularly when a user is none the wiser about the use of FCMs and existentials. I have made use of this trick to package together Irmin stores with implementations . You could not, however, simply return the value that was packed away inside the existential. # let woops Set _ , s = s Line 1 , characters 26 - 27 : Error : The value s has type $ a but an expression was expected of type 'a The type constructor $ a would escape its scope Hint : $ a is an existential type bound by the constructor Set . to mention the need for the extra allocation to create the existential in the first place. This will likely not be too significant, but is worth considering. Patrick Ferris https://patrick.sirref.org/abs-modular-explicits/ abs-modular-explicits /abs-modular-explicits/ Modular Explicits OCaml 5.5 has module-dependent function types also known as explicits modular . The type of a function can now depend on types that come from a module that is also part of the function definition. # let set_length : module S : Set . S -> S . t -> int = fun module S v -> S . cardinal v ; ; val set_length : module S : Set . S -> S . t -> int = < fun > # set_length module IntSet IntSet . empty ; ; - : int = 0 # set_length module StringSet StringSet . empty ; ; - : int = 0 In fact, we can now return values of a particular type that depends on a particular module! # let empty_set : module S : Set . S -> S . t = fun module S -> S . empty ; ; val empty_set : module S : Set . S -> S . t = < fun > # empty_set module IntSet ; ; - : IntSet . t = < abstr > # empty_set module StringSet ; ; - : StringSet . t = < abstr > Or, we can dynamically select implementations and values. # let with_impl : config : string -> module S : Set . S -> S . t -> 'a -> 'a = fun ~ config fn -> match String . lowercase_ascii config with | " string " -> fn module StringSet StringSet . empty | " int " -> fn module IntSet IntSet . empty | c -> invalid_arg c val with_impl : config : string -> module S : Set . S -> S . t -> 'a -> 'a = < fun > This is like a more usable version of the FCM-existential trick, though does require more manual type annotations on functions. For instance, I tried switching out OBuilder's use of existentials with modular explicits: builder - let log tag msg = match tag with | `Heading -> Fmt.pr "%a@." Fmt.(styled (`Fg (`Hi `Blue)) string) msg | `Note -> Fmt.pr "%a@." Fmt.(styled (`Fg `Yellow) string) msg | `Output -> output_string stdout msg; flush stdout -let create_builder store_spec conf = +let create_builder : + Store_spec.store Lwt.t -> Native_sandbox.config -> + ((module B : Obuilder.BUILDER) -> B.t -> 'a) -> 'a Lwt.t = + fun store_spec conf fn -> store_spec >>= fun (Store_spec.Store ((module Store), store)) -> let module Builder = Obuilder.Builder (Store) (Native_sandbox) (Docker_extract) in Native_sandbox.create ~state_dir:(Store.state_dir store / "sandbox") conf >|= fun sandbox -> let builder = Builder.v ~store ~sandbox in - Builder ((module Builder), builder) + fn (module Builder) builder]]> I think the code is more pragmatic; there is no longer an indirection via a type called builder. The extra type annotations do add a little noise. Roll on modular implicits . Patrick Ferris 2025 8 7 https://patrick.sirref.org/irmin-retro/ irmin-retro /irmin-retro/ Irmin Retrospective Irmin is an OCaml library for building branchable and mergeable data stores. The data is mergeable in the sense of mergeable replicated data types . I have been using Irmin for over five years to build different kinds of interesting data stores including: A simple markdown-based note-taking web application . A content-addressed database . Mentoring an intern who worked on Irmin in the browser . Most recently, an Irmin-backed shell session manager . I was asked to provide some feedback recently on Irmin , so I thought writing a little retrospective here would be a good way to do that. The remit for the retrospective was about improving Irmin , so the content is focussed on pain points and areas of improvement. An in-memory Irmin store module Store = Irmin_mem . KV . Make Irmin . Contents . String Patrick Ferris 2025 8 7 What is Irmin? Irmin , at its simplest, is a key-value database. Users associate keys with values and can query and update these bindings. Additionally, this database supports versioning. This means independent snapshots of the database can coexist and users can switch between them and update them without fear of interfering with other versions. Different versions of the database can be combined by merging . When you set up your instance of an Irmin database, you also provide it with a merge function . # # show_type Irmin . Merge . f ;; type nonrec 'a f = old : 'a Irmin . Merge . promise -> 'a -> 'a -> 'a , Irmin . Merge . conflict result Lwt . t Patrick Ferris 2025 8 7 Design and API Irmin 's API is very git-inspired. There is a large overlap of shared vocabulary and concepts: repositories , branches , commits , heads etc. Probably the most confusing aspect of this is the notion of a Store. When I was describing Irmin above, I used the term database to help distinguish between some of these concepts. In Irmin's documentation, it is used for multiple Creating a Store " stores refer to the entire database, whereas in the Irmin API docs we have that: There are two kinds of store in Irmin: the ones based on persistent named branches and the ones based temporary detached heads. For Irmin library users, a Store.t can be thought of as a checkout of the database at a particular revision. This may be from a branch or a specific commit. # Store . of_branch ; ; - : Store . repo -> string -> Store . t Lwt . t = < fun > # Store . of_commit ; ; - : Store . commit -> Store . t Lwt . t = < fun > The overloading of the term is confusing. I think it leaves users unsure about what other people might mean when they say "store". Being careful with these terms, and the contexts in which they are used, would help avoid this confusion. Patrick Ferris 2025 8 7 Module and Functor Soup Undoubtedly for a majority of use-cases and users, Irmin is over-functorised. Nearly every module requires that you must apply some functor to access any useful code. In general, a user will have to interact with the Schema module when describing the types they want to instantiate their store with. To counteract this, Irmin has plenty of KV modules that provide a Make functor that only requires a user to provide a suitable content module # # show_module Irmin_mem . KV ;; module KV : sig type endpoint = unit type metadata = unit type hash type info = Store.info type contents_key = 'h type 'h node_key = 'h type 'h commit_key = 'h module ]]> sig ... end end I think it is fair to say the documentation is hard to follow as it is a module and functor soup. For example, looking at irmin.3.11.0, starting at the toplevel documentation page our path to finding this paricular module and functor is as follows: We jump into Irmin_mem from the nicely written landing page. We scroll down to find the KV module . We now make sense of the module's signature by following the KV_maker link . We see a at the end of the module, and we navigate through that . Finally we have come to our journey's end, and find the schema module with type constraints like type Branch.t = string. There is a big assumption there that you know what the Schema module is telling you and how it relates to your "store". There is a counter-argument here, in that Irmin is incredibly flexible in terms of what you can use as types for your keys, branches, hashes etc. I was quite easily able to produce for example. I think there is a middle ground. For Irmin to be usable it needs to reduce the complexity of the API. The complexity is due to, in large part, over-functorisation. If the functors cannot be removed then perhaps better documentation or more introductory libraries like irmin-containers would be helpful. Perhaps a standalone library that acts as an interface to Irmin stores would be helpful. I find myself time and again implementing something like that . Patrick Ferris 2025 8 7 Backends Galore Irmin has plenty of backends including a git-compatible one , a MirageOS backend , an in-browser IndexedDB backend and even a FoundationDB backend . Irmin needs to have fewer, better tested backends. Of course, some of these are likely driven by funding and use-case specific details. However, I think by focusing on a few key backends and striving for solid performance but also strong consitency guarantees, Irmin would become a more viable candidate for potential users. I also believe the browser backend plays a key part to making this work in order to facilate local-first applications . Patrick Ferris 2025 8 7 Syncing Remote Stores The synchronisation mechanisms in Irmin are powerful, but often the API is very confusing. For example, with the git-compatible Unix backend, nested deep in the documentation is the function required to create a remote endpoint . Once they have found this function, using it is not easy as it requires them to learn about the Mimic.ctx, which is an abstraction of the networking stack! Patrick Ferris 2025 8 7 Do Fewer Things and Do Them Well Irmin is simultaneously a database that supports: content-addressing, merging, key-value lookup, branches, transactions, complete OS portability etc. I think, in short, it tries to do too many things and comes up short on some of them in ways that really matter. When building the content-addressed database I needed strong guarantees about some atomic actions to perform on the underlying store. For example, setting the value and accessing the commit associated with it which was not possible with the API at that time in an atomic way. This kind of missing functionality is probably only understood after users stumble across it, but it also makes the library feel less focused and ready for production use-cases. Patrick Ferris 2025 8 7 Documentation It will probably come as no surprise that one of the main limitations of using Irmin is the lack of documentation and tutorials. This is a particular pain point as the API is not very straightforward. A while back, I started an Irmin book intending to help document the kinds of things real-world users would need in order to use Irmin in earnest. For example: ways to deal with type migrations or primers on runtime types . I still believe this sort of work would be invaluable, but there is a large cost to completing it, and it is not clear if there is enough interest in the project to warrant such an effort. The tutorials on the irmin website are still good starting points for most new users. But they quickly lack the depth for real-world scenarios. Patrick Ferris 2025 8 7 Feature Wishlist What follows are additional ideas for improving Irmin . Patrick Ferris 2025 8 7 Heterogeneous Stores missing feature for lots of first-time users of the library: you can only store values of a single type. This leads to a few common workarounds: Storing a serealised version of your values, essentially escaping the type-system and implementing a form of dynamic typing. Growing your value to hold lots of different types via some large variant type. Both of these options are feasible and have been used in practice. However, they are workarounds. A long time ago CraigFe created an RFC for heterogeneous stores , which I think about a lot. The main idea is to augment Irmin to be GADTs that carry type information about the kinds of values they access heterogeneuous variants of other data structures API would make Irmin more appealing as a library for persistent data storage. Patrick Ferris 2025 8 7 Real-world Retrospectives This echos some thoughts from the Documentation section. There are some very real-world Irmin Tezos blockchain were using Irmin which uses the irmin-pack backend. Does this make irmin-pack the best tested backend and perhaps should be the default backend for new users? Or is it hyper-specific to the Tezos use case? There are blogs on tarides.com about the irmin-pack backend , but they might be outdated and should not be the first place to find advice on which Irmin backend to use. Patrick Ferris 2025 8 7 Active Development and Engagement As far as I know, Irmin was at the forefront of technologies that have come to be described as local-first . There is a growing interest Ink & Switch's essay on the matter . And it would be great to see more research via Irmin at things like the lofi unconference and the PLF workshop at SPLASH 2023 ! Thank you for reading! And thank you to all the Irmin contributors. I hope this might be useful in the future for building the next-generation Irmin ! 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 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 PRs to the opam-repository 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 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, 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 Hazel is a research programming language with typed holes written completely in OCaml 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 Typed Debugging for Hazel 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 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 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. 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 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 5 19 https://patrick.sirref.org/open-trace/ open-trace /open-trace/ Opentrace 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 opentrace can remain programming language agnostic. However, less information is none about the user's intent compared to something like pyshark. Patrick Ferris 2025 5 19 Monitoring the System opentrace has an all command that will trace the entire system. The --flags=O_WRONLY argument filters the events where the O_WRONLY flag was set in the call to open. comm-wrapped is an artefact of using Nix. Patrick Ferris 2025 5 19 Tracing an Executable The primary use case for this tool is to inspect what files your program might be reading and writing. The "flags" argument can specify a small boolean formula for checking the open flags of a particular event with |&~ Parentheses can be used for precedence. Patrick Ferris 2025 5 19 Spawning Subprocesses One feature opentrace 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 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. Patrick Ferris 2025 5 19 Eio's Process API 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 forkclone 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 into that group. Patrick Ferris 2025 5 19 Limitations: Io_uring 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 eio 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. It is interesting to note two things here: We can tell that forester reads the configuration file probably using something like In_channelit 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. do_filp_openio_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! Patrick Ferris 2025 5 9 https://patrick.sirref.org/try-oxcaml/ try-oxcaml /try-oxcaml/ Try OxCaml https://patrick.sirref.org/oxcaml This week, I have been trying out Janestreet's Oxidised OCaml ICFP paper This adds a system of modes to OCaml for expressing things like locality. ...we’re introducing a system of modes, which track properties like the locality and uniqueness of OCaml values. Modes allow the compiler to emit better, lower-allocation code, empower users to write safer APIs, and with the advent of multicore, statically guarantee data race freedom—all in a lightweight way that only affects those in need. For example, you might say: let is_empty s : string @@ local = String . length s = 0 To get a feel for how this changes the language, you can follow their instructions on their custom opam-repository . Alternatively, you can give it a go in your browser ! This is a full toplevel running with Base Happy OxCamling! Patrick Ferris 2025 5 2 https://patrick.sirref.org/hazel-of-ocaml/ hazel-of-ocaml /hazel-of-ocaml/ A Transpiler from OCaml to Hazel 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 . Patrick Ferris 2025 5 2 Typed Holes 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. fun xs -> case xs | [] => [] | x :: xs => f (x) :: map(f)(xs) end in map(fun x -> ?)([1, 2, 3])]]> ? 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? -> Int. Patrick Ferris 2025 5 2 From OCaml to Hazel 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 Patrick Ferris 2025 5 2 A Transformation of Syntax 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. 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 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. 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 taking a look at how to handle polymorphism in Hazel. Patrick Ferris 2025 5 2 Explicit Polymorphism 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 Craig's excellent blogpost on the matter ... type variables in signatures are implicitly universally-quantified So in reality, we have that map has the following type. 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 forall b -> (a -> b) -> [a] -> [b] = typfun a -> typfun b -> fun f -> fun xs -> case xs | [] => [] | x :: xs => f (x) :: map@@(f)(xs) end in map@@(fun x -> ?)([1, 2, 3])]]> forall introduces a universally quantified type variable into our type annotation, and typfun 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. Patrick Ferris 2025 5 2 Propagating OCaml Types into Hazel Most often, OCaml users interact with prenex 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. [a] -> Int = typfun a -> fun x1 -> case x1 | [] => 0 | _ :: xs => 1 + length@(xs) end in let int_len : Int = length@(1 :: 2 :: [3]) in let str_len : Int = length@("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. Patrick Ferris 2025 5 2 A Corpus of Hazel Programs 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 . Patrick Ferris 2025 5 2 Future Work 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 cmi 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? 2025 2 18 https://patrick.sirref.org/ppxlib-5-2/ ppxlib-5-2 /ppxlib-5-2/ Bumping Ppxlib's AST to 5.2 Ppxlib is a libary for building OCaml preprocessors. Users can generate OCaml code from At the core of ppxlib is the OCaml parsetree; a data structure in the compiler that represents OCaml source code. Ppxlib ppxlib some In ppxlib.0.36.0 , the internal AST was bumped from the AST of OCaml 4.14.0, to the AST of OCaml 5.2.0. This post discusses some of the internal mechanisms that allows ppxlib to support multiple compilers and how this bump broke a lot of backwards compatibility. 2025 2 18 Manual Migrations in Ppxlib It is possible to perform migrations between parsetrees manually using the ppxlib.ast package. open Ppxlib open Ast_builder . Make struct let loc = Location . none end let pp_expr = Format . printf " " Pp_ast . expression ? config : None let loc = Location . none let str txt = txt ; loc module To_408 = Ppxlib_ast . Convert Ppxlib_ast . Js Ppxlib_ast__ . Versions . OCaml_408 The Ppxlib_ast.Js module is ppxlib ppxlib.0.36.0 provided by the Convert functor to copy parts of the AST from one version to another. Usually there is very little difference between the versioned parsetrees. let func arg body = pexp_fun Nolabel None ppat_var str arg body let add_expr = func " x " func " y " eapply evar " Int.add " evar " x " ; evar " y " The above excerpt of OCaml code produces an expression in the language. More accurately, a function that has two arguments. If we print the simplified AST we can see that the arguments are stored together in a list. # pp_expr add_expr ; ; Pexp_function pparam_loc = __loc ; pparam_desc = Pparam_val Nolabel , None , Ppat_var " x " ; pparam_loc = __loc ; pparam_desc = Pparam_val Nolabel , None , Ppat_var " y " , None , Pfunction_body Pexp_apply Pexp_ident Ldot Lident " Int " , " add " , Nolabel , Pexp_ident Lident " x " ; Nolabel , Pexp_ident Lident " y " - : unit = This is a relatively new feature of the parsetree, if we migrate the expression down to 4.08 we see it is represented differently. The output is a little too verbose to be readable, and there's no good way to show the AST, so you will just have to take my word. let expr_408 = To_408 . copy_expression add_expr 2025 2 18 Function Arity in the OCaml Parsetree OCaml 5.2 introduced a new way to represent functions. Before 5.2, all functions were represented with single arity. Functions with multiple arguments would return functions as their body. For example: let add x y = x + y A two-argument add function would be represented in the parsetree as fun x -> fun y -> x + y. Now, since OCaml 5.2, functions can be expressed with their syntactic arity intact. The function add would look something like . Both arguments are stored as a list. The constructor Pexp_function was not a new addition to the parsetree. It used to represent pattern-matching expressions like function A -> 1. This can now be expressed as a function body of Pfunction_cases. Whilst these additions and modifications to the parsetree are arguably a better design, it has caused chaos in terms of ppxlib 's reverse dependencies! 2025 2 18 Coalescing Function Arguments In ppxlib 0.36.0, functions from Ast_builder will help ppx authors produce maximum arity functions. However, functions from Ast_helper will not do this. let ast_helper_func arg body = Ast_helper . Exp . fun_ Nolabel None ppat_var str arg body let ast_helper_expr = ast_helper_func " x " ast_helper_func " y " eapply evar " Int.equal " evar " x " ; evar " y " Here we recreate our helper function to create function expressions using Ast_helper.Exp.fun_ instead. We can now show the difference between the expr expression and the ast_helper_expr expression. # pp_expr add_expr ; ; Pexp_function pparam_loc = __loc ; pparam_desc = Pparam_val Nolabel , None , Ppat_var " x " ; pparam_loc = __loc ; pparam_desc = Pparam_val Nolabel , None , Ppat_var " y " , None , Pfunction_body Pexp_apply Pexp_ident Ldot Lident " Int " , " add " , Nolabel , Pexp_ident Lident " x " ; Nolabel , Pexp_ident Lident " y " - : unit = Above, we see a function expression with arity two . Below, we see nested function expressions each with arity one . # pp_expr ast_helper_expr ; ; Pexp_function pparam_loc = __loc ; pparam_desc = Pparam_val Nolabel , None , Ppat_var " x " , None , Pfunction_body Pexp_function pparam_loc = __loc ; pparam_desc = Pparam_val Nolabel , None , Ppat_var " y " , None , Pfunction_body Pexp_apply Pexp_ident Ldot Lident " Int " , " equal " , Nolabel , Pexp_ident Lident " x " ; Nolabel , Pexp_ident Lident " y " - : unit = 2025 2 18 Ramifications for Ppxlib Users Many, many, many ppxes broke after migrating the ppxlib AST to 5.2. Mostly, this is due to pattern-matching. Downstream users of ppxlib frequently pattern-match directly against the parsetree. However, nodes such as Pexp_fun no longer exist and Pexp_function has completely different constructor arguments! Pexp_fun xfunction Here is a non-exhaustive list of some of the ppxes that have been patched. It may useful for other ppx authors who wish to upgrade to ppxlib.0.36.0: https://github.com/patricoferris/ppx_deriving_yaml/pull/58 https://github.com/mirage/ocaml-rpc/pull/181 https://gitlab.com/o-labs/ppx_deriving_jsoo/-/merge_requests/1 https://github.com/ocaml-community/sedlex/pull/160 https://github.com/ocaml-sys/config.ml/pull/22 https://github.com/vogler/ppx_distr_guards/pull/2 https://github.com/ocaml-gospel/gospel/pull/424 https://github.com/teamwalnut/graphql-ppx/pull/299 https://github.com/ocsigen/lwt/pull/1033 https://gitlab.com/gopiandcode/ppx-inline-alcotest/-/merge_requests/3 https://github.com/dariusf/ppx_interact/pull/3 https://github.com/ocaml-ppx/ppx_deriving_yojson/pull/160 https://github.com/LPCIC/elpi/pull/276 https://github.com/mirage/repr/pull/110 https://github.com/ProgramingIsTheFuture/ppx_default/pull/3 https://github.com/codinuum/vlt/pull/3 https://github.com/sim642/ppx_deriving_hash/pull/6 https://github.com/ocsigen/tyxml/pull/340 https://github.com/sim642/ppx_viewpattern/pull/2 https://github.com/little-arhat/ppx_jsobject_conv/pull/11 https://github.com/robur-coop/lun/pull/1 https://gitlab.com/o-labs/ppx_deriving_encoding/-/merge_requests/4 https://github.com/francoisthire/bam/pull/12 https://github.com/xguerin/bitstring/pull/36 tension is not new . 2025 2 18 The Future of Ppxlib Ppxlib is not going anywhere anytime soon. We have committed to trying to keep the internal AST up to date with the latest OCaml release. In fact, there is already a PR for the 5.3 bump ! There have been many discussions about simplifying this process, but so far the time and effort to build a ppxlib an attempt using views Thank you to Tarides and Jane Street for funding my time to work on this release and all the chaos it has caused. Happy preprocessing! 2025 2 28 https://patrick.sirref.org/vpnkit-upgrade/ vpnkit-upgrade /vpnkit-upgrade/ Defunctorising VPNKit VPNKit is a core part of the Docker for Mac/Windows stack. It is tasked with translating network activity from the host to the Linux container. This short post discusses some of the recent changes to MirageOS and how they impact VPNKit . https://patrick.sirref.org/dune-virt-libs/ dune-virt-libs /dune-virt-libs/ Dune Virtual Libraries Dune, for quite some time, has supported virtual libraries module type S = struct ... end This feature has been quite widely used in OCaml. For example, the excellent digestif library of hashing algorithms provides both C and OCaml implementations individually packaged into digestif.c and digestif.ocamlIrmin in the browser js_of_ocaml will happily compile for us. There are two distinct limitations to this approach. Applications can only link exactly one implementation. dune#5597 2025 2 28 Functors in MirageOS MirageOS unix, hvt Recently, there has been a push to use dune virtual libraries instead. 2025 2 28 VPNKit's Fake Clock & Time For VPNkit, we need this to port some of the DNS forwarding tests which use a fake clock and sleep module. Previously, they could be provided directly as functor arguments to the various DNS modules e.g. let module R = Dns_forward . Resolver . Make Proto_client Fake . Time Fake . Clock In this new world of dune virtual libraries , our Resolver is significantly more simple. let module R = Dns_forward . Resolver . Make Proto_client The first thing I had to do was make the mirage-mtime and mirage-sleep virtual libraries unwrapped. This allowed me to make a new private dune library called fake_time with two public modules: the normal interface required by mirage-mtime and another module called Fake_time_state containing: let timeofday = ref 0L let c = Lwt_condition . create let advance nsecs = timeofday : = Int64 . add ! timeofday nsecs ; Lwt_condition . broadcast c let reset = timeofday : = 0L ; Lwt_condition . broadcast c I did the same for fake_sleep which depended on the state from the fake_time Whereas before, all the tests could live in the same test suite, now they had to be split into those that needed the fake implementation and those that did not. This is another slight limitation of this virtual library approach. Thus, the dune rules for running the dns_forward test suite became: ; Uses the default implementations provided by virtual libraries executable name test libraries dns_forward alcotest ; Override the defaults and select the private fake libraries executable name test_fake libraries dns_forward alcotest fake_sleep fake_time With all of this machinery in place, I could now update test_fake to use the new modules that would provide the custom functionality for the clock and time implementations. The bad server should be marked offline and no-one will wait for it Fake_time_state . reset ; avoid the timeouts winning the race with the actual result Fake_time_state . advance Duration . of_ms 500 ; 2025 2 28 Conclusion The switch to dune virtual libraries seems to be a good one. A majority of applications only require a single clock or time implementation. However, it has become harder to inject custom implementations of various resources into your MirageOS-based applications. The pull request to upgrade VPNKit is open . Patrick Ferris 2023 8 9 https://patrick.sirref.org/ocaml-rtree/ ocaml-rtree /ocaml-rtree/ Reviving OCaml R-Tree Fast-forwarding a decade-old OCaml implementation of the R-Tree geospatial data-structure. Antonin Guttman first proposed R-Trees in 1984 . In order to handle spatial data efficiently, as required in computer aided design and geo-data applications, a database system needs an index mechanism that will help it retrieve data items quickly according to their spatial locations... R*-trees blog post seems to be the most popular introduction. Patrick Ferris https://patrick.sirref.org/ocaml-rtree-001/ ocaml-rtree-001 /ocaml-rtree-001/ An OCaml Rtree Implementation In the Energy and Environment Group we are working on various geospatial tools, one of which is written in OCaml and needed a geospatial index to speedup spatial queries over a dataset. Searching Github revealed an implementation in pure OCaml . After reaching out to the author, they were more than happy for their work to be maintained in the Geocaml organisation , updated to the latest OCaml tools and hopefully released on opam . The dunification Patrick Ferris https://patrick.sirref.org/ocaml-rtree-002/ ocaml-rtree-002 /ocaml-rtree-002/ OCaml Rtree Examples Rectangle envelope. module Line = struct type t = p0 : float * float ; p1 : float * float let t = let open Repr in record " line " fun p0 p1 -> p0 ; p1 |+ field " p0 " pair float float fun t -> t . p0 |+ field " p1 " pair float float fun t -> t . p1 |> sealr type envelope = Rtree . Rectangle . t let envelope p0 = x1 , y1 ; p1 = x2 , y2 = let x0 = Float . min x1 x2 in let x1 = Float . max x1 x2 in let y0 = Float . min y1 y2 in let y1 = Float . max y1 y2 in Rtree . Rectangle . v ~ x0 ~ y0 ~ x1 ~ y1 end In addition to the envelope function, we also need to provide a runtime representation using Repr. In the example, we've constructed it by hand but I recommend using ppx_repr to derive it for you. From here we can create our index. module R = Rtree . Make Rtree . Rectangle Line Patrick Ferris https://patrick.sirref.org/ocaml-rtree-003/ ocaml-rtree-003 /ocaml-rtree-003/ Inserting Values There are two ways to insert values into the index -- either by inserting them one-by-one or by bulk loading them. The latter is the recommended approach where possible as the resulting layout of the R-Tree is much better. # let r = let i = R . empty 4 in R . insert i Line . p0 = 1. , 1. ; p1 = 2. , 2. ; ; val r : R . t = < abstr > As you might expect, bulk loading takes a list of values instead. # let r = let lines = Line . p0 = 0. , 0. ; p1 = 1. , 1. ; p0 = 1. , 1. ; p1 = 2. , 2. ; p0 = 2. , 2. ; p1 = 3. , 3. ; p0 = 3. , 3. ; p1 = 4. , 4. ; in R . load ~ max_node_load : 4 lines ; ; val r : R . t = < abstr > Patrick Ferris https://patrick.sirref.org/ocaml-rtree-004/ ocaml-rtree-004 /ocaml-rtree-004/ Spatial Queries The lines we loaded into the last index are really just four segments of the line y = x. Let's spatially query for the first two segments. # R . find r Rtree . Rectangle . v ~ x0 : 0. ~ y0 : 0. ~ x1 : 2. ~ y1 : 2. ; ; - : Line . t list = Line . p0 = 0. , 0. ; p1 = 1. , 1. ; Line . p0 = 1. , 1. ; p1 = 2. , 2. ; Line . p0 = 2. , 2. ; p1 = 3. , 3. You can see the query actually returned the first three, that's because all bound checks are inclusive. The source repository includes a fairly simple example of rendering an R-Tree using Vg . This can be done outside of the repository using the tree representation. # # show R . tree ;; val tree : R . t -> R . tree type nonrec tree = R . tree = Node of Line . envelope * R . tree list | Leaf of Line . envelope * Line . t list | Empty Here is an example rendering from the repository showing some random lines and the envelopes of the R-tree. Weeklies A collection of less thoughtfully crafted reports, focused on OCaml-related activities. Patrick Ferris 2026 3 10 https://patrick.sirref.org/ocaml-roundup-march-2026/ ocaml-roundup-march-2026 /ocaml-roundup-march-2026/ OCaml Roundup: March 2026 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 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 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 10 https://patrick.sirref.org/ocaml-roundup-february-2026/ ocaml-roundup-february-2026 /ocaml-roundup-february-2026/ OCaml Roundup: February 2026 Much like last month, this month has been busy with lots of work on ocaml-tiff thanks to Tambe Salome and ppxlib . Patrick Ferris 2026 3 10 Outreachy Tambe Salome , at the time of writing, has completed her internship! We will be hosting the biannual Demo Day celebration for this round's interns, so please do come along. Since the last update Tambe Salome and I have been slowly making progress on making the write-functionality more feature-complete. As I eluded last month , TIFF files are hard to write. Their internal structure is all over the place: directories, immediate and non-immediate values, 32-bit or 64-bit, little or big endian etc. We're tackling this bit by bit, evolving the library to be able to support writing to files. In the early days, when we only read TIFF files there was no need to something like in memory before writing them to the file, so we will need some kind of API change internally to support this. This month, I also bumped up against well-meaning contributions to ocaml-tiff that made use of AI. This left me with some thoughts that I am adding below. Patrick Ferris 2026 3 9 https://patrick.sirref.org/vibecoding-etiquette/ vibecoding-etiquette /vibecoding-etiquette/ Vibecoding Etiquette I am yet to jump head-first into using LLM-based tools like Claude Code or even ChatGPT to help with my programming in any serious way. After using some "free"-tier tools to try to better understand some tricky eBPF problems and to make sense of the semantics of POSIX shells, I was left unimpressed and ultimately felt my time was wasted. I feel unwilling to send my money to the corporations behind these tools. This is for a variety of reasons including the deeply worrying relationship with the "opertational planning" vs. "operational decision-making" given the leaders of the US DoW environmental concerns, exploitation of human labour and a fast-and-loose approach to copyright laws. All of which leaves the entire industry very unattractive to me. Everybody can draw their line where they want, and I am, in good faith, trying to find where mine should be. Unfortunately, my own decision to not use these tools is not completely in my control. What do I do about pull requests to open-source libraries I maintain that include agentic code? I am, after all, participating in the idea of open-source and inclusive collaboration. Agentic code also comes in many flavours, from blatant LLM-generated pull requests to heavily edited, human-in-the-loop contributions. An immediate first step for me is to develop some policy for contributions to projects I maintain regarding code where LLMs have had a hand. This can help set expectations for contributors. Eio's "AI-generated Code" subsection is succinct and prohibits contributions that solely use AI. It obfuscates how you think. Purely AI-generated code tells us little about how you think and the problems you might be having. This makes it harder to provide good feedback on PRs and issues. It is often more work to review. Particularly for the OCaml ecosystem and libraries like Eio, it seems that these tools are not very good and generate a lot of believable code that is in actual fact completely wrong. PR comments and the code submitted with them can say completely different things. It is a grey area for licensing. Models like ChatGPT have been trained on lots of code with different licenses and has been known to simply copy code as an answer to a prompt. We would like to avoid this headache as best we can. Additionally, I plead to other developers to own their use of these tools. Please, make Claude a co-author of the commits where Claude has generated any code at the very least. This is the bare minimum. I have strong feelings about the etiquette of sending AI summarised information or LLM-generated code to other people, but before any of that, the decent thing is to remove any uncertainty in the receiving party's mind of how the information they are receiving came to be. In this way, I appreciate the OCaml compiler's AI notice which includes: If a significant portion of your code, PR description, review comments or messages has been AI-generated, this must be disclosed, stating which tool was used and for what. Reviewing AI-produced code can require a different approach from human-written code. You should have an acknowledgement anyway even if another human contributor did some of the work. Patrick Ferris 2026 3 10 Ppxlib We are still in the throes of supporting OCaml 5.5 in ppxlib . Some of the features have proved more complicated to encode into migrations that "roundtrip". Roundtripping, in the context of ppxlib , is when a modern feature of OCaml must be preserved in older abstract syntax trees which may have no specific node to encode them. Patrick Ferris 2026 3 10 Bugs in Ptyp_open Unfortunately, Jon Sterling was bitten by this recently . module M = struct type t = int end type t = M . t list Here, the is an example of a new AST node called Ptyp_open. I had hastily not migrated Ptyp_open to older compilers. So what happened? Jon had used this new feature in his project with a 5.3 compiler with ppxlib 0.35. This is before our internal bump to the 5.2 AST so the code migrated all the way down to OCaml 4.14! Whilst passing from 5.2 to 5.1 we raised an error saying: Error: migration error: module open in types is not supported before OCaml 5.02 patched this issue and have planned to release a patched 0.35 in the not too distant future. If you can bump to ppxlib>=0.36 then you will not face this issue. Patrick Ferris 2026 3 10 Modular Explicits We ran into a fairly interesting corner of the OCaml language whilst trying to migrate modular explicits in ppxlib . As a recap, in OCaml 5.5 you can now right function types that depend on modules. For example: module type Add = sig type t val add : t -> t -> t end let add : module A : Add -> A . t -> A . t -> A . t = fun module A : Add a b -> A . add a b We have to tell the compiler the module type of A by adding : Add. However, there is another very similar syntax for these kinds of constraints, . And in fact, the former syntax was syntactic sugar for the latter. In OCaml 5.5, these two were distinguished from one another and is the required syntax for modular explicits. As you might imagine, this is a bit of a headache in terms of ppxlib migrations. In the end, we try to preserve the original intent of the author where possible, and you can read more about it on the PR for distinguishing the two. Patrick Ferris 2026 3 10 Six Month Retrospective This month marks the end of six months of work with Tarides . I wrote a small retrospective and plan for my next six months below. 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 . Patrick Ferris 2026 2 5 https://patrick.sirref.org/ocaml-roundup-january-2026/ ocaml-roundup-january-2026 /ocaml-roundup-january-2026/ OCaml Roundup: January 2026 A new year, another roundup in my open-source, OCaml activities. This month has been busy with lots of work on ocaml-tiff thanks to Tambe Salome and ppxlib . Patrick Ferris 2026 2 5 Outreachy Tambe Salome has been working to add write support to ocaml-tiff ; and we are well on the way to having good support. Writing TIFF files is no easy business. Unlike many formats, TIFF files have an internal metadata-data split, with values sometimes being place nowhere near the metadata describing the values in the file. In practice, it means if you do not wish to read the whole file into memory, then your IO layer must support seeking to file offsets. This means when you come to write the file, you might need to write at different offsets too. This becomes particularly complicated when you do not know ahead of time how copying a TIFF file we have access to but pushing all the copying via the OCaml values we read from our source TIFF. And Salome has made excellent progress: input.exif $ dune exec -- example/copy.exe test/data/uniform.tiff example.tiff $ exiftool example.tiff > output.exif $ patdiff input.exif output.exif ------ input.exif ++++++ output.exif @|-1,23 +1,23 ============================================================ |ExifTool Version Number : 13.39 -|File Size : 66 kB +|File Size : 118 bytes |File Permissions : -rw-r--r-- |File Type : TIFF |File Type Extension : tif |MIME Type : image/tiff |Exif Byte Order : Little-endian (Intel, II) |Image Width : 256 |Image Height : 256 |Bits Per Sample : 8 |Compression : Uncompressed |Photometric Interpretation : BlackIsZero |Strip Offsets : 122 |Rows Per Strip : 256 |Strip Byte Counts : 65536 |Planar Configuration : Chunky |Image Size : 256x256 |Megapixels : 0.066]]> From this slightly edited output, we can see that the OCaml libary is doing a good job at preserving the metadata. The file size discrepancy is because we have not tackled writing the raw data to the file yet! This is next on our TODOs, write data to a file given a strip-offset plan. Once we do that we can then start to think how might we come up with such a plan if we did not have one. Patrick Ferris 2026 2 5 Ppxlib The first cut of OCaml 5.5 is not too far away, at which point the ppxlib of new features sneaked into this release including: modular explicits, external type declarations and the previously mentioned local structure items. All of these features require migrations to support newer compilers. In this migration process, ppxlib lowers modern ASTs to the AST of OCaml 5.2. New migration process. We have a few PRs to support 5.5 fully: The initial PR to support 5.5 . Supporting all the new local structure items . Full support for modular explicits and external type declarations . Additionally, Nathan has been adding better feature encoding support for the 5.3 and 5.4. Unlike the migration code above, these also include Ast_builder and Ast_pattern functions for building and matching on the new AST nodes. This allows ppx authors to use those nodes in their ppxes. OCaml 5.3 effect syntax . OCaml 5.4 labeled tuples . Finally, we have an important bug fix where long identifiers were given a Location.none. Unfortunately, it is not possible to fully recover a long Read more about that fix here . Patrick Ferris 2026 1 11 https://patrick.sirref.org/ocaml-roundup-december-2025/ ocaml-roundup-december-2025 /ocaml-roundup-december-2025/ OCaml Roundup: December 2025 December was busy, both personally and in my OCaml-related work. If you haven't been following along have a read of my October and November roundups. Patrick Ferris 2026 1 11 https://patrick.sirref.org/outreachy-december-round/ outreachy-december-round /outreachy-december-round/ Outreachy December Round December marks the beginning of a new Outreachy round, and I am very excited to be working with Tambe Salome on improving the ocaml-tiff library. In particular, we will be adding the ability to write TIFF files from raw data. I imagine an API something like: val write : 'repr , 'kind Tiff . Data . t -> Tiff . File . wo -> unit So far Salome has been making excellent progress churning through some issues that are blocking us from starting the main project, including: Adding a sublibrary for Eio . Using the new Nx library . Uncovering bugs in our decoding of the number of bits per pixel Preliminary work on write support ! As a community, the mentors and mentees also had a community call to introduce ourselves to eachother. I am very excited about the Yocaml project and the two Raven projects. Patrick Ferris 2026 1 11 Ppxlib If you recall, I discussed the new representation for local constructs in the AST. Patrick Ferris 2025 12 9 https://patrick.sirref.org/ppxlib-5.5-support ppxlib-5.5-support /ppxlib-5.5-support Initial OCaml 5.5 Support We recently merged 5.5 support into our main some new features coming to OCaml including the ability to define any structure locally . Most OCaml programmers will be familiar with things like let open M in External types have also landed, bringing with them a new "type kind" in the AST. You can read about them in the manual . It transpires that since then, changes have landed in the compiler that mean just about any OCaml structure item can now be declared locally. I have documented an issue on ppxlib about the problems this will cause for us: Currently, we only migrate the previously supported structure items that let open allows anything to appear in a local let-binding except additional let let ...let include change had a 1:1 mapping from a 5.4 AST node to a 5.5 AST node, now we do not and will need to think of a suitable encoding. Patrick Ferris 2026 1 11 Migration Woes gasche for the report Longidents are used in OCaml to identify values like Foo.bar or . In OCaml 5.4, each segment of the identifier was given a precise location. This means that in the migration from 5.3 to 5.4 we have to reconstruct those locations. However, our code didn't do this, instead it simply defaulted to Location.none causing some imprecise error codes in preprocessed OCaml files. For example: let = NonExistingModule . run Has the following error: I have proposed a fix which partially brings back locations during the migration. Part of the problem, I think, is that the exact location of long identifiers containing applications Patrick Ferris 2025 12 9 https://patrick.sirref.org/ocaml-roundup-november-2025/ ocaml-roundup-november-2025 /ocaml-roundup-november-2025/ OCaml Roundup: November 2025 A roundup of some of my OCaml activity this month! Patrick Ferris 2025 12 9 Ppxlib Updates I managed to get a substantial amount of ppxlib work completed in the last month. Some of those bits are described in detail below. Patrick Ferris 2025 12 9 https://patrick.sirref.org/ppxlib-5.5-support ppxlib-5.5-support /ppxlib-5.5-support Initial OCaml 5.5 Support We recently merged 5.5 support into our main some new features coming to OCaml including the ability to define any structure locally . Most OCaml programmers will be familiar with things like let open M in External types have also landed, bringing with them a new "type kind" in the AST. You can read about them in the manual . Patrick Ferris 2025 12 9 Bug Fixes for Attribute Rewriters 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. Patrick Ferris 2025 12 9 Value binding constraints in ppxlib 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 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 ; : intpvb_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.0.20.0repr transforms instances of let x : t = ... into ! This limitation does not seem inherent to the parsetree, the printer or the migrations. But I have Patrick Ferris 2025 12 9 Outreachy 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 ! Patrick Ferris 2025 12 9 https://patrick.sirref.org/graft-and-bib-update/ graft-and-bib-update /graft-and-bib-update/ 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 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 We're making two 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 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 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 # ; unboxed values You can see how this impacted the library ! 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 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 it detailing exactly what the numbers that tell you where something is mean. One of the most common being WGS84 will need to convert between them. That is what PROJ does. Patrick Ferris 2025 11 5 WKT in OCaml 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 9 2 https://patrick.sirref.org/ocaml-weekly-2025-w32-w35/ ocaml-weekly-2025-w32-w35 /ocaml-weekly-2025-w32-w35/ OCaml Weekly 2025 w32 to w35 I have been working on a few different OCaml-related projects over the last few weeks. This is also coinciding with my partner and I moving across the UK, which has made finding time to write weeklies and posts a little tricky. Nevertheless, here are some of the things I have been thinking about and working on! I managed to publish one signficant post this month: a retrospective on Irmin . Patrick Ferris 2025 9 2 Eio Increasingly, I'm feeling the dream of a unified framework for asynchronous IO slipping through the OCaml community's fingers. It is perhaps not such a bad thing, and I think with the right library authoring we can at least get to a place where it isn't so bad, for example providing read functions as opposed to using an opinionated IO library directly. That being said, I am a very happy user of Eio when those choices do not matter, Shelter I have spent a good bit of time upstreaming support for various missing pieces in Eio's API including: Setuid and setgid fork action's for the process API. Set process group support for job control in the process API. Responding to Buf_write.of_flow request, and tinkering with the example there. I think this does highlight the awkwardness of making code portable across concurrency mechanisms, particularly with Eio's structured concurrency. I did some investigating into EINTR bug which seems to be stemming from a known-issue on Uring in that writes are not buffered which usually does not matter except perhaps when there are parallel writes to stdout. Spent some time thinking about the fiber local storage across domains issue , I've passed on some thoughts to folks working on this. Patrick Ferris 2025 9 2 Vpnkit You might recall I was interested in using vpnkit . Hannes has done an amazing amount of work much better and could be considered soon for merging. This defunctorisation is actually very useful for the Eio port I wrote a long time ago. Patrick Ferris 2025 9 2 Papers and Talks at ICFP Somehow, I have ended up on lots of papers and talks at ICFP and the co-located events in October. The vaguely OCaml-related ones include: Essentially a Vpnkit Experience Report . An extended abstract on generating a corpus of ill-typed Hazel programs was accepted into TyDe workshop . Relatedly, the work that project supported was accepted into HATRA which was the Part II project I supervised : Decomposable Type Highlighting for Bidirectional Type and Cast Systems . And two PROPL talks ! Patrick Ferris 2025 9 2 Outreachy We have come to the end of another Outreachy round! I will write more on this soon in its own separate post. But for now I am very grateful to this round's mentors gridbugs and mdales , and also our fantastic interns. If you are interested, please do watch our demo day presentations. The next round is fast approaching and we still need to work out the logistics. But I had a good conversation with mdales about possible Geocaml projects that I intend to submit! 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 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 I tried using x-ocaml with Irmin and something is not see the issue in case you can help 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 the dune-project file, 2. run dune build to update the opam file, 3. remove all occurences of the library from dune 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 Patrick Ferris 2025 7 15 https://patrick.sirref.org/ocaml-weekly-2025-w29/ ocaml-weekly-2025-w29 /ocaml-weekly-2025-w29/ OCaml Weekly 2025 w29 Patrick Ferris 2025 7 15 Ppxlib 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 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 in ppxlib.0.36.0 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. Patrick Ferris 2025 7 15 Outreachy 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. Patrick Ferris 2025 7 15 Graft 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 . https://patrick.sirref.org/ppxlib/ ppxlib /ppxlib/ Ppxlib https://github.com/ocaml-ppx/ppxlib An OCaml library and ecosystem for preprocessing. https://patrick.sirref.org/shelter/ shelter /shelter/ Shelter https://tangled.sh/@patrick.sirref.org/shelter A shim between a user and the shell that provides greater reproducibility and insight into what your code is doing! Give me Shelter! Shelter combines the caching and reproducibility of Dockerfiles with the ease of interaction of the shell. I'd be interested to use Ryan 's package management work as a means to specify the base environment users want. Here is an example of running Shelter and making use of the branchable sessions. shelter> echo hello > hello.txt sheltermain#bbd691arw @ session exp-1]]>shelterexp-1#bbd691arw echo world >> hello.txt]]>shelterexp-1#dd5bab8rw @ session main]]>sheltermain#bbd691arw echo "to the" >> hello.txt]]>sheltermain#b9abef4rw cat hello.txt hello to the]]>sheltermain#d1c1728rw @ session exp-1]]>shelterexp-1#dd5bab8rw @ replay main]]>shelterexp-1#df1b4e1rw cat hello.txt hello to the world]]>shelterexp-1#67162b7rw]]> The @ character allows the user to interact directly with Shelter's build-in operators. @ session exp-1 creates a new session as there is no exp-1 session. git checkout -b exp-1 From there we make changes and switch between branches until we perform a replay. A replay is a rebase without any merge conflict checks, it simply finds the least common ancestor exp-1main Under the hood Shelter is a light-weight, reimplementation of obuilder . ZFS Void Between the low-level store and the interactive shell parts, there is an Irmin store. https://patrick.sirref.org/talex5/ talex5 /talex5/ Thomas Leonard person https://roscidus.com/ Patrick Ferris 2025 11 5 https://patrick.sirref.org/icfp-oxcaml-uring/ icfp-oxcaml-uring /icfp-oxcaml-uring/ OCaml Roundup: October 2025 › 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 # ; unboxed values You can see how this impacted the library ! 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 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. Contributions