OCaml Weekly News

OCaml Compiler

• Relocatable OCaml Merged
  

  In December 2025, the final piece of the Relocatable OCaml puzzle was merged, enabling opam to clone switches instead of recompiling them. This feature will be available in the first alpha release of OCaml 5.5.

  What Relocatable OCaml Enables:

  This feature allows the OCaml compiler and its associated tools to be moved to different filesystem locations after installation without breaking functionality. Key benefits include:

  • Binary distributions that work regardless of installation path
  • Improved flexibility for package managers organizing OCaml installations
  • Bundling of specific OCaml versions by developer tools without path conflicts
  • Simplified cross-platform distribution

  The implementation is the culmination of work by David Allsopp, with review from Samuel Hym, Jonah Beckford, and others. See the announcement on Discuss and the merged PRs (#14244, #14245, #14246, #14247) for technical details.

Build System

• Dune
  

  Dune 3.21.0 (January 16, 2026) is a large release including dozens of fixes, improvements, and additions from many contributors.

  Notable Additions:

  • (Experimental): library_parameter stanza for the OxCaml compiler
  • Copy-on-write (COW) when copying files on supporting filesystems (Btrfs, ZFS, XFS) under Linux
  • Support for Tangled ATproto-based code repositories
  • New (dir ..) field on packages to filter stanzas with --only-packages
  • dune promotion show command to preview corrected files
  • New (lang rocq) build mode for Rocq 9.0 and later
  • Support for instantiating OxCaml parameterised libraries
  • dune describe tests to list tests in the workspace
  • Horizontal scrolling in TUI
  • Support for expanding variables in (promote (into ..))

  Notable Fixes:

  • Fixed include_subdirs qualified picking the furthest module instead of the closest
  • Improved error messages for invalid version formats with non-ASCII characters
  • Fixed crash when running dune build @check on libraries with virtual modules
  • Allow $ dune init to work on absolute paths
  • Stop hiding the root_module from the include path

  For the complete list of changes, see the full release on GitHub.

  Dune Maintained by: Rudi Grinberg (@rgrinberg, Jane Street), Nicolás Ojeda Bär (@nojb, LexiFi), Marek Kubica (@Leonidas-from-XIV, Tarides), Ali Caglayan (@Alizter, Tarides), Stephen Sherratt (@gridbugs, Tarides), Antonio Nuno Monteiro (@anmonteiro), Sudha Parimala (@Sudha247, Tarides), Ambre Suhamy (@ElectreAAS, Tarides), Puneeth Chaganti (@punchagan, Tarides)

Package Management

• opam
  

  opam 2.5.0 (November 27, 2025) is a major release with significant performance and usability improvements.

  Key Features:

  • Massive speedup for opam update: Thanks to @arozovyk, opam update now loads opam files incrementally, only parsing files that have changed since the last update. For typical small diffs, this means operations that took seconds now complete in milliseconds
  • Improved shell integration: Fixed issues where parts of a previous environment were kept in the current environment, causing various problems
  • Changed default shell integration file: Now writes to .bashrc instead of .profile when bash is detected, preventing infinite loop issues
  • AppArmor profile: The install script now installs an appropriate apparmor profile on systems configured with apparmor (enabled by default on Ubuntu)
  • macOS sandbox improvements: Allow writing to /var/folders/ and /var/db/mds/ directories as required by some macOS core tools

  Build Changes:

  • Ecosystem cmdliner 2.0.0 compatibility: opam no longer depends on cmdliner, removing a key blocker that prevented cmdliner 1.x and 2.0.0 from being co-installable. This change enables ecosystem-wide migration to cmdliner 2.0.0
  • OCaml 5.5 (trunk) support when using dune's dev profile
  • The release archive (opam-full-*.tar.gz) is now reproducible
  • OpenBSD binary is now a full static binary

  For more details, see the official opam 2.5.0 announcement blog and full release notes on GitHub.

  opam Maintained by: Raja Boujbel (@rjbou, OCamlPro), Kate Deplaix (@kit-ty-kate, Ahrefs), David Allsopp (@dra27, Tarides)

• opam-publish
  

  opam-publish 2.7.1 (November 18, 2025) fixes bugs related to the GitHub API token permissions introduced in version 2.7.0.

  Changes:

  • Advertise the need, and check, for the workflow scope for GitHub personal access tokens
  • Enforce the git remote used to push branches to users' fork to be used instead of the SSH method
  • Avoid potential previously used tokens with wrong permissions to be used instead of the new one
  • Add support for the opam 2.5 API

  Maintained by: Raja Boujbel (@rjbou, OCamlPro), Kate Deplaix (@kit-ty-kate, Ahrefs)

• opam-repository Archival: January 1, 2026
  

  On January 1, 2026, the opam-repository completed its scheduled archival run, removing 3,264 package versions (881 unique packages) marked with x-maintenance-intent: archival. This ongoing maintenance process helps keep the repository manageable by removing unmaintained or obsolete package versions.

  What This Means:

  • Archived packages are no longer available in the default opam repository
  • CI systems and lock files may need updates if they depend on archived versions
  • The archival is based on metadata explicitly set by package maintainers

  Background:

  The archival process was announced by Hannes Mehnert in December 2025 (see the Discuss announcement), giving package maintainers and users time to prepare. Packages can be restored from archival by maintainers upon request.

  This maintenance practice ensures the opam repository remains sustainable and focused on actively maintained packages.

• dune-release
  

  dune-release 2.2.0 (January 29, 2026) brings full compatibility with cmdliner 2.0.0, continuing the ecosystem-wide migration that began with opam 2.5.0's removal of the cmdliner dependency.

  Why This Matters:

  cmdliner 2.0.0 introduced stricter requirements that improve CLI reliability but prevent co-installability with cmdliner 1.x. Following opam 2.5.0's lead in removing this conflict, dune-release 2.2.0 updates to use cmdliner 2.0.0 exclusively. This coordinated effort across the platform tools enables users to upgrade without encountering dependency conflicts.

  Important Changes:

  • Breaking: Following cmdliner 2.0's stricter requirements, prefix-matching for command options is no longer supported
  • Users must now provide the full wording for all flags (for example, --skip-tests instead of --skip-test)
  • This change ensures compatibility with cmdliner 2.0.0 and aligns with modern CLI best practices
  • If you have automation scripts using shortened flags, update them before upgrading

  For technical details, see the cmdliner 2.0.0 documentation and the dune-release PR #512.

  Maintained by: Tarides

Editor Tools

• OCaml-LSP Server
  

  OCaml-LSP 1.25.0 (December 20, 2025) introduces support for .mlx files and new custom requests.

  Features:

  • .mlx Support: Added support for .mlx files, including diagnostics, code actions, hover, and formatting via ocamlformat-mlx
  • New Custom Requests: Added typeExpression, locate, and phrase requests to the server
  • Code-Lens Configuration: Code-lens for nested let bindings is now configurable

  Fixes:

  • The server now falls back to .merlin configuration if a dune-project file is missing, provided dot-merlin-reader is installed
  • Improved precision of timestamps for collected metrics
• Merlin
  

  Merlin 5.6.1-504 (December 20, 2025) brings performance optimizations and improved stability.

  Key Improvements:

  • Smarter Signature Help: Now triggers correctly on unfinished let ... in bindings and no longer appears redundantly on function names
  • More Reliable Completion: Fixed issues with completion for inlined record labels
  • Improved Performance: Optimized buffer indexing and path calculations
  • Bug Fixes: Resolved a bug where the document command concatenated labels and variants incorrectly

  OCaml LSP Server maintained by: Ulysse Gérard (@voodoos, Tarides), Xavier Van de Woestyne (@xvw, Tarides), Rudi Grinberg (@rgrinberg, Jane Street)

  Merlin maintained by: Ulysse Gérard (@voodoos, Tarides), Xavier Van de Woestyne (@xvw, Tarides), Muluh Godson (@PizieDust, Tarides)

• OCaml-eglot
  

  We have drastically modified the xref backend (allowing navigation from definition to definition) to make it more suitable for OCaml.

  In addition, we added the ability to annotate the type of an enclosing active_ (used with the ocaml-eglot-type-enclosing command) or simply to type the selection.

  These updates have been merged into main, making them available via a MELPA update.

  ocaml-eglot maintained by: Xavier Van de Woestyne (@xvw, Tarides)

Platform Infrastructure

• OCaml Infrastructure: How the opam-repository Works
  

  The November 5, 2025 article on the opam-repository provides an in-depth look at this critical piece of OCaml infrastructure. The opam repository serves as the central package registry for the OCaml ecosystem, hosting over 4,500 packages. It relies on dedicated volunteer maintainers who review every submission.

  —

  As always, we encourage feedback and contributions from the community as we continue to improve the OCaml Platform ecosystem.

mkernel

• A new workflow and targets
  

  This release is experimental: Xen is not yet supported, and Unikraft support is partial. However, both platforms are on our roadmap, so if you are interested in them, please let us know! We are also developing tooling to streamline the workflow, with the guiding principle that these tools should never dictate which dependencies your unikernel uses or which build system you choose.

mnet

let run (ipv4, gateway, ipv6) =
  Mkernel.(run [ rng; Mnet.stack ~name:"service" ?gateway ~ipv6 ipv4 ])
  @@ fun rng (daemon, tcp, udp) ->
  let@ () = fun () -> Mnet.kill daemon in
  let@ () = fun () -> Mirage_crypto_rng_mkernel.kill rng in
  let listen = Mnet.TCP.listen tcp 9000 in
  let flow = Mnet.TCP.accept tcp listen in
  let buf = Bytes.create 4096 in
  let rec echo () =
    let len = Mnet.TCP.read flow buf in
    if len > 0 then begin
      Mnet.TCP.write flow (Bytes.sub_string buf 0 len);
      echo ()
    end
  in
  let@ () = fun () -> Mnet.TCP.close flow in
  echo ()

• Developing and deploying unikernels
  

  This library also marks a turning point in our approach to unikernels. We no longer treat a regular OCaml application as something that can be transparently turned into a unikernel. Developing a unikernel now means developing a unikernel from the start. In our experience, trying to retrofit an existing application into a unikernel was neither practical nor worthwhile.

  Our tutorial therefore covers both the development and deployment of unikernels. On the deployment side, we also recommend exploring Albatross.

utcp

• Unikernels and proofs
  

  utcp also reflects our ambition to incorporate, as time permits, proof-based implementations. Other examples include:

  • miou with its priority queue (see the VOCAL project)
  • mirage-crypto with certain cryptographic primitives (see the fiat project)
  • and now utcp (see netsem)

  This effort is still in its early stages, but we welcome improvements and collaborations in this area.

mhttp

• Ecosystem and composability
  

  Although mhttp is a small project, it illustrates a broader principle behind our cooperative's approach to library development. As we have stated before, everything we build is designed to be reusable with schedulers other than Miou and in contexts other than unikernels. Regardless of which scheduler one prefers, we believe in a pragmatic, cooperative approach to contributing to the community.

  Our primary focus remains unikernel development, but years of experience building protocol and format libraries have taught us that keeping them scheduler-agnostic is in everyone's best interest, for maintainability, testability, and reliability alike. Wherever possible, you will find a general-purpose library alongside its unikernel counterpart: httpcats and mhttp are one such pair.

Vifu

let hello_world req _server () =
  let open Vifu.Response.Syntax in
  let* () = Vifu.Response.with_text req "Hello World!\n" in
  Vifu.Response.respond `OK

let run (ipv4, gateway, ipv6) =
  Mkernel.(run [ rng; Mnet.stack ~name:"service" ?gateway ~ipv6 ipv4 ])
  @@ fun rng (daemon, tcp, udp) ->
  let@ () = fun () -> Mnet.kill daemon in
  let@ () = fun () -> Mirage_crypto_rng_mkernel.kill rng in
  let cfg = Vifu.Config.v 80 in
  let routes = Vifu.[ get (rel /?? any) --> hello_world ] in
  Vifu.run ~cfg tcp routes ()

Unikernels

Conclusion

Installation Instructions

Fine-Tuned Compiler Configuration

opam update
opam switch create <switch_name> ocaml-variants.5.5.0~alpha1+options <option_list>

opam switch create 5.5.0~alpha1+flambda+nffa ocaml-variants.5.5.0~alpha1+options ocaml-option-flambda ocaml-option-no-flat-float-array

Why?

Current Status

Important Dates