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What does an OCaml version mean?

OCaml releases follow a linux-like scheme for their version string. The OCaml version string consists in three numbers, optionally followed by either a prerelease or development tag (%i.%i.%i[~alpha%i|~beta%i|~rc%i|+%s]). For example, 4.14.1, 5.1.0~alpha2 and 5.3.0+dev0-2023-12-22 are valid OCaml versions.

  • The first version number (4 in 4.14.1) is the major version of OCaml. This version number is updated when major new features are added to the OCaml language. For instance, OCaml 5 added shared memory parallelism and effect handlers and OCaml 4 introduced GADTs (Generalised Abstract Data Types).

  • The second version number (14 in 4.14.1) is the minor version of OCaml. This number is increased for every new release of OCaml. In particular, a new minor version of OCaml can contain breaking changes. However, we strive to maintain backward compatibility as much as possible.

  • The last number (1 in 4.14.1) is the bugfix number. Updating to the latest bugfix release is always safe, those bugfix versions are meant to be completely backward compatible and only contain important or very safe bug fixes.

  • The prerelease tag ~alpha%i, ~beta%i, ~rc%i (alpha2 in 5.1.0alpha2) describes a prerelease version of the compiler that is currently being tested. See [below](## Prerelease versions) for a more thorough explanation.

  • The development tag +tag indicates a development or experimental version of the compiler. +dev0-2023-12-22 in 5.3.0+dev0-2023-12-22 is an example of the tags of the form +dev%i-%date used by the compiler for its development versions.

When are new versions released?

Since OCaml 4.03, we are using a time-based release schedule: a new minor version of OCaml is released every six months.

For instance, at the date of writing, the next planned releases of OCaml are:

  • OCaml 5.2: around April 2024
  • OCaml 5.3: around October 2024

The timing is approximate, as we often delay a release to ensure quality when unforeseen issues come up. In consequence, releases are often late, typically by up to two months.

We may release bugfix releases at any time.

What happens between minor releases?

Feature freeze

All PRs go to the development branch of the compiler, called 'trunk'. (This was the standard name in the SVN era, and it remains more descriptive than 'main'.)

Three months before a new release, that is, at the half of the time window between two releases, we create a separate branch for the next version of OCaml. The intention (there are always exceptions) is that the release should correspond to the state of 'trunk' at the time this branch was created, but we wait three more months for quality analysis, to get feedback and integrate bugfixes.

We do not integrate new features in the release branch, to avoid unplanned regressions coming from last-minute changes. Only bugfixes and documentation improvements go to the release branch -- by cherry-picking them from 'trunk'.

We do not have the resources to maintain more than one dev branch, one prelease branch, and [one exceptional LTS branch](# Exceptional LTS versions).

Example

5.1.0 was released in September 2023, and the feature freeze for 5.2 happened on December 2023, for a 5.2 release planned around April 2024.

Prerelease versions

Once a new version branch has been created, and the feature set has been stabilised, we start publishing prerelease versions of this branch.

For instance, after branching OCaml 5.2, we start publishing alpha versions: for instance 5.2.0~alpha1, then 5.2.0~alpha2.

Once core development tools have been ported to work on those alpha versions, we switch to releasing beta versions (e.g. 5.2.0~beta1).

At last, just before the official release, we publish a release candidate (e.g. 5.2.0~rc1) to have one last check with the hopefully final version of the branch.

Our idea for alpha, beta, and rc releases is to have releases with increasing stability guarantees, and which can be tested by an increasingly wider audience:

  • alpha releases:

    • nearly stable internal compiler-libs API, only API fixes accepted
    • no new features, problematic features might be removed at this stage
    • bug fixes very welcome
    • documentation PRs still accepted
    • intended for core development tools (merlin, ppxlib, dune) to unlock the rest of the opam ecosystem

  • beta releases:

    • stable internal compiler-libs API
    • stable set of features
    • bug fixes very welcome
    • documentation PRs still accepted
    • intended for early adopters: opam library authors should be able to test their libraries at this point.

  • rc releases:

    • stable internal compiler-libs API
    • stable feature sets
    • only emergency bug fixes
    • documentation PRs postponed to after the release
    • intended for wide testing (and detecting deployment or production issues among large private code base)

Starting from the first alpha release, there is a small team effort to try to build and test every package available on opam with the new release, with the bulk of the work done by Kate Deplaix. This has been incredibly useful in the past to catch bugs or usability regressions.

Bugfix versions

Bugfix versions are published if we discover issues that significantly impede the use of the initially released version. In that situation, it is not uncommon that we backport safe bug fixes that were integrated in the trunk after the release.

Most bugfix releases are M.m.1 releases that happened one or two months after the M.m.0 minor release, to fix an important issue that was not found during prerelease testing.

Users are strongly encouraged to switch to the last bugfix versions as soon as possible. We make this easy by doing our best to avoid any regression there.

Exceptional LTS versions

Switching from OCaml 4 to OCaml 5 required a full rewrite of the OCaml runtime. This has negatively affected the stability of the releases of OCaml 5 in term of

  • supported architectures
  • supported OS
  • performance stability
  • number of runtime bugs

To keep a stable version easily available, we are exceptionally maintaining OCaml 4.14 as a long term support version of OCaml. New bugfix versions of OCaml 4.14 will be released in the future until OCaml 5 is considered mature enough.

User feedback is welcome on which fixes from OCaml 5 should be also included in 4.14.

Once OCaml 5 is stabilized, this extended support of OCaml 4.14 will stop. Currently, we expect to support OCaml 4.14 until OCaml 5.4 (around April 2025).

How are new versions of OCaml released?

The release process is documented in the release howto