OCaml 4.13.0, second alpha release

The release of OCaml 4.13.0 is approaching. We have released a second alpha version to help fellow hackers join us early in our bug hunting and opam ecosystem fixing fun (see below for the installation instructions). You can see the progress on this front at https://github.com/ocaml/opam-repository/issues/18791 .

Beyond the usual bug fixes (see the full list below), this second alpha integrates a new feature for native code: poll points. Those poll points currently fixes some issues with signals in non-allocating loops
in native code. More importantly, they are prerequisite for the multicore runtime.

Another change is the removal of the removal of interbranch propagation of type information.
The feature, already postponed from 4.12, has been removed to focus for now on better error
message in the -principal mode.

If you find any bugs, please report them here:


The first beta release may follow soon since the opam ecosystem is in quite good shape;
and we are on track for a full release in September.

Happy hacking,
Florian Angeletti for the OCaml team.

Installation instructions

The base compiler can be installed as an opam switch with the following commands

opam update
opam switch create 4.13.0~alpha2 --repositories=default,beta=git+https://github.com/ocaml/ocaml-beta-repository.git

If you want to tweak the configuration of the compiler, you can switch to the option variant with:

opam update
opam switch create <switch_name> --packages=ocaml-variants.4.13.0~alpha2+options,<option_list> --repositories=default,beta=git+https://github.com/ocaml/ocaml-beta-repository.git

where <option_list> is a comma separated list of ocaml-option-* packages. For instance, for a flambda and no-flat-float-array switch:

opam switch create 4.13.0~alpha2+flambda+nffa --packages=ocaml-variants.4.13.0~alpha2+options,ocaml-option-flambda,ocaml-option-no-flat-float-array --repositories=default,beta=git+https://github.com/ocaml/ocaml-beta-repository.git

All available options can be listed with “opam search ocaml-option”.

If you want to test this version, it is advised to install the alpha opam repository


opam repo add alpha git://github.com/kit-ty-kate/opam-alpha-repository.git

This alpha repository contains various fixes in the process of being upstreamed.

The source code for the alpha is also available at these addresses:


Changes since the first alpha release

New feature

  • 10039: Safepoints
    Add poll points to native generated code. These are effectively
    zero-sized allocations and fix some signal and remembered set
    issues. Also multicore prerequisite.
    (Sadiq Jaffer, Stephen Dolan, Damien Doligez, Xavier Leroy,
    Anmol Sahoo, Mark Shinwell, review by Damien Doligez, Xavier Leroy,
    and Mark Shinwell)

New bug fixes

  • 10449: Fix major GC work accounting (the GC was running too fast).
    (Damien Doligez, report by Stephen Dolan, review by Nicolás Ojeda Bär and
    Sadiq Jaffer)

  • 10454: Check row_more in nondep_type_rec.
    (Leo White, review by Thomas Refis)

  • 10468: Correctly pretty print local type substitution, e.g. type t := …,
    with -dsource
    (Matt Else, review by Florian Angeletti)

  • 10461, 10498: caml_send* helper functions take derived pointers
    as arguments. Those must be declared with type Addr instead of Val.
    Moreover, poll point insertion must be disabled for caml_send*,
    otherwise the derived pointer is live across a poll point.
    (Vincent Laviron and Xavier Leroy, review by Xavier Leroy and Sadiq Jaffer)

  • 10478: Fix segfault under Windows due to a mistaken initialization of thread
    ID when a thread starts.
    (David Allsopp, Nicolás Ojeda Bär, review by Xavier Leroy)

  • 9525, 10402: ocamldoc only create paragraphq at the toplevel of
    documentation comments
    (Florian Angeletti, report by Hendrik Tews, review by Gabriel Scherer)

  • 10206: Split labels and polymorphic variants tutorials
    Splits the labels and polymorphic variants tutorial into two. Moves the GADTs
    tutorial from the Language Extensions chapter to the tutorials.
    (John Whitington, review by Florian Angeletti and Xavier Leroy)

Removed feature

  • [breaking change] 9811: remove propagation from previous branches
    Type information inferred from previous branches was propagated in
    non-principal mode. Revert this for better compatibility with
    -principal mode.
    For the time being, infringing code should result in a principality warning.
    (Jacques Garrigue, review by Thomas Refis and Gabriel Scherer)

The up-to-date list of changes for OCaml 4.13 is available at https://github.com/ocaml/ocaml/blob/4.13/Changes .