OCaml 4.14.0, second alpha release

The release of OCaml 4.14.0 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/20501 .

If you find any bugs, please report them here:


Compared to the last alpha release, we have exceptionally a new feature for writing future-compatible early thread exit in 4.14.0 .
The second notable change is a major fix in the typechecker for escaping existential types when using GADTs, that might break existing code that relied on the broken types. However, there are currently no known instance of package broken by this change in the opam repository.
The other fixes are more usual and should be fairly safe.

Changes compared to the first alpha

The full list of new features and bug fixes, is available at:


New feature

  • 10951: Introduce the Thread.Exit exception as an alternative way to
    terminate threads prematurely. This alternative way will become
    the standard way in 5.00.
    (Xavier Leroy, review by Florian Angeletti)

Major new bug fix

  • 10907, 10959: Wrong type inferred from existential types
    (Jacques Garrigue and Gabriel Scherer, report by @dyzsr, review by Leo White)

New bug fixes

  • [additional fixes] 10596, +10978: Add with_open_bin, with_open_text and with_open_gen to
    In_channel and Out_channel. Also, add In_channel.input_all.
    (Nicolás Ojeda Bär, review by Daniel Bünzli, Jérémie Dimino, Damien Doligez
    and Xavier Leroy)
  • 10839: Fix regression of #show when printing class type
    (Élie Brami, review by Florian Angeletti)

  • 10836, 10952: avoid internal typechecker errors when checking signature
    inclusion in presence of incompatible types.
    (Florian Angeletti, report by Craig Ferguson, review by Gabriel Scherer)

Deprecation fine tuning

  • [additional fixes] 10675, +10937: Emit deprecation warnings when old C runtime function names
    are used. This will break C stub code that uses these old names and
    treats warnings as errors. The workaround is to use the new names.
    (Xavier Leroy and David Allsopp, review by Sébastien Hinderer and
    Damien Doligez)

Tools fine tuning

  • 10846: add the -shape command-line option to ocamlobjinfo. When reading a
    cmt file, shape information will only be shown if that option is used.
    (Ulysse Gérard, review by Florian Angeletti)

Installation instructions

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

opam update
opam switch create 4.14.0~alpha2 --repositories=default,beta=git+https://github.com/ocaml/ocaml-beta-repository.git

With opam 2.1, the previous command line can be simplified to

opam update
opam switch create 4.14.0~alpha2

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.4.14.0~alpha2+options,<option_list> --repositories=default,beta=git+https://github.com/ocaml/ocaml-beta-repository.git

or with opam 2.1:

opam update
opam switch create <switch_name> ocaml-variants.4.14.0~alpha2+options <option_list>

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 4.14.0~alpha2+flambda+nffa ocaml-variants.4.14.0~alpha2+options ocaml-option-flambda ocaml-option-no-flat-float-array

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: