Installing a Specific OCaml Compiler Version

TL;DR

Use opam switch set to manually select the switch to use and use dune-workspace to automatically run commands in different environment.

Compilation environments are managed with opam switches. The typical workflow is to have a local opam switch for the project, but you may need to select a different compilation environment (i.e. a different compiler version) sometimes. For instance, to run unit tests on an older/newer version of OCaml.

To do this, you'll need to create global opam switches. To create an opam switch with a given version of the compiler, you can use:

opam switch create 4.14.0 ocaml-base-compiler.4.14.0

This will create a new switch called 4.14.0 with the compiler version 4.14.0.

The list of available compiler version can be retrieved with:

opam switch list-available

This will list the available compiler version for all of the configured Opam repositories.

Once you've created a switch (or you already have a switch you'd like to use), you can run:

opam switch set <switch_name>
eval $(opam env)

to configure the current environment with this switch.

If it is a new switch, you will need to reinstall your dependencies (see "Installing dependencies") with opam install . --deps-only.

Alternatively, you may want to automatically run commands in a given set of compilation environments. To do this, you can create a file dune-workspace at the root of your project and list the opam switches you'd like to use there:

(lang dune 2.0)
(context (opam (switch 4.11.0)))
(context (opam (switch 4.12.0)))
(context (opam (switch 4.13.0)))

All the Dune commands you will run, will be run on all of the switches listed. For instance with the definition above:

dune runtest --workspace dune-workspace

Dune will run the tests for OCaml 4.11.0, 4.12.0 and 4.13.0.