Doc page about using opam by default with dune pkg

Signed-off-by: Stephen Sherratt <[email protected]>

Author: gridbugs

doc/howto/index.rst

@@ -24,3 +24,4 @@ These guides will help you use Dune's features in your project.
    rule-generation
    override-default-entrypoint
    release-binaries-with-github-action
+   use-opam-alongside-dune-package-management

doc/howto/use-opam-alongside-dune-package-management.rst

@@ -0,0 +1,47 @@
+How to Use Opam Alongside Dune Package Management
+==================================================
+
+This guide will show you a workflow for gradually adopting Dune Package
+Management, while retaining the use of an Opam switch as the default way of
+resolving dependencies of a project. It assumes you have a project with
+dependencies, and are managing those dependencies with an Opam switch.
+By the end of this guide you'll have enabled Dune Package Management for your
+project, however by default Dune will still use an Opam switch to resolve your
+project's dependencies. By setting an environment variable or passing an extra
+argument to commands like ``dune build`` you'll be able to toggle between an
+Opam-based workflow and a purely Dune-based workflow.
+
+Create a file named ``dune-workspace`` at the root of your project, with the following
+contents:
+
+.. code:: scheme
+
+   (lang dune 3.20)
+
+   (pkg disabled)
+
+Since ``dune-workspace`` is the default workspace file, this tells Dune that by
+default, don't use its internal package management mechanism to resolve the
+project's dependencies, even in the presence of a lockdir.
+
+Now create a second file in the project root named ``dune-workspace.pkg``. The
+name of this file isn't important, but conventionally alternative workspace
+files begin with the ``dune-workspace.`` prefix. Add the following to this new
+file:
+
+.. code:: scheme
+
+   (lang dune 3.20)
+
+   (pkg enabled)
+
+With these two files in place, by default Dune will use the specified Opam
+switch to resolve your project's dependencies. However, setting the environment
+variable ``DUNE_WORKSPACE=dune-workspace.pkg`` will cause Dune to use
+its internal package management mechanism to resolve dependencies instead. Unset
+the ``DUNE_WORKSPACE`` environment variable to return to an Opam-based workflow.
+
+Alternatively to setting the ``DUNE_WORKSPACE`` environment variable, invoking
+commands like ``dune build``, ``dune exec``, and ``dune pkg lock`` with the
+argument ``--workspace=dune-workspace.pkg`` also has the effect of changing
+which workspace file Dune will use to the one which enables package management.