***************
Advanced topics
***************

This section describes some details of dune for advanced users.

META file generation
====================

Dune uses ``META`` files from the `findlib library
manager <http://projects.camlcity.org/projects/findlib.html>`__ in order
to interoperate with the rest of the world when installing libraries. It
is able to generate them automatically. However, for the rare cases
where you would need a specific ``META`` file, or to ease the transition
of a project to dune, it is allowed to write/generate a specific
one.

In order to do that, write or setup a rule to generate a
``META.<package>.template`` file in the same directory as the
``<package>.opam`` file. Dune will generate a ``META.<package>``
file from the ``META.<package>.template`` file by replacing lines of
the form ``# JBUILDER_GEN`` by the contents of the ``META`` it would
normally generate.

For instance if you want to extend the ``META`` file generated by
dune you can write the folliwing ``META.foo.template`` file:

.. code::

   # JBUILDER_GEN
   blah = "..."

Findlib integration and limitations
===================================

Dune uses ``META`` files to support external libraries. However, it
doesn't export the full power of findlib to the user, and especially
it doesn't let the user specify *predicates*.

The reason for this limitation is that so far they haven't been
needed, and adding full support for them would complicate things quite
a lot. In particular, complex ``META`` files are often hand-written and
the various features they offer are only available once the package is
installed, which goes against the root ideas dune is built on.

In practice, dune interprets ``META`` files assuming the following
set of predicates:

- ``mt``: what this means is that using a library that can be used
  with or without threads with dune will force the threaded
  version

- ``mt_posix``: forces the use of posix threads rather than VM
  threads. VM threads are deprecated and are likely to go away soon

- ``ppx_driver``: when a library acts differently depending on whether
  it is linked as part of a driver or meant to add a ``-ppx`` argument
  to the compiler, choose the former behavior

Dynamic loading of packages
===========================

Dune supports the ``findlib.dynload`` package from `findlib
<http://projects.camlcity.org/projects/findlib.html>`_ that allows to
dynamically load packages and their dependencies (using OCaml Dynlink module).
So adding the ability for an application to have plugins just requires to add
``findlib.dynload`` to the set of library dependencies:

.. code:: scheme

    (library
      (name mytool)
      (public_name mytool)
      (modules ...)
    )

    (executable
      (name main)
      (public_name mytool)
      (libraries mytool findlib.dynload)
      (modules ...)
    )


Then you could use in your application ``Fl_dynload.load_packages l``
that will load the list ``l`` of packages. The packages are loaded
only once. So trying to load a package statically linked does nothing.

A plugin creator just need to link to your library:

.. code:: scheme

    (library
      (name mytool_plugin_a)
      (public_name mytool-plugin-a)
      (libraries mytool)
    )

By choosing some naming convention, for example all the plugins of
``mytool`` should start with ``mytool-plugin-``. You can automatically
load all the plugins installed for your tool by listing the existing packages:

.. code:: ocaml

    let () = Findlib.init ()
    let () =
      let pkgs = Fl_package_base.list_packages () in
      let pkgs =
        List.filter
          (fun pkg -> 14 <= String.length pkg && String.sub pkg 0 14 = "mytool-plugin-")
          pkgs
      in
      Fl_dynload.load_packages pkgs

.. _advanced-cross-compilation:

Cross Compilation
=================

Dune allows for cross compilation by defining build contexts with
multiple targets. Targets are specified by adding a ``targets`` field
to the definition of a build context.

``targets`` takes a list of target name. It can be either:

- ``native`` which means using the native tools that can build
  binaries that run on the machine doing the build

- the name of an alternative toolchain

Note that at the moment, there is no official support for
cross-compilation in OCaml. Dune supports the opam-cross-x
repositories from the `ocaml-cross organization on github
<https://github.com/ocaml-cross/>`_, such as:

- `opam-cross-windows <https://github.com/ocaml-cross/opam-cross-windows>`_
- `opam-cross-android <https://github.com/ocaml-cross/opam-cross-android>`_
- `opam-cross-ios <https://github.com/ocaml-cross/opam-cross-ios>`_

In particular:

- to build Windows binaries using opam-cross-windows, write ``windows``
  in the list of targets
- to build Android binaries using opam-cross-android, write
  ``android`` in the list of targets
- to build IOS binaries using opam-cross-ios, write ``ios`` in the
  list of targets

For example, the following workspace file defines three different
targets for the ``default`` build context:

.. code:: scheme

    (context (default (targets (native windows android))))

This configuration defines three build contexts:

- ``default``
- ``default.windows``
- ``default.android``

Note that the ``native`` target is always implicitly added when not
present. However, when implicitly added ``dune build @install``
will skip this context, i.e. ``default`` will only be used for
building executables needed by the other contexts.

With such a setup, calling ``dune build @install`` will build all
the packages three times.

Note that instead of writing a ``dune-workspace`` file, you can also
use the ``-x`` command line option. Passing ``-x foo`` to ``dune``
without having a ``dune-workspace`` file is the same as writing the
following ``dune-workspace`` file:

.. code:: scheme

   (context (default (targets (foo))))

If you have a ``dune-workspace`` and pass a ``-x foo`` option,
``foo`` will be added as target of all context stanzas.

How does it work?
-----------------

In such a setup, binaries that need to be built and executed in the
``default.windows`` or ``default.android`` contexts as part of the
build, will no longer be executed. Instead, all the binaries that will
be executed will come from the ``default`` context. One consequence of
this is that all preprocessing (ppx or otherwise) will be done using
binaries built in the ``default`` context.

To clarify this with an example, let's assume that you have the following
``src/dune`` file:

.. code:: scheme

    (executable (name foo))
    (rule (with-stdout-to blah (run ./foo.exe)))

When building ``_build/default/src/blah``, dune will resolve ``./foo.exe`` to
``_build/default/src/foo.exe`` as expected. However, for
``_build/default.windows/src/blah`` dune will resolve ``./foo.exe`` to
``_build/default/src/foo.exe``

Assuming that the right packages are installed or that your workspace
has no external dependencies, dune will be able to cross-compile a
given package without doing anything special.

Some packages might still have to be updated to support cross-compilation. For
instance if the ``foo.exe`` program in the previous example was using
``Sys.os_type``, it should instead take it as a command line argument:

.. code:: scheme

  (rule (with-stdout-to blah (run ./foo.exe -os-type %{os_type})))

Classical ppx
=============

*classical ppx* refers to running ppx using the -ppx compiler option, which is
composed using Findlib. Even though this is useful to run some (usually old)
ppx's which don't support drivers, dune does not support preprocessing with
ppx this way. but a workaround exists using the `ppxfind
<https://github.com/diml/ppxfind>`_ tool.
