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Findlib Integration
===================
Dune integrates with `findlib`_ so that it is possible to use a dependency
built with Dune in a project that does not use Dune, or vice versa.
.. _findlib: https://github.com/ocaml/ocamlfind
.. seealso:: :doc:`../explanation/ocaml-ecosystem` explains the role of
findlib and its relation with Dune.
To do so, Dune both interprets and generates ``META`` files.
How Dune Interprets ``META`` files
----------------------------------
``META`` files use the concept of *predicates*, which can be used to change the
interpretation of the directives the files contain. However, Dune does not
expose this to the user.
Instead, Dune interprets ``META`` files assuming the following set of
predicates:
- ``mt``: refers to a library that can be used with or without threads. Dune
will force the threaded version.
- ``mt_posix``: forces the use of POSIX threads rather than VM threads. VM
threads are deprecated and will soon be obsolete.
- ``ppx_driver``: when a library acts differently depending on whether
it's linked as part of a driver or meant to add a ``-ppx`` argument
to the compiler, choose the former behavior.
The Special Case of the OCaml Compiler
--------------------------------------
Libraries installed by the compiler are a special case: when the OCaml compiler
is older than version 5.0, it does not include ``META`` files. In that
situation, Dune uses its own internal database.
How Dune Generates ``META`` Files
---------------------------------
When Dune builds a library, it generates a corresponding ``META`` file
automatically. Usually, there is nothing to do. However, for the rare cases
where a specific ``META`` file is needed, or to ease the transition of a
project to Dune, it is possible to write/generate a specific one.
If a ``META.<package>.template`` is found in the same directory as
``<package>.OPAM``, the generated ``META.<package>`` file will be produced by
starting from the contents of ``META.<package>.template`` and replacing lines
of the form ``# DUNE_GEN`` with the contents of the ``META`` it would normally
generate.
For instance, to add ``field = "..."`` to the generated ``META`` file of
package ``pkg``, you can create a file named ``META.pkg.template`` with the
following contents::
# DUNE_GEN
blah = "..."
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