[Doc] Add explanation for sites in its own file

doc/advanced-topics.rst

@@ -57,91 +57,6 @@ set of predicates:
   it is linked as part of a driver or meant to add a ``-ppx`` argument
   to the compiler, choose the former behavior
 
-.. _plugins:
-
-Plugins and dynamic loading of packages
-========================================
-
-Dune allows to define and load plugins without having to deal with specific
-compilation, installation directories, dependencies or the module `Dynlink`.
-Here we show an example of an executable which can be extended using plugins,
-and the definition of one plugin in another package.
-
-Example
--------
-
-Main executable (C)
-^^^^^^^^^^^^^^^^^^^^^
-
-- ``dune-project`` file:
-
-.. code:: scheme
-
-   (lang dune 2.8)
-   (name c)
-   (package (name c) (sites (lib plugins)))
-
-
-- ``dune`` file:
-
-.. code:: scheme
-
-   (executable
-    (public_name c)
-    (modules sites c)
-    (libraries c.register dune-site dune-site.plugins))
-
-   (library
-    (public_name c.register)
-    (name c_register)
-    (modules c_register))
-
-   (generate_module (module sites)  (plugins (c plugins)))
-
-- The module ``c_register.ml`` of the library ``c.register``:
-
-.. code:: ocaml
-
-   let todo = Queue.create ()
-
-- The code of the exectuable ``c.ml``:
-
-.. code:: ocaml
-
-   (* load all the available plugins *)
-   let () = Sites.Plugins.Plugins.load_all ()
-   (* Execute the code registered by the plugins *)
-   let () = Queue.iter (fun f -> f ()) !C_register.todo
-
-One plugin (B)
-^^^^^^^^^^^^^^
-
-- ``dune-project`` file:
-
-.. code:: scheme
-
-   (lang dune 2.8)
-   (name b)
-
-- ``dune`` file:
-
-.. code:: scheme
-
-  (library
-   (public_name b)
-   (libraries c.register))
-
-  (plugin
-   (name b)
-   (libraries b)
-   (site (c plugins)))
-
-- The code of the plugin ``b.ml``:
-
-.. code:: ocaml
-
-   let () = Queue.add (fun () -> print_endline "B is doing something") C_register.todo
-
 
 Dynamic loading of packages with findlib
 ========================================

doc/concepts.rst

@@ -1048,8 +1048,6 @@ the installation directory is either guessed or can be manually
 specified by the user.  This is described more in detail in the last
 section of this page.
 
-.. _sites:
-
 Sites of a package
 ------------------
 
@@ -1058,7 +1056,7 @@ could be hard to find. Moreover some packages could add resources to another
 package, for example in the case of plugins. These location are called sites in
 dune. One package can define them. During execution one site corresponds to a
 list of directories. They are like layers, the first directories have an higher
-priority.
+priority. Examples and precisions are available at :ref:`sites`.
 
 
 Libraries

doc/dune-files.rst

@@ -260,6 +260,9 @@ It contains the following fields:
   <url>)``, ``(documentation <url>)`` are the same (and take precedence over)
   the corresponding global fields. These fields are available since Dune 2.0.
 
+- ``(sites (<section> <name>) ...)`` define a site named ``<name>`` in the
+  section ``<section>``.
+
 Adding libraries to different packages is done via  ``public_name`` field. See
 :ref:`library` section for details.
 

doc/index.rst

@@ -19,6 +19,7 @@ Welcome to dune's documentation!
    foreign-code
    documentation
    jsoo
+   sites
    opam
    variants
    formatting

doc/sites.rst

@@ -0,0 +1,219 @@
+.. _sites:
+
+*****************
+Sites and Plugins
+*****************
+
+Sites
+=====
+
+Sites are a way for a package to define particular sub-directories in a :ref:`section<install>`
+(``lib``, ``share``, ``etc``, ... ) directory. The main difficulty is that those directories
+are not at the same location at different time:
+
+- When the package is available locally, the location is inside ``_build``
+- When the package is installed, the location is inside the install prefix
+- If a local package want to install files to the site of another installed
+  package the location is at the same time in ``_build`` and in the install prefix
+  of the second package.
+
+With the last example we see that the location of a site is not always uniq but
+could be layers of directories: ``["dir1";"dir2"]``. So a lookup must first look
+into "dir1", then into "dir2".
+
+Defining a site
+---------------
+
+A site is defined a package :ref:`package` in the ``dune-project`` file.
+
+.. code:: scheme
+
+   (lang gune 2.8)
+   (name mygui)
+   (package (name mygui) (sites (share themes)))
+
+Adding files to a site
+----------------------
+
+
+Here the package ``mygui`` defines a sites named ``themes`` it will be located
+in the section ``share``. This package can add files to this ``sites`` using the
+:ref:`install stanza<install>`:
+
+.. code:: scheme
+
+   (install
+     (section (site c themes))
+     (files
+       (layout.css as default/layout.css)
+       (ok.png  as default/ok.png)
+       (ko.png  as default/ko.png)
+     )
+   )
+
+Another package ``mygui_material_theme`` can install files inside ``mygui`` directory for adding a new
+theme. Inside the scope of ``mygui_material_theme`` the ``dune`` file:
+
+.. code:: scheme
+
+   (install
+     (section (site c themes))
+     (files
+       (layout.css as material/layout.css)
+       (ok.png  as material/ok.png)
+       (ko.png  as material/ko.png)
+     )
+   )
+
+The package ``mygui`` must be present in the workspace or installed.
+
+Getting the locations of a site at runtime
+------------------------------------------
+
+The executable ``mygui`` will be able to get the locations of the ``themes``
+site using the :ref:`generate module stanza<generate_module>`
+
+.. code:: scheme
+
+   (executable
+      (name mygui)
+      (module mygui mysites)
+   )
+
+   (generate_module (name mysites) (sites c))
+
+Then inside the ``mygui.ml`` module the locations can be recovered and used:
+
+.. code:: ocaml
+
+   (** Locations of the site for the themes *)
+   let themes_locations : string list = Mysites.Sites.themes
+
+   (** Merge the content of the directories in [dirs] *)
+   let rec readdirs dirs =
+     List.concat
+       (List.map
+          (fun dir -> Array.to_list (Sys.readdir dir))
+          (List.filter Sys.file_exists dirs))
+
+   (** Get the lists of the available themes  *)
+   let find_available_themes () : string list = lookup_dirs themes_locations
+
+   (** Lookup a file in the directories, priority on the first *)
+   let rec lookup_file filename = function
+     | [] -> raise Not_found
+     | dir::dirs ->
+        let filename' = Filename.concat dir filename in
+        if Sys.file_exists filename' then filename'
+        else lookup_file filename dirs
+
+   (** [lookup_theme_file theme file] get the [file] of the [theme] *)
+   let lookup_theme_file file theme =
+     lookup_file (Filename.concat theme file) themes_locations
+
+   let get_layout_css = lookup_theme_file "layout.css"
+   let get_ok_ico = lookup_theme_file "ok.png"
+   let get_ko_ico = lookup_theme_file "ko.png"
+
+
+Tests
+-----
+
+During tests the files are copied into the sites through the dependency
+``(package mygui)`` and ``(package mygui_material_theme)``.
+
+
+Installation
+------------
+
+Installation is done simply with ``dune install``, however if one want to
+install this tool such as it is relocatable. One can use ``dune
+install --relocatable --prefix $dir``. The files will be copied to the directory
+``$dir`` but the binary ``$dir/bin/mygui`` will get the site location relatively
+to its location. So even if the directory ``$dir`` is moved, ``themese_locations`` will
+be correct.
+
+.. _plugins:
+
+Plugins and dynamic loading of packages
+========================================
+
+Dune allows to define and load plugins without having to deal with specific
+compilation, installation directories, dependencies or the module `Dynlink`.
+Here we show an example of an executable which can be extended using plugins,
+and the definition of one plugin in another package.
+
+Example
+-------
+
+Main executable (C)
+^^^^^^^^^^^^^^^^^^^^^
+
+- ``dune-project`` file:
+
+.. code:: scheme
+
+   (lang dune 2.8)
+   (name c)
+   (package (name c) (sites (lib plugins)))
+
+
+- ``dune`` file:
+
+.. code:: scheme
+
+   (executable
+    (public_name c)
+    (modules sites c)
+    (libraries c.register dune-site dune-site.plugins))
+
+   (library
+    (public_name c.register)
+    (name c_register)
+    (modules c_register))
+
+   (generate_module (module sites)  (plugins (c plugins)))
+
+- The module ``c_register.ml`` of the library ``c.register``:
+
+.. code:: ocaml
+
+   let todo = Queue.create ()
+
+- The code of the exectuable ``c.ml``:
+
+.. code:: ocaml
+
+   (* load all the available plugins *)
+   let () = Sites.Plugins.Plugins.load_all ()
+   (* Execute the code registered by the plugins *)
+   let () = Queue.iter (fun f -> f ()) !C_register.todo
+
+One plugin (B)
+^^^^^^^^^^^^^^
+
+- ``dune-project`` file:
+
+.. code:: scheme
+
+   (lang dune 2.8)
+   (name b)
+
+- ``dune`` file:
+
+.. code:: scheme
+
+  (library
+   (public_name b)
+   (libraries c.register))
+
+  (plugin
+   (name b)
+   (libraries b)
+   (site (c plugins)))
+
+- The code of the plugin ``b.ml``:
+
+.. code:: ocaml
+
+   let () = Queue.add (fun () -> print_endline "B is doing something") C_register.todo