

.. _tutorial_directives:

Optionality and directives
==========================
In this chapter we're going to go over some of the more flexible constructs
which BuildStream offers for :ref:`optionality <project_options>`, and
show how we can use :ref:`directives <format_directives>` in the BuildStream
YAML format.

.. note::

   This example is distributed with BuildStream
   in the `doc/examples/directives
   <https://github.com/apache/buildstream/tree/master/doc/examples/directives>`_
   subdirectory.


Overview
--------
This chapter's example will build another ``hello.c`` program which much
resembles the program in the :ref:`running commands <tutorial_running_commands>` example,
but here we're going to make the greeting string *configurable* using the C
preprocessor.

We'll be compiling the following C file:


``files/src/hello.c``
~~~~~~~~~~~~~~~~~~~~~
.. literalinclude:: ../../examples/directives/files/src/hello.c
   :language: c

And we're going to build it using ``make``, using the following Makefile:


``files/src/Makefile``
~~~~~~~~~~~~~~~~~~~~~~

.. literalinclude:: ../../examples/directives/files/src/Makefile
   :language: Makefile

Notice the addition of ``-DGREETING_MESSAGE="\"${GREETING}\""`` in the above
Makefile, this will allow us to configure the greeting message from the
``hello.bst`` element declaration.

We will need to add support to our project for *optionality*, and we'll
have to make *conditional statements* to resolve what kind of greeting
we want from the hello world program.


Project structure
-----------------
Since this project has much the same structure as the
:ref:`running commands <tutorial_running_commands>` chapter did, we won't go over all of
these elements in detail. Instead let's focus on the addition of the new
:ref:`project options <project_options>` in ``project.conf``, the added
file in the ``include/`` project subdirectory, and how these come together
in the the ``hello.bst`` element.


``project.conf``
~~~~~~~~~~~~~~~~

.. literalinclude:: ../../examples/directives/project.conf
   :language: yaml

Here, our ``project.conf`` declares a project option called ``flavor``, and this
will inform what kind of greeting message we want to use when building the project.


``elements/hello.bst``
~~~~~~~~~~~~~~~~~~~~~~

.. literalinclude:: ../../examples/directives/elements/hello.bst
   :language: yaml

Notice the ``(@)`` symbol we've added in the ``variables:`` section, this
symbol is used to invoke the :ref:`include directive <format_directives_include>`,
which can be useful for code sharing between elements or simply to improve readability.

In this case, we are compositing the content of ``include/greeting.bst`` into the
:ref:`variables <format_variables>` section of the element declaration, directives
can however be used virtually anywhere in the BuildStream YAML format.


``include/greeting.bst``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. literalinclude:: ../../examples/directives/include/greeting.bst
   :language: yaml

Here we can see the dictionary which will be composited into the ``variables:``
section of the ``hello.bst`` element described above.

Note the usage of the ``(?)`` symbol at the toplevel of the YAML dictionary,
this is how we perform :ref:`conditional statements <format_directives_conditional>`
in the BuildStream YAML format.

This include file uses the ``flavor`` project option we declared in ``project.conf`` to
decide what value will end up being assigned to the ``%{greeting}`` variable, which
will ultimately be used in the ``hello.bst`` element.


Using the project
-----------------
Now that we have a project which uses options and conditional statements,
lets build the project with a few different options and observe the outputs.


Building hello.bst element with options
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Since the :ref:`flavor option <project_options>` we've declared above
has a default, we can build it the first time using :ref:`bst build <invoking_build>`
without any special command line options:

.. raw:: html
   :file: ../sessions/directives-build-normal.html

If we want to build the ``somber`` flavor, we just need to specify the
additional ``--option`` command line option to :ref:`bst <invoking_bst>`
in order to inform BuildStream of the options we want.

.. raw:: html
   :file: ../sessions/directives-build-somber.html

Note that the ``--option`` option can be specified many times on the
``bst`` command line, so as to support projects which have multiple
options.

Finally lets get the ``excited`` flavor built as well:

.. raw:: html
   :file: ../sessions/directives-build-excited.html

If you observe the cache keys above, you will notice that while
we have only three elements in the pipeline, counting ``base/alpine.bst``,
``base.bst`` and ``hello.bst``, we have actually built *five artifacts*,
because the ``hello.bst`` is built differently each time, it has a
different cache key and is stored separately in the artifact cache.


Run the hello world program with options
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Since the ``--option`` command line option to :ref:`bst <invoking_bst>`
is a main option, it can be used in any command.

Let's run the ``hello`` program using :ref:`bst shell <invoking_shell>`
three times in a row, each time using a different option so we can
observe the results.


.. raw:: html
   :file: ../sessions/directives-shell-normal.html


.. raw:: html
   :file: ../sessions/directives-shell-somber.html


.. raw:: html
   :file: ../sessions/directives-shell-excited.html


Summary
-------
In this chapter we've demonstrated how to declare :ref:`project options <project_options>`,
how to use :ref:`conditional directives <format_directives_conditional>`, and also
how to use :ref:`include directives <format_directives_include>`.

To get more familliar with these concepts, you may want to explore the remaining
:ref:`directives <format_directives>` in the BuildStream YAML format, and also take
a look at the various :ref:`types of project options <project_options>` that
are also supported.