Validating a variable's value
================================

.. objectives:: Objectives

   We will see how to create strong validations for the values ​​of our variables, 
   that is, validations that go beyond those inherent in the initial type definition.

.. prerequisites:: Prerequisites

    - We assume that Rougail's library is :ref:`installed <installation>` on your computer.

    - It is possible to retrieve the current state of the various Rougail files manipulated in this tutorial step
      by checking out the corresponding tag of the `rougail-tutorials` git repository.
      Each tag corresponds to a stage of progress in the tutorial.
      Of course, you can also decide to copy/paste or download the tutorial files contents while following the tutorial steps.

    If you want to follow this tutorial with the help of the corresponding :tutorial:`rougail-tutorials git repository <src/branch/v1.1>`,
    this workshop page corresponds to the tag :tutorial:`v1.1_170 <src/tag/v1.1_170/README.md>`
    in the repository.

    ::

       git clone https://forge.cloud.silique.fr/stove/rougail-tutorials.git
       git switch --detach v1.1_170

A variable with custom validation
-----------------------------------

First, we would like the :term:`operator` to remember not to forget the http protocol, that is to put `http://` at the beginning of our `web_address` variable's value. 
Actually the `web_address`\ 's type requires that the address include the HTTP protocol, namely:

::

    http(s)://<domain_name><directory>


However, in our use case, we want *to force* the HTTPS protocol. 
We don't want `http://`, only `https://`.

.. note:: This URL is for DNS over HTTPS, providing greater privacy than using the ISP's DNS, 
    which can filter traffic. If it uses `http://`, it doesn't make sense.

But there's no existing type for this specific use case (forcing HTTPS).

We just said that a valid value must start with `https://` instead of `http://`.
It is possible, instead of creating a new type, to add custom validation of the variable's value.
 
We call it a :term:`validator`.

.. glossary::

   validator
   
       A validator is a Jinja code that parses a variable's value and checks
       that this value corresponds to some stated criteria.
       This is a `validator` parameter of our variable, and another `jinja` sub-parameter
       contains validation code for the variable's value.

.. extinclude:: https://forge.cloud.silique.fr/stove/rougail-tutorials/raw/commit/v1.1_170/firefox/60-dns_over_https.yml
   :language: yaml
   :caption: The :file:`firefox/60-dns_over_https.yml` with the jinja validator