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 HTTPS protocol, that is to put `https://` at the beginning of our `web_address` variable's value. 
Actually the `web_address`\ 's variable existence *requires* that the address include the HTTPS 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 or types parameters 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

Here we can see that the validation code is:

.. code-block:: jinja

    {{ _.custom_dns_url.startswith("http://") }}


A validator can return a string, in fact it is its default behavior.
If it returns a string of characters, it indicates the validation error.

In this case we see that we have specified the `return_type parameter`, which 
specifies that the Jinja code shall return a boolean value.

The validator's `description` parameter serves two purposes:

- it does, of course, have a string function for documenting validation,
- since the `return_type` parameter is set to boolean, 
  the error message contains this description's string.

.. type-along:: example with `http://` in the user data

Now, let's try with incorrect user data, containing `http://` and not `https://`

.. extinclude:: https://forge.cloud.silique.fr/stove/rougail-tutorials/raw/tag/v1.1_170/config/03/config.yml
   :language: yaml
   :caption: A :file:`config/03/config.yml` user data file containing `http://` 

..
    ---
    dns_over_https:
      enable_dns_over_https: true
      provider: Custom
      custom_dns_url: http://dns.net


Let's run the Rougail CLI: 

.. raw:: html
   :class: terminal
   :url: https://forge.cloud.silique.fr/stove/rougail-tutorials/raw/tag/v1.1_170/config/03/cmd_ro.txt

..
    rougail -m firefox/ --types types/proxy --modes_level basic standard advanced -u yaml -yf config/03/config.yml

The result is:

.. raw:: html
  :class: output 
  :url: https://forge.cloud.silique.fr/stove/rougail-tutorials/raw/tag/v1.1_170/config/03/output_ro.html

The warning indicates that the validator did not allow an HTTP protocol to pass:

::

        ┃   the value "http://dns.net" is an invalid URL, must starts with 
        ┃   'https://' only, it will be ignored when loading from the YAML file 
        ┃   "config/03/config.yml"

.. keypoints:: Let's review the key points

   We explained the use of validations to refine the type definition of a variable.

   The variable's parameter is `validator`.

