feat: document frontmatter (#58)

docs/docs/using-components.md

@@ -103,3 +103,204 @@ Similar to a [`fragment`](/docs/template-tags-filters#fragment) tag, a component
 Assigning component output to a variable cannot be done inside a `with` block.
 
 :::
+
+## Define types and defaults with front matter
+
+As [shown in Component context](#component-context) you can define
+default values by using the `var` tag:
+
+```twig title="button.html"
+{% var class=class|default:"btn btn-primary" %}
+<button {% attrs class %}>{{ children }}</button>
+```
+
+If the defaults are more complex structures, using the `default` filter may not be enough. In such case you would need to define a custom tag or a filter to generate the defaults.
+
+Another limit of using the `var` tag is that it doesn't check for types.
+
+In these cases, you can add a front matter with custom Python code to define the types and defaults of the component's props.
+
+Front matter is a block of code at the beginning of the file, which starts and ends with lines with 3 dashes:
+
+```twig
+---
+props.types = {
+    'required_string': str,
+    'optional_number': Optional[int],
+    'default_number': int,
+}
+props.defaults = {
+    'default_number': 10,
+}
+
+props['new_number'] = props['default_number'] * 2
+---
+
+The context contains:
+
+Required string: {{ required_string }}
+Optional number: {{ optional_number }}
+Default number: {{ default_number }}
+New number: {{ new_number }}
+```
+
+The code in the front matter is evaluated as a regular Python code.
+
+It exposes the `props` variable (instance of `slippers.props.Props`),
+which has 3 special features:
+
+### 1. `props.types` - If defined, component props are checked against types in `props.types` and raise `PropError` on mismatch.
+
+Using the previous example, if we set `required_string` to `int`,
+or it was `None`, this would raise `PropError`.
+
+Since the type for `optional_number` is `Optional`, we CAN set `optional_number` to `None`, but setting it to other type like `str` would raise `PropError`.
+
+Although `default_number` is NOT `Optional`, we provided a default value, so we won't raise an error if we don't provide the value (or provide `None`).
+
+However, if the default value of `default_number` was NOT `int` this would raise `PropError`.
+
+For convenience with defining the types, the front matter is already prepopulated with the `typing` module, effectively the same as calling:
+
+```py
+from typing import *
+```
+
+### 2. `props.defaults` - If defined, variables will fall back to these defaults if `None`.
+
+So the button example from [Component context](#component-context) can be rewritten as:
+
+```twig title="button.html"
+---
+props.defaults = {
+    'class': 'btn btn-primary'
+}
+---
+<button {% attrs class %}>{{ children }}</button>
+```
+
+And used as:
+
+```twig
+{# Usage #}
+{% button %}
+{# Output #}
+<button class="btn btn-primary"></button>
+
+{# Usage #}
+{% button class="btn btn-secondary" %}
+{# Output #}
+<button class="btn btn-secondary"></button>
+```
+
+### 3. Props mapping - Add or modify props with `props['prop_name']`
+
+To assign variables to the component's context, simply use:
+```python
+props['var_name'] = my_value
+```
+
+See the [next section](#preprocess-and-extend-component-context-with-front-matter)
+which delves deeper into this.
+
+## Preprocess and extend component context with front matter
+
+Sometimes, you may want to define some data internal to the component,
+like a static set of items in a menu.
+
+Normally, you'd have to define the items one by one:
+
+```twig title="menu.html"
+<div class="menu">
+  <h3>Menu</h3>
+  <div class="menu-item">
+    <a href="/path1">Link 1</a>
+    <a href="/path2">Link 2</a>
+    <a href="/path3">Link 3</a>
+  </div>
+</div>
+```
+
+Or pass them as props:
+
+```twig title="menu.html"
+<div class="menu">
+  <h3>Menu</h3>
+  <div class="menu-item">
+    {% for name, href in items %}
+    <a href="{{href}}">{{ name }}</a>
+    {% endfor %}
+  </div>
+</div>
+```
+
+However, if these variables are never meant to be changed, then providing them through the props
+doesn't make sense, as they will always be the same.
+
+It would clutter the component's interface, and make a room for error, as you now have to make sure
+to pass in all "static" variables every time you use the component:
+
+```twig
+{# You have to provide `items` every single time #}
+{% menu items=items %}
+{% menu items=items class="my_class" %}
+```
+
+Instead, you can make use of the front matter again, this time to
+preprocess, define, or even import variables:
+
+```twig title="menu.html"
+---
+from my_app.constants.icons import icon_bars_3
+
+props.types = {
+    'title_level': Optional[int],
+}
+props.defaults = {
+    'title_level': 3,
+}
+
+props['items'] = [
+  ('Link 1', '/path1'),
+  ('Link 2', '/path2'),
+  ('Link 3', '/path3')
+]
+
+props['title_tag'] = f'h{props['title_level']}'
+---
+<div class="menu">
+  {{icon_bars_3}}
+  <{{title_tag}}>Menu</{{title_tag}}>
+  <div class="menu-item">
+    {% for name, href in items %}
+    <a href="{{href}}">{{ name }}</a>
+    {% endfor %}
+  </div>
+</div>
+```
+
+Which can be used as:
+```twig
+{# Usage #}
+{% menu title_tag=5 %}
+
+{# Output #}
+<div class="menu">
+  <svg>
+    <path><!-- ... --></path>
+  </svg>
+  <h5>Menu</h5>
+  <div class="menu-item">
+    <a href="/path1">Link 1</a>
+    <a href="/path2">Link 2</a>
+    <a href="/path3">Link 3</a>
+  </div>
+</div>
+```
+
+To assign variables to the component's context, simply use:
+```python
+props['var_name'] = my_value
+```
+
+And since the code in the front matter is evaluated as a regular Python code, we can import other modules.