- Finalizing the documentation before tagging 1.0 release

docs/02-control-structure-function.md

@@ -0,0 +1,42 @@
+# Control structure - `function`
+
+Sometimes comes a need to have a small subset of code which is gonna be used twice (or more)
+in the same view. Instead of simply duplicating the code (with everything that can then go wrong if you ever need
+to play with it later), you could create a `function`.
+
+Functions are declared exactly like vanilla PHP.
+
+**[PICEA]** So, using this code:
+
+```html
+
+{% use Psr\Http\Message\ServerRequestInterface %}
+
+{% title "My generic title" %}
+
+{% function printCustomTitle(ServerRequestInterface $request) : bool %}
+    {% if $request->getAttribute('lean.route')->name === 'home' %}
+        <h4 class="title">This is a custom title !</h4>
+
+        {% return true %}
+    {% endif %}
+
+    {% return false %}
+{% endfunction %}
+
+{% if ! printCustomTitle($request) %}
+    <h1>{{ title() }}</h1>
+{% endif %}
+```
+
+**[HTML]** Would yield:
+
+*page **is** 'home'*
+```php
+<h4>This is a custom title !</h4>
+```
+
+*page **is not** 'home'*
+```php
+<h4>My generic title</h4>
+```