Adding the extension page to the documentation

docs/00-intro.md

@@ -9,9 +9,9 @@ Once installed you will be able to do the following:
 use Bakame\Http\StructuredFields\DataType;
 
 $fieldValue = 'text/html, application/xhtml+xml, application/xml;q=0.9, image/webp, */*;q=0.8';
-$field = DataType::List->parse($fieldValue);
-$field[1]->value()->toString(); // returns 'application/xhtml+xml'
-$field[1]->parameterByKey(key: 'q', default: 1.0); // returns 1.0 if the parameter is not defined
+$container = DataType::List->parse($fieldValue);
+$container[1]->value()->toString(); // returns 'application/xhtml+xml'
+$container[1]->parameterByKey(key: 'q', default: 1.0); // returns 1.0 if the parameter is not defined
 ```
 
 ## Motivation

docs/01-basic-usage.md

@@ -27,6 +27,8 @@ $permissions['picture-in-picture']->isEmpty(); // returns true because the list
 count($permissions['geolocation']);            // returns 2 the 'geolocation' feature has 2 values associated to it via a list
 $permissions['geolocation'][-1]->value();      // returns the last value of the list 'https://example.com/'
 $permissions['camera']->value();               // returns '*' the sole value attached to the 'camera' feature
+isset($permissions['yolo']);                   // returns false this permission does not exust
+$permissions->isEmpty();                       // returns false the dictionary contains some permissions
 ```
 
 > [!WARNING]

docs/02-parsing-serializing.md

@@ -107,7 +107,7 @@ $permissions['picture-in-picture']->isEmpty(); // picture-in-picture returned va
 count($permissions['geolocation']);            // geolocation returned value is an InnerList of 3 Items
 $permissions['geolocation'][1]->value();       // returns "https://example.com/"
 $permissions['camera']->value();               // camera only value is a single Item
-$permissions->getByIndex(2);                   // returns a tuple as n array ['camera', Item::fromPair(['*', []])]
+$permissions->getByIndex(2);                   // returns an array ['camera', Item::fromPair(['*', []])]
 ```
 
 In the example above:

docs/03-value-types.md

@@ -116,4 +116,4 @@ $displayString->type(); // returns Type::DisplayString
 > Values are not directly accessible. They can only be retrieved from an Item
 > Data type.
 
-← [Parsing and Serializing](02-parsing-serializing.md)  |  [Item API](04-item.md) →
+← [Parsing and Serializing](02-parsing-serializing.md)  |  [Item](04-item.md) →

docs/04-item.md

@@ -69,3 +69,8 @@ By default, you can access the member `Item` of a parameters using the following
 - `Item::parameters` returns a `Parameters` instance;
 - `Item::parameterByKey` returns the value of the bare item instance attached to the supplied `key`;
 - `Item::parameterByIndex` returns the value of the bare item instance attached to the supplied `index`;
+
+It is possible to alter and modify the `Parameters` attached to an `Item` but this section
+will be explored in the next section about the containers.
+
+← [Value Types](03-value-types.md)  |  [Containers](05-containers.md) →

docs/05-containers.md

@@ -457,4 +457,4 @@ echo InnerList::new('foo', 'bar')
 // ("foo" "bar");expire=@1681538756;path="/";max-age=2500
 ```
 
-← [Item](04-item.md)
+← [Item](04-item.md)  |  [Validation](06-validation.md) →

docs/06-validation.md

@@ -427,4 +427,4 @@ class it becomes easier to reuse it to validate your data.
 
 To show how this can be achieved you can check the codebase from [HTTP Cache Status](https://github.com/bakame-php/http-cache-status)
 
-← [Containers](05-containers.md) 
+← [Containers](05-containers.md)  | [Extending the package functionalities](07-extensions.md)

docs/07-extensions.md

@@ -0,0 +1,82 @@
+# Extension
+
+## Everything is final
+
+As of now everything is done within the package. But what about interacting with the
+PHP ecosystem as a whole. As a rule of thumb all the classes from this package are
+final and immutable and expose no particular interface.
+
+> [!IMPORTANT]
+> The `StructuredField` interface should be seen as an internal implementation detail 
+> and should not be implemented outside the package. While PHP does not prohibit such 
+> action. **You should simply not implement or rely on that interface.**
+
+The reason for disallowing the `StructuredField` interface is simple. We want to ensure
+that in any situation the RFC is being respected. The contract is between the RFC and your
+code the package only acts as a link between both parties.
+
+## Closed for extension opened for composition
+
+On the other hand to allow composition the package does expose the `StructuredFieldProvider` interface.
+
+```php
+interface StructuredFieldProvider
+{
+    /**
+     * Returns an object implementing the StructuredField interface.
+     */
+    public function toStructuredField(): Item|InnerList|OuterList|Dictionary|Parameters;
+}
+```
+
+This interface should return one of the DataType class and this is the interface that needs
+to be implemented. All the containers are able to work with a `StructuredFieldProvider`.
+
+Imagine you have an `AcceptHeaderItem` class and you want to take advantage of the package. You
+only have to implemente the `StructuredFieldProvider`. Once done, your class will be able to
+work with the `OuterList` class.
+
+```php
+class readonly AcceptHeaderItem
+{
+    public function __construct(
+        public string $value;
+        public float $quality = 1.0;
+    ) {}
+}
+```
+
+To use the package you will have to add the missing interface
+
+```php
+class readonly AcceptHeaderItem implements ServiceProvider
+{
+    public function __construct(
+        public string $value;
+        public float $quality = 1.0;
+    ) {}
+
+    public function toStructuredField(): Item
+    {
+        return Iten::fromToken($value)
+            ->withParaneters(
+                Parameter::new()
+                ->append('q', $this->quality),
+                ->filter(fn (array $pair): bool => $par[0] !== 'q' && 1.0 !== $pair[1]->value())
+            );
+    }
+}
+```
+
+Now this class can be used with the `OuterList` class to properly serialize an `Accept` header
+as mandated by the RFC.
+
+In the example provided we added the interface on the class itself but of course you are free to use
+a different approach, as long as you end up have a class that implements the `StructuredFieldProvider`
+contract.
+
+To show how this can be achieved you can check the codebase from [HTTP Cache Status](https://github.com/bakame-php/http-cache-status)
+which uses the interface. Of note by using this interface you can completely hide the presence of 
+this package to your end users if needed.
+
+← [Containers](06-validation.md)