introduced Storage interface

More/Repository/MemoryStorage.php

@@ -2,7 +2,7 @@
 
 namespace DesignPatterns\More\Repository;
 
-class MemoryStorage
+class MemoryStorage implements Storage
 {
     /**
      * @var array

More/Repository/PostRepository.php

@@ -14,11 +14,11 @@ namespace DesignPatterns\More\Repository;
 class PostRepository
 {
     /**
-     * @var MemoryStorage
+     * @var Storage
      */
     private $persistence;
 
-    public function __construct(MemoryStorage $persistence)
+    public function __construct(Storage $persistence)
     {
         $this->persistence = $persistence;
     }

More/Repository/Storage.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace DesignPatterns\More\Repository;
+
+interface Storage
+{
+    public function persist(array $data): int;
+
+    public function retrieve(int $id): array;
+
+    public function delete(int $id);
+}