diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 04b188a..af44b43 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -36,3 +36,6 @@ jobs:
 
             -   name: "Psalm"
                 run: ./bin/psalm
+
+            -   name: "CodeSniffer"
+                run: ./bin/phpcs
diff --git a/composer.json b/composer.json
index 06cd05d..57a3469 100644
--- a/composer.json
+++ b/composer.json
@@ -28,7 +28,8 @@
         "phpunit/phpunit": "^9.0",
         "phpstan/extension-installer": "^1.0",
         "vimeo/psalm": "^4.0",
-        "friends-of-phpspec/phpspec-code-coverage": "^6.0"
+        "friends-of-phpspec/phpspec-code-coverage": "^6.0",
+        "squizlabs/php_codesniffer": "^3.6"
     },
     "autoload": {
         "psr-4": {
diff --git a/phpcs.xml b/phpcs.xml
new file mode 100644
index 0000000..b41d2c9
--- /dev/null
+++ b/phpcs.xml
@@ -0,0 +1,17 @@
+<?xml version="1.0"?>
+<ruleset name="Gowork">
+    <description>The coding standard of Gowork.</description>
+
+    <file>src</file>
+
+    <rule ref="PSR1"/>
+    <rule ref="PSR2">
+        <exclude name="PSR2.ControlStructures.ControlStructureSpacing" />
+        <exclude name="PSR2.Namespaces.UseDeclaration" />
+    </rule>
+    <rule ref="PSR12">
+        <exclude name="PSR12.Files.OpenTag" />
+        <exclude name="PSR12.Files.FileHeader" />
+    </rule>
+
+</ruleset>
diff --git a/src/ArrayValue.php b/src/ArrayValue.php
index 6327181..d5c9b38 100644
--- a/src/ArrayValue.php
+++ b/src/ArrayValue.php
@@ -121,9 +121,9 @@ public function offsetExists($offset): bool;
 
     /**
      * @param int $offset
-     * @phpstan-return TValue
+     * @return ?TValue
      */
-    public function offsetGet($offset);
+    public function offsetGet($offset): mixed;
 
     /**
      * @param int $offset
diff --git a/src/AssocArray.php b/src/AssocArray.php
index c97b204..c0da603 100644
--- a/src/AssocArray.php
+++ b/src/AssocArray.php
@@ -343,7 +343,7 @@ public function offsetExists($offset): bool
      * @param TKey $offset
      * @return ?TValue
      */
-    public function offsetGet($offset)
+    public function offsetGet($offset): mixed
     {
         return $this->items->toAssocArray()[$offset];
     }
diff --git a/src/AssocValue.php b/src/AssocValue.php
index 4e1e2e1..5afe328 100644
--- a/src/AssocValue.php
+++ b/src/AssocValue.php
@@ -70,10 +70,10 @@ public function reverse(): AssocValue;
     public function offsetExists($offset): bool;
 
     /**
-     * @phpstan-param TKey $offset
+     * @param TKey $offset
      * @return ?TValue
      */
-    public function offsetGet($offset);
+    public function offsetGet($offset): mixed;
 
     /**
      * @phpstan-param TKey $offset
diff --git a/src/InfiniteIterableValue.php b/src/InfiniteIterableValue.php
index 462c35e..51296f2 100644
--- a/src/InfiniteIterableValue.php
+++ b/src/InfiniteIterableValue.php
@@ -51,7 +51,7 @@ public function each(callable $callback): InfiniteIterableValue
     }
 
     /**
-     * @param (callable(TValue,TValue):int) | null $comparator
+     * @phpstan-param (callable(TValue,TValue):int) | null $comparator
      * @phpstan-return InfiniteIterableValue<TKey, TValue>
      */
     public function unique(?callable $comparator = null): InfiniteIterableValue
@@ -505,7 +505,9 @@ public function first()
     public function last()
     {
         $value = null;
-        foreach ($this->stack->iterate() as $value) {}
+        foreach ($this->stack->iterate() as $value) {
+            // iterating until the end
+        }
 
         return $value;
     }
diff --git a/src/PlainArray.php b/src/PlainArray.php
index 8025b7b..b7eb729 100644
--- a/src/PlainArray.php
+++ b/src/PlainArray.php
@@ -259,7 +259,7 @@ public function shift(&$value = null): PlainArray
         $items = $this->toArray();
         $value = array_shift($items);
 
-        return new self(new JustArray($items));;
+        return new self(new JustArray($items));
     }
 
     /**
@@ -413,7 +413,7 @@ public function offsetExists($offset): bool
      * @param int $offset
      * @return ?TValue
      */
-    public function offsetGet($offset)
+    public function offsetGet($offset): mixed
     {
         return $this->items->toArray()[$offset];
     }
diff --git a/src/PlainStringsArray.php b/src/PlainStringsArray.php
index e0b43b7..1f5373e 100644
--- a/src/PlainStringsArray.php
+++ b/src/PlainStringsArray.php
@@ -259,7 +259,7 @@ public function offsetExists($offset): bool
     /**
      * @param int $offset
      */
-    public function offsetGet($offset): StringValue
+    public function offsetGet($offset): ?StringValue
     {
         return $this->strings->offsetGet($offset);
     }
diff --git a/src/StringsArray.php b/src/StringsArray.php
index 5e8ae3f..4a2be75 100644
--- a/src/StringsArray.php
+++ b/src/StringsArray.php
@@ -103,7 +103,7 @@ public function offsetExists($offset): bool;
     /**
      * @param int $offset
      */
-    public function offsetGet($offset): StringValue;
+    public function offsetGet($offset): ?StringValue;
 
     /**
      * @param int $offset