diff --git a/types/All.php b/types/All.php
index 0aba72b..80c802d 100644
--- a/types/All.php
+++ b/types/All.php
@@ -19,7 +19,7 @@ final class All implements Semigroup, Setoid, Monoid {
     public $value;
 
     public static function empty(): Monoid {
-        return new static(true);
+        return new self(true);
     }
 
     public function __construct(bool $value) {
diff --git a/types/Any.php b/types/Any.php
index db26532..876476f 100644
--- a/types/Any.php
+++ b/types/Any.php
@@ -19,7 +19,7 @@ final class Any implements Semigroup, Setoid, Monoid {
     public $value;
 
     public static function empty(): Monoid {
-        return new static(false);
+        return new self(false);
     }
 
     public function __construct(bool $value) {
diff --git a/types/Max.php b/types/Max.php
index 7d9ba69..520a2f7 100644
--- a/types/Max.php
+++ b/types/Max.php
@@ -19,7 +19,7 @@ final class Max implements Semigroup, Setoid, Monoid {
     public $value;
 
     public static function empty(): Monoid {
-        return new static(PHP_INT_MIN);
+        return new self(PHP_INT_MIN);
     }
 
     public function __construct(float $value) {
diff --git a/types/Min.php b/types/Min.php
index 53667ed..1e2bbc1 100644
--- a/types/Min.php
+++ b/types/Min.php
@@ -19,7 +19,7 @@ final class Min implements Semigroup, Setoid, Monoid {
     public $value;
 
     public static function empty(): Monoid {
-        return new static(PHP_INT_MAX);
+        return new self(PHP_INT_MAX);
     }
 
     public function __construct(float $value) {
diff --git a/types/Product.php b/types/Product.php
index e56b5d5..d5d0c7f 100644
--- a/types/Product.php
+++ b/types/Product.php
@@ -19,7 +19,7 @@ final class Product implements Semigroup, Setoid, Monoid {
     public $value;
 
     public static function empty(): Monoid {
-        return new static(1);
+        return new self(1);
     }
 
     public function __construct(float $value) {
@@ -34,7 +34,7 @@ public function concat(Semigroup $that): Semigroup {
         if (!$that instanceof self) {
             throw new \LogicException('Semigroup cannot concatenate two distinct types.');
         }
-        return new Product($this->value * $that->value);
+        return new self($this->value * $that->value);
     }
 
 }
diff --git a/types/StringM.php b/types/StringM.php
index 29a8837..37073d7 100644
--- a/types/StringM.php
+++ b/types/StringM.php
@@ -19,7 +19,7 @@ final class StringM implements Ord, Semigroup, Monoid {
     public $value;
 
     public static function empty(): Monoid {
-        return new static('');
+        return new self('');
     }
 
     public function __construct(string $value) {
diff --git a/types/Sum.php b/types/Sum.php
index a1e0627..2cf2a70 100644
--- a/types/Sum.php
+++ b/types/Sum.php
@@ -19,7 +19,7 @@ final class Sum implements Semigroup, Setoid, Monoid {
     public $value;
 
     public static function empty(): Monoid {
-        return new static(0);
+        return new self(0);
     }
 
     public function __construct(float $value) {
@@ -34,7 +34,7 @@ public function concat(Semigroup $that): Semigroup {
         if (!$that instanceof self) {
             throw new \LogicException('Semigroup cannot concatenate two distinct types.');
         }
-        return new Sum($this->value + $that->value);
+        return new self($this->value + $that->value);
     }
 
 }