Replace static with self

All these classes are final so static and self are always the same
class. The word self is easier to read.

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) {

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) {

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) {

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) {

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);
     }
 
 }

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) {

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);
     }
 
 }