feat: implemented filter method

src/AbstractOptional.php

@@ -63,6 +63,25 @@ public function equals(mixed $obj): bool
         return ($obj === null || static::isSupported($obj)) && $this->value == $obj;
     }
 
+    /**
+     * @param callable(T): bool $predicate
+     *
+     * @return static
+     */
+    public function filter(callable $predicate): static
+    {
+        if ($this->value !== null) {
+            $matches = $predicate($this->value);
+            if (!is_bool($matches)) {
+                throw new InvalidArgumentException('Predicate must return boolean.');
+            }
+            if (!$matches) {
+                return static::empty();
+            }
+        }
+        return $this;
+    }
+
     /**
      * @template U of mixed
      *

tests/OptionalTest.php

@@ -76,6 +76,23 @@ public static function dataMethodEqualsWorks(): array
         ];
     }
 
+    #[DataProvider('dataMethodFilterWorks')]
+    public function testMethodFilterWorks(Optional $optional, bool $expected): void
+    {
+        self::assertEquals(
+            $expected ? $optional : Optional::empty(),
+            $optional->filter(static fn (string $value): bool => $value === self::VALUE),
+        );
+    }
+
+    public static function dataMethodFilterWorks(): array
+    {
+        return [
+            self::VALUE => [Optional::of(self::VALUE), true],
+            self::OTHER => [Optional::of(self::OTHER), false],
+        ];
+    }
+
     #[DataProvider('dataMethodFlatMapWorks')]
     public function testMethodFlatMapWorks(Optional $optional, Optional $expectedResult): void
     {