Add proveNull function

Author: klimick

composer.json

@@ -86,6 +86,7 @@
             "src/Fp/Functions/Collection/Tail.php",
             "src/Fp/Functions/Collection/Zip.php",
             "src/Fp/Functions/Collection/MkString.php",
+            "src/Fp/Functions/Evidence/ProveNull.php",
             "src/Fp/Functions/Evidence/ProveArray.php",
             "src/Fp/Functions/Evidence/ProveBool.php",
             "src/Fp/Functions/Evidence/ProveFloat.php",

src/Fp/Functions/Evidence/ProveNull.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Fp\Evidence;
+
+use Fp\Functional\Option\Option;
+
+/**
+ * Prove that subject is null.
+ *
+ * ```php
+ * >>> proveNull(null);
+ * => Some(null)
+ *
+ * >>> proveNull(1);
+ * => None
+ * ```
+ *
+ * @return Option<null>
+ */
+function proveNull(mixed $potential): Option
+{
+    return null === $potential ? Option::some($potential) : Option::none();
+}

tests/Runtime/Functions/Evidence/ProveNullTest.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Runtime\Functions\Evidence;
+
+use Fp\Functional\Option\Option;
+use PHPUnit\Framework\TestCase;
+
+use function Fp\Evidence\proveNull;
+
+final class ProveNullTest extends TestCase
+{
+    public function testProveNull(): void
+    {
+        $this->assertEquals(Option::none(), proveNull(1));
+        $this->assertEquals(Option::some(null), proveNull(null));
+    }
+}