Add bind() to option and result

tests/ResultTest.php

@@ -74,6 +74,31 @@ class ResultTest extends TestCase
         $this->assertFalse($result->isError(), 'The check for "Error" should have returned false');
     }
 
+    #[TestDox('Bind succeeds for OK with OK')]
+    public function testBindSucceedsForOKWithOK(): void
+    {
+        $result = Result::OK('one')->bind(fn($it) => Result::OK("$it two"));
+        $this->assertTrue($result->isOK(), 'The result should have been OK');
+        $this->assertEquals('one two', $result->getOK(), 'The bound function was not called');
+    }
+
+    #[TestDox('Bind succeeds for OK with Error')]
+    public function testBindSucceedsForOKWithError(): void
+    {
+        $result = Result::OK('three')->bind(fn($it) => Result::Error('back to two'));
+        $this->assertTrue($result->isError(), 'The result should have been Error');
+        $this->assertEquals('back to two', $result->getError(), 'The bound function was not called');
+    }
+
+    #[TestDox('Bind succeeds for Error')]
+    public function testBindSucceedsForError(): void
+    {
+        $original = Result::Error('oops');
+        $result   = $original->bind(fn($it) => Result::OK('never mind - it worked!'));
+        $this->assertTrue($result->isError(), 'The result should have been Error');
+        $this->assertSame($original, $result, 'The same Error result should have been returned');
+    }
+
     #[TestDox('Map succeeds for OK result')]
     public function testMapSucceedsForOKResult(): void
     {