interface Student
{
    public function id(): StudentId;
    public function isActive(): bool;
    public function name(): FullName;
}

enum ClassEnrollmentError: string {
    case InactiveStudent = 'inactive_student';
    case StudentAlreadyEnrolled = 'student_already_enrolled';
    case NoSeatsAvailable = 'no_seats_available';
}

class Class_
{
    private ClassId $id;
    private bool $finished;
    private int $numberOfSeats;
    /** @var StudentId[] */
    private array $students;
    
    /** @return Result<null, ClassEnrollmentError> */
    public function enroll(Student $student): Result
    {
        if (!$student->isActive()) {
            return err(ClassEnrollmentError::InactiveStudent);
        }
        if (in_iterable($student->id(), $this->students)) {
            return err(ClassEnrollmentError::StudentAlreadyEnrolled);
        }
        if (count($this->students) >= $this->numberOfSeats) {
            return err(ClassEnrollmentError::NoSeatsAvailable);
        }
        
        $this->students[] = $student->id();
        
        return ok();
    }
}

class ClassController
{
    // PUT /classes/{classId}/students/{studentId}
    public function enrollToClass(string $classId, string $studentId)
    {
        // ... fetch the class and the student
        return $class->enroll($student)
            ->andThen(fn() => $this->created()) // returns 201 if the result is ok
            ->getOrElse(fn(ClassEnrollmentError $error) => match ($error) {
                ClassEnrollmentError::StudentAlreadyEnrolled => $this->noContent(), // returns 204
                default => $this->badRequest($error), // returns 400 with the error
            });
    }
}

interface UserRepositoryInterface
{
    /** @return Option<User> */
    public function find(string $username): Option;
}

class UserController
{
    public function __construct(private UserRepositoryInterface $userRepository) {}

    // GET /users/{username}
    public function getUser(string $username)
    {
        return $this->userRepository->find($username)
            ->andThen(fn(User $user) => $this->userData($user)) // maps the user to its resource representation
            ->getOrElse(fn() => $this->notFound()); // returns 404 response in case of the "none" option
    }
}

/** @implements Equatable<Phone> */
class Phone implements Equatable
{
    public function __construct(private string $value) {}
    
    public function equals(self $other): bool
    {
        return $this->value === $other->value;
    }
}

assert(true === new Phone('(321) 456-1234')->equals(new Phone('(321) 456-1234')));
assert(false === new Phone('(321) 456-1234')->equals(new Phone('(454) 456-1234')));