Forcing a class to be unit-testable

[olleharstedt]

A class can be unit-tested if:

• All methods are pure; or
• All effectful calls are mockable (injected dependencies or wrappers, and no static access)

Could it be interesting to add an option to Psalm to force a class or folder of a project to be unit-testable?

Example:

/**
 * @psalm-unit-testable (or maybe just @psalm-testable)
 */
class UserService
{
  public function __construct($logger, $io)
  {
    $this->logger = $logger
    $this->io = $io;
  }

  public function notifyUser(User $user)
  {
    $this->logger('We are notifying a user');
    if (!emailIsValid($user->email)) {  // User-defined pure function
      throw new Exception("No valid email");  // Exceptions are ok
    }
    $this->io->email($user->email, 'Heyooo');
    // Doing things like file_get_contents($file) or echo("bla") would break unit-testability
    // Same with e.g. Logger::trace("Oh noe").
    // And of course super-global access, $_GET, etc.
  }
}

This would require that all built-in functions to PHP are marked as either pure or effectful. Don't know if this effort has been done already? Alternatively, it can assume that everything is effectful by default, unless tagged as pure.

Use-case of this functionality is to be able to force a divide in your code-base between blackbox and whitebox testing. Controller classes, for example, are often easier to blackbox test, while service classes with business logic could be whitebox tested. (Slightly related to "functional core, imperative shell" architectural pattern.)