if ( !Auth::guest() )
{
  return Redirect::to('/dashboard');
}

class Auth extends Facade {
  /**
   * Get the registered name of the component.
   *
   * @return string
   */
  protected static function getFacadeAccessor() { return 'auth'; }
}

$this->app->bindShared('auth', function($app)
{
  // Once the authentication service has actually been requested by the developer
  // we will set a variable in the application indicating such. This helps us
  // know that we need to set any queued cookies in the after event later.
  $app['auth.loaded'] = true;
  return new AuthManager($app);
});

Illuminate\Support\Facades\Auth::guest()
Illuminate\Support\Facades\Facade::__callStatic
Illuminate\Auth\AuthManager->guest()
Illuminate\Support\Manager->__call
public function __call($method, $parameters)
{
    return call_user_func_array(array($this->driver(), $method), $parameters);
}

public function driver($driver = null)
{
    $driver = $driver ?: $this->getDefaultDriver();
    // If the given driver has not been created before, we will create the instances
    // here and cache it so we can return it next time very quickly. If there is
    // already a driver created by this name, we'll just return that instance.
    if ( ! isset($this->drivers[$driver]))
    {
      $this->drivers[$driver] = $this->createDriver($driver);
    }
    return $this->drivers[$driver];
}

/**
* Get the default authentication driver name.
*
* @return string
*/
public function getDefaultDriver()
{
    return $this->app['config']['auth.driver'];
}

'driver' => 'eloquent'

public function createEloquentDriver()
{
    $provider = $this->createEloquentProvider();
    return new Guard($provider, $this->app['session.store']);
}

public function user()
{
    if ($this->loggedOut) return;
    // If we have already retrieved the user for the current request we can just
    // return it back immediately. We do not want to pull the user data every
    // request into the method because that would tremendously slow an app.
    if ( ! is_null($this->user))
    {
      return $this->user;
    }
    $id = $this->session->get($this->getName());
    // First we will try to load the user using the identifier in the session if
    // one exists. Otherwise we will check for a "remember me" cookie in this
    // request, and if one exists, attempt to retrieve the user using that.
    $user = null;
    if ( ! is_null($id))
    {
      //provider為EloquentUserProvider
     $user = $this->provider->retrieveByID($id);
    }
    // If the user is null, but we decrypt a "recaller" cookie we can attempt to
    // pull the user data on that cookie which serves as a remember cookie on
    // the application. Once we have a user we can return it to the caller.
    $recaller = $this->getRecaller();
    if (is_null($user) && ! is_null($recaller))
    {
      $user = $this->getUserByRecaller($recaller);
    }
    return $this->user = $user;
}