namespace Symfony\Component\DependencyInjection\Loader\Configurator;

return static function (ContainerConfigurator $container) {
    $container->parameters()
        // ...
        ->set('mailer.transport', 'sendmail')
    ;

    $container->services()
        ->set('mailer', 'Mailer')
            ->args(['%mailer.transport%'])

    $services->set('mailer', 'Mailer')
        // the param() method was introduced in Symfony 5.2.
        ->args([param('mailer.transport')])
    ;

    $services->set('newsletter_manager', 'NewsletterManager')
        // In versions earlier to Symfony 5.1 the service() function was called ref()
        ->call('setMailer', [service('mailer')])
    ;
};