I don't know if this actually makes sense as a way to make defining services easier. The idea is to be able to define a service only in a config. Without the need to define a class.

Example in yml:

var_dump_logger: >
        new class extends \Psr\Log\AbstractLogger {
        
            public function log($level, $message, array $context = array())
            {
                var_dump(['level' => $level, 'message' => $message, 'context' => $context]);
            }
        };

I don't think is a good way to define services, rather a quick way to play around with the DI component.