code: private function resolveRootValue(ServerConfig $config, OperationParams $params, DocumentNode $doc, $operationType)
      {
          $root = $config->getRootValue();
  
          if (is_callable($root)) {
              $root = $root($params, $doc, $operationType);
          }
  
          return $root;
      }
docs: @param string $operationType
  
  @return mixed