code: private function parseOperationType()
      {
          $operationToken = $this->expect(Token::NAME);
          switch ($operationToken->value) {
              case 'query':
                  return 'query';
              case 'mutation':
                  return 'mutation';
              case 'subscription':
                  return 'subscription';
          }
  
          throw $this->unexpected($operationToken);
      }
docs: @return string
  
  @throws SyntaxError