code: func trimFn(fn func(string, string) string, cutset string) BuiltinFunc {
  	return func(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  		if args[0].IsNull() {
  			return d, nil
  		}
  		str, err := args[0].ToString()
  		if err != nil {
  			return d, errors.Trace(err)
  		}
  		d.SetString(fn(str, cutset))
  		return d, nil
  	}
  }
docs: // For LTRIM & RTRIM
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_ltrim
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_rtrim