parsing_list_example.cpp

template<typename Fn, typename ForEach>
std::optional<ParseError> ParseListConjugate(Fn fn, ForEach for_each,
                                    Token pair_sep = Token::Colon,
                                    Token next_pair = Token::Comma) {
  int index = 0;
  while (true) {
    auto first = (this->*fn)();
    if (!first) return first.error();
    if (!ConsumeToken(pair_sep.type))
      return ParseError(ParseError::Kind::MissingToken, "Requiring separator",
                        pair_sep);
    auto second = (this->*fn)();
    if (!second) return second.error();
    for_each(index, std::move(*first), std::move(*second));
    if (stream.PeekCur().type == next_pair.type) {
      index++;
      stream.PopCur();
    } else {
      break;
    }
  }
  return std::nullopt;
}

template<typename Fn, typename ForEach>
std::optional<ParseError> ParseList(Fn fn, ForEach for_each,
                                    Token continuer = Token::Comma) {
  int index = 0;
  while (true) {
    auto res = (this->*fn)();
    if (!res) return res.error();
    for_each(index, std::move(*res));
    if (stream.PeekCur().type == continuer.type) {
      index++;
      stream.PopCur();
    } else {
      break;
    }
  }
  return std::nullopt;
}