template<typename T>
std::string variant_container<T>::to_string_impl(bool annotate, bool annotate_empty, std::function<std::string(variant&)> mod_func) const;
{
    std::ostringstream ss;

    if(annotate) {
        ss << "[";
    }

    bool first_time = true;

    for(const auto& member : container_) {
        if(!first_time) {
            ss << ",";
        }

        first_time = false;

        if(type_ == TYPE_LIST) {
            ss << mod_func(member);
        } else if(type_ == TYPE_MAP) {
            ss << mod_func(member.first);
            ss << "->";
            ss << mod_func(member.second);
        }
    }

    // TODO: evaluate if this really needs to be separately conditional.
    if(annotate_empty && container_->empty()) {
        ss << "->";
    }

    if(annotate) {
        ss << "]";
    }

    return ss.str();
}