else if (is_binary_op(std::get<std::string>(tok)))
            {
                auto right = stack.top();
                stack.pop();
                if (!stack.empty())
                {
                    auto left = stack.top();
                    stack.pop();
                    auto op = std::get<std::string>(tok);
                    stack.push([left, right, op](double var_value) {return compute_binary_ops(left(var_value), right(var_value), op);});
                }
                else if(std::get<std::string>(tok) == "-")
                {
                    stack.push([right](double var_value) {return -(right(var_value));});
                }
                else
                {
                    throw parse_error("error in build func");
                }
            }