If you iterate your data types, you should iterate your rules.
#include <iostream>
#include <string>
#include <boost/variant.hpp>
#include <boost/spirit/include/karma.hpp>
namespace karma = boost::spirit::karma;
typedef boost::variant<int, std::string> Item;
typedef std::vector<Item> ParameterList;
typedef boost::variant<int, std::string, ParameterList> Parameter;
int main()
{
using karma::int_;
using boost::spirit::ascii::string;
using karma::eol;
using karma::lit;
std::string generated;
std::back_insert_iterator<std::string> sink(generated);
karma::rule<std::back_insert_iterator<std::string>, Item()> itemRule =
int_ | (lit('"') << string << lit('"'));
karma::rule<std::back_insert_iterator<std::string>, ParameterList()>
parameterListRule = itemRule % lit(", ");
karma::rule<std::back_insert_iterator<std::string>, Parameter()>
parameterRule = (int_ | (lit('"') << string << lit('"')) | parameterListRule) << eol;
karma::generate(sink, parameterRule, 1);
karma::generate(sink, parameterRule, "foo");
karma::generate(sink, parameterRule, Parameter(ParameterList {1, "foo"}));
std::cout << generated;
return 0;
}