#include <iostream>
#include <boost/fusion/include/adapted.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/transform.hpp>
#include <boost/fusion/include/as_vector.hpp>
#include <boost/fusion/include/size.hpp>
#include <boost/variant/variant.hpp>

struct params {
  double       d1;
  double       d2;
  double       d3;
  unsigned int i4;
  unsigned int i5;
  unsigned int i6;
  unsigned int i7;
};

BOOST_FUSION_ADAPT_STRUCT(
  params,
  (double, d1)
  (double, d2)
  (double, d3)
  (unsigned int, i4)
  (unsigned int, i5)
  (unsigned int, i6)
  (unsigned int, i7)
)


template <typename T1>
struct range {
  T1 min; 
  T1 max; 
  T1 inc;
};

template <typename T>
struct fixed_or_range {
  typedef boost::variant<T, range<T> > type;
};


struct make_fixed_or_range
{
	template<typename Sig> struct result;

	template<typename Self, typename Arg>
	struct result<Self(Arg)>
	{
    typedef typename fixed_or_range<Arg>::type type;
	};

	template<class Arg>
	typename result<make_fixed_or_range(Arg)>::type
	operator()(Arg);
};


typedef boost::fusion::result_of::as_vector<
  boost::fusion::result_of::transform<
    params,
    make_fixed_or_range
  >::type
>::type params_fixed_or_range;





int main()
{
  ...
  params_fixed_or_range pfr;   // now contains a sequence of fixed values or ranges
}