C++ Template Hanoi

hanoi.cpp

#include <iostream>
#include <utility>

template <int, typename T, T a, T b, T c>
struct hanoi;

template <typename seq1, typename seq2>
struct concat;

template <typename... Args>
struct sequence {};

template <typename T, T a, T b>
struct pair {
  using type = T;
  static constexpr auto first = a;
  static constexpr auto second = b;
};

template <typename... Args1, typename... Args2>
struct concat<sequence<Args1...>, sequence<Args2...>> {
  using type = sequence<Args1..., Args2...>;
};

template <typename T, T a, T b, T c>
struct hanoi<1, T, a, b, c> {
  using type = sequence<pair<T, a, c>>;
};

template <int n, typename T, T a, T b, T c>
struct hanoi {
  using type = typename concat<
                 typename concat<typename hanoi<n - 1, T, a, c, b>::type,
                                 sequence<pair<T, a, c>>
                                >::type,
                 typename hanoi<n - 1, T, b, a, c>::type
               >::type;
};

// prepend values before integer sequence
template <typename seq, typename seq::value_type...>
struct prepend;

template <typename T, T... x1, T... x2>
struct prepend<std::integer_sequence<T, x1...>, x2...> {
  using type = std::integer_sequence<T, x2..., x1...>;
};

template <typename T>
struct seq2str {};

template <typename T>
struct seq2str<sequence<T>> {
  using type = std::integer_sequence<typename T::type,
      T::first, '-', '>', T::second, '\n'>;
};

template <typename T, typename... Args>
struct seq2str<sequence<T, Args...>> {
  using type = typename prepend<
      typename seq2str<sequence<Args...>>::type,
      T::first, '-', '>', T::second, '\n'
      >::type;
};

template <typename T, T... xs>
void print_seq(std::integer_sequence<T, xs...>) {
    bool Do[] = { (std::cout << xs, true)... };
    (void)Do;
}

int main(int, const char*[]) {
  print_seq(typename seq2str<
              typename hanoi<3, char, '1', '2', '3'>
              ::type
            >::type ());
}