text: // Copyright Louis Dionne 2013-2017
  // Distributed under the Boost Software License, Version 1.0.
  // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
  
  #include <boost/mpl/equal_to.hpp>
  #include <boost/mpl/int.hpp>
  #include <boost/mpl/integral_c.hpp>
  #include <boost/mpl/minus.hpp>
  #include <boost/mpl/multiplies.hpp>
  #include <boost/mpl/pair.hpp>
  #include <boost/mpl/plus.hpp>
  
  #include <boost/hana/assert.hpp>
  #include <boost/hana/concept/constant.hpp>
  #include <boost/hana/equal.hpp>
  #include <boost/hana/integral_constant.hpp>
  #include <boost/hana/minus.hpp>
  #include <boost/hana/mult.hpp>
  #include <boost/hana/pair.hpp>
  #include <boost/hana/plus.hpp>
  
  #include <type_traits>
  namespace hana = boost::hana;
  
  
  namespace support {
  template <typename T, typename = std::enable_if_t<
    !hana::Constant<T>::value
  >>
  constexpr T sqrt(T x) {
    T inf = 0, sup = (x == 1 ? 1 : x/2);
    while (!((sup - inf) <= 1 || ((sup*sup <= x) && ((sup+1)*(sup+1) > x)))) {
      T mid = (inf + sup) / 2;
      bool take_inf = mid*mid > x ? 1 : 0;
      inf = take_inf ? inf : mid;
      sup = take_inf ? mid : sup;
    }
  
    return sup*sup <= x ? sup : inf;
  }
  
  template <typename T, typename = std::enable_if_t<
    hana::Constant<T>::value
  >>
  constexpr auto sqrt(T const&) {
    return hana::integral_c<typename T::value_type, sqrt(T::value)>;
  }
  } // end namespace support
  
  
  namespace then {
  namespace mpl = boost::mpl;
  
  template <typename N>
  struct sqrt
    : mpl::integral_c<typename N::value_type, support::sqrt(N::value)>
  { };
  
  template <typename X, typename Y>
  struct point {
    using x = X;
    using y = Y;
  };
  
  //! [distance-mpl]
  template <typename P1, typename P2>
  struct distance {
    using xs = typename mpl::minus<typename P1::x,
                                   typename P2::x>::type;
    using ys = typename mpl::minus<typename P1::y,
                                   typename P2::y>::type;
    using type = typename sqrt<
      typename mpl::plus<
        typename mpl::multiplies<xs, xs>::type,
        typename mpl::multiplies<ys, ys>::type
      >::type
    >::type;
  };
  
  static_assert(mpl::equal_to<
    distance<point<mpl::int_<3>, mpl::int_<5>>,
             point<mpl::int_<7>, mpl::int_<2>>>::type,
    mpl::int_<5>
  >::value, "");
  //! [distance-mpl]
  }
  
  
  namespace now {
  namespace hana = boost::hana;
  using namespace hana::literals;
  
  template <typename X, typename Y>
  struct _point {
    X x;
    Y y;
  };
  template <typename X, typename Y>
  constexpr _point<X, Y> point(X x, Y y) { return {x, y}; }
  
  using support::sqrt; // avoid conflicts with ::sqrt
  
  //! [distance-hana]
  template <typename P1, typename P2>
  constexpr auto distance(P1 p1, P2 p2) {
    auto xs = p1.x - p2.x;
    auto ys = p1.y - p2.y;
    return sqrt(xs*xs + ys*ys);
  }
  
  BOOST_HANA_CONSTANT_CHECK(distance(point(3_c, 5_c), point(7_c, 2_c)) == 5_c);
  //! [distance-hana]
  
  void test() {
  
  //! [distance-dynamic]
  auto p1 = point(3, 5); // dynamic values now
  auto p2 = point(7, 2); //
  BOOST_HANA_RUNTIME_CHECK(distance(p1, p2) == 5); // same function works!
  //! [distance-dynamic]
  
  }
  }
  
  
  int main() {
    now::test();
  }
  