Function distance

example/tutorial/integral.cpp:105–109

//! [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]