text: /*
  @copyright Louis Dionne 2014
  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/hana/functional.hpp>
  
  #include <boost/hana/detail/assert.hpp>
  #include <boost/hana/detail/constexpr.hpp>
  #include <boost/hana/integral.hpp>
  using namespace boost::hana;
  
  
  BOOST_HANA_CONSTEXPR_LAMBDA auto fact = fix(
      [](auto fact, auto n) {
          return eval_if(n == int_<0>,
              always(int_<1>),
              [=](auto id) { return n * fact(n - id(int_<1>)); }
          );
      }
  );
  
  constexpr unsigned long long reference(unsigned long long n)
  { return n == 0 ? 1 : n * reference(n - 1); }
  
  template <int n>
  constexpr void test() {
      BOOST_HANA_CONSTANT_ASSERT(fact(ullong<n>) == ullong<reference(n)>);
      test<n - 1>();
  }
  
  template <> constexpr void test<-1>() { }
  
  int main() {
      test<15>();
  }
  