ceph/src/boost/libs/hana/example/monadic_fold_right.cpp

   1 // Copyright Louis Dionne 2013-2017
   2 // Distributed under the Boost Software License, Version 1.0.
   3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   4
   5 #include <boost/hana/assert.hpp>
   6 #include <boost/hana/config.hpp>
   7 #include <boost/hana/div.hpp>
   8 #include <boost/hana/equal.hpp>
   9 #include <boost/hana/eval_if.hpp>
  10 #include <boost/hana/integral_constant.hpp>
  11 #include <boost/hana/lazy.hpp>
  12 #include <boost/hana/monadic_fold_right.hpp>
  13 #include <boost/hana/optional.hpp>
  14 #include <boost/hana/tuple.hpp>
  15 namespace hana = boost::hana;
  16
  17
  18 int main() {
  19     BOOST_HANA_CONSTEXPR_LAMBDA auto safe_div = [](auto x, auto y) {
  20         return hana::eval_if(y == hana::int_c<0>,
  21             hana::make_lazy(hana::nothing),
  22             [=](auto _) {
  23                 return hana::just(_(x) / y);
  24             }
  25         );
  26     };
  27
  28     // with an initial state
  29     BOOST_HANA_CONSTANT_CHECK(
  30         hana::monadic_fold_right<hana::optional_tag>(
  31             hana::tuple_c<int, 1000, 8, 4>, hana::int_c<2>, safe_div
  32         )
  33             ==
  34         hana::just(hana::int_c<1000> / (hana::int_c<8> / (hana::int_c<4> / hana::int_c<2>)))
  35     );
  36
  37     BOOST_HANA_CONSTANT_CHECK(
  38         hana::monadic_fold_right<hana::optional_tag>(
  39             hana::tuple_c<int, 1000, 8, 4>, hana::int_c<0>, safe_div
  40         )
  41             ==
  42         hana::nothing
  43     );
  44
  45     // without an initial state
  46     BOOST_HANA_CONSTANT_CHECK(
  47         hana::monadic_fold_right<hana::optional_tag>(
  48             hana::tuple_c<int, 1000, 8, 4, 2>, safe_div
  49         )
  50             ==
  51         hana::just(hana::int_c<1000> / (hana::int_c<8> / (hana::int_c<4> / hana::int_c<2>)))
  52     );
  53
  54     BOOST_HANA_CONSTANT_CHECK(
  55         hana::monadic_fold_right<hana::optional_tag>(
  56             hana::tuple_c<int, 1000, 8, 4, 0>, safe_div
  57         )
  58             ==
  59         hana::nothing
  60     );
  61 }