1// Copyright Louis Dionne 2013-2022
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>
15namespace hana = boost::hana;
16
17
18int 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}
62

source code of boost/libs/hana/example/monadic_fold_right.cpp