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/at.hpp>
7#include <boost/hana/bool.hpp>
8#include <boost/hana/drop_front.hpp>
9#include <boost/hana/eval.hpp>
10#include <boost/hana/front.hpp>
11#include <boost/hana/functional/fix.hpp>
12#include <boost/hana/functional/iterate.hpp>
13#include <boost/hana/fwd/at.hpp>
14#include <boost/hana/fwd/empty.hpp>
15#include <boost/hana/fwd/prepend.hpp>
16#include <boost/hana/integral_constant.hpp>
17#include <boost/hana/is_empty.hpp>
18#include <boost/hana/lazy.hpp>
19#include <boost/hana/not.hpp>
20#include <boost/hana/value.hpp>
21
22#include <cstddef>
23namespace hana = boost::hana;
24
25
26// A naive implementation of a lazy compile-time list, i.e. a tuple that can
27// potentially have an infinite number of elements (but that infinity must be
28// determinable at compile-time).
29
30struct LazyList;
31
32template <typename X, typename Xs>
33struct lazy_cons_type {
34 X x;
35 Xs xs;
36 using hana_tag = LazyList;
37};
38
39auto lazy_cons = [](auto x, auto xs) {
40 return lazy_cons_type<decltype(x), decltype(xs)>{x, xs};
41};
42
43struct lazy_nil_type { using hana_tag = LazyList; };
44
45constexpr lazy_nil_type lazy_nil{};
46
47auto repeat = hana::fix([](auto repeat, auto x) {
48 return lazy_cons(x, hana::make_lazy(repeat)(x));
49});
50
51namespace boost { namespace hana {
52 //////////////////////////////////////////////////////////////////////////
53 // Iterable
54 //////////////////////////////////////////////////////////////////////////
55 template <>
56 struct at_impl<LazyList> {
57 template <typename Xs, typename N>
58 static constexpr auto apply(Xs&& lcons, N const& n) {
59 return hana::drop_front(lcons, n).x;
60 }
61 };
62
63 namespace detail {
64 struct eval_tail {
65 template <typename Xs>
66 constexpr auto operator()(Xs const& lcons) const {
67 return hana::eval(lcons.xs);
68 }
69
70 constexpr auto operator()(lazy_nil_type const&) const {
71 return lazy_nil;
72 }
73 };
74 }
75
76 template <>
77 struct drop_front_impl<LazyList> {
78 template <typename Xs, typename N>
79 static constexpr auto apply(Xs&& lcons, N const&) {
80 return hana::iterate<N::value>(detail::eval_tail{}, lcons);
81 }
82 };
83
84 template <>
85 struct is_empty_impl<LazyList> {
86 template <typename Xs>
87 static constexpr auto apply(Xs const&)
88 { return hana::false_c; }
89
90 static constexpr auto apply(lazy_nil_type const&)
91 { return hana::true_c; }
92 };
93
94 //////////////////////////////////////////////////////////////////////////
95 // MonadPlus
96 //////////////////////////////////////////////////////////////////////////
97 template <>
98 struct prepend_impl<LazyList> {
99 template <typename Xs, typename X>
100 static constexpr auto apply(Xs xs, X x)
101 { return lazy_cons(x, hana::make_lazy(xs)); }
102 };
103
104 template <>
105 struct empty_impl<LazyList> {
106 static constexpr auto apply()
107 { return lazy_nil; }
108 };
109}}
110
111
112int main() {
113 BOOST_HANA_CONSTANT_CHECK(!hana::is_empty(repeat(1)));
114 BOOST_HANA_CONSTEXPR_CHECK(hana::front(repeat(1)) == 1);
115 BOOST_HANA_CONSTEXPR_CHECK(hana::at(repeat(1), hana::size_c<10>) == 1);
116}
117

source code of boost/libs/hana/example/misc/infinite_list.cpp