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61 lines
1.5 KiB
C++
61 lines
1.5 KiB
C++
/*
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@copyright Louis Dionne 2014
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#include <boost/hana/detail/sandbox/lazy.hpp>
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#include <boost/hana/detail/static_assert.hpp>
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#include <boost/hana/functional.hpp>
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#include <boost/hana/integral.hpp>
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#include <boost/hana/iterable.hpp>
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using namespace boost::hana;
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struct LazyList;
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template <typename X, typename Xs>
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struct lcons_type {
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X x;
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Xs xs;
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using hana_datatype = LazyList;
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};
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auto lcons = [](auto x, auto xs) {
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return lcons_type<decltype(x), decltype(xs)>{x, xs};
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};
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auto repeat = fix([](auto repeat, auto x) {
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return lcons(x, sandbox::lazy<>(repeat)(x));
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});
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namespace boost { namespace hana {
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template <>
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struct Iterable<LazyList> : defaults<Iterable>::with<LazyList> {
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template <typename Xs>
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static constexpr auto head_impl(Xs lcons)
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{ return lcons.x; }
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template <typename Xs>
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static constexpr auto tail_impl(Xs lcons)
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{ return lcons.xs.eval(); }
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template <typename Xs>
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static constexpr auto is_empty_impl(Xs lcons)
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{ return false_; }
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};
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template <>
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constexpr bool foldable_from_iterable<LazyList> = true;
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template <>
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constexpr bool comparable_from_iterable<LazyList> = true;
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}}
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int main() {
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BOOST_HANA_STATIC_ASSERT(!is_empty(repeat(1)));
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BOOST_HANA_STATIC_ASSERT(head(repeat(1)) == 1);
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BOOST_HANA_STATIC_ASSERT(at(int_<10>, repeat(1)) == 1);
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}
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