| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::drop_back`. |
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| 4 | |
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| 5 | Copyright Louis Dionne 2013-2022 |
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| 6 | Distributed under the Boost Software License, Version 1.0. |
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| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 8 | */ |
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| 9 | |
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| 10 | #ifndef BOOST_HANA_FWD_DROP_BACK_HPP |
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| 11 | #define BOOST_HANA_FWD_DROP_BACK_HPP |
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| 12 | |
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| 13 | #include <boost/hana/config.hpp> |
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| 14 | #include <boost/hana/core/when.hpp> |
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| 15 | |
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| 16 | |
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| 17 | namespace boost { namespace hana { |
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| 18 | //! Drop the last `n` elements of a finite sequence, and return the rest. |
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| 19 | //! @ingroup group-Sequence |
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| 20 | //! |
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| 21 | //! Given a finite `Sequence` `xs` with a linearization of `[x1, ..., xm]` |
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| 22 | //! and a non-negative `IntegralConstant` `n`, `drop_back(xs, n)` is a |
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| 23 | //! sequence with the same tag as `xs` whose linearization is |
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| 24 | //! `[x1, ..., xm-n]`. If `n` is not given, it defaults to an |
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| 25 | //! `IntegralConstant` with a value equal to `1`. |
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| 26 | //! |
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| 27 | //! In case `length(xs) <= n`, `drop_back` will simply drop the whole |
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| 28 | //! sequence without failing, thus returning an empty sequence. |
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| 29 | //! |
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| 30 | //! |
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| 31 | //! @param xs |
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| 32 | //! The sequence from which elements are dropped. |
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| 33 | //! |
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| 34 | //! @param n |
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| 35 | //! A non-negative `IntegralConstant` representing the number of elements |
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| 36 | //! to be dropped from the end of the sequence. If `n` is not given, it |
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| 37 | //! defaults to an `IntegralConstant` with a value equal to `1`. |
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| 38 | //! |
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| 39 | //! |
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| 40 | //! Example |
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| 41 | //! ------- |
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| 42 | //! @include example/drop_back.cpp |
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| 43 | #ifdef BOOST_HANA_DOXYGEN_INVOKED |
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| 44 | constexpr auto drop_back = [](auto&& xs[, auto const& n]) { |
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| 45 | return tag-dispatched; |
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| 46 | }; |
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| 47 | #else |
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| 48 | template <typename S, typename = void> |
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| 49 | struct drop_back_impl : drop_back_impl<S, when<true>> { }; |
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| 50 | |
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| 51 | struct drop_back_t { |
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| 52 | template <typename Xs, typename N> |
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| 53 | constexpr auto operator()(Xs&& xs, N const& n) const; |
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| 54 | |
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| 55 | template <typename Xs> |
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| 56 | constexpr auto operator()(Xs&& xs) const; |
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| 57 | }; |
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| 58 | |
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| 59 | BOOST_HANA_INLINE_VARIABLE constexpr drop_back_t drop_back{}; |
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| 60 | #endif |
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| 61 | }} // end namespace boost::hana |
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| 62 | |
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| 63 | #endif // !BOOST_HANA_FWD_DROP_BACK_HPP |
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| 64 | |
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