| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::prepend`. |
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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_PREPEND_HPP |
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| 11 | #define BOOST_HANA_FWD_PREPEND_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 | //! Prepend an element to a monadic structure. |
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| 19 | //! @ingroup group-MonadPlus |
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| 20 | //! |
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| 21 | //! Given a monadic structure `xs` and an element `x`, `prepend` returns |
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| 22 | //! a new monadic structure which is the result of lifting `x` into the |
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| 23 | //! monadic structure and then combining that (to the left) with `xs`. |
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| 24 | //! In other words, |
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| 25 | //! @code |
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| 26 | //! prepend(xs, x) == concat(lift<Xs>(x), xs) |
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| 27 | //! @endcode |
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| 28 | //! |
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| 29 | //! For sequences, this has the intuitive behavior of simply prepending |
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| 30 | //! an element to the beginning of the sequence, hence the name. |
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| 31 | //! |
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| 32 | //! > #### Rationale for not calling this `push_front` |
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| 33 | //! > While `push_front` is the de-facto name used in the standard library, |
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| 34 | //! > it also strongly suggests mutation of the underlying sequence, which |
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| 35 | //! > is not the case here. The author also finds that `push_front` |
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| 36 | //! > suggests too strongly the sole interpretation of putting an |
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| 37 | //! > element to the front of a sequence, whereas `prepend` is slightly |
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| 38 | //! > more nuanced and bears its name better for e.g. `hana::optional`. |
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| 39 | //! |
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| 40 | //! |
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| 41 | //! Signature |
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| 42 | //! --------- |
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| 43 | //! Given a MonadPlus `M`, the signature is |
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| 44 | //! @f$ \mathtt{prepend} : M(T) \times T \to M(T) @f$. |
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| 45 | //! |
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| 46 | //! @param xs |
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| 47 | //! A monadic structure that will be combined to the right of the element. |
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| 48 | //! |
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| 49 | //! @param x |
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| 50 | //! An element to combine to the left of the monadic structure. |
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| 51 | //! |
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| 52 | //! |
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| 53 | //! Example |
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| 54 | //! ------- |
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| 55 | //! @include example/prepend.cpp |
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| 56 | #ifdef BOOST_HANA_DOXYGEN_INVOKED |
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| 57 | constexpr auto prepend = [](auto&& xs, auto&& x) { |
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| 58 | return tag-dispatched; |
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| 59 | }; |
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| 60 | #else |
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| 61 | template <typename M, typename = void> |
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| 62 | struct prepend_impl : prepend_impl<M, when<true>> { }; |
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| 63 | |
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| 64 | struct prepend_t { |
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| 65 | template <typename Xs, typename X> |
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| 66 | constexpr auto operator()(Xs&& xs, X&& x) const; |
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| 67 | }; |
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| 68 | |
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| 69 | BOOST_HANA_INLINE_VARIABLE constexpr prepend_t prepend{}; |
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| 70 | #endif |
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| 71 | }} // end namespace boost::hana |
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| 72 | |
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| 73 | #endif // !BOOST_HANA_FWD_PREPEND_HPP |
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| 74 | |
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