hana/fwd/append.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::append`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_APPEND_HPP
0011 #define BOOST_HANA_FWD_APPEND_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Append an element to a monadic structure.
0019     //! @ingroup group-MonadPlus
0020     //!
0021     //! Given an element `x` and a monadic structure `xs`, `append` returns a
0022     //! new monadic structure which is the result of lifting `x` into the
0023     //! monadic structure and then combining that (to the right) with `xs`.
0024     //! In other words,
0025     //! @code
0026     //!     append(xs, x) == concat(xs, lift<Xs>(x))
0027     //! @endcode
0028     //! where `Xs` is the tag of `xs`. For sequences, this has the intuitive
0029     //! behavior of simply appending an element to the end of the sequence,
0030     //! hence the name.
0031     //!
0032     //! > #### Rationale for not calling this `push_back`
0033     //! > See the rationale for using `prepend` instead of `push_front`.
0034     //!
0035     //!
0036     //! Signature
0037     //! ---------
0038     //! Given a MonadPlus `M`, the signature is
0039     //! @f$ \mathtt{append} : M(T) \times T \to M(T) @f$.
0040     //!
0041     //! @param xs
0042     //! A monadic structure that will be combined to the left of the element.
0043     //!
0044     //! @param x
0045     //! An element to combine to the right of the monadic structure.
0046     //!
0047     //!
0048     //! Example
0049     //! -------
0050     //! @include example/append.cpp
0051 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0052     constexpr auto append = [](auto&& xs, auto&& x) {
0053         return tag-dispatched;
0054     };
0055 #else
0056     template <typename M, typename = void>
0057     struct append_impl : append_impl<M, when<true>> { };
0058
0059     struct append_t {
0060         template <typename Xs, typename X>
0061         constexpr auto operator()(Xs&& xs, X&& x) const;
0062     };
0063
0064     BOOST_HANA_INLINE_VARIABLE constexpr append_t append{};
0065 #endif
0066 }} // end namespace boost::hana
0067
0068 #endif // !BOOST_HANA_FWD_APPEND_HPP