hana/fwd/cycle.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::cycle`.
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_CYCLE_HPP
0011 #define BOOST_HANA_FWD_CYCLE_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Combine a monadic structure with itself `n` times.
0019     //! @ingroup group-MonadPlus
0020     //!
0021     //! Given a monadic structure `xs` and a non-negative number `n`,
0022     //! `cycle` returns a new monadic structure which is the result of
0023     //! combining `xs` with itself `n` times using the `concat` operation.
0024     //! In other words,
0025     //! @code
0026     //!     cycle(xs, n) == concat(xs, concat(xs, ... concat(xs, xs)))
0027     //!                                       // ^^^^^ n times total
0028     //! @endcode
0029     //!
0030     //! Also note that since `concat` is required to be associative, we
0031     //! could also have written
0032     //! @code
0033     //!     cycle(xs, n) == concat(concat(... concat(xs, xs), xs), xs)
0034     //!                               // ^^^^^ n times total
0035     //! @endcode
0036     //!
0037     //! If `n` is zero, then the identity of `concat`, `empty`, is returned.
0038     //! In the case of sequences, this boils down to returning a sequence
0039     //! containing `n` copies of itself; for other models it might differ.
0040     //!
0041     //!
0042     //! Signature
0043     //! ---------
0044     //! Given an `IntegralConstant` `C` and a `MonadPlus` `M`, the signature is
0045     //! @f$ \mathrm{cycle} : M(T) \times C \to M(T) @f$.
0046     //!
0047     //! @param xs
0048     //! A monadic structure to combine with itself a certain number of times.
0049     //!
0050     //! @param n
0051     //! A non-negative `IntegralConstant` representing the number of times to
0052     //! combine the monadic structure with itself. If `n` is zero, `cycle`
0053     //! returns `empty`.
0054     //!
0055     //!
0056     //! Example
0057     //! -------
0058     //! @include example/cycle.cpp
0059 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0060     constexpr auto cycle = [](auto&& xs, auto const& n) {
0061         return tag-dispatched;
0062     };
0063 #else
0064     template <typename M, typename = void>
0065     struct cycle_impl : cycle_impl<M, when<true>> { };
0066
0067     struct cycle_t {
0068         template <typename Xs, typename N>
0069         constexpr auto operator()(Xs&& xs, N const& n) const;
0070     };
0071
0072     BOOST_HANA_INLINE_VARIABLE constexpr cycle_t cycle{};
0073 #endif
0074 }} // end namespace boost::hana
0075
0076 #endif // !BOOST_HANA_FWD_CYCLE_HPP