hana/fwd/replicate.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::replicate`.
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_REPLICATE_HPP
0011 #define BOOST_HANA_FWD_REPLICATE_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Create a monadic structure by combining a lifted value with itself
0019     //! `n` times.
0020     //! @ingroup group-MonadPlus
0021     //!
0022     //! Given a value `x`, a non-negative `IntegralConstant` `n` and the tag
0023     //! of a monadic structure `M`, `replicate` creates a new monadic structure
0024     //! which is the result of combining `x` with itself `n` times inside the
0025     //! monadic structure. In other words, `replicate` simply `lift`s `x` into
0026     //! the monadic structure, and then combines that with itself `n` times:
0027     //! @code
0028     //!     replicate<M>(x, n) == cycle(lift<M>(x), n)
0029     //! @endcode
0030     //!
0031     //! If `n` is zero, then the identity of the `concat` operation is returned.
0032     //! In the case of sequences, this corresponds to creating a new sequence
0033     //! holding `n` copies of `x`.
0034     //!
0035     //!
0036     //! Signature
0037     //! ---------
0038     //! Given an `IntegralConstant` `C` and MonadPlus `M`, the signature is
0039     //! @f$ \mathtt{replicate}_M : T \times C \to M(T) @f$.
0040     //!
0041     //! @tparam M
0042     //! The tag of the returned monadic structure. It must be a
0043     //! model of the MonadPlus concept.
0044     //!
0045     //! @param x
0046     //! The value to lift into a monadic structure and then combine with
0047     //! itself.
0048     //!
0049     //! @param n
0050     //! A non-negative `IntegralConstant` representing the number of times to
0051     //! combine `lift<M>(x)` with itself. If `n == 0`, `replicate` returns
0052     //! `empty<M>()`.
0053     //!
0054     //!
0055     //! Example
0056     //! -------
0057     //! @include example/replicate.cpp
0058 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0059     template <typename M>
0060     constexpr auto replicate = [](auto&& x, auto const& n) {
0061         return tag-dispatched;
0062     };
0063 #else
0064     template <typename M, typename = void>
0065     struct replicate_impl : replicate_impl<M, when<true>> { };
0066
0067     template <typename M>
0068     struct replicate_t {
0069         template <typename X, typename N>
0070         constexpr auto operator()(X&& x, N const& n) const;
0071     };
0072
0073     template <typename M>
0074     BOOST_HANA_INLINE_VARIABLE constexpr replicate_t<M> replicate{};
0075 #endif
0076 }} // end namespace boost::hana
0077
0078 #endif // !BOOST_HANA_FWD_REPLICATE_HPP