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- /*!
- @file
- Forward declares `boost::hana::unfold_right`.
- @copyright Louis Dionne 2013-2017
- Distributed under the Boost Software License, Version 1.0.
- (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
- */
- #ifndef BOOST_HANA_FWD_UNFOLD_RIGHT_HPP
- #define BOOST_HANA_FWD_UNFOLD_RIGHT_HPP
- #include <boost/hana/config.hpp>
- #include <boost/hana/core/when.hpp>
- BOOST_HANA_NAMESPACE_BEGIN
- //! Dual operation to `fold_right` for sequences.
- //! @ingroup group-Sequence
- //!
- //! While `fold_right` reduces a structure to a summary value from the
- //! right, `unfold_right` builds a sequence from a seed value and a
- //! function, starting from the right.
- //!
- //!
- //! Signature
- //! ---------
- //! Given a `Sequence` `S`, an initial value `state` of tag `I`, an
- //! arbitrary Product `P` and a function \f$ f : I \to P(T, I) \f$,
- //! `unfold_right<S>` has the following signature:
- //! \f[
- //! \mathtt{unfold\_right}_S : I \times (I \to P(T, I)) \to S(T)
- //! \f]
- //!
- //! @tparam S
- //! The tag of the sequence to build up.
- //!
- //! @param state
- //! An initial value to build the sequence from.
- //!
- //! @param f
- //! A function called as `f(state)`, where `state` is an initial value,
- //! and returning
- //! 1. `nothing` if it is done producing the sequence.
- //! 2. otherwise, `just(make<P>(x, state))`, where `state` is the new
- //! initial value used in the next call to `f`, `x` is an element to
- //! be prepended to the resulting sequence, and `P` is an arbitrary
- //! `Product`.
- //!
- //!
- //! Fun fact
- //! ---------
- //! In some cases, `unfold_right` can undo a `fold_right` operation:
- //! @code
- //! unfold_right<S>(fold_right(xs, state, f), g) == xs
- //! @endcode
- //!
- //! if the following holds
- //! @code
- //! g(f(x, y)) == just(make_pair(x, y))
- //! g(state) == nothing
- //! @endcode
- //!
- //!
- //! Example
- //! -------
- //! @include example/unfold_right.cpp
- #ifdef BOOST_HANA_DOXYGEN_INVOKED
- template <typename S>
- constexpr auto unfold_right = [](auto&& state, auto&& f) {
- return tag-dispatched;
- };
- #else
- template <typename S, typename = void>
- struct unfold_right_impl : unfold_right_impl<S, when<true>> { };
- template <typename S>
- struct unfold_right_t;
- template <typename S>
- constexpr unfold_right_t<S> unfold_right{};
- #endif
- BOOST_HANA_NAMESPACE_END
- #endif // !BOOST_HANA_FWD_UNFOLD_RIGHT_HPP
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