projects / ceph.git / blame
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|---|---|---|
| 7c673cae FG |
1 | /*! |
| 2 | @file | |
| 3 | Forward declares `boost::hana::unfold_left`. | |
| 4 | ||
| 5 | @copyright Louis Dionne 2013-2016 | |
| 6 | Distributed under the Boost Software License, Version 1.0. | |
| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) | |
| 8 | */ | |
| 9 | ||
| 10 | #ifndef BOOST_HANA_FWD_UNFOLD_LEFT_HPP | |
| 11 | #define BOOST_HANA_FWD_UNFOLD_LEFT_HPP | |
| 12 | ||
| 13 | #include <boost/hana/config.hpp> | |
| 14 | #include <boost/hana/core/when.hpp> | |
| 15 | ||
| 16 | ||
| 17 | BOOST_HANA_NAMESPACE_BEGIN | |
| 18 | //! Dual operation to `fold_left` for sequences. | |
| 19 | //! @ingroup group-Sequence | |
| 20 | //! | |
| 21 | //! While `fold_left` reduces a structure to a summary value from the left, | |
| 22 | //! `unfold_left` builds a sequence from a seed value and a function, | |
| 23 | //! starting from the left. | |
| 24 | //! | |
| 25 | //! | |
| 26 | //! Signature | |
| 27 | //! --------- | |
| 28 | //! Given a `Sequence` `S`, an initial value `state` of tag `I`, an | |
| 29 | //! arbitrary Product `P` and a function \f$ f : I \to P(I, T) \f$, | |
| 30 | //! `unfold_left<S>` has the following signature: | |
| 31 | //! \f[ | |
| 32 | //! \mathtt{unfold\_left}_S : I \times (I \to P(I, T)) \to S(T) | |
| 33 | //! \f] | |
| 34 | //! | |
| 35 | //! @tparam S | |
| 36 | //! The tag of the sequence to build up. | |
| 37 | //! | |
| 38 | //! @param state | |
| 39 | //! An initial value to build the sequence from. | |
| 40 | //! | |
| 41 | //! @param f | |
| 42 | //! A function called as `f(state)`, where `state` is an initial value, | |
| 43 | //! and returning | |
| 44 | //! 1. `nothing` if it is done producing the sequence. | |
| 45 | //! 2. otherwise, `just(make<P>(state, x))`, where `state` is the new | |
| 46 | //! initial value used in the next call to `f`, `x` is an element to | |
| 47 | //! be appended to the resulting sequence, and `P` is an arbitrary | |
| 48 | //! `Product`. | |
| 49 | //! | |
| 50 | //! | |
| 51 | //! Fun fact | |
| 52 | //! --------- | |
| 53 | //! In some cases, `unfold_left` can undo a `fold_left` operation: | |
| 54 | //! @code | |
| 55 | //! unfold_left<S>(fold_left(xs, state, f), g) == xs | |
| 56 | //! @endcode | |
| 57 | //! | |
| 58 | //! if the following holds | |
| 59 | //! @code | |
| 60 | //! g(f(x, y)) == just(make_pair(x, y)) | |
| 61 | //! g(state) == nothing | |
| 62 | //! @endcode | |
| 63 | //! | |
| 64 | //! | |
| 65 | //! Example | |
| 66 | //! ------- | |
| 67 | //! @include example/unfold_left.cpp | |
| 68 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
| 69 | template <typename S> | |
| 70 | constexpr auto unfold_left = [](auto&& state, auto&& f) { | |
| 71 | return tag-dispatched; | |
| 72 | }; | |
| 73 | #else | |
| 74 | template <typename S, typename = void> | |
| 75 | struct unfold_left_impl : unfold_left_impl<S, when<true>> { }; | |
| 76 | ||
| 77 | template <typename S> | |
| 78 | struct unfold_left_t; | |
| 79 | ||
| 80 | template <typename S> | |
| 81 | constexpr unfold_left_t<S> unfold_left{}; | |
| 82 | #endif | |
| 83 | BOOST_HANA_NAMESPACE_END | |
| 84 | ||
| 85 | #endif // !BOOST_HANA_FWD_UNFOLD_LEFT_HPP |