1/*!
2@file
3Forward declares `boost::hana::unfold_left`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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
17namespace boost { namespace hana {
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 BOOST_HANA_INLINE_VARIABLE constexpr unfold_left_t<S> unfold_left{};
82#endif
83}} // end namespace boost::hana
84
85#endif // !BOOST_HANA_FWD_UNFOLD_LEFT_HPP
86

source code of boost/libs/hana/include/boost/hana/fwd/unfold_left.hpp