1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
|
/*!
@file
Forward declares `boost::hana::unfold_right`.
@copyright Louis Dionne 2013-2016
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
|
