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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::while_`. | |
4 | ||
b32b8144 | 5 | @copyright Louis Dionne 2013-2017 |
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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_WHILE_HPP | |
11 | #define BOOST_HANA_FWD_WHILE_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
1e59de90 | 17 | namespace boost { namespace hana { |
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18 | //! Apply a function to an initial state while some predicate is satisfied. |
19 | //! @ingroup group-Logical | |
20 | //! | |
21 | //! This method is a natural extension of the `while` language construct | |
22 | //! to manipulate a state whose type may change from one iteration to | |
23 | //! another. However, note that having a state whose type changes from | |
24 | //! one iteration to the other is only possible as long as the predicate | |
25 | //! returns a `Logical` whose truth value is known at compile-time. | |
26 | //! | |
27 | //! Specifically, `while_(pred, state, f)` is equivalent to | |
28 | //! @code | |
29 | //! f(...f(f(state))) | |
30 | //! @endcode | |
31 | //! where `f` is iterated as long as `pred(f(...))` is a true-valued | |
32 | //! `Logical`. | |
33 | //! | |
34 | //! | |
35 | //! @param pred | |
36 | //! A predicate called on the state or on the result of applying `f` a | |
37 | //! certain number of times to the state, and returning whether `f` | |
38 | //! should be applied one more time. | |
39 | //! | |
40 | //! @param state | |
41 | //! The initial state on which `f` is applied. | |
42 | //! | |
43 | //! @param f | |
44 | //! A function that is iterated on the initial state. Note that the | |
45 | //! return type of `f` may change from one iteration to the other, | |
46 | //! but only while `pred` returns a compile-time `Logical`. In other | |
47 | //! words, `decltype(f(stateN))` may differ from `decltype(f(stateN+1))`, | |
48 | //! but only if `pred(f(stateN))` returns a compile-time `Logical`. | |
49 | //! | |
50 | //! | |
51 | //! Example | |
52 | //! ------- | |
53 | //! @include example/while.cpp | |
54 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
55 | constexpr auto while_ = [](auto&& pred, auto&& state, auto&& f) -> decltype(auto) { | |
56 | return tag-dispatched; | |
57 | }; | |
58 | #else | |
59 | template <typename L, typename = void> | |
60 | struct while_impl : while_impl<L, when<true>> { }; | |
61 | ||
62 | struct while_t { | |
63 | template <typename Pred, typename State, typename F> | |
64 | constexpr decltype(auto) operator()(Pred&& pred, State&& state, F&& f) const; | |
65 | }; | |
66 | ||
1e59de90 | 67 | BOOST_HANA_INLINE_VARIABLE constexpr while_t while_{}; |
7c673cae | 68 | #endif |
1e59de90 | 69 | }} // end namespace boost::hana |
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70 | |
71 | #endif // !BOOST_HANA_FWD_WHILE_HPP |