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1 | /*! | |
2 | @file | |
3 | Forward declares `boost::hana::span`. | |
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_SPAN_HPP | |
11 | #define BOOST_HANA_FWD_SPAN_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | #include <boost/hana/detail/nested_by_fwd.hpp> | |
16 | ||
17 | ||
18 | BOOST_HANA_NAMESPACE_BEGIN | |
19 | //! Returns a `Product` containing the longest prefix of a sequence | |
20 | //! satisfying a predicate, and the rest of the sequence. | |
21 | //! @ingroup group-Sequence | |
22 | //! | |
23 | //! The first component of the returned `Product` is a sequence for which | |
24 | //! all elements satisfy the given predicate. The second component of the | |
25 | //! returned `Product` is a sequence containing the remainder of the | |
26 | //! argument. Both or either sequences may be empty, depending on the | |
27 | //! input argument. More specifically, | |
28 | //! @code | |
29 | //! span(xs, predicate) == make_pair(take_while(xs, predicate), | |
30 | //! drop_while(xs, predicate)) | |
31 | //! @endcode | |
32 | //! except that `make_pair` may be an arbitrary `Product`. | |
33 | //! | |
34 | //! | |
35 | //! Signature | |
36 | //! --------- | |
37 | //! Given a `Sequence` `S(T)`, a `Logical` `Bool` and a predicate | |
38 | //! \f$ T \to Bool \f$, `span` has the following signature: | |
39 | //! \f[ | |
40 | //! \mathtt{span} : S(T) \times (T \to Bool) \to S(T) \times S(T) | |
41 | //! \f] | |
42 | //! | |
43 | //! @param xs | |
44 | //! The sequence to break into two parts. | |
45 | //! | |
46 | //! @param predicate | |
47 | //! A function called as `predicate(x)`, where `x` is an element of the | |
48 | //! sequence, and returning a `Logical. In the current implementation of | |
49 | //! the library, `predicate` has to return a compile-time `Logical`. | |
50 | //! | |
51 | //! | |
52 | //! Syntactic sugar (`span.by`) | |
53 | //! --------------------------- | |
54 | //! `span` can be called in an alternate way, which provides a nice syntax | |
55 | //! in some cases where the predicate is short: | |
56 | //! @code | |
57 | //! span.by(predicate, xs) == span(xs, predicate) | |
58 | //! span.by(predicate) == span(-, predicate) | |
59 | //! @endcode | |
60 | //! | |
61 | //! where `span(-, predicate)` denotes the partial application of | |
62 | //! `span` to `predicate`. | |
63 | //! | |
64 | //! | |
65 | //! Example | |
66 | //! ------- | |
67 | //! @include example/span.cpp | |
68 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
69 | constexpr auto span = [](auto&& xs, auto&& predicate) { | |
70 | return tag-dispatched; | |
71 | }; | |
72 | #else | |
73 | template <typename S, typename = void> | |
74 | struct span_impl : span_impl<S, when<true>> { }; | |
75 | ||
76 | struct span_t : detail::nested_by<span_t> { | |
77 | template <typename Xs, typename Pred> | |
78 | constexpr auto operator()(Xs&& xs, Pred&& pred) const; | |
79 | }; | |
80 | ||
81 | constexpr span_t span{}; | |
82 | #endif | |
83 | BOOST_HANA_NAMESPACE_END | |
84 | ||
85 | #endif // !BOOST_HANA_FWD_SPAN_HPP |