1 /*!
2 @file
3 Forward declares `boost::hana::span`.
4 
5 @copyright Louis Dionne 2013-2017
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
__anon47cb15830102(auto&& xs, auto&& predicate) 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
86