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::span`.
@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_SPAN_HPP
#define BOOST_HANA_FWD_SPAN_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
#include <boost/hana/detail/nested_by_fwd.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Returns a `Product` containing the longest prefix of a sequence
//! satisfying a predicate, and the rest of the sequence.
//! @ingroup group-Sequence
//!
//! The first component of the returned `Product` is a sequence for which
//! all elements satisfy the given predicate. The second component of the
//! returned `Product` is a sequence containing the remainder of the
//! argument. Both or either sequences may be empty, depending on the
//! input argument. More specifically,
//! @code
//! span(xs, predicate) == make_pair(take_while(xs, predicate),
//! drop_while(xs, predicate))
//! @endcode
//! except that `make_pair` may be an arbitrary `Product`.
//!
//!
//! Signature
//! ---------
//! Given a `Sequence` `S(T)`, a `Logical` `Bool` and a predicate
//! \f$ T \to Bool \f$, `span` has the following signature:
//! \f[
//! \mathtt{span} : S(T) \times (T \to Bool) \to S(T) \times S(T)
//! \f]
//!
//! @param xs
//! The sequence to break into two parts.
//!
//! @param predicate
//! A function called as `predicate(x)`, where `x` is an element of the
//! sequence, and returning a `Logical. In the current implementation of
//! the library, `predicate` has to return a compile-time `Logical`.
//!
//!
//! Syntactic sugar (`span.by`)
//! ---------------------------
//! `span` can be called in an alternate way, which provides a nice syntax
//! in some cases where the predicate is short:
//! @code
//! span.by(predicate, xs) == span(xs, predicate)
//! span.by(predicate) == span(-, predicate)
//! @endcode
//!
//! where `span(-, predicate)` denotes the partial application of
//! `span` to `predicate`.
//!
//!
//! Example
//! -------
//! @include example/span.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto span = [](auto&& xs, auto&& predicate) {
return tag-dispatched;
};
#else
template <typename S, typename = void>
struct span_impl : span_impl<S, when<true>> { };
struct span_t : detail::nested_by<span_t> {
template <typename Xs, typename Pred>
constexpr auto operator()(Xs&& xs, Pred&& pred) const;
};
constexpr span_t span{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_SPAN_HPP
|
