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| 7c673cae FG |
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 |