ceph/src/boost/boost/hana/fwd/span.hpp

   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
  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