ceph/src/boost/libs/hana/include/boost/hana/fwd/unique.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::unique`.
   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_UNIQUE_HPP
  11 #define BOOST_HANA_FWD_UNIQUE_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     //! Removes all consecutive duplicate elements from a Sequence.
  20     //! @ingroup group-Sequence
  21     //!
  22     //! Given a `Sequence` and an optional binary predicate, `unique` returns
  23     //! a new sequence containing only the first element of every subrange
  24     //! of the original sequence whose elements are all equal. In other words,
  25     //! it turns a sequence of the form `[a, a, b, c, c, c, d, d, d, a]` into
  26     //! a sequence `[a, b, c, d, a]`. The equality of two elements is
  27     //! determined by the provided `predicate`, or by `equal` if no
  28     //! `predicate` is provided.
  29     //!
  30     //!
  31     //! Signature
  32     //! ---------
  33     //! Given a `Sequence` `S(T)`, a `Logical` `Bool` and a binary predicate
  34     //! \f$ T \times T \to Bool \f$, `unique` has the following signature:
  35     //! \f[
  36     //!     \mathtt{unique} : S(T) \times (T \times T \to Bool) \to S(T)
  37     //! \f]
  38     //!
  39     //! @param xs
  40     //! The sequence from which to remove consecutive duplicates.
  41     //!
  42     //! @param predicate
  43     //! A function called as `predicate(x, y)`, where `x` and `y` are adjacent
  44     //! elements of the sequence, and returning a `Logical` representing
  45     //! whether `x` and `y` should be considered equal. `predicate` should
  46     //! define an [equivalence relation][1] over the elements of the sequence.
  47     //! In the current implementation of the library, `predicate` has to
  48     //! return a compile-time `Logical`. This parameter is optional; it
  49     //! defaults to `equal` if it is not provided, which then requires the
  50     //! elements of the sequence to be compile-time `Comparable`.
  51     //!
  52     //!
  53     //! Syntactic sugar (`unique.by`)
  54     //! -----------------------------
  55     //! `unique` can be called in an alternate way, which provides a nice
  56     //! syntax, especially in conjunction with the `comparing` combinator:
  57     //! @code
  58     //!     unique.by(predicate, xs) == unique(xs, predicate)
  59     //!     unique.by(predicate) == unique(-, predicate)
  60     //! @endcode
  61     //!
  62     //! where `unique(-, predicate)` denotes the partial application of
  63     //! `unique` to `predicate`.
  64     //!
  65     //!
  66     //! Example
  67     //! -------
  68     //! @include example/unique.cpp
  69     //!
  70     //! [1]: http://en.wikipedia.org/wiki/Equivalence_relation#Definition
  71 #if defined(BOOST_HANA_DOXYGEN_INVOKED)
  72     constexpr auto unique = [](auto&& xs[, auto&& predicate]) {
  73         return tag-dispatched;
  74     };
  75 #else
  76     template <typename S, typename = void>
  77     struct unique_impl : unique_impl<S, when<true>> { };
  78
  79     struct unique_t : detail::nested_by<unique_t> {
  80         template <typename Xs>
  81         constexpr auto operator()(Xs&& xs) const;
  82
  83         template <typename Xs, typename Predicate>
  84         constexpr auto operator()(Xs&& xs, Predicate&& predicate) const;
  85     };
  86
  87     constexpr unique_t unique{};
  88 #endif
  89 BOOST_HANA_NAMESPACE_END
  90
  91 #endif // !BOOST_HANA_FWD_UNIQUE_HPP