hana/fwd/unique.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::unique`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_UNIQUE_HPP
0011 #define BOOST_HANA_FWD_UNIQUE_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015 #include <boost/hana/detail/nested_by_fwd.hpp>
0016
0017
0018 namespace boost { namespace hana {
0019     //! Removes all consecutive duplicate elements from a Sequence.
0020     //! @ingroup group-Sequence
0021     //!
0022     //! Given a `Sequence` and an optional binary predicate, `unique` returns
0023     //! a new sequence containing only the first element of every subrange
0024     //! of the original sequence whose elements are all equal. In other words,
0025     //! it turns a sequence of the form `[a, a, b, c, c, c, d, d, d, a]` into
0026     //! a sequence `[a, b, c, d, a]`. The equality of two elements is
0027     //! determined by the provided `predicate`, or by `equal` if no
0028     //! `predicate` is provided.
0029     //!
0030     //!
0031     //! Signature
0032     //! ---------
0033     //! Given a `Sequence` `S(T)`, a `Logical` `Bool` and a binary predicate
0034     //! \f$ T \times T \to Bool \f$, `unique` has the following signature:
0035     //! \f[
0036     //!     \mathtt{unique} : S(T) \times (T \times T \to Bool) \to S(T)
0037     //! \f]
0038     //!
0039     //! @param xs
0040     //! The sequence from which to remove consecutive duplicates.
0041     //!
0042     //! @param predicate
0043     //! A function called as `predicate(x, y)`, where `x` and `y` are adjacent
0044     //! elements of the sequence, and returning a `Logical` representing
0045     //! whether `x` and `y` should be considered equal. `predicate` should
0046     //! define an [equivalence relation][1] over the elements of the sequence.
0047     //! In the current implementation of the library, `predicate` has to
0048     //! return a compile-time `Logical`. This parameter is optional; it
0049     //! defaults to `equal` if it is not provided, which then requires the
0050     //! elements of the sequence to be compile-time `Comparable`.
0051     //!
0052     //!
0053     //! Syntactic sugar (`unique.by`)
0054     //! -----------------------------
0055     //! `unique` can be called in an alternate way, which provides a nice
0056     //! syntax, especially in conjunction with the `comparing` combinator:
0057     //! @code
0058     //!     unique.by(predicate, xs) == unique(xs, predicate)
0059     //!     unique.by(predicate) == unique(-, predicate)
0060     //! @endcode
0061     //!
0062     //! where `unique(-, predicate)` denotes the partial application of
0063     //! `unique` to `predicate`.
0064     //!
0065     //!
0066     //! Example
0067     //! -------
0068     //! @include example/unique.cpp
0069     //!
0070     //! [1]: http://en.wikipedia.org/wiki/Equivalence_relation#Definition
0071 #if defined(BOOST_HANA_DOXYGEN_INVOKED)
0072     constexpr auto unique = [](auto&& xs[, auto&& predicate]) {
0073         return tag-dispatched;
0074     };
0075 #else
0076     template <typename S, typename = void>
0077     struct unique_impl : unique_impl<S, when<true>> { };
0078
0079     struct unique_t : detail::nested_by<unique_t> {
0080         template <typename Xs>
0081         constexpr auto operator()(Xs&& xs) const;
0082
0083         template <typename Xs, typename Predicate>
0084         constexpr auto operator()(Xs&& xs, Predicate&& predicate) const;
0085     };
0086
0087     BOOST_HANA_INLINE_VARIABLE constexpr unique_t unique{};
0088 #endif
0089 }} // end namespace boost::hana
0090
0091 #endif // !BOOST_HANA_FWD_UNIQUE_HPP