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/*!
@file
Forward declares `boost::hana::unique`.
Copyright Louis Dionne 2013-2022
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_UNIQUE_HPP
#define BOOST_HANA_FWD_UNIQUE_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
#include <boost/hana/detail/nested_by_fwd.hpp>
namespace boost { namespace hana {
//! Removes all consecutive duplicate elements from a Sequence.
//! @ingroup group-Sequence
//!
//! Given a `Sequence` and an optional binary predicate, `unique` returns
//! a new sequence containing only the first element of every subrange
//! of the original sequence whose elements are all equal. In other words,
//! it turns a sequence of the form `[a, a, b, c, c, c, d, d, d, a]` into
//! a sequence `[a, b, c, d, a]`. The equality of two elements is
//! determined by the provided `predicate`, or by `equal` if no
//! `predicate` is provided.
//!
//!
//! Signature
//! ---------
//! Given a `Sequence` `S(T)`, a `Logical` `Bool` and a binary predicate
//! \f$ T \times T \to Bool \f$, `unique` has the following signature:
//! \f[
//! \mathtt{unique} : S(T) \times (T \times T \to Bool) \to S(T)
//! \f]
//!
//! @param xs
//! The sequence from which to remove consecutive duplicates.
//!
//! @param predicate
//! A function called as `predicate(x, y)`, where `x` and `y` are adjacent
//! elements of the sequence, and returning a `Logical` representing
//! whether `x` and `y` should be considered equal. `predicate` should
//! define an [equivalence relation][1] over the elements of the sequence.
//! In the current implementation of the library, `predicate` has to
//! return a compile-time `Logical`. This parameter is optional; it
//! defaults to `equal` if it is not provided, which then requires the
//! elements of the sequence to be compile-time `Comparable`.
//!
//!
//! Syntactic sugar (`unique.by`)
//! -----------------------------
//! `unique` can be called in an alternate way, which provides a nice
//! syntax, especially in conjunction with the `comparing` combinator:
//! @code
//! unique.by(predicate, xs) == unique(xs, predicate)
//! unique.by(predicate) == unique(-, predicate)
//! @endcode
//!
//! where `unique(-, predicate)` denotes the partial application of
//! `unique` to `predicate`.
//!
//!
//! Example
//! -------
//! @include example/unique.cpp
//!
//! [1]: http://en.wikipedia.org/wiki/Equivalence_relation#Definition
#if defined(BOOST_HANA_DOXYGEN_INVOKED)
constexpr auto unique = [](auto&& xs[, auto&& predicate]) {
return tag-dispatched;
};
#else
template <typename S, typename = void>
struct unique_impl : unique_impl<S, when<true>> { };
struct unique_t : detail::nested_by<unique_t> {
template <typename Xs>
constexpr auto operator()(Xs&& xs) const;
template <typename Xs, typename Predicate>
constexpr auto operator()(Xs&& xs, Predicate&& predicate) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr unique_t unique{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_UNIQUE_HPP
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