/*!
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@file
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Forward declares `boost::hana::index_if`.
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@copyright Louis Dionne 2013-2017
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@copyright Jason Rice 2017
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_INDEX_IF_HPP
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#define BOOST_HANA_FWD_INDEX_IF_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Finds the value associated to the first key satisfying a predicate.
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//! @ingroup group-Iterable
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//!
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//! Given an `Iterable` structure `xs` and a predicate `pred`,
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//! `index_if(xs, pred)` returns a `hana::optional` containing an `IntegralConstant`
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//! of the index of the first element that satisfies the predicate or nothing
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//! if no element satisfies the predicate.
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//!
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//!
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//! @param xs
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//! The structure to be searched.
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//!
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//! @param predicate
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//! A function called as `predicate(x)`, where `x` is an element of the
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//! `Iterable` structure and returning whether `x` is the element being
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//! searched for. In the current version of the library, the predicate
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//! has to return an `IntegralConstant` holding a value that can be
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//! converted to `bool`.
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//!
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//!
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//! Example
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//! -------
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//! @include example/index_if.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto index_if = [](auto&& xs, auto&& predicate) {
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return tag-dispatched;
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};
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#else
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template <typename S, typename = void>
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struct index_if_impl : index_if_impl<S, when<true>> { };
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struct index_if_t {
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template <typename Xs, typename Pred>
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constexpr auto operator()(Xs&& xs, Pred&& pred) const;
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};
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constexpr index_if_t index_if{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_INDEX_IF_HPP
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