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+/*!
+@file
+Forward declares `boost::hana::at` and `boost::hana::at_c`.
+
+@copyright Louis Dionne 2013-2016
+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_AT_HPP
+#define BOOST_HANA_FWD_AT_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+#include <cstddef>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Returns the `n`th element of an iterable.
+    //! @ingroup group-Iterable
+    //!
+    //! Given an `Iterable` and an `IntegralConstant` index, `at` returns the
+    //! element located at the index in the linearization of the iterable.
+    //! Specifically, given an iterable `xs` with a linearization of
+    //! `[x1, ..., xN]`, `at(xs, k)` is equivalent to `xk`.
+    //!
+    //! If the `Iterable` actually stores the elements it contains, `at` is
+    //! required to return a lvalue reference, a lvalue reference to const
+    //! or a rvalue reference to the matching element, where the type of
+    //! reference must match that of the iterable passed to `at`. If the
+    //! `Iterable` does not store the elements it contains (i.e. it generates
+    //! them on demand), this requirement is dropped.
+    //!
+    //!
+    //! @param xs
+    //! The iterable in which an element is retrieved. The iterable must
+    //! contain at least `n + 1` elements.
+    //!
+    //! @param n
+    //! A non-negative `IntegralConstant` representing the 0-based index of
+    //! the element to return. It is an error to call `at` with an index that
+    //! out of bounds of the iterable.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/at.cpp
+    //!
+    //!
+    //! Benchmarks
+    //! ----------
+    //! <div class="benchmark-chart"
+    //!      style="min-width: 310px; height: 400px; margin: 0 auto"
+    //!      data-dataset="benchmark.at.compile.json">
+    //! </div>
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto at = [](auto&& xs, auto const& n) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename It, typename = void>
+    struct at_impl : at_impl<It, when<true>> { };
+
+    struct at_t {
+        template <typename Xs, typename N>
+        constexpr decltype(auto) operator()(Xs&& xs, N const& n) const;
+    };
+
+    constexpr at_t at{};
+#endif
+
+    //! Equivalent to `at`; provided for convenience.
+    //! @ingroup group-Iterable
+    //!
+    //!
+    //! @note
+    //! `hana::at_c<n>` is an overloaded function, not a function object.
+    //! Hence, it can't be passed to higher-order algorithms. This is done
+    //! for compile-time performance reasons.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/at_c.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    template <std::size_t n>
+    constexpr auto at_c = [](auto&& xs) {
+        return hana::at(forwarded(xs), hana::size_c<n>);
+    };
+#else
+    template <std::size_t n, typename Xs>
+    constexpr decltype(auto) at_c(Xs&& xs);
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_AT_HPP