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diff --git a/boost/hana/scan_left.hpp b/boost/hana/scan_left.hpp
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+++ b/boost/hana/scan_left.hpp
@@ -0,0 +1,133 @@
+/*!
+@file
+Defines `boost::hana::scan_left`.
+
+@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_SCAN_LEFT_HPP
+#define BOOST_HANA_SCAN_LEFT_HPP
+
+#include <boost/hana/fwd/scan_left.hpp>
+
+#include <boost/hana/at.hpp>
+#include <boost/hana/concept/sequence.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+#include <boost/hana/core/make.hpp>
+#include <boost/hana/empty.hpp>
+#include <boost/hana/length.hpp>
+#include <boost/hana/prepend.hpp>
+
+#include <cstddef>
+#include <utility>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @cond
+    template <typename Xs, typename F>
+    constexpr auto scan_left_t::operator()(Xs&& xs, F const& f) const {
+        using S = typename hana::tag_of<Xs>::type;
+        using ScanLeft = BOOST_HANA_DISPATCH_IF(scan_left_impl<S>,
+            hana::Sequence<S>::value
+        );
+
+#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+        static_assert(hana::Sequence<S>::value,
+        "hana::scan_left(xs, f) requires 'xs' to be a Sequence");
+#endif
+
+        return ScanLeft::apply(static_cast<Xs&&>(xs), f);
+    }
+
+    template <typename Xs, typename State, typename F>
+    constexpr auto scan_left_t::operator()(Xs&& xs, State&& state, F const& f) const {
+        using S = typename hana::tag_of<Xs>::type;
+        using ScanLeft = BOOST_HANA_DISPATCH_IF(scan_left_impl<S>,
+            hana::Sequence<S>::value
+        );
+
+#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+        static_assert(hana::Sequence<S>::value,
+        "hana::scan_left(xs, state, f) requires 'xs' to be a Sequence");
+#endif
+
+        return ScanLeft::apply(static_cast<Xs&&>(xs),
+                               static_cast<State&&>(state), f);
+    }
+    //! @endcond
+
+    template <typename S, bool condition>
+    struct scan_left_impl<S, when<condition>> : default_ {
+        // Without initial state
+        template <typename Xs, typename F, std::size_t n1, std::size_t n2, std::size_t ...ns>
+        static constexpr auto
+        apply1_impl(Xs&& xs, F const& f, std::index_sequence<n1, n2, ns...>) {
+            static_assert(n1 == 0, "logic error in Boost.Hana: file a bug report");
+
+            // Use scan_left with the first element as an initial state.
+            return scan_left_impl::apply_impl(
+                static_cast<Xs&&>(xs),
+                hana::at_c<0>(static_cast<Xs&&>(xs)),
+                f, std::index_sequence<n2, ns...>{}
+            );
+        }
+
+        template <typename Xs, typename F, std::size_t n>
+        static constexpr auto apply1_impl(Xs&& xs, F const&, std::index_sequence<n>) {
+            return hana::make<S>(hana::at_c<n>(static_cast<Xs&&>(xs)));
+        }
+
+        template <typename Xs, typename F>
+        static constexpr auto apply1_impl(Xs&&, F const&, std::index_sequence<>) {
+            return hana::empty<S>();
+        }
+
+        template <typename Xs, typename F>
+        static constexpr auto apply(Xs&& xs, F const& f) {
+            constexpr std::size_t Len = decltype(hana::length(xs))::value;
+            return scan_left_impl::apply1_impl(static_cast<Xs&&>(xs),
+                                               f, std::make_index_sequence<Len>{});
+        }
+
+
+        // With initial state
+        template <typename Xs, typename State, typename F,
+                  std::size_t n1, std::size_t n2, std::size_t ...ns>
+        static constexpr auto
+        apply_impl(Xs&& xs, State&& state, F const& f,
+                   std::index_sequence<n1, n2, ns...>)
+        {
+            auto rest = scan_left_impl::apply_impl(
+                                static_cast<Xs&&>(xs),
+                                f(state, hana::at_c<n1>(static_cast<Xs&&>(xs))),
+                                f, std::index_sequence<n2, ns...>{});
+            return hana::prepend(std::move(rest), static_cast<State&&>(state));
+        }
+
+        template <typename Xs, typename State, typename F, std::size_t n>
+        static constexpr auto
+        apply_impl(Xs&& xs, State&& state, F const& f, std::index_sequence<n>) {
+            auto new_state = f(state, hana::at_c<n>(static_cast<Xs&&>(xs)));
+            return hana::make<S>(static_cast<State&&>(state), std::move(new_state));
+        }
+
+        template <typename Xs, typename State, typename F>
+        static constexpr auto
+        apply_impl(Xs&&, State&& state, F const&, std::index_sequence<>) {
+            return hana::make<S>(static_cast<State&&>(state));
+        }
+
+        template <typename Xs, typename State, typename F>
+        static constexpr auto apply(Xs&& xs, State&& state, F const& f) {
+            constexpr std::size_t Len = decltype(hana::length(xs))::value;
+            return scan_left_impl::apply_impl(static_cast<Xs&&>(xs),
+                                              static_cast<State&&>(state),
+                                              f, std::make_index_sequence<Len>{});
+        }
+    };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_SCAN_LEFT_HPP