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diff --git a/boost/hana/fold_left.hpp b/boost/hana/fold_left.hpp
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+/*!
+@file
+Defines `boost::hana::fold_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_FOLD_LEFT_HPP
+#define BOOST_HANA_FOLD_LEFT_HPP
+
+#include <boost/hana/fwd/fold_left.hpp>
+
+#include <boost/hana/concept/foldable.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+#include <boost/hana/detail/variadic/foldl1.hpp>
+#include <boost/hana/functional/partial.hpp>
+#include <boost/hana/unpack.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @cond
+    template <typename Xs, typename State, typename F>
+    constexpr decltype(auto) fold_left_t::operator()(Xs&& xs, State&& state, F&& f) const {
+        using S = typename hana::tag_of<Xs>::type;
+        using FoldLeft = BOOST_HANA_DISPATCH_IF(fold_left_impl<S>,
+            hana::Foldable<S>::value
+        );
+
+    #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+        static_assert(hana::Foldable<S>::value,
+        "hana::fold_left(xs, state, f) requires 'xs' to be Foldable");
+    #endif
+
+        return FoldLeft::apply(static_cast<Xs&&>(xs),
+                               static_cast<State&&>(state),
+                               static_cast<F&&>(f));
+    }
+
+    template <typename Xs, typename F>
+    constexpr decltype(auto) fold_left_t::operator()(Xs&& xs, F&& f) const {
+        using S = typename hana::tag_of<Xs>::type;
+        using FoldLeft = BOOST_HANA_DISPATCH_IF(fold_left_impl<S>,
+            hana::Foldable<S>::value
+        );
+
+    #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+        static_assert(hana::Foldable<S>::value,
+        "hana::fold_left(xs, f) requires 'xs' to be Foldable");
+    #endif
+
+        return FoldLeft::apply(static_cast<Xs&&>(xs), static_cast<F&&>(f));
+    }
+    //! @endcond
+
+    namespace detail {
+        template <typename F, typename State>
+        struct variadic_foldl1 {
+            F& f;
+            State& state;
+            template <typename ...T>
+            constexpr decltype(auto) operator()(T&& ...t) const {
+                return detail::variadic::foldl1(
+                    static_cast<F&&>(f),
+                    static_cast<State&&>(state),
+                    static_cast<T&&>(t)...
+                );
+            }
+        };
+    }
+
+    template <typename T, bool condition>
+    struct fold_left_impl<T, when<condition>> : default_ {
+        // with state
+        template <typename Xs, typename S, typename F>
+        static constexpr decltype(auto) apply(Xs&& xs, S&& s, F&& f) {
+            return hana::unpack(static_cast<Xs&&>(xs),
+                detail::variadic_foldl1<F, S>{f, s}
+            );
+        }
+
+        // without state
+        template <typename Xs, typename F>
+        static constexpr decltype(auto) apply(Xs&& xs, F&& f) {
+            return hana::unpack(static_cast<Xs&&>(xs),
+                hana::partial(
+                    detail::variadic::foldl1,
+                    static_cast<F&&>(f)
+                )
+            );
+        }
+    };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FOLD_LEFT_HPP