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+/*!
+@file
+Defines `boost::hana::on`.
+
+@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_FUNCTIONAL_ON_HPP
+#define BOOST_HANA_FUNCTIONAL_ON_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/detail/create.hpp>
+#include <boost/hana/functional/infix.hpp>
+
+#include <utility>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @ingroup group-functional
+    //! Invoke a function with the result of invoking another function on
+    //! each argument.
+    //!
+    //! Specifically, `on(f, g)` is a function such that
+    //! @code
+    //!     on(f, g)(x...) == f(g(x)...)
+    //! @endcode
+    //!
+    //! For convenience, `on` also supports infix application as provided
+    //! by `infix`.
+    //!
+    //!
+    //! @note
+    //! `on` is associative, i.e. `on(f, on(g, h))` is equivalent to
+    //! `on(on(f, g), h)`.
+    //!
+    //! @internal
+    //! ### Proof of associativity
+    //!
+    //! @code
+    //!     on(f, on(g, h))(xs...) == f(on(g, h)(xs)...)
+    //!                            == f(g(h(xs))...)
+    //!
+    //!     on(on(f, g), h)(xs...) == on(f, g)(h(xs)...)
+    //!                            == f(g(h(xs))...)
+    //! @endcode
+    //! @endinternal
+    //!
+    //!
+    //! ### Example
+    //! @include example/functional/on.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto on = infix([](auto&& f, auto&& g) {
+        return [perfect-capture](auto&& ...x) -> decltype(auto) {
+            return forwarded(f)(g(forwarded(x))...);
+        };
+    });
+#else
+    template <typename F, typename G>
+    struct on_t {
+        F f; G g;
+        template <typename ...X>
+        constexpr decltype(auto) operator()(X&& ...x) const& {
+            return f(g(static_cast<X&&>(x))...);
+        }
+
+        template <typename ...X>
+        constexpr decltype(auto) operator()(X&& ...x) & {
+            return f(g(static_cast<X&&>(x))...);
+        }
+
+        template <typename ...X>
+        constexpr decltype(auto) operator()(X&& ...x) && {
+            return std::move(f)(g(static_cast<X&&>(x))...);
+        }
+    };
+
+    constexpr auto on = infix(detail::create<on_t>{});
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FUNCTIONAL_ON_HPP