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Diffstat (limited to 'boost/hana/functional/on.hpp')
-rw-r--r-- | boost/hana/functional/on.hpp | 83 |
1 files changed, 83 insertions, 0 deletions
diff --git a/boost/hana/functional/on.hpp b/boost/hana/functional/on.hpp new file mode 100644 index 0000000000..ff40c0db5c --- /dev/null +++ b/boost/hana/functional/on.hpp @@ -0,0 +1,83 @@ +/*! +@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 |