84 lines
2.3 KiB
C++
84 lines
2.3 KiB
C++
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/*!
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@file
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Defines `boost::hana::on`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FUNCTIONAL_ON_HPP
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#define BOOST_HANA_FUNCTIONAL_ON_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/detail/create.hpp>
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#include <boost/hana/functional/infix.hpp>
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#include <utility>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @ingroup group-functional
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//! Invoke a function with the result of invoking another function on
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//! each argument.
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//!
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//! Specifically, `on(f, g)` is a function such that
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//! @code
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//! on(f, g)(x...) == f(g(x)...)
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//! @endcode
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//!
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//! For convenience, `on` also supports infix application as provided
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//! by `infix`.
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//!
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//!
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//! @note
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//! `on` is associative, i.e. `on(f, on(g, h))` is equivalent to
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//! `on(on(f, g), h)`.
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//!
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//! @internal
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//! ### Proof of associativity
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//!
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//! @code
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//! on(f, on(g, h))(xs...) == f(on(g, h)(xs)...)
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//! == f(g(h(xs))...)
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//!
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//! on(on(f, g), h)(xs...) == on(f, g)(h(xs)...)
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//! == f(g(h(xs))...)
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//! @endcode
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//! @endinternal
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//!
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//!
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//! ### Example
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//! @include example/functional/on.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto on = infix([](auto&& f, auto&& g) {
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return [perfect-capture](auto&& ...x) -> decltype(auto) {
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return forwarded(f)(g(forwarded(x))...);
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};
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});
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#else
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template <typename F, typename G>
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struct on_t {
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F f; G g;
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template <typename ...X>
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constexpr decltype(auto) operator()(X&& ...x) const& {
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return f(g(static_cast<X&&>(x))...);
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}
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template <typename ...X>
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constexpr decltype(auto) operator()(X&& ...x) & {
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return f(g(static_cast<X&&>(x))...);
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}
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template <typename ...X>
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constexpr decltype(auto) operator()(X&& ...x) && {
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return std::move(f)(g(static_cast<X&&>(x))...);
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}
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};
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constexpr auto on = infix(detail::create<on_t>{});
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FUNCTIONAL_ON_HPP
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