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 Hu Chunming
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/*!
@file
Defines `boost::hana::on`.
Copyright Louis Dionne 2013-2022
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>
namespace boost { namespace hana {
//! @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))...);
}
};
BOOST_HANA_INLINE_VARIABLE constexpr auto on = infix(detail::create<on_t>{});
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FUNCTIONAL_ON_HPP
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