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/*!
@file
Defines `boost::hana::lockstep`.
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_LOCKSTEP_HPP
#define BOOST_HANA_FUNCTIONAL_LOCKSTEP_HPP
#include <boost/hana/basic_tuple.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/detail/decay.hpp>
#include <cstddef>
#include <utility>
namespace boost { namespace hana {
//! @ingroup group-functional
//! Invoke a function with the result of invoking other functions on its
//! arguments, in lockstep.
//!
//! Specifically, `lockstep(f)(g1, ..., gN)` is a function such that
//! @code
//! lockstep(f)(g1, ..., gN)(x1, ..., xN) == f(g1(x1), ..., gN(xN))
//! @endcode
//!
//! Since each `g` is invoked on its corresponding argument in lockstep,
//! the number of arguments must match the number of `g`s.
//!
//!
//! Example
//! -------
//! @include example/functional/lockstep.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto lockstep = [](auto&& f, auto&& ...g) {
return [perfect-capture](auto&& ...x) -> decltype(auto) {
return forwarded(f)(forwarded(g)(forwarded(x))...);
};
};
#else
template <typename Indices, typename F, typename ...G>
struct lockstep_t;
template <typename F>
struct pre_lockstep_t;
struct make_pre_lockstep_t {
struct secret { };
template <typename F>
constexpr pre_lockstep_t<typename detail::decay<F>::type> operator()(F&& f) const {
return {static_cast<F&&>(f)};
}
};
template <std::size_t ...n, typename F, typename ...G>
struct lockstep_t<std::index_sequence<n...>, F, G...> {
template <typename ...T>
constexpr lockstep_t(make_pre_lockstep_t::secret, T&& ...t)
: storage_{static_cast<T&&>(t)...}
{ }
basic_tuple<F, G...> storage_;
template <typename ...X>
constexpr decltype(auto) operator()(X&& ...x) const& {
return hana::at_c<0>(storage_)(
hana::at_c<n+1>(storage_)(static_cast<X&&>(x))...
);
}
template <typename ...X>
constexpr decltype(auto) operator()(X&& ...x) & {
return hana::at_c<0>(storage_)(
hana::at_c<n+1>(storage_)(static_cast<X&&>(x))...
);
}
template <typename ...X>
constexpr decltype(auto) operator()(X&& ...x) && {
return static_cast<F&&>(hana::at_c<0>(storage_))(
static_cast<G&&>(hana::at_c<n+1>(storage_))(static_cast<X&&>(x))...
);
}
};
template <typename F>
struct pre_lockstep_t {
F f;
template <typename ...G>
constexpr lockstep_t<std::make_index_sequence<sizeof...(G)>, F,
typename detail::decay<G>::type...>
operator()(G&& ...g) const& {
return {make_pre_lockstep_t::secret{}, this->f, static_cast<G&&>(g)...};
}
template <typename ...G>
constexpr lockstep_t<std::make_index_sequence<sizeof...(G)>, F,
typename detail::decay<G>::type...>
operator()(G&& ...g) && {
return {make_pre_lockstep_t::secret{}, static_cast<F&&>(this->f),
static_cast<G&&>(g)...};
}
};
BOOST_HANA_INLINE_VARIABLE constexpr make_pre_lockstep_t lockstep{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FUNCTIONAL_LOCKSTEP_HPP
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