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 Hu Chunming
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/*!
@file
Defines `boost::hana::partial`.
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_PARTIAL_HPP
#define BOOST_HANA_FUNCTIONAL_PARTIAL_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
//! Partially apply a function to some arguments.
//!
//! Given a function `f` and some arguments, `partial` returns a new
//! function corresponding to the partially applied function `f`. This
//! allows providing some arguments to a function and letting the rest
//! of the arguments be provided later. Specifically, `partial(f, x...)`
//! is a function such that
//! @code
//! partial(f, x...)(y...) == f(x..., y...)
//! @endcode
//!
//! @note
//! The arity of `f` must match the total number of arguments passed to
//! it, i.e. `sizeof...(x) + sizeof...(y)`.
//!
//!
//! Example
//! -------
//! @include example/functional/partial.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto partial = [](auto&& f, auto&& ...x) {
return [perfect-capture](auto&& ...y) -> decltype(auto) {
return forwarded(f)(forwarded(x)..., forwarded(y)...);
};
};
#else
template <typename Indices, typename F, typename ...X>
struct partial_t;
struct make_partial_t {
struct secret { };
template <typename F, typename ...X>
constexpr partial_t<
std::make_index_sequence<sizeof...(X)>,
typename detail::decay<F>::type,
typename detail::decay<X>::type...
>
operator()(F&& f, X&& ...x) const {
return {secret{}, static_cast<F&&>(f), static_cast<X&&>(x)...};
}
};
template <std::size_t ...n, typename F, typename ...X>
struct partial_t<std::index_sequence<n...>, F, X...> {
partial_t() = default;
template <typename ...T>
constexpr partial_t(make_partial_t::secret, T&& ...t)
: storage_{static_cast<T&&>(t)...}
{ }
basic_tuple<F, X...> storage_;
template <typename ...Y>
constexpr decltype(auto) operator()(Y&& ...y) const& {
return hana::at_c<0>(storage_)(
hana::at_c<n+1>(storage_)...,
static_cast<Y&&>(y)...
);
}
template <typename ...Y>
constexpr decltype(auto) operator()(Y&& ...y) & {
return hana::at_c<0>(storage_)(
hana::at_c<n+1>(storage_)...,
static_cast<Y&&>(y)...
);
}
template <typename ...Y>
constexpr decltype(auto) operator()(Y&& ...y) && {
return static_cast<F&&>(hana::at_c<0>(storage_))(
static_cast<X&&>(hana::at_c<n+1>(storage_))...,
static_cast<Y&&>(y)...
);
}
};
BOOST_HANA_INLINE_VARIABLE constexpr make_partial_t partial{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FUNCTIONAL_PARTIAL_HPP
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