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/*!
@file
Forward declares `boost::hana::ap`.
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_FWD_AP_HPP
#define BOOST_HANA_FWD_AP_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Lifted application.
//! @ingroup group-Applicative
//!
//! Specifically, `ap` applies a structure containing functions to a
//! structure containing values, and returns a new structure containing
//! values. The exact way in which the functions are applied to the values
//! depends on the `Applicative`.
//!
//! `ap` can be called with two arguments or more; the functions in the `f`
//! structure are curried and then applied to the values in each `x...`
//! structure using the binary form of `ap`. Note that this requires the
//! number of `x...` must match the arity of the functions in the `f`
//! structure. In other words, `ap(f, x1, ..., xN)` is equivalent to
//! @code
//! ((curry(f) ap x1) ap x2) ... ap xN
//! @endcode
//! where `x ap y` is just `ap(x, y)` written in infix notation to
//! emphasize the left associativity.
//!
//!
//! Signature
//! ---------
//! Given an Applicative `A`, the signature is
//! @f$ \mathtt{ap} : A(T_1 \times \cdots \times T_n \to U)
//! \times A(T_1) \times \cdots \times A(T_n)
//! \to A(U) @f$.
//!
//! @param f
//! A structure containing function(s).
//!
//! @param x...
//! Structure(s) containing value(s) and on which `f` is applied. The
//! number of structures must match the arity of the functions in the
//! `f` structure.
//!
//!
//! Example
//! -------
//! @include example/ap.cpp
//!
//! @todo
//! Consider giving access to all the arguments to the tag-dispatched
//! implementation for performance purposes.
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto ap = [](auto&& f, auto&& ...x) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename A, typename = void>
struct ap_impl : ap_impl<A, when<true>> { };
struct ap_t {
template <typename F, typename X>
constexpr decltype(auto) operator()(F&& f, X&& x) const;
template <typename F, typename ...Xs>
constexpr decltype(auto) operator()(F&& f, Xs&& ...xs) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr ap_t ap{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_AP_HPP
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