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