/*=============================================================================
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Copyright (c) 2015 Paul Fultz II
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fold.h
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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==============================================================================*/
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#ifndef BOOST_HOF_GUARD_FOLD_H
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#define BOOST_HOF_GUARD_FOLD_H
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/// fold
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/// ========
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///
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/// Description
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/// -----------
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///
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/// The `fold` function adaptor uses a binary function to apply a
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/// [fold](https://en.wikipedia.org/wiki/Fold_%28higher-order_function%29)
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/// operation to the arguments passed to the function. Additionally, an
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/// optional initial state can be provided, otherwise the first argument is
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/// used as the initial state.
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///
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/// The arguments to the binary function, take first the state and then the
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/// argument.
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///
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/// Synopsis
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/// --------
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///
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/// template<class F, class State>
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/// constexpr fold_adaptor<F, State> fold(F f, State s);
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///
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/// template<class F>
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/// constexpr fold_adaptor<F> fold(F f);
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///
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/// Semantics
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/// ---------
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///
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/// assert(fold(f, z)() == z);
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/// assert(fold(f, z)(x, xs...) == fold(f, f(z, x))(xs...));
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/// assert(fold(f)(x) == x);
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/// assert(fold(f)(x, y, xs...) == fold(f)(f(x, y), xs...));
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///
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/// Requirements
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/// ------------
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///
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/// State must be:
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///
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/// * CopyConstructible
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///
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/// F must be:
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///
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/// * [BinaryInvocable](BinaryInvocable)
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/// * MoveConstructible
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///
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/// Example
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/// -------
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///
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/// #include <boost/hof.hpp>
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/// #include <cassert>
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///
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/// struct max_f
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/// {
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/// template<class T, class U>
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/// constexpr T operator()(T x, U y) const
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/// {
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/// return x > y ? x : y;
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/// }
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/// };
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/// int main() {
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/// assert(boost::hof::fold(max_f())(2, 3, 4, 5) == 5);
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/// }
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///
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/// References
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/// ----------
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///
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/// * [Fold](https://en.wikipedia.org/wiki/Fold_(higher-order_function))
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/// * [Variadic sum](<Variadic sum>)
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///
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#include <boost/hof/detail/callable_base.hpp>
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#include <boost/hof/detail/delegate.hpp>
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#include <boost/hof/detail/compressed_pair.hpp>
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#include <boost/hof/detail/move.hpp>
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#include <boost/hof/detail/make.hpp>
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#include <boost/hof/detail/static_const_var.hpp>
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namespace boost { namespace hof { namespace detail {
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struct v_fold
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{
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BOOST_HOF_RETURNS_CLASS(v_fold);
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template<class F, class State, class T, class... Ts>
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constexpr BOOST_HOF_SFINAE_MANUAL_RESULT(const v_fold&, id_<const F&>, result_of<const F&, id_<State>, id_<T>>, id_<Ts>...)
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operator()(const F& f, State&& state, T&& x, Ts&&... xs) const BOOST_HOF_SFINAE_MANUAL_RETURNS
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(
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(*BOOST_HOF_CONST_THIS)(f, f(BOOST_HOF_FORWARD(State)(state), BOOST_HOF_FORWARD(T)(x)), BOOST_HOF_FORWARD(Ts)(xs)...)
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);
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template<class F, class State>
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constexpr State operator()(const F&, State&& state) const noexcept
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{
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return BOOST_HOF_FORWARD(State)(state);
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}
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};
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}
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template<class F, class State=void>
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struct fold_adaptor
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: detail::compressed_pair<detail::callable_base<F>, State>
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{
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typedef detail::compressed_pair<detail::callable_base<F>, State> base_type;
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BOOST_HOF_INHERIT_CONSTRUCTOR(fold_adaptor, base_type)
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template<class... Ts>
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constexpr const detail::callable_base<F>& base_function(Ts&&... xs) const noexcept
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{
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return this->first(xs...);
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}
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template<class... Ts>
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constexpr State get_state(Ts&&... xs) const noexcept
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{
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return this->second(xs...);
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}
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BOOST_HOF_RETURNS_CLASS(fold_adaptor);
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template<class... Ts>
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constexpr BOOST_HOF_SFINAE_RESULT(detail::v_fold, id_<const detail::callable_base<F>&>, id_<State>, id_<Ts>...)
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operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
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(
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detail::v_fold()(
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BOOST_HOF_MANGLE_CAST(const detail::callable_base<F>&)(BOOST_HOF_CONST_THIS->base_function(xs...)),
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BOOST_HOF_MANGLE_CAST(State)(BOOST_HOF_CONST_THIS->get_state(xs...)),
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BOOST_HOF_FORWARD(Ts)(xs)...
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)
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)
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};
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template<class F>
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struct fold_adaptor<F, void>
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: detail::callable_base<F>
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{
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BOOST_HOF_INHERIT_CONSTRUCTOR(fold_adaptor, detail::callable_base<F>)
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template<class... Ts>
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constexpr const detail::callable_base<F>& base_function(Ts&&... xs) const noexcept
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{
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return boost::hof::always_ref(*this)(xs...);
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}
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BOOST_HOF_RETURNS_CLASS(fold_adaptor);
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template<class... Ts>
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constexpr BOOST_HOF_SFINAE_RESULT(detail::v_fold, id_<const detail::callable_base<F>&>, id_<Ts>...)
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operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
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(
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detail::v_fold()(
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BOOST_HOF_MANGLE_CAST(const detail::callable_base<F>&)(BOOST_HOF_CONST_THIS->base_function(xs...)),
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BOOST_HOF_FORWARD(Ts)(xs)...
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)
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)
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};
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BOOST_HOF_DECLARE_STATIC_VAR(fold, detail::make<fold_adaptor>);
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}} // namespace boost::hof
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#endif
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