summaryrefslogtreecommitdiff
path: root/boost/hof/reverse_fold.hpp
diff options
context:
space:
mode:
Diffstat (limited to 'boost/hof/reverse_fold.hpp')
-rw-r--r--boost/hof/reverse_fold.hpp173
1 files changed, 173 insertions, 0 deletions
diff --git a/boost/hof/reverse_fold.hpp b/boost/hof/reverse_fold.hpp
new file mode 100644
index 0000000000..771cb95c41
--- /dev/null
+++ b/boost/hof/reverse_fold.hpp
@@ -0,0 +1,173 @@
+/*=============================================================================
+ Copyright (c) 2015 Paul Fultz II
+ reverse_fold.h
+ Distributed under the Boost Software License, Version 1.0. (See accompanying
+ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+==============================================================================*/
+
+#ifndef BOOST_HOF_GUARD_REVERSE_FOLD_H
+#define BOOST_HOF_GUARD_REVERSE_FOLD_H
+
+/// reverse_fold
+/// ========
+///
+/// Description
+/// -----------
+///
+/// The `reverse_fold` function adaptor uses a binary function to apply a
+/// reverse [fold]
+/// (https://en.wikipedia.org/wiki/Fold_%28higher-order_function%29)(ie right
+/// fold in functional programming terms) operation to the arguments passed to
+/// the function. Additionally, an optional initial state can be provided,
+/// otherwise the first argument is used as the initial state.
+///
+/// The arguments to the binary function, take first the state and then the
+/// argument.
+///
+/// Synopsis
+/// --------
+///
+/// template<class F, class State>
+/// constexpr reverse_fold_adaptor<F, State> reverse_fold(F f, State s);
+///
+/// template<class F>
+/// constexpr reverse_fold_adaptor<F> reverse_fold(F f);
+///
+/// Semantics
+/// ---------
+///
+/// assert(reverse_fold(f, z)() == z);
+/// assert(reverse_fold(f, z)(x, xs...) == f(reverse_fold(f, z)(xs...), x));
+/// assert(reverse_fold(f)(x) == x);
+/// assert(reverse_fold(f)(x, xs...) == f(reverse_fold(f)(xs...), x));
+///
+/// Requirements
+/// ------------
+///
+/// State must be:
+///
+/// * CopyConstructible
+///
+/// F must be:
+///
+/// * [BinaryInvocable](BinaryInvocable)
+/// * MoveConstructible
+///
+/// Example
+/// -------
+///
+/// #include <boost/hof.hpp>
+/// #include <cassert>
+///
+/// struct max_f
+/// {
+/// template<class T, class U>
+/// constexpr T operator()(T x, U y) const
+/// {
+/// return x > y ? x : y;
+/// }
+/// };
+///
+/// int main() {
+/// assert(boost::hof::reverse_fold(max_f())(2, 3, 4, 5) == 5);
+/// }
+///
+/// References
+/// ----------
+///
+/// * [Projections](Projections)
+/// * [Variadic print](<Variadic print>)
+///
+
+#include <boost/hof/detail/callable_base.hpp>
+#include <boost/hof/detail/delegate.hpp>
+#include <boost/hof/detail/compressed_pair.hpp>
+#include <boost/hof/detail/move.hpp>
+#include <boost/hof/detail/make.hpp>
+#include <boost/hof/detail/static_const_var.hpp>
+
+namespace boost { namespace hof { namespace detail {
+
+struct v_reverse_fold
+{
+ BOOST_HOF_RETURNS_CLASS(v_reverse_fold);
+ template<class F, class State, class T, class... Ts>
+ constexpr BOOST_HOF_SFINAE_MANUAL_RESULT(const F&, result_of<const v_reverse_fold&, id_<const F&>, id_<State>, id_<Ts>...>, id_<T>)
+ operator()(const F& f, State&& state, T&& x, Ts&&... xs) const BOOST_HOF_SFINAE_MANUAL_RETURNS
+ (
+ f((*BOOST_HOF_CONST_THIS)(f, BOOST_HOF_FORWARD(State)(state), BOOST_HOF_FORWARD(Ts)(xs)...), BOOST_HOF_FORWARD(T)(x))
+ );
+
+ template<class F, class State>
+ constexpr State operator()(const F&, State&& state) const noexcept
+ {
+ return BOOST_HOF_FORWARD(State)(state);
+ }
+};
+
+}
+
+template<class F, class State=void>
+struct reverse_fold_adaptor
+: detail::compressed_pair<detail::callable_base<F>, State>
+{
+ typedef detail::compressed_pair<detail::callable_base<F>, State> base_type;
+ BOOST_HOF_INHERIT_CONSTRUCTOR(reverse_fold_adaptor, base_type)
+
+ template<class... Ts>
+ constexpr const detail::callable_base<F>& base_function(Ts&&... xs) const noexcept
+ {
+ return this->first(xs...);
+ }
+
+ template<class... Ts>
+ constexpr State get_state(Ts&&... xs) const noexcept
+ {
+ return this->second(xs...);
+ }
+
+ BOOST_HOF_RETURNS_CLASS(reverse_fold_adaptor);
+
+ template<class... Ts>
+ constexpr BOOST_HOF_SFINAE_RESULT(detail::v_reverse_fold, id_<const detail::callable_base<F>&>, id_<State>, id_<Ts>...)
+ operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
+ (
+ detail::v_reverse_fold()(
+ BOOST_HOF_MANGLE_CAST(const detail::callable_base<F>&)(BOOST_HOF_CONST_THIS->base_function(xs...)),
+ BOOST_HOF_MANGLE_CAST(State)(BOOST_HOF_CONST_THIS->get_state(xs...)),
+ BOOST_HOF_FORWARD(Ts)(xs)...
+ )
+ )
+};
+
+
+template<class F>
+struct reverse_fold_adaptor<F, void>
+: detail::callable_base<F>
+{
+ BOOST_HOF_INHERIT_CONSTRUCTOR(reverse_fold_adaptor, detail::callable_base<F>)
+
+ template<class... Ts>
+ constexpr const detail::callable_base<F>& base_function(Ts&&... xs) const noexcept
+ {
+ return boost::hof::always_ref(*this)(xs...);
+ }
+
+ BOOST_HOF_RETURNS_CLASS(reverse_fold_adaptor);
+
+ template<class... Ts>
+ constexpr BOOST_HOF_SFINAE_RESULT(detail::v_reverse_fold, id_<const detail::callable_base<F>&>, id_<Ts>...)
+ operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
+ (
+ detail::v_reverse_fold()(
+ BOOST_HOF_MANGLE_CAST(const detail::callable_base<F>&)(BOOST_HOF_CONST_THIS->base_function(xs...)),
+ BOOST_HOF_FORWARD(Ts)(xs)...
+ )
+ )
+};
+
+BOOST_HOF_DECLARE_STATIC_VAR(reverse_fold, detail::make<reverse_fold_adaptor>);
+
+}} // namespace boost::hof
+
+#endif