mirror of
https://github.com/ecency/ecency-mobile.git
synced 2024-12-23 05:13:04 +03:00
89 lines
3.0 KiB
C++
89 lines
3.0 KiB
C++
/*!
|
|
@file
|
|
Forward declares `boost::hana::fold_left`.
|
|
|
|
@copyright Louis Dionne 2013-2016
|
|
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_FOLD_LEFT_HPP
|
|
#define BOOST_HANA_FWD_FOLD_LEFT_HPP
|
|
|
|
#include <boost/hana/config.hpp>
|
|
#include <boost/hana/core/when.hpp>
|
|
|
|
|
|
BOOST_HANA_NAMESPACE_BEGIN
|
|
//! Left-fold of a structure using a binary operation and an optional
|
|
//! initial reduction state.
|
|
//! @ingroup group-Foldable
|
|
//!
|
|
//! `fold_left` is a left-associative fold using a binary operation.
|
|
//! Given a structure containing `x1, ..., xn`, a function `f` and
|
|
//! an optional initial state, `fold_left` applies `f` as follows
|
|
//! @code
|
|
//! f(... f(f(f(x1, x2), x3), x4) ..., xn) // without state
|
|
//! f(... f(f(f(f(state, x1), x2), x3), x4) ..., xn) // with state
|
|
//! @endcode
|
|
//!
|
|
//! When the structure is empty, two things may arise. If an initial
|
|
//! state was provided, it is returned as-is. Otherwise, if the no-state
|
|
//! version of the function was used, an error is triggered. When the
|
|
//! stucture contains a single element and the no-state version of the
|
|
//! function was used, that single element is returned as is.
|
|
//!
|
|
//!
|
|
//! Signature
|
|
//! ---------
|
|
//! Given a `Foldable` `F` and an optional initial state of tag `S`,
|
|
//! the signatures for `fold_left` are
|
|
//! \f[
|
|
//! \mathtt{fold\_left} : F(T) \times S \times (S \times T \to S) \to S
|
|
//! \f]
|
|
//!
|
|
//! for the variant with an initial state, and
|
|
//! \f[
|
|
//! \mathtt{fold\_left} : F(T) \times (T \times T \to T) \to T
|
|
//! \f]
|
|
//!
|
|
//! for the variant without an initial state.
|
|
//!
|
|
//! @param xs
|
|
//! The structure to fold.
|
|
//!
|
|
//! @param state
|
|
//! The initial value used for folding.
|
|
//!
|
|
//! @param f
|
|
//! A binary function called as `f(state, x)`, where `state` is the
|
|
//! result accumulated so far and `x` is an element in the structure.
|
|
//! For left folds without an initial state, the function is called as
|
|
//! `f(x1, x2)`, where `x1` and `x2` are elements of the structure.
|
|
//!
|
|
//!
|
|
//! Example
|
|
//! -------
|
|
//! @include example/fold_left.cpp
|
|
#ifdef BOOST_HANA_DOXYGEN_INVOKED
|
|
constexpr auto fold_left = [](auto&& xs[, auto&& state], auto&& f) -> decltype(auto) {
|
|
return tag-dispatched;
|
|
};
|
|
#else
|
|
template <typename T, typename = void>
|
|
struct fold_left_impl : fold_left_impl<T, when<true>> { };
|
|
|
|
struct fold_left_t {
|
|
template <typename Xs, typename State, typename F>
|
|
constexpr decltype(auto) operator()(Xs&& xs, State&& state, F&& f) const;
|
|
|
|
template <typename Xs, typename F>
|
|
constexpr decltype(auto) operator()(Xs&& xs, F&& f) const;
|
|
};
|
|
|
|
constexpr fold_left_t fold_left{};
|
|
#endif
|
|
BOOST_HANA_NAMESPACE_END
|
|
|
|
#endif // !BOOST_HANA_FWD_FOLD_LEFT_HPP
|