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 Hu Chunming
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/*!
@file
Forward declares `boost::hana::while_`.
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_WHILE_HPP
#define BOOST_HANA_FWD_WHILE_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Apply a function to an initial state while some predicate is satisfied.
//! @ingroup group-Logical
//!
//! This method is a natural extension of the `while` language construct
//! to manipulate a state whose type may change from one iteration to
//! another. However, note that having a state whose type changes from
//! one iteration to the other is only possible as long as the predicate
//! returns a `Logical` whose truth value is known at compile-time.
//!
//! Specifically, `while_(pred, state, f)` is equivalent to
//! @code
//! f(...f(f(state)))
//! @endcode
//! where `f` is iterated as long as `pred(f(...))` is a true-valued
//! `Logical`.
//!
//!
//! @param pred
//! A predicate called on the state or on the result of applying `f` a
//! certain number of times to the state, and returning whether `f`
//! should be applied one more time.
//!
//! @param state
//! The initial state on which `f` is applied.
//!
//! @param f
//! A function that is iterated on the initial state. Note that the
//! return type of `f` may change from one iteration to the other,
//! but only while `pred` returns a compile-time `Logical`. In other
//! words, `decltype(f(stateN))` may differ from `decltype(f(stateN+1))`,
//! but only if `pred(f(stateN))` returns a compile-time `Logical`.
//!
//!
//! Example
//! -------
//! @include example/while.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto while_ = [](auto&& pred, auto&& state, auto&& f) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename L, typename = void>
struct while_impl : while_impl<L, when<true>> { };
struct while_t {
template <typename Pred, typename State, typename F>
constexpr decltype(auto) operator()(Pred&& pred, State&& state, F&& f) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr while_t while_{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_WHILE_HPP
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