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