hana/fwd/while.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::while_`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_WHILE_HPP
0011 #define BOOST_HANA_FWD_WHILE_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Apply a function to an initial state while some predicate is satisfied.
0019     //! @ingroup group-Logical
0020     //!
0021     //! This method is a natural extension of the `while` language construct
0022     //! to manipulate a state whose type may change from one iteration to
0023     //! another. However, note that having a state whose type changes from
0024     //! one iteration to the other is only possible as long as the predicate
0025     //! returns a `Logical` whose truth value is known at compile-time.
0026     //!
0027     //! Specifically, `while_(pred, state, f)` is equivalent to
0028     //! @code
0029     //!     f(...f(f(state)))
0030     //! @endcode
0031     //! where `f` is iterated as long as `pred(f(...))` is a true-valued
0032     //! `Logical`.
0033     //!
0034     //!
0035     //! @param pred
0036     //! A predicate called on the state or on the result of applying `f` a
0037     //! certain number of times to the state, and returning whether `f`
0038     //! should be applied one more time.
0039     //!
0040     //! @param state
0041     //! The initial state on which `f` is applied.
0042     //!
0043     //! @param f
0044     //! A function that is iterated on the initial state. Note that the
0045     //! return type of `f` may change from one iteration to the other,
0046     //! but only while `pred` returns a compile-time `Logical`. In other
0047     //! words, `decltype(f(stateN))` may differ from `decltype(f(stateN+1))`,
0048     //! but only if `pred(f(stateN))` returns a compile-time `Logical`.
0049     //!
0050     //!
0051     //! Example
0052     //! -------
0053     //! @include example/while.cpp
0054 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0055     constexpr auto while_ = [](auto&& pred, auto&& state, auto&& f) -> decltype(auto) {
0056         return tag-dispatched;
0057     };
0058 #else
0059     template <typename L, typename = void>
0060     struct while_impl : while_impl<L, when<true>> { };
0061
0062     struct while_t {
0063         template <typename Pred, typename State, typename F>
0064         constexpr decltype(auto) operator()(Pred&& pred, State&& state, F&& f) const;
0065     };
0066
0067     BOOST_HANA_INLINE_VARIABLE constexpr while_t while_{};
0068 #endif
0069 }} // end namespace boost::hana
0070
0071 #endif // !BOOST_HANA_FWD_WHILE_HPP