hana/fwd/remove_if.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::remove_if`.
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_REMOVE_IF_HPP
0011 #define BOOST_HANA_FWD_REMOVE_IF_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015
0016
0017 namespace boost { namespace hana {
0018     //! Remove all the elements of a monadic structure that satisfy some
0019     //! predicate.
0020     //! @ingroup group-MonadPlus
0021     //!
0022     //! Given a monadic structure `xs` and a unary predicate, `remove_if`
0023     //! returns a new monadic structure equal to `xs` without all its elements
0024     //! that satisfy the predicate. This is equivalent to `filter` with a
0025     //! negated predicate, i.e.
0026     //! @code
0027     //!     remove_if(xs, predicate) == filter(xs, negated predicated)
0028     //! @endcode
0029     //!
0030     //!
0031     //! Signature
0032     //! ---------
0033     //! Given a MonadPlus `M` and a predicate of type \f$ T \to Bool \f$ for
0034     //! some compile-time Logical `Bool`, the signature is
0035     //! \f$
0036     //!     \mathrm{remove\_if} : M(T) \times (T \to Bool) \to M(T)
0037     //! \f$
0038     //!
0039     //! @param xs
0040     //! A monadic structure to remove some elements from.
0041     //!
0042     //! @param predicate
0043     //! A unary predicate called as `predicate(x)`, where `x` is an element
0044     //! of the structure, and returning whether `x` should be removed from
0045     //! the structure. In the current version of the library, `predicate`
0046     //! must return a compile-time Logical.
0047     //!
0048     //!
0049     //! Example
0050     //! -------
0051     //! @include example/remove_if.cpp
0052 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0053     constexpr auto remove_if = [](auto&& xs, auto&& predicate) {
0054         return tag-dispatched;
0055     };
0056 #else
0057     template <typename M, typename = void>
0058     struct remove_if_impl : remove_if_impl<M, when<true>> { };
0059
0060     struct remove_if_t {
0061         template <typename Xs, typename Pred>
0062         constexpr auto operator()(Xs&& xs, Pred&& pred) const;
0063     };
0064
0065     BOOST_HANA_INLINE_VARIABLE constexpr remove_if_t remove_if{};
0066 #endif
0067 }} // end namespace boost::hana
0068
0069 #endif // !BOOST_HANA_FWD_REMOVE_IF_HPP