/*!
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@file
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Forward declares `boost::hana::filter`.
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@copyright Louis Dionne 2013-2017
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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_FILTER_HPP
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#define BOOST_HANA_FWD_FILTER_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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//! Filter a monadic structure using a custom predicate.
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//! @ingroup group-MonadPlus
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//!
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//! Given a monadic structure and a predicate, `filter` returns a new
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//! monadic structure containing only those elements that satisfy the
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//! predicate. This is a generalization of the usual `filter` function
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//! for sequences; it works for any MonadPlus. Intuitively, `filter` is
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//! somewhat equivalent to:
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//! @code
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//! filter(xs, pred) == flatten(transform(xs, [](auto x) {
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//! return pred(x) ? lift<Xs>(x) : empty<Xs>();
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//! })
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//! @endcode
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//! In other words, we basically turn a monadic structure containing
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//! `[x1, ..., xn]` into a monadic structure containing
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//! @code
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//! [
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//! pred(x1) ? [x1] : [],
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//! pred(x2) ? [x2] : [],
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//! ...
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//! pred(xn) ? [xn] : []
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//! ]
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//! @endcode
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//! and we then `flatten` that.
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//!
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//!
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//! Signature
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//! ---------
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//! Given a `MonadPlus` `M` and an `IntegralConstant` `Bool` holding a
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//! value of type `bool`, the signature is
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//! @f$ \mathtt{filter} : M(T) \times (T \to \mathtt{Bool}) \to M(T) @f$.
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//!
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//! @param xs
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//! The monadic structure to filter.
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//!
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//! @param pred
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//! A function called as `pred(x)` for each element `x` in the monadic
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//! structure and returning whether that element should be __kept__ in
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//! the resulting structure. In the current version of the library, the
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//! predicate has to return an `IntegralConstant` holding a value
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//! convertible to a `bool`.
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//!
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//!
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//! Example
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//! -------
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//! @include example/filter.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto filter = [](auto&& xs, auto&& pred) {
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return tag-dispatched;
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};
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#else
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template <typename M, typename = void>
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struct filter_impl : filter_impl<M, when<true>> { };
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struct filter_t {
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template <typename Xs, typename Pred>
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constexpr auto operator()(Xs&& xs, Pred&& pred) const;
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};
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constexpr filter_t filter{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_FILTER_HPP
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