1/*!
2@file
3Forward declares `boost::hana::filter`.
4
5Copyright Louis Dionne 2013-2022
6Distributed under the Boost Software License, Version 1.0.
7(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
8 */
9
10#ifndef BOOST_HANA_FWD_FILTER_HPP
11#define BOOST_HANA_FWD_FILTER_HPP
12
13#include <boost/hana/config.hpp>
14#include <boost/hana/core/when.hpp>
15
16
17namespace boost { namespace hana {
18 //! Filter a monadic structure using a custom predicate.
19 //! @ingroup group-MonadPlus
20 //!
21 //! Given a monadic structure and a predicate, `filter` returns a new
22 //! monadic structure containing only those elements that satisfy the
23 //! predicate. This is a generalization of the usual `filter` function
24 //! for sequences; it works for any MonadPlus. Intuitively, `filter` is
25 //! somewhat equivalent to:
26 //! @code
27 //! filter(xs, pred) == flatten(transform(xs, [](auto x) {
28 //! return pred(x) ? lift<Xs>(x) : empty<Xs>();
29 //! })
30 //! @endcode
31 //! In other words, we basically turn a monadic structure containing
32 //! `[x1, ..., xn]` into a monadic structure containing
33 //! @code
34 //! [
35 //! pred(x1) ? [x1] : [],
36 //! pred(x2) ? [x2] : [],
37 //! ...
38 //! pred(xn) ? [xn] : []
39 //! ]
40 //! @endcode
41 //! and we then `flatten` that.
42 //!
43 //!
44 //! Signature
45 //! ---------
46 //! Given a `MonadPlus` `M` and an `IntegralConstant` `Bool` holding a
47 //! value of type `bool`, the signature is
48 //! @f$ \mathtt{filter} : M(T) \times (T \to \mathtt{Bool}) \to M(T) @f$.
49 //!
50 //! @param xs
51 //! The monadic structure to filter.
52 //!
53 //! @param pred
54 //! A function called as `pred(x)` for each element `x` in the monadic
55 //! structure and returning whether that element should be __kept__ in
56 //! the resulting structure. In the current version of the library, the
57 //! predicate has to return an `IntegralConstant` holding a value
58 //! convertible to a `bool`.
59 //!
60 //!
61 //! Example
62 //! -------
63 //! @include example/filter.cpp
64#ifdef BOOST_HANA_DOXYGEN_INVOKED
65 constexpr auto filter = [](auto&& xs, auto&& pred) {
66 return tag-dispatched;
67 };
68#else
69 template <typename M, typename = void>
70 struct filter_impl : filter_impl<M, when<true>> { };
71
72 struct filter_t {
73 template <typename Xs, typename Pred>
74 constexpr auto operator()(Xs&& xs, Pred&& pred) const;
75 };
76
77 BOOST_HANA_INLINE_VARIABLE constexpr filter_t filter{};
78#endif
79}} // end namespace boost::hana
80
81#endif // !BOOST_HANA_FWD_FILTER_HPP
82

source code of boost/libs/hana/include/boost/hana/fwd/filter.hpp