ceph/src/boost/libs/hana/include/boost/hana/fwd/filter.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::filter`.
   4
   5 @copyright Louis Dionne 2013-2016
   6 Distributed 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
  17 BOOST_HANA_NAMESPACE_BEGIN
  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     constexpr filter_t filter{};
  78 #endif
  79 BOOST_HANA_NAMESPACE_END
  80
  81 #endif // !BOOST_HANA_FWD_FILTER_HPP