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

   1 /*!
   2 @file
   3 Forward declares `boost::hana::prepend`.
   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_PREPEND_HPP
  11 #define BOOST_HANA_FWD_PREPEND_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15
  16
  17 BOOST_HANA_NAMESPACE_BEGIN
  18     //! Prepend an element to a monadic structure.
  19     //! @ingroup group-MonadPlus
  20     //!
  21     //! Given a monadic structure `xs` and an element `x`, `prepend` returns
  22     //! a new monadic structure which is the result of lifting `x` into the
  23     //! monadic structure and then combining that (to the left) with `xs`.
  24     //! In other words,
  25     //! @code
  26     //!     prepend(xs, x) == concat(lift<Xs>(x), xs)
  27     //! @endcode
  28     //!
  29     //! For sequences, this has the intuitive behavior of simply prepending
  30     //! an element to the beginning of the sequence, hence the name.
  31     //!
  32     //! > #### Rationale for not calling this `push_front`
  33     //! > While `push_front` is the de-facto name used in the standard library,
  34     //! > it also strongly suggests mutation of the underlying sequence, which
  35     //! > is not the case here. The author also finds that `push_front`
  36     //! > suggests too strongly the sole interpretation of putting an
  37     //! > element to the front of a sequence, whereas `prepend` is slightly
  38     //! > more nuanced and bears its name better for e.g. `hana::optional`.
  39     //!
  40     //!
  41     //! Signature
  42     //! ---------
  43     //! Given a MonadPlus `M`, the signature is
  44     //! @f$ \mathtt{prepend} : M(T) \times T \to M(T) @f$.
  45     //!
  46     //! @param xs
  47     //! A monadic structure that will be combined to the right of the element.
  48     //!
  49     //! @param x
  50     //! An element to combine to the left of the monadic structure.
  51     //!
  52     //!
  53     //! Example
  54     //! -------
  55     //! @include example/prepend.cpp
  56 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  57     constexpr auto prepend = [](auto&& xs, auto&& x) {
  58         return tag-dispatched;
  59     };
  60 #else
  61     template <typename M, typename = void>
  62     struct prepend_impl : prepend_impl<M, when<true>> { };
  63
  64     struct prepend_t {
  65         template <typename Xs, typename X>
  66         constexpr auto operator()(Xs&& xs, X&& x) const;
  67     };
  68
  69     constexpr prepend_t prepend{};
  70 #endif
  71 BOOST_HANA_NAMESPACE_END
  72
  73 #endif // !BOOST_HANA_FWD_PREPEND_HPP