text: /*!
  @file
  Forward declares `boost::hana::prepend`.
  
  @copyright Louis Dionne 2013-2017
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   */
  
  #ifndef BOOST_HANA_FWD_PREPEND_HPP
  #define BOOST_HANA_FWD_PREPEND_HPP
  
  #include <boost/hana/config.hpp>
  #include <boost/hana/core/when.hpp>
  
  
  namespace boost { namespace hana {
      //! Prepend an element to a monadic structure.
      //! @ingroup group-MonadPlus
      //!
      //! Given a monadic structure `xs` and an element `x`, `prepend` returns
      //! a new monadic structure which is the result of lifting `x` into the
      //! monadic structure and then combining that (to the left) with `xs`.
      //! In other words,
      //! @code
      //!     prepend(xs, x) == concat(lift<Xs>(x), xs)
      //! @endcode
      //!
      //! For sequences, this has the intuitive behavior of simply prepending
      //! an element to the beginning of the sequence, hence the name.
      //!
      //! > #### Rationale for not calling this `push_front`
      //! > While `push_front` is the de-facto name used in the standard library,
      //! > it also strongly suggests mutation of the underlying sequence, which
      //! > is not the case here. The author also finds that `push_front`
      //! > suggests too strongly the sole interpretation of putting an
      //! > element to the front of a sequence, whereas `prepend` is slightly
      //! > more nuanced and bears its name better for e.g. `hana::optional`.
      //!
      //!
      //! Signature
      //! ---------
      //! Given a MonadPlus `M`, the signature is
      //! @f$ \mathtt{prepend} : M(T) \times T \to M(T) @f$.
      //!
      //! @param xs
      //! A monadic structure that will be combined to the right of the element.
      //!
      //! @param x
      //! An element to combine to the left of the monadic structure.
      //!
      //!
      //! Example
      //! -------
      //! @include example/prepend.cpp
  #ifdef BOOST_HANA_DOXYGEN_INVOKED
      constexpr auto prepend = [](auto&& xs, auto&& x) {
          return tag-dispatched;
      };
  #else
      template <typename M, typename = void>
      struct prepend_impl : prepend_impl<M, when<true>> { };
  
      struct prepend_t {
          template <typename Xs, typename X>
          constexpr auto operator()(Xs&& xs, X&& x) const;
      };
  
      BOOST_HANA_INLINE_VARIABLE constexpr prepend_t prepend{};
  #endif
  }} // end namespace boost::hana
  
  #endif // !BOOST_HANA_FWD_PREPEND_HPP
  