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 Hu Chunming
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/*!
@file
Forward declares `boost::hana::prepend`.
Copyright Louis Dionne 2013-2022
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
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