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 Hu Chunming
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/*!
@file
Forward declares `boost::hana::prefix`.
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_PREFIX_HPP
#define BOOST_HANA_FWD_PREFIX_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Inserts a value before each element of a monadic structure.
//! @ingroup group-MonadPlus
//!
//! Given a monadic structure `xs` and a value `z` called the prefix,
//! `prefix` returns a new monadic structure. `prefix` satisfies
//! @code
//! prefix(xs, z) == flatten(transform(xs, [](auto x) {
//! return concat(lift<M>(z), lift<M>(x));
//! }))
//! @endcode
//!
//! For sequences, this simply corresponds to inserting the prefix before
//! each element of the sequence. For example, given a sequence
//! `[x1, ..., xn]`, `prefix` will return
//! @code
//! [z, x1, z, x2, ..., z, xn]
//! @endcode
//! As explained above, this can be generalized to other MonadPlus models,
//! with various levels of interest.
//!
//!
//! Signature
//! ---------
//! Given a MonadPlus `M`, the signature is
//! @f$ \mathrm{prefix} : M(T) \times T \to M(T) @f$.
//!
//! @param xs
//! A monadic structure.
//!
//! @param pref
//! A value (the prefix) to insert before each element of a monadic
//! structure.
//!
//!
//! Example
//! -------
//! @include example/prefix.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto prefix = [](auto&& xs, auto&& pref) {
return tag-dispatched;
};
#else
template <typename M, typename = void>
struct prefix_impl : prefix_impl<M, when<true>> { };
struct prefix_t {
template <typename Xs, typename Pref>
constexpr auto operator()(Xs&& xs, Pref&& pref) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr prefix_t prefix{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_PREFIX_HPP
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