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 Hu Chunming
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/*!
@file
Forward declares `boost::hana::flatten`.
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_FLATTEN_HPP
#define BOOST_HANA_FWD_FLATTEN_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Collapse two levels of monadic structure into a single level.
//! @ingroup group-Monad
//!
//! Given a monadic value wrapped into two levels of monad, `flatten`
//! removes one such level. An implementation of `flatten` must satisfy
//! @code
//! flatten(xs) == chain(xs, id)
//! @endcode
//!
//! For `Sequence`s, this simply takes a `Sequence` of `Sequence`s, and
//! returns a (non-recursively) flattened `Sequence`.
//!
//!
//! Signature
//! ---------
//! For a `Monad` `M`, the signature of `flatten` is
//! @f$
//! \mathtt{flatten} : M(M(T)) \to M(T)
//! @f$
//!
//! @param xs
//! A value with two levels of monadic structure, which should be
//! collapsed into a single level of structure.
//!
//!
//! Example
//! -------
//! @include example/flatten.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto flatten = [](auto&& xs) {
return tag-dispatched;
};
#else
template <typename M, typename = void>
struct flatten_impl : flatten_impl<M, when<true>> { };
struct flatten_t {
template <typename Xs>
constexpr auto operator()(Xs&& xs) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr flatten_t flatten{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_FLATTEN_HPP
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