73 lines
2.1 KiB
C++
73 lines
2.1 KiB
C++
|
/*!
|
||
|
|
@file
|
||
|
|
Forward declares `boost::hana::prefix`.
|
||
|
|
|
||
|
|
@copyright Louis Dionne 2013-2016
|
||
|
|
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>
|
||
|
|
|
||
|
|
|
||
|
|
BOOST_HANA_NAMESPACE_BEGIN
|
||
|
|
//! 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;
|
||
|
|
};
|
||
|
|
|
||
|
|
constexpr prefix_t prefix{};
|
||
|
|
#endif
|
||
|
|
BOOST_HANA_NAMESPACE_END
|
||
|
|
|
||
|
|
#endif // !BOOST_HANA_FWD_PREFIX_HPP
|