74 lines
2.4 KiB
C++
74 lines
2.4 KiB
C++
/*!
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@file
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Forward declares `boost::hana::prepend`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_PREPEND_HPP
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#define BOOST_HANA_FWD_PREPEND_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Prepend an element to a monadic structure.
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//! @ingroup group-MonadPlus
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//!
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//! Given a monadic structure `xs` and an element `x`, `prepend` returns
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//! a new monadic structure which is the result of lifting `x` into the
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//! monadic structure and then combining that (to the left) with `xs`.
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//! In other words,
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//! @code
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//! prepend(xs, x) == concat(lift<Xs>(x), xs)
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//! @endcode
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//!
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//! For sequences, this has the intuitive behavior of simply prepending
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//! an element to the beginning of the sequence, hence the name.
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//!
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//! > #### Rationale for not calling this `push_front`
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//! > While `push_front` is the de-facto name used in the standard library,
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//! > it also strongly suggests mutation of the underlying sequence, which
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//! > is not the case here. The author also finds that `push_front`
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//! > suggests too strongly the sole interpretation of putting an
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//! > element to the front of a sequence, whereas `prepend` is slightly
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//! > more nuanced and bears its name better for e.g. `hana::optional`.
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//!
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//!
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//! Signature
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//! ---------
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//! Given a MonadPlus `M`, the signature is
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//! @f$ \mathtt{prepend} : M(T) \times T \to M(T) @f$.
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//!
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//! @param xs
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//! A monadic structure that will be combined to the right of the element.
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//!
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//! @param x
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//! An element to combine to the left of the monadic structure.
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//!
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//!
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//! Example
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//! -------
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//! @include example/prepend.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto prepend = [](auto&& xs, auto&& x) {
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return tag-dispatched;
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};
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#else
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template <typename M, typename = void>
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struct prepend_impl : prepend_impl<M, when<true>> { };
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struct prepend_t {
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template <typename Xs, typename X>
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constexpr auto operator()(Xs&& xs, X&& x) const;
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};
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constexpr prepend_t prepend{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_PREPEND_HPP
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