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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::prepend`. | |
4 | ||
5 | @copyright Louis Dionne 2013-2016 | |
6 | Distributed under the Boost Software License, Version 1.0. | |
7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) | |
8 | */ | |
9 | ||
10 | #ifndef BOOST_HANA_FWD_PREPEND_HPP | |
11 | #define BOOST_HANA_FWD_PREPEND_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! Prepend an element to a monadic structure. | |
19 | //! @ingroup group-MonadPlus | |
20 | //! | |
21 | //! Given a monadic structure `xs` and an element `x`, `prepend` returns | |
22 | //! a new monadic structure which is the result of lifting `x` into the | |
23 | //! monadic structure and then combining that (to the left) with `xs`. | |
24 | //! In other words, | |
25 | //! @code | |
26 | //! prepend(xs, x) == concat(lift<Xs>(x), xs) | |
27 | //! @endcode | |
28 | //! | |
29 | //! For sequences, this has the intuitive behavior of simply prepending | |
30 | //! an element to the beginning of the sequence, hence the name. | |
31 | //! | |
32 | //! > #### Rationale for not calling this `push_front` | |
33 | //! > While `push_front` is the de-facto name used in the standard library, | |
34 | //! > it also strongly suggests mutation of the underlying sequence, which | |
35 | //! > is not the case here. The author also finds that `push_front` | |
36 | //! > suggests too strongly the sole interpretation of putting an | |
37 | //! > element to the front of a sequence, whereas `prepend` is slightly | |
38 | //! > more nuanced and bears its name better for e.g. `hana::optional`. | |
39 | //! | |
40 | //! | |
41 | //! Signature | |
42 | //! --------- | |
43 | //! Given a MonadPlus `M`, the signature is | |
44 | //! @f$ \mathtt{prepend} : M(T) \times T \to M(T) @f$. | |
45 | //! | |
46 | //! @param xs | |
47 | //! A monadic structure that will be combined to the right of the element. | |
48 | //! | |
49 | //! @param x | |
50 | //! An element to combine to the left of the monadic structure. | |
51 | //! | |
52 | //! | |
53 | //! Example | |
54 | //! ------- | |
55 | //! @include example/prepend.cpp | |
56 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
57 | constexpr auto prepend = [](auto&& xs, auto&& x) { | |
58 | return tag-dispatched; | |
59 | }; | |
60 | #else | |
61 | template <typename M, typename = void> | |
62 | struct prepend_impl : prepend_impl<M, when<true>> { }; | |
63 | ||
64 | struct prepend_t { | |
65 | template <typename Xs, typename X> | |
66 | constexpr auto operator()(Xs&& xs, X&& x) const; | |
67 | }; | |
68 | ||
69 | constexpr prepend_t prepend{}; | |
70 | #endif | |
71 | BOOST_HANA_NAMESPACE_END | |
72 | ||
73 | #endif // !BOOST_HANA_FWD_PREPEND_HPP |