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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::prefix`. | |
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_PREFIX_HPP | |
11 | #define BOOST_HANA_FWD_PREFIX_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/core/when.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! Inserts a value before each element of a monadic structure. | |
19 | //! @ingroup group-MonadPlus | |
20 | //! | |
21 | //! Given a monadic structure `xs` and a value `z` called the prefix, | |
22 | //! `prefix` returns a new monadic structure. `prefix` satisfies | |
23 | //! @code | |
24 | //! prefix(xs, z) == flatten(transform(xs, [](auto x) { | |
25 | //! return concat(lift<M>(z), lift<M>(x)); | |
26 | //! })) | |
27 | //! @endcode | |
28 | //! | |
29 | //! For sequences, this simply corresponds to inserting the prefix before | |
30 | //! each element of the sequence. For example, given a sequence | |
31 | //! `[x1, ..., xn]`, `prefix` will return | |
32 | //! @code | |
33 | //! [z, x1, z, x2, ..., z, xn] | |
34 | //! @endcode | |
35 | //! As explained above, this can be generalized to other MonadPlus models, | |
36 | //! with various levels of interest. | |
37 | //! | |
38 | //! | |
39 | //! Signature | |
40 | //! --------- | |
41 | //! Given a MonadPlus `M`, the signature is | |
42 | //! @f$ \mathrm{prefix} : M(T) \times T \to M(T) @f$. | |
43 | //! | |
44 | //! @param xs | |
45 | //! A monadic structure. | |
46 | //! | |
47 | //! @param pref | |
48 | //! A value (the prefix) to insert before each element of a monadic | |
49 | //! structure. | |
50 | //! | |
51 | //! | |
52 | //! Example | |
53 | //! ------- | |
54 | //! @include example/prefix.cpp | |
55 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
56 | constexpr auto prefix = [](auto&& xs, auto&& pref) { | |
57 | return tag-dispatched; | |
58 | }; | |
59 | #else | |
60 | template <typename M, typename = void> | |
61 | struct prefix_impl : prefix_impl<M, when<true>> { }; | |
62 | ||
63 | struct prefix_t { | |
64 | template <typename Xs, typename Pref> | |
65 | constexpr auto operator()(Xs&& xs, Pref&& pref) const; | |
66 | }; | |
67 | ||
68 | constexpr prefix_t prefix{}; | |
69 | #endif | |
70 | BOOST_HANA_NAMESPACE_END | |
71 | ||
72 | #endif // !BOOST_HANA_FWD_PREFIX_HPP |