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1 | /*! |
2 | @file | |
3 | Forward declares `boost::hana::lazy`. | |
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_LAZY_HPP | |
11 | #define BOOST_HANA_FWD_LAZY_HPP | |
12 | ||
13 | #include <boost/hana/config.hpp> | |
14 | #include <boost/hana/fwd/core/make.hpp> | |
15 | ||
16 | ||
17 | BOOST_HANA_NAMESPACE_BEGIN | |
18 | //! @ingroup group-datatypes | |
19 | //! `hana::lazy` implements superficial laziness via a monadic interface. | |
20 | //! | |
21 | //! It is important to understand that the laziness implemented by `lazy` | |
22 | //! is only superficial; only function applications made inside the `lazy` | |
23 | //! monad can be made lazy, not all their subexpressions. | |
24 | //! | |
25 | //! | |
26 | //! @note | |
27 | //! The actual representation of `hana::lazy` is completely | |
28 | //! implementation-defined. Lazy values may only be created through | |
29 | //! `hana::make_lazy`, and they can be stored in variables using | |
30 | //! `auto`, but any other assumption about the representation of | |
31 | //! `hana::lazy<...>` should be avoided. In particular, one should | |
32 | //! not rely on the fact that `hana::lazy<...>` can be pattern-matched | |
33 | //! on, because it may be a dependent type. | |
34 | //! | |
35 | //! | |
36 | //! Modeled concepts | |
37 | //! ---------------- | |
38 | //! 1. `Functor`\n | |
39 | //! Applying a function over a lazy value with `transform` returns the | |
40 | //! result of applying the function, as a lazy value. | |
41 | //! @include example/lazy/functor.cpp | |
42 | //! | |
43 | //! 2. `Applicative`\n | |
44 | //! A normal value can be lifted into a lazy value by using `lift<lazy_tag>`. | |
45 | //! A lazy function can be lazily applied to a lazy value by using `ap`. | |
46 | //! | |
47 | //! 3. `Monad`\n | |
48 | //! The `lazy` monad allows combining lazy computations into larger | |
49 | //! lazy computations. Note that the `|` operator can be used in place | |
50 | //! of the `chain` function. | |
51 | //! @include example/lazy/monad.cpp | |
52 | //! | |
53 | //! 4. `Comonad`\n | |
54 | //! The `lazy` comonad allows evaluating a lazy computation to get its | |
55 | //! result and lazily applying functions taking lazy inputs to lazy | |
56 | //! values. This [blog post][1] goes into more details about lazy | |
57 | //! evaluation and comonads. | |
58 | //! @include example/lazy/comonad.cpp | |
59 | //! | |
60 | //! | |
61 | //! @note | |
62 | //! `hana::lazy` only models a few concepts because providing more | |
63 | //! functionality would require evaluating the lazy values in most cases. | |
64 | //! Since this raises some issues such as side effects and memoization, | |
65 | //! the interface is kept minimal. | |
66 | //! | |
67 | //! | |
68 | //! [1]: http://ldionne.com/2015/03/16/laziness-as-a-comonad | |
69 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
70 | template <typename implementation_defined> | |
71 | struct lazy { | |
72 | //! Equivalent to `hana::chain`. | |
73 | template <typename ...T, typename F> | |
74 | friend constexpr auto operator|(lazy<T...>, F); | |
75 | }; | |
76 | #else | |
77 | // We do not _actually_ define the lazy<...> type. Per the documentation, | |
78 | // users can't rely on it being anything, and so they should never use | |
79 | // it explicitly. The implementation in <boost/hana/lazy.hpp> is much | |
80 | // simpler if we use different types for lazy calls and lazy values. | |
81 | #endif | |
82 | ||
83 | //! Tag representing `hana::lazy`. | |
84 | //! @relates hana::lazy | |
85 | struct lazy_tag { }; | |
86 | ||
87 | //! Lifts a normal value to a lazy one. | |
88 | //! @relates hana::lazy | |
89 | //! | |
90 | //! `make<lazy_tag>` can be used to lift a normal value or a function call | |
91 | //! into a lazy expression. Precisely, `make<lazy_tag>(x)` is a lazy value | |
92 | //! equal to `x`, and `make<lazy_tag>(f)(x1, ..., xN)` is a lazy function | |
93 | //! call that is equal to `f(x1, ..., xN)` when it is `eval`uated. | |
94 | //! | |
95 | //! @note | |
96 | //! It is interesting to note that `make<lazy_tag>(f)(x1, ..., xN)` is | |
97 | //! equivalent to | |
98 | //! @code | |
99 | //! ap(make<lazy_tag>(f), lift<lazy_tag>(x1), ..., lift<lazy_tag>(xN)) | |
100 | //! @endcode | |
101 | //! which in turn is equivalent to `make<lazy_tag>(f(x1, ..., xN))`, except | |
102 | //! for the fact that the inner call to `f` is evaluated lazily. | |
103 | //! | |
104 | //! | |
105 | //! Example | |
106 | //! ------- | |
107 | //! @include example/lazy/make.cpp | |
108 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
109 | template <> | |
110 | constexpr auto make<lazy_tag> = [](auto&& x) { | |
111 | return lazy<implementation_defined>{forwarded(x)}; | |
112 | }; | |
113 | #endif | |
114 | ||
115 | //! Alias to `make<lazy_tag>`; provided for convenience. | |
116 | //! @relates hana::lazy | |
117 | //! | |
118 | //! Example | |
119 | //! ------- | |
120 | //! @include example/lazy/make.cpp | |
121 | constexpr auto make_lazy = make<lazy_tag>; | |
122 | BOOST_HANA_NAMESPACE_END | |
123 | ||
124 | #endif // !BOOST_HANA_FWD_LAZY_HPP |