1/*!
2@file
3Defines `boost::hana::fix`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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_FUNCTIONAL_FIX_HPP
11#define BOOST_HANA_FUNCTIONAL_FIX_HPP
12
13#include <boost/hana/config.hpp>
14#include <boost/hana/detail/create.hpp>
15
16#include <utility>
17
18
19namespace boost { namespace hana {
20 //! @ingroup group-functional
21 //! Return a function computing the fixed point of a function.
22 //!
23 //! `fix` is an implementation of the [Y-combinator][], also called the
24 //! fixed-point combinator. It encodes the idea of recursion, and in fact
25 //! any recursive function can be written in terms of it.
26 //!
27 //! Specifically, `fix(f)` is a function such that
28 //! @code
29 //! fix(f)(x...) == f(fix(f), x...)
30 //! @endcode
31 //!
32 //! This definition allows `f` to use its first argument as a continuation
33 //! to call itself recursively. Indeed, if `f` calls its first argument
34 //! with `y...`, it is equivalent to calling `f(fix(f), y...)` per the
35 //! above equation.
36 //!
37 //! Most of the time, it is more convenient and efficient to define
38 //! recursive functions without using a fixed-point combinator. However,
39 //! there are some cases where `fix` provides either more flexibility
40 //! (e.g. the ability to change the callback inside `f`) or makes it
41 //! possible to write functions that couldn't be defined recursively
42 //! otherwise.
43 //!
44 //! @param f
45 //! A function called as `f(self, x...)`, where `x...` are the arguments
46 //! in the `fix(f)(x...)` expression and `self` is `fix(f)`.
47 //!
48 //! ### Example
49 //! @include example/functional/fix.cpp
50 //!
51 //! [Y-combinator]: http://en.wikipedia.org/wiki/Fixed-point_combinator
52#ifdef BOOST_HANA_DOXYGEN_INVOKED
53 constexpr auto fix = [](auto&& f) {
54 return [perfect-capture](auto&& ...x) -> decltype(auto) {
55 return forwarded(f)(fix(f), forwarded(x)...);
56 };
57 };
58#else
59 template <typename F>
60 struct fix_t;
61
62 BOOST_HANA_INLINE_VARIABLE constexpr detail::create<fix_t> fix{};
63
64 template <typename F>
65 struct fix_t {
66 F f;
67
68 template <typename ...X>
69 constexpr decltype(auto) operator()(X&& ...x) const&
70 { return f(fix(f), static_cast<X&&>(x)...); }
71
72 template <typename ...X>
73 constexpr decltype(auto) operator()(X&& ...x) &
74 { return f(fix(f), static_cast<X&&>(x)...); }
75
76 template <typename ...X>
77 constexpr decltype(auto) operator()(X&& ...x) &&
78 { return std::move(f)(fix(f), static_cast<X&&>(x)...); }
79 };
80#endif
81}} // end namespace boost::hana
82
83#endif // !BOOST_HANA_FUNCTIONAL_FIX_HPP
84

source code of boost/libs/hana/include/boost/hana/functional/fix.hpp