84 lines
2.8 KiB
C++
84 lines
2.8 KiB
C++
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/*!
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@file
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Defines `boost::hana::fix`.
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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_FUNCTIONAL_FIX_HPP
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#define BOOST_HANA_FUNCTIONAL_FIX_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/detail/create.hpp>
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#include <utility>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @ingroup group-functional
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//! Return a function computing the fixed point of a function.
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//!
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//! `fix` is an implementation of the [Y-combinator][], also called the
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//! fixed-point combinator. It encodes the idea of recursion, and in fact
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//! any recursive function can be written in terms of it.
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//!
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//! Specifically, `fix(f)` is a function such that
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//! @code
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//! fix(f)(x...) == f(fix(f), x...)
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//! @endcode
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//!
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//! This definition allows `f` to use its first argument as a continuation
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//! to call itself recursively. Indeed, if `f` calls its first argument
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//! with `y...`, it is equivalent to calling `f(fix(f), y...)` per the
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//! above equation.
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//!
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//! Most of the time, it is more convenient and efficient to define
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//! recursive functions without using a fixed-point combinator. However,
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//! there are some cases where `fix` provides either more flexibility
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//! (e.g. the ability to change the callback inside `f`) or makes it
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//! possible to write functions that couldn't be defined recursively
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//! otherwise.
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//!
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//! @param f
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//! A function called as `f(self, x...)`, where `x...` are the arguments
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//! in the `fix(f)(x...)` expression and `self` is `fix(f)`.
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//!
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//! ### Example
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//! @include example/functional/fix.cpp
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//!
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//! [Y-combinator]: http://en.wikipedia.org/wiki/Fixed-point_combinator
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto fix = [](auto&& f) {
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return [perfect-capture](auto&& ...x) -> decltype(auto) {
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return forwarded(f)(fix(f), forwarded(x)...);
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};
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};
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#else
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template <typename F>
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struct fix_t;
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constexpr detail::create<fix_t> fix{};
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template <typename F>
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struct fix_t {
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F f;
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template <typename ...X>
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constexpr decltype(auto) operator()(X&& ...x) const&
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{ return f(fix(f), static_cast<X&&>(x)...); }
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template <typename ...X>
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constexpr decltype(auto) operator()(X&& ...x) &
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{ return f(fix(f), static_cast<X&&>(x)...); }
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template <typename ...X>
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constexpr decltype(auto) operator()(X&& ...x) &&
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{ return std::move(f)(fix(f), static_cast<X&&>(x)...); }
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};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FUNCTIONAL_FIX_HPP
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