stabilize build system: depends, installer, boost/bdb fixes, cross targets groundwork

2026-02-24 18:38:47 +00:00
parent da8c28aaeb
commit 65cb2619a7
13106 changed files with 2484322 additions and 1804 deletions
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+/*!
+@file
+Defines `boost::hana::fix`.
+
+@copyright Louis Dionne 2013-2017
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#ifndef BOOST_HANA_FUNCTIONAL_FIX_HPP
+#define BOOST_HANA_FUNCTIONAL_FIX_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/detail/create.hpp>
+
+#include <utility>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @ingroup group-functional
+    //! Return a function computing the fixed point of a function.
+    //!
+    //! `fix` is an implementation of the [Y-combinator][], also called the
+    //! fixed-point combinator. It encodes the idea of recursion, and in fact
+    //! any recursive function can be written in terms of it.
+    //!
+    //! Specifically, `fix(f)` is a function such that
+    //! @code
+    //!     fix(f)(x...) == f(fix(f), x...)
+    //! @endcode
+    //!
+    //! This definition allows `f` to use its first argument as a continuation
+    //! to call itself recursively. Indeed, if `f` calls its first argument
+    //! with `y...`, it is equivalent to calling `f(fix(f), y...)` per the
+    //! above equation.
+    //!
+    //! Most of the time, it is more convenient and efficient to define
+    //! recursive functions without using a fixed-point combinator. However,
+    //! there are some cases where `fix` provides either more flexibility
+    //! (e.g. the ability to change the callback inside `f`) or makes it
+    //! possible to write functions that couldn't be defined recursively
+    //! otherwise.
+    //!
+    //! @param f
+    //! A function called as `f(self, x...)`, where `x...` are the arguments
+    //! in the `fix(f)(x...)` expression and `self` is `fix(f)`.
+    //!
+    //! ### Example
+    //! @include example/functional/fix.cpp
+    //!
+    //! [Y-combinator]: http://en.wikipedia.org/wiki/Fixed-point_combinator
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto fix = [](auto&& f) {
+        return [perfect-capture](auto&& ...x) -> decltype(auto) {
+            return forwarded(f)(fix(f), forwarded(x)...);
+        };
+    };
+#else
+    template <typename F>
+    struct fix_t;
+
+    constexpr detail::create<fix_t> fix{};
+
+    template <typename F>
+    struct fix_t {
+        F f;
+
+        template <typename ...X>
+        constexpr decltype(auto) operator()(X&& ...x) const&
+        { return f(fix(f), static_cast<X&&>(x)...); }
+
+        template <typename ...X>
+        constexpr decltype(auto) operator()(X&& ...x) &
+        { return f(fix(f), static_cast<X&&>(x)...); }
+
+        template <typename ...X>
+        constexpr decltype(auto) operator()(X&& ...x) &&
+        { return std::move(f)(fix(f), static_cast<X&&>(x)...); }
+    };
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FUNCTIONAL_FIX_HPP