feat():initial version

install/boost_1_75_0/include/boost/hof/fix.hpp

@@ -0,0 +1,242 @@
+/*=============================================================================
+    Copyright (c) 2014 Paul Fultz II
+    fix.h
+    Distributed under the Boost Software License, Version 1.0. (See accompanying
+    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+==============================================================================*/
+
+#ifndef BOOST_HOF_GUARD_FUNCTION_FIX_H
+#define BOOST_HOF_GUARD_FUNCTION_FIX_H
+
+/// fix
+/// ===
+/// 
+/// Description
+/// -----------
+/// 
+/// The `fix` function adaptor implements a fixed-point combinator. This can be
+/// used to write recursive functions. 
+/// 
+/// When using `constexpr`, a function can recurse to a depth that is defined by
+/// `BOOST_HOF_RECURSIVE_CONSTEXPR_DEPTH`(default is 16). There is no limitiation on
+/// recursion depth for non-constexpr functions. In addition, due to the
+/// eagerness of `constexpr` to instantiation templates, in some cases, an
+/// explicit return type must be specified in order to avoid reaching the
+/// recursion limits of the compiler. This can be accomplished using
+/// [`boost::hof::result`](/include/boost/hof/result):
+/// 
+///     int r = boost::hof::result<int>(factorial)(5);
+/// 
+/// Synopsis
+/// --------
+/// 
+///     template<class F>
+///     constexpr fix_adaptor<F> fix(F f);
+/// 
+/// Semantics
+/// ---------
+/// 
+///     assert(fix(f)(xs...) == f(fix(f), xs...));
+/// 
+/// Requirements
+/// ------------
+/// 
+/// F must be:
+/// 
+/// * [ConstFunctionObject](ConstFunctionObject)
+/// * MoveConstructible
+/// 
+/// Example
+/// -------
+/// 
+///     #include <boost/hof.hpp>
+///     #include <cassert>
+/// 
+///     int main() {
+///         auto factorial = boost::hof::fix(
+///             [](auto recurse, auto x) -> decltype(x) { 
+///                 return x == 0 ? 1 : x * recurse(x-1); 
+///             }
+///         );
+///         int r = boost::hof::result<int>(factorial)(5);
+///         assert(r == 5*4*3*2*1);
+///     }
+/// 
+/// References
+/// ----------
+/// 
+/// * [Fixed-point combinator](https://en.wikipedia.org/wiki/Fixed-point_combinator)
+/// * [Recursive](Recursive)
+/// 
+
+#include <boost/hof/always.hpp>
+#include <boost/hof/detail/callable_base.hpp>
+#include <boost/hof/reveal.hpp>
+#include <boost/hof/detail/delegate.hpp>
+#include <boost/hof/detail/move.hpp>
+#include <boost/hof/detail/make.hpp>
+#include <boost/hof/detail/static_const_var.hpp>
+#include <boost/hof/indirect.hpp>
+#include <boost/hof/result.hpp>
+#include <boost/hof/detail/recursive_constexpr_depth.hpp>
+
+
+namespace boost { namespace hof {
+
+namespace detail{
+
+template<class F>
+struct compute_indirect_ref
+{ typedef indirect_adaptor<const F*> type; };
+
+template<class F>
+struct compute_indirect_ref<indirect_adaptor<F*>>
+{ typedef indirect_adaptor<F*> type; };
+
+template<class F>
+constexpr indirect_adaptor<const F*> make_indirect_ref(const F& f) noexcept
+{
+    return indirect_adaptor<const F*>(&f);
+}
+
+template<class F>
+constexpr indirect_adaptor<const F*> make_indirect_ref(const indirect_adaptor<F*>& f) noexcept
+{
+    return f;
+}
+
+template<class F, class=void>
+struct fix_result
+{
+    template<class... Ts>
+    struct apply
+    { 
+        typedef decltype(std::declval<F>()(std::declval<Ts>()...)) type; 
+    };
+};
+
+template<class F>
+struct fix_result<F, typename holder<
+    typename F::result_type
+>::type>
+{
+    template<class...>
+    struct apply
+    { 
+        typedef typename F::result_type type; 
+    };
+    
+};
+
+template<class F, class Result, int N>
+struct fix_adaptor_base : F
+{
+    BOOST_HOF_INHERIT_CONSTRUCTOR(fix_adaptor_base, F);
+
+    typedef typename compute_indirect_ref<F>::type base_ref_type;
+    typedef fix_adaptor_base<base_ref_type, Result, N-1> derived;
+
+
+    template<class... Ts>
+    constexpr const F& base_function(Ts&&... xs) const noexcept
+    {
+        return boost::hof::always_ref(*this)(xs...);
+    }
+
+    template<class... Ts>
+    constexpr derived derived_function(Ts&&... xs) const noexcept
+    {
+        return derived(boost::hof::detail::make_indirect_ref(this->base_function(xs...)));
+    }
+
+    struct fix_failure
+    {
+        template<class Failure>
+        struct apply
+        {
+            template<class... Ts>
+            struct of
+            : Failure::template of<derived, Ts...>
+            {};
+        };
+    };
+
+    struct failure
+    : failure_map<fix_failure, F>
+    {};
+
+
+    BOOST_HOF_RETURNS_CLASS(fix_adaptor_base);
+
+    template<class... Ts>
+    constexpr BOOST_HOF_SFINAE_RESULT(const F&, id_<derived>, id_<Ts>...) 
+    operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
+    (
+        BOOST_HOF_MANGLE_CAST(const F&)(BOOST_HOF_CONST_THIS->base_function(xs...))
+            (
+                BOOST_HOF_MANGLE_CAST(derived)(BOOST_HOF_CONST_THIS->derived_function(xs...)), 
+                BOOST_HOF_FORWARD(Ts)(xs)...
+            )
+    );
+};
+
+template<class F, class Result>
+struct fix_adaptor_base<F, Result, 0> : F
+{
+    BOOST_HOF_INHERIT_CONSTRUCTOR(fix_adaptor_base, F);
+
+    template<class... Ts>
+    const F& base_function(Ts&&...) const noexcept
+    {
+        return *this;
+    }
+
+    struct fix_failure
+    {
+        template<class Failure>
+        struct apply
+        {
+            template<class... Ts>
+            struct of
+            : Failure::template of<fix_adaptor_base, Ts...>
+            {};
+        };
+    };
+
+    struct failure
+    : failure_map<fix_failure, F>
+    {};
+
+
+    BOOST_HOF_RETURNS_CLASS(fix_adaptor_base);
+
+    template<class... Ts>
+    typename Result::template apply<fix_adaptor_base, Ts...>::type
+    operator()(Ts&&... xs) const
+    {
+        return this->base_function(xs...)(*this, BOOST_HOF_FORWARD(Ts)(xs)...);
+    }
+};
+}
+
+template<class F>
+struct fix_adaptor : detail::fix_adaptor_base<F, detail::fix_result<F>, BOOST_HOF_RECURSIVE_CONSTEXPR_DEPTH>
+{
+    typedef fix_adaptor fit_rewritable1_tag;
+    typedef detail::fix_adaptor_base<F, detail::fix_result<F>, BOOST_HOF_RECURSIVE_CONSTEXPR_DEPTH> base;
+    BOOST_HOF_INHERIT_CONSTRUCTOR(fix_adaptor, base);
+};
+
+template<class Result, class F>
+struct result_adaptor<Result, fix_adaptor<F>>
+: fix_adaptor<result_adaptor<Result, F>>
+{
+    typedef fix_adaptor<result_adaptor<Result, F>> base;
+    BOOST_HOF_INHERIT_CONSTRUCTOR(result_adaptor, base)
+};
+
+BOOST_HOF_DECLARE_STATIC_VAR(fix, detail::make<fix_adaptor>);
+
+}} // namespace boost::hof
+
+#endif