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