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132 lines
3.1 KiB
C++
132 lines
3.1 KiB
C++
/*=============================================================================
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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 FIT_GUARD_FUNCTION_FIX_H
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#define FIT_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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/// Note: Compilers are too eager to instantiate templates when using
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/// constexpr, which causes the compiler to reach its internal instantiation
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/// limit. So, unfortunately, `fix` cannot be used for `constexpr` functions.
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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(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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/// * [FunctionObject](concepts.md#functionobject)
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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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/// int r = fit::fix([](auto s, auto x) -> decltype(x) { return x == 0 ? 1 : x * s(x-1); })(5);
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/// assert(r == 5*4*3*2*1);
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///
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#include <fit/always.h>
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#include <fit/detail/result_of.h>
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#include <fit/reveal.h>
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#include <fit/detail/delegate.h>
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#include <fit/detail/move.h>
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#include <fit/detail/make.h>
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#include <fit/detail/static_const_var.h>
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#ifndef FIT_FIX_HAS_CONSTEXPR
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#define FIT_FIX_HAS_CONSTEXPR 0
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#endif
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#if FIT_FIX_HAS_CONSTEXPR
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#define FIT_FIX_CONSTEXPR constexpr
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#else
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#define FIT_FIX_CONSTEXPR
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#endif
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namespace fit {
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namespace detail{
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template<class Derived, class F>
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struct fix_adaptor_base : F
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{
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FIT_INHERIT_CONSTRUCTOR(fix_adaptor_base, F);
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template<class... Ts>
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FIT_FIX_CONSTEXPR const F& base_function(Ts&&... xs) const
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{
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return always_ref(*this)(xs...);
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}
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template<class... Ts>
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FIT_FIX_CONSTEXPR const Derived& derived_function(Ts&&... xs) const
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{
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return static_cast<const Derived&>(always_ref(*this)(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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FIT_RETURNS_CLASS(fix_adaptor_base);
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template<class... Ts>
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FIT_FIX_CONSTEXPR FIT_SFINAE_RESULT(const F&, id_<const Derived&>, id_<Ts>...)
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operator()(Ts&&... xs) const FIT_SFINAE_RETURNS
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(
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FIT_MANGLE_CAST(const F&)(FIT_CONST_THIS->base_function(xs...))
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(FIT_MANGLE_CAST(const Derived&)(FIT_CONST_THIS->derived_function(xs...)), fit::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<fix_adaptor<F>, F>
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{
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typedef fix_adaptor fit_rewritable1_tag;
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typedef detail::fix_adaptor_base<fix_adaptor<F>, F> base;
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FIT_INHERIT_CONSTRUCTOR(fix_adaptor, base);
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};
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FIT_DECLARE_STATIC_VAR(fix, detail::make<fix_adaptor>);
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}
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#endif
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