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87 lines
2.3 KiB
C++
87 lines
2.3 KiB
C++
/*=============================================================================
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Copyright (c) 2015 Paul Fultz II
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eval.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_EVAL_H
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#define BOOST_HOF_GUARD_EVAL_H
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/// eval
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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 `eval` function will evaluate a "thunk". This can be either a nullary
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/// function or it can be a unary function that takes the identity function as
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/// the first parameter(which is helpful to delay compile-time checking).
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/// Also, additional parameters can be passed to `eval` to delay
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/// compiliation(so that result can depend on template parameters).
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///
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/// Synopsis
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/// --------
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///
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/// template<class F, class... Ts>
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/// constexpr auto eval(F&& f, Ts&&...);
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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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/// * [EvaluatableFunctionObject](EvaluatableFunctionObject)
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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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/// assert(boost::hof::eval([]{ return 3; }) == 3);
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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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/// * [POO51](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0051r2.pdf) - Proposal for C++
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/// Proposal for C++ generic overload function
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/// * [static_if](static_if)
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/// * [Ordering evaluation of arguments](<Ordering evaluation of arguments>)
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///
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#include <boost/hof/always.hpp>
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#include <boost/hof/identity.hpp>
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#include <boost/hof/first_of.hpp>
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#include <boost/hof/detail/result_of.hpp>
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namespace boost { namespace hof {
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namespace detail {
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struct simple_eval
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{
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template<class F, class... Ts>
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constexpr BOOST_HOF_SFINAE_RESULT(F)
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operator()(F&& f, Ts&&...xs) const BOOST_HOF_SFINAE_RETURNS
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(boost::hof::always_ref(f)(xs...)());
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};
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struct id_eval
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{
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template<class F, class... Ts>
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constexpr BOOST_HOF_SFINAE_RESULT(F, id_<decltype(boost::hof::identity)>)
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operator()(F&& f, Ts&&...xs) const BOOST_HOF_SFINAE_RETURNS
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(boost::hof::always_ref(f)(xs...)(boost::hof::identity));
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};
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}
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BOOST_HOF_DECLARE_STATIC_VAR(eval, boost::hof::first_of_adaptor<detail::simple_eval, detail::id_eval>);
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}} // namespace boost::hof
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#endif
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