flip.hpp

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#pragma once
#include <functional> // std::invoke
#include <mpc/functional/partial.hpp>
 
// clang-format off
 
namespace mpc {
  namespace detail {
    struct flip_op {
      struct closure {
        template<class Fn, class T, class U, class... Args>
        constexpr auto operator()(Fn&& f, T&& t, U&& u, Args&&... args) const noexcept(
          noexcept(   std::invoke(std::forward<Fn>(f), std::forward<U>(u), std::forward<T>(t), std::forward<Args>(args)...)))
          -> decltype(std::invoke(std::forward<Fn>(f), std::forward<U>(u), std::forward<T>(t), std::forward<Args>(args)...))
          { return    std::invoke(std::forward<Fn>(f), std::forward<U>(u), std::forward<T>(t), std::forward<Args>(args)...); }
      };
 
      // NOTE: You cannot write as `partial<closure>{}(std::forward<Fn>(f))`.
      template<class Fn>
      constexpr auto operator()(Fn&& f) const noexcept(
        noexcept(   partial(closure{}, std::forward<Fn>(f))))
        -> decltype(partial(closure{}, std::forward<Fn>(f)))
        { return    partial(closure{}, std::forward<Fn>(f)); }
    };
  } // namespace detail
 
  inline namespace cpo {
    inline constexpr partial<detail::flip_op> flip;
  }
} // namespace mpc
 
// clang-format on