functor.hpp

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#pragma once
#include <mpc/functional/partial.hpp>
#include <mpc/prelude/compose.hpp>
#include <mpc/prelude/constant.hpp>
#include <mpc/prelude/flip.hpp>
 
namespace mpc {
  // functor
  // https://hackage.haskell.org/package/base-4.16.0.0/docs/Control-Monad.html#t:Functor
 
  template <class>
  struct functor_traits;
 
  template <class F>
  concept functor = requires {
    functor_traits<std::remove_cvref_t<F>>::fmap;
    functor_traits<std::remove_cvref_t<F>>::replace2nd;
  };
 
  // Methods required for the class definition.
 
  namespace detail {
    struct fmap_op {
      template <class Fn, class Fa>
      constexpr auto operator()(Fn&& fn, Fa&& fa) const noexcept(
      noexcept(   functor_traits<std::remove_cvref_t<Fa>>::fmap(std::forward<Fn>(fn), std::forward<Fa>(fa))))
      -> decltype(functor_traits<std::remove_cvref_t<Fa>>::fmap(std::forward<Fn>(fn), std::forward<Fa>(fa)))
      { return    functor_traits<std::remove_cvref_t<Fa>>::fmap(std::forward<Fn>(fn), std::forward<Fa>(fa)); }
    };
 
    struct replace2nd_op {
      template <class A, class Fb>
      constexpr auto operator()(A&& a, Fb&& fb) const noexcept(
      noexcept(   functor_traits<std::remove_cvref_t<Fb>>::replace2nd(std::forward<A>(a), std::forward<Fb>(fb))))
      -> decltype(functor_traits<std::remove_cvref_t<Fb>>::replace2nd(std::forward<A>(a), std::forward<Fb>(fb)))
      { return    functor_traits<std::remove_cvref_t<Fb>>::replace2nd(std::forward<A>(a), std::forward<Fb>(fb)); }
    };
  } // namespace detail
 
  inline namespace cpo {
    inline constexpr partial<detail::fmap_op> fmap{};
 
    inline constexpr partial<detail::replace2nd_op> replace2nd{};
  } // namespace cpo
 
  namespace functors {
    inline constexpr auto replace2nd = compose(mpc::fmap, constant);
  } // namespace functors
 
  // Grobal methods
 
  inline namespace cpo {
    inline constexpr auto replace1st = flip % mpc::replace2nd;
  } // namespace cpo
} // namespace mpc