#ifndef TMP_TRAITS_H
#define TMP_TRAITS_H

#include <type_traits>
#include "pack.h"

namespace tmp {

/**
 * void_t
 *
 * as of C++17, use std::void_t instead
 */
template<typename...> using void_t = void;

/**
 * Void
 *
 * use to replace `void` when an instance is required
 */
struct Void {};

// TODO: remove it?
template<typename Ostream>
Ostream &operator<<(Ostream &os, Void const&) {
	return os << "Void{}";
}

/**
 * ReturnType
 */
template<typename> struct ReturnTypeImpl;
template<typename R, typename... Ts>
struct ReturnTypeImpl<R(Ts...)> {
	using type = R;
};

template<typename T>
using ReturnType = typename ReturnTypeImpl<T>::type;

/**
 * Parameters
 */
template<typename> struct ParametersImpl;
template<typename R, typename... Ps>
struct ParametersImpl<R(Ps...)> {
	using type = Pack<Ps...>;
};

template<typename T>
using Parameters = typename ParametersImpl<T>::type;

/**
 * invoke_result
 * as of C++17, use std::invoke_result instead
 */
template<typename F, typename... Args>
using invoke_result = std::result_of<F(Args...)>;

template<typename F, typename... Args>
using invoke_result_t = typename invoke_result<F, Args...>::type;

/**
 * FunctionCat
 */
template<typename, typename> struct FunctionCatImpl;

template<typename Ret, typename... Ts, typename... Us>
struct FunctionCatImpl<Ret(Ts...), Ret(Us...)> {
	using type = Ret(Ts..., Us...);
};

template<typename F1, typename F2>
using FunctionCat = typename FunctionCatImpl<F1, F2>::type;

}

#endif