or_else.hpp

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//          Copyright Dominic Koepke 2019 - 2023.
// Distributed under the Boost Software License, Version 1.0.
//    (See accompanying file LICENSE_1_0.txt or copy at
//          https://www.boost.org/LICENSE_1_0.txt)
 
#ifndef SL_UTILITY_NULLABLES_OR_ELSE_HPP
#define SL_UTILITY_NULLABLES_OR_ELSE_HPP
 
#pragma once
 
#include "Simple-Utility/functional/bind_back.hpp"
#include "Simple-Utility/nullables/base.hpp"
 
namespace sl::nullables::detail
{
    template <nullable TNullable, std::invocable TFunc>
        requires std::is_void_v<std::invoke_result_t<TFunc>>
                || std::constructible_from<std::remove_cvref_t<TNullable>, std::invoke_result_t<TFunc>>
    [[nodiscard]]
    constexpr std::remove_cvref_t<TNullable> or_else(TNullable&& nullableObj, TFunc&& func)
    {
        using result_t = std::remove_cvref_t<TNullable>;
 
        if (nullableObj == null_v<TNullable>)
        {
            if constexpr (std::is_void_v<std::invoke_result_t<TFunc>>)
            {
                std::invoke(std::forward<TFunc>(func));
                return result_t{ null_v<TNullable> };
            }
            else
            {
                return result_t{ std::invoke(std::forward<TFunc>(func)) };
            }
        }
        return std::forward<TNullable>(nullableObj);
    }
 
    class or_else_caller_fn
    {
    public:
        template <nullable TNullable, std::invocable TFunc>
        [[nodiscard]]
        constexpr std::remove_cvref_t<TNullable> operator ()
        (
            TNullable&& nullableObj,
            TFunc&& func
        ) const
        noexcept(noexcept(or_else(std::declval<TNullable>(), std::declval<TFunc>())))
        {
            return or_else(
                std::forward<TNullable>(nullableObj),
                std::forward<TFunc>(func)
            );
        }
    };
}
 
namespace sl::nullables
{
    inline constexpr auto or_else = []<class Fn>(Fn&& fn) noexcept(std::is_nothrow_constructible_v<std::remove_cvref_t<Fn>, Fn>)
    {
        return Algorithm{functional::bind_back(detail::or_else_caller_fn{}, std::forward<Fn>(fn))};
    };
    
}
 
#endif