and_then.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_AND_THEN_HPP
#define SL_UTILITY_NULLABLES_AND_THEN_HPP
 
#pragma once
 
#include "Simple-Utility/functional/bind_back.hpp"
#include "Simple-Utility/nullables/base.hpp"
 
namespace sl::nullables::detail
{
    template <input_nullable TInputNullable, std::invocable<dereference_result_t<TInputNullable>> TFunc>
        requires nullable<std::invoke_result_t<TFunc, dereference_result_t<TInputNullable>>>
    [[nodiscard]]
    constexpr std::invoke_result_t<TFunc, dereference_result_t<TInputNullable>> and_then(TInputNullable&& inputNullable, TFunc&& func)
    {
        using result_nullable_t = std::invoke_result_t<TFunc, dereference_result_t<TInputNullable>>;
 
        if (inputNullable != null_v<TInputNullable>)
        {
            return std::invoke(std::forward<TFunc>(func), unwrap(std::forward<TInputNullable>(inputNullable)));
        }
        return result_nullable_t{null_v<result_nullable_t>};
    }
 
    class and_then_caller_fn
    {
    public:
        template <input_nullable TInputNullable, std::invocable<dereference_result_t<TInputNullable>> TFunc>
            requires nullable<std::invoke_result_t<TFunc, dereference_result_t<TInputNullable>>>
        [[nodiscard]]
        constexpr auto operator ()(
            TInputNullable&& inputNullable,
            TFunc&& func
        ) const
            noexcept(noexcept(and_then(std::declval<TInputNullable>(), std::declval<TFunc>())))
        {
            return and_then(
                std::forward<TInputNullable>(inputNullable),
                std::forward<TFunc>(func));
        }
    };
}
 
namespace sl::nullables
{
    inline constexpr auto and_then = []<class Fn>(Fn&& fn) noexcept(std::is_nothrow_constructible_v<std::remove_cvref_t<Fn>, Fn>)
    {
        return Algorithm{functional::bind_back(detail::and_then_caller_fn{}, std::forward<Fn>(fn))};
    };
 
}
 
#endif