OrElse.hpp

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//          Copyright Dominic (DNKpp) Koepke 2025-2026.
// 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 GIMO_ALGORITHM_OR_ELSE_HPP
#define GIMO_ALGORITHM_OR_ELSE_HPP
 
#pragma once
 
#include "gimo/Common.hpp"
#include "gimo/Pipeline.hpp"
#include "gimo/algorithm/BasicAlgorithm.hpp"
 
#include <concepts>
#include <functional>
#include <tuple>
#include <type_traits>
#include <utility>
 
namespace gimo::detail::or_else
{
    template <typename Nullable, typename Action>
    consteval Nullable& print_diagnostics()
    {
        if constexpr (!std::is_invocable_v<Action>)
        {
            static_assert(always_false_v<Nullable>, "The or_else algorithm requires an action invocable without any arguments.");
        }
        else if constexpr (!std::same_as<Nullable, std::invoke_result_t<Action>> && !std::is_void_v<std::invoke_result_t<Action>>)
        {
            static_assert(always_false_v<Nullable>, "The or_else algorithm requires an action returning the same nullable type or void.");
        }
 
        return std::declval<Nullable&>();
    }
 
    template <typename Action, nullable Nullable>
    [[nodiscard]]
    constexpr std::remove_cvref_t<Nullable> on_value(Action&& /*action*/, Nullable&& opt)
    {
        return std::forward<Nullable>(opt);
    }
 
    template <typename Action, nullable Nullable, typename Next, typename... Steps>
    [[nodiscard]]
    constexpr auto on_value(
        Action&& /*action*/,
        Nullable&& opt,
        Next&& next,
        Steps&&... steps)
    {
        return std::forward<Next>(next).on_value(
            std::forward<Nullable>(opt),
            std::forward<Steps>(steps)...);
    }
 
    template <typename Action, nullable Nullable>
    [[nodiscard]]
    constexpr std::remove_cvref_t<Nullable> on_null(Action&& action, Nullable&& /*opt*/)
    {
        if constexpr (std::is_void_v<std::invoke_result_t<Action>>)
        {
            std::invoke(std::forward<Action>(action));
            return detail::construct_empty<std::remove_cvref_t<Nullable>>();
        }
        else
        {
            return std::invoke(std::forward<Action>(action));
        }
    }
 
    template <nullable Nullable, typename Action, typename Next, typename... Steps>
    [[nodiscard]]
    constexpr auto on_null(Action&& action, Nullable&& opt, Next&& next, Steps&&... steps)
    {
        return std::invoke(
            std::forward<Next>(next),
            or_else::on_null(std::forward<Action>(action), std::forward<Nullable>(opt)),
            std::forward<Steps>(steps)...);
    }
 
    template <nullable Nullable, typename Action, typename Next, typename... Steps>
        requires std::is_void_v<std::invoke_result_t<Action>>
    [[nodiscard]]
    constexpr auto on_null(Action&& action, Nullable&& opt, Next&& next, Steps&&... steps)
    {
        return std::forward<Next>(next).on_null(
            or_else::on_null(std::forward<Action>(action), std::forward<Nullable>(opt)),
            std::forward<Steps>(steps)...);
    }
 
    struct traits
    {
        template <nullable Nullable, typename Action>
        static constexpr bool is_applicable_on = requires {
            requires std::same_as<
                         std::remove_cvref_t<Nullable>,
                         std::remove_cvref_t<std::invoke_result_t<Action>>>
                         || std::is_void_v<std::invoke_result_t<Action>>;
        };
 
        template <typename Action, nullable Nullable, typename... Steps>
        [[nodiscard]]
        static constexpr auto on_value(Action&& action, Nullable&& opt, Steps&&... steps)
        {
            GIMO_ASSERT(detail::has_value(opt), "Nullable is empty while it's expected to contain a value.");
 
            if constexpr (is_applicable_on<Nullable, Action>)
            {
                return or_else::on_value(
                    std::forward<Action>(action),
                    std::forward<Nullable>(opt),
                    std::forward<Steps>(steps)...);
            }
            else
            {
                return or_else::print_diagnostics<Nullable, Action>();
            }
        }
 
        template <typename Action, nullable Nullable, typename... Steps>
        [[nodiscard]]
        static constexpr auto on_null(Action&& action, Nullable&& opt, Steps&&... steps)
        {
            GIMO_ASSERT(!detail::has_value(opt), "Nullable contains a value while it's expected to be empty.");
 
            if constexpr (is_applicable_on<Nullable, Action>)
            {
                return or_else::on_null(
                    std::forward<Action>(action),
                    std::forward<Nullable>(opt),
                    std::forward<Steps>(steps)...);
            }
            else
            {
                return or_else::print_diagnostics<Nullable, Action>();
            }
        }
    };
}
 
namespace gimo
{
    namespace detail
    {
        template <typename Action>
        using or_else_t = BasicAlgorithm<or_else::traits, std::remove_cvref_t<Action>>;
    }

    template <typename Action>
    [[nodiscard]]
    constexpr auto or_else(Action&& action)
    {
        using Algorithm = detail::or_else_t<Action>;
 
        return Pipeline{std::tuple<Algorithm>{std::forward<Action>(action)}};
    }
}
 
#endif