either.h

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/**********************************************************************
 * LeechCraft - modular cross-platform feature rich internet client.
 * Copyright (C) 2006-2014  Georg Rudoy
 *
 * Distributed under the Boost Software License, Version 1.0.
 * (See accompanying file LICENSE or copy at https://www.boost.org/LICENSE_1_0.txt)
 **********************************************************************/

#pragma once

#include <expected>
#include <optional>
#include <type_traits>
#include "overloaded.h"

namespace LC::Util
{
    template<typename T>
    struct Left
    {
        T Value_;
    };

    template<>
    struct Left<void> {};

    constexpr auto AsLeft = Left<void> {};

    inline struct FromStdExpected_t {} FromStdExpected;

    template<typename L, typename R>
    class Either
    {
        std::expected<R, L> This_;
    public:
        using L_t = L;
        using R_t = R;

        Either (FromStdExpected_t, std::expected<R, L>&& ex)
        : This_ { std::move (ex) }
        {
        }

        Either () = delete;

        Either (R&& r)
        : This_ { std::move (r) }
        {
        }

        Either (const R& r)
        : This_ { r }
        {
        }

        Either (Left<void>, const L& l)
        : This_ { std::unexpect, l }
        {
        }

        explicit Either (const L& l) requires (!std::is_same_v<L, R>)
        : This_ { std::unexpect, l }
        {
        }

        Either (Left<L>&& left)
        : This_ { std::unexpect, std::move (left.Value_) }
        {
        }

        template<typename LL>
            requires std::is_constructible_v<L, LL&&>
        Either (Left<LL>&& left)
        : This_ { std::unexpect, L { std::move (left.Value_) } }
        {
        }

        Either (const Either&) = default;
        Either (Either&&) = default;
        Either& operator= (const Either&) = default;
        Either& operator= (Either&&) = default;

        bool IsLeft () const
        {
            return !IsRight ();
        }

        bool IsRight () const
        {
            return This_.has_value ();
        }

        const L& GetLeft () const
        {
            if (!IsLeft ())
                throw std::runtime_error { "Tried accessing Left for a Right Either" };
            return This_.error ();
        }

        L& GetLeft ()
        {
            if (!IsLeft ())
                throw std::runtime_error { "Tried accessing Left for a Right Either" };
            return This_.error ();
        }

        const R& GetRight () const
        {
            if (!IsRight ())
                throw std::runtime_error { "Tried accessing Right for a Left Either" };
            return This_.value ();
        }

        R& GetRight ()
        {
            if (!IsRight ())
                throw std::runtime_error { "Tried accessing Right for a Left Either" };
            return This_.value ();
        }

        std::optional<L> MaybeLeft () const
        {
            if (!IsLeft ())
                return {};
            return GetLeft ();
        }

        std::optional<R> MaybeRight () const
        {
            if (!IsRight ())
                return {};
            return GetRight ();
        }

        template<typename F>
        R ToRight (F&& f) const
        {
            return IsRight () ? GetRight () : std::forward<F> (f) (GetLeft ());
        }

        template<typename F>
        auto MapLeft (F&& f) const
        {
            using Result = Either<std::invoke_result_t<F, L>, R>;
            return Result { FromStdExpected, This_.transform_error (std::forward<F> (f)) };
        }

        template<typename F>
        auto MapRight (F&& f) const
        {
            using Result = Either<L, std::invoke_result_t<F, R>>;
            return Result { This_.transform (std::forward<F> (f)) };
        }

        friend bool operator== (const Either& e1, const Either& e2)
        {
            return e1.This_ == e2.This_;
        }

        friend bool operator!= (const Either& e1, const Either& e2)
        {
            return !(e1 == e2);
        }
    };

    template<template<typename> class Cont, typename L, typename R>
    std::pair<Cont<L>, Cont<R>> Partition (const Cont<Either<L, R>>& eithers)
    {
        std::pair<Cont<L>, Cont<R>> result;
        for (const auto& either : eithers)
            if (either.IsLeft ())
                result.first.push_back (either.GetLeft ());
            else
                result.second.push_back (either.GetRight ());

        return result;
    }

    template<typename Left, typename Right, typename... Args>
    auto Visit (const Either<Left, Right>& either, Args&&... args)
    {
        Overloaded visitor { std::forward<Args> (args)... };
        return either.IsRight () ?
                std::move (visitor) (either.GetRight ()) :
                std::move (visitor) (either.GetLeft ());
    }

    template<typename Left, typename Right, typename... Args>
    auto Visit (Either<Left, Right>&& either, Args&&... args)
    {
        Overloaded visitor { std::forward<Args> (args)... };
        return either.IsRight () ?
                std::move (visitor) (std::move (either.GetRight ())) :
                std::move (visitor) (std::move (either.GetLeft ()));
    }
}