functor.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 <functional>
#include <optional>
#include "typeclassutil.h"
#include "void.h"
#include "either.h"
 
namespace LC
{
namespace Util
{
    template<typename T>
    struct InstanceFunctor
    {
        using UndefinedTag = void;

        template<typename F>
        using FmapResult_t = detail::ImplementationType;

        template<typename F>
        static FmapResult_t<F> Apply (const T& functor, const F& function);
    };
 
    namespace detail
    {
        template<typename T>
        constexpr bool IsFunctorImpl (int, typename InstanceFunctor<T>::UndefinedTag* = nullptr)
        {
            return false;
        }
 
        template<typename T>
        constexpr bool IsFunctorImpl (float)
        {
            return true;
        }
    }

    template<typename T>
    constexpr bool IsFunctor ()
    {
        return detail::IsFunctorImpl<T> (0);
    }

    template<typename T, typename F>
    using FmapResult_t = typename InstanceFunctor<T>::template FmapResult_t<F>;

    template<typename T, typename F, typename = std::enable_if_t<IsFunctor<T> ()>>
    FmapResult_t<T, F> Fmap (const T& functor, const F& function)
    {
        return InstanceFunctor<T>::Apply (functor, function);
    }

    template<typename T, typename F>
    auto operator* (const F& function, const T& functor) -> decltype (Fmap (functor, function))
    {
        return Fmap (functor, function);
    }

    template<typename T, typename F>
    auto operator* (const T& functor, const F& function) -> decltype (Fmap (functor, function))
    {
        return Fmap (functor, function);
    }
 
    namespace detail
    {
        template<typename T>
        struct WrapVoidResult
        {
            using Type = T;
        };
 
        template<>
        struct WrapVoidResult<void>
        {
            using Type = Void;
        };
 
        template<typename T>
        using WrapVoidResult_t = typename WrapVoidResult<T>::Type;
    }

    template<typename T>
    struct InstanceFunctor<std::optional<T>>
    {
        template<typename F>
        using FmapResult_t = std::optional<detail::WrapVoidResult_t<std::decay_t<std::invoke_result_t<F, T>>>>;
 
        template<typename F>
        static FmapResult_t<F> Apply (const std::optional<T>& t, const F& f)
        {
            if (!t)
                return {};
 
            if constexpr (std::is_same_v<FmapResult_t<F>, std::optional<Void>>)
            {
                std::invoke (f, *t);
                return { Void {} };
            }
            else
                return { std::invoke (f, *t) };
        }
    };
 
    template<typename L, typename R>
    struct InstanceFunctor<Either<L, R>>
    {
        template<typename F>
        using FmapResult_t = Either<L, std::invoke_result_t<F, R>>;
 
        template<typename F>
        static FmapResult_t<F> Apply (const Either<L, R>& either, const F& f)
        {
            if (either.IsLeft ())
                return FmapResult_t<F>::Left (either.GetLeft ());
 
            return FmapResult_t<F>::Right (f (either.GetRight ()));
        }
    };
}
}