monadplus.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 <numeric>
#include <optional>
 
namespace LC
{
namespace Util
{
    template<typename T, typename SFINAE = void>
    struct InstanceMonadPlus
    {
        using UndefinedTag = void;
    };
 
    namespace detail
    {
        template<typename T>
        constexpr bool IsMonadPlusImpl (int, typename InstanceMonadPlus<T>::UndefinedTag* = nullptr)
        {
            return false;
        }
 
        template<typename T>
        constexpr bool IsMonadPlusImpl (float)
        {
            return true;
        }
    }
 
    template<typename T>
    constexpr bool IsMonadPlus ()
    {
        return detail::IsMonadPlusImpl<T> (0);
    }
 
    template<typename MP>
    MP Mzero ()
    {
        return InstanceMonadPlus<MP>::Mzero ();
    }
 
    const struct
    {
        template<typename MP>
        auto operator() (const MP& m1) const
        {
            return [m1] (const MP& m2) { return InstanceMonadPlus<MP>::Mplus (m1, m2); };
        }
    } Mplus {};
 
    template<typename MP>
    auto operator+ (const MP& m1, const MP& m2) -> decltype (Mplus (m1) (m2))
    {
        return Mplus (m1) (m2);
    }
 
    const struct
    {
        template<typename Vec>
        auto operator() (Vec&& vec) const
        {
            using std::begin;
            using std::end;
            using MP = typename Vec::value_type;
            return std::accumulate (begin (vec), end (vec), Mzero<MP> (), &operator+<MP>);
        }
 
        template<typename T>
        auto operator() (std::initializer_list<T> vec) const
        {
            using std::begin;
            using std::end;
            return std::accumulate (begin (vec), end (vec), Mzero<T> (), &operator+<T>);
        }
    } Msum {};
 
    template<typename T>
    struct InstanceMonadPlus<std::optional<T>>
    {
        static std::optional<T> Mzero ()
        {
            return {};
        }
 
        static std::optional<T> Mplus (const std::optional<T>& t1, const std::optional<T>& t2)
        {
            return t1 ? t1 : t2;
        }
    };
}
}