monad.h

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/**********************************************************************
 * LeechCraft - modular cross-platform feature rich internet client.
 * Copyright (C) 2006-2014  Georg Rudoy
 *
 * Boost Software License - Version 1.0 - August 17th, 2003
 *
 * Permission is hereby granted, free of charge, to any person or organization
 * obtaining a copy of the software and accompanying documentation covered by
 * this license (the "Software") to use, reproduce, display, distribute,
 * execute, and transmit the Software, and to prepare derivative works of the
 * Software, and to permit third-parties to whom the Software is furnished to
 * do so, all subject to the following:
 *
 * The copyright notices in the Software and this entire statement, including
 * the above license grant, this restriction and the following disclaimer,
 * must be included in all copies of the Software, in whole or in part, and
 * all derivative works of the Software, unless such copies or derivative
 * works are solely in the form of machine-executable object code generated by
 * a source language processor.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
 * SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
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 **********************************************************************/

#pragma once

#include <boost/optional.hpp>
#include "typelist.h"
#include "applicative.h"

namespace LeechCraft
{
namespace Util
{
    template<typename T>
    struct InstanceMonad;

    template<template<typename...> class Monad, typename... Args, typename V>
    auto Return (const V& v)
    {
        return Pure<Monad, Args...> (v);
    }

    template<typename MV, typename F>
    using BindResult_t = typename InstanceMonad<MV>::template BindResult_t<F>;

    namespace detail
    {
        template<template<typename...> class Monad, typename... Args1, typename... Args2>
        constexpr bool IsCompatibleMonadImpl (const Monad<Args1...>*, const Monad<Args2...>*, int)
        {
            return std::is_same<
                        decltype (Init (Typelist<Args1...> {})),
                        decltype (Init (Typelist<Args2...> {}))
                    >::value;
        }

        template<typename T1, typename T2>
        constexpr bool IsCompatibleMonadImpl (const T1*, const T2*, ...)
        {
            return false;
        }

        template<typename T>
        constexpr T* declptr () noexcept
        {
            return nullptr;
        }

        template<typename T1, typename T2>
        constexpr bool IsCompatibleMonad ()
        {
            return IsCompatibleMonadImpl (detail::declptr<T1> (), detail::declptr<T2> (), 0);
        }
    }

    template<typename MV, typename F>
    BindResult_t<MV, F> Bind (const MV& value, const F& f)
    {
        static_assert (detail::IsCompatibleMonad<MV, BindResult_t<MV, F>> (),
                "Incompatible function return type");
        return InstanceMonad<MV>::Bind (value, f);
    }

    template<typename MV, typename F>
    auto operator>> (const MV& value, const F& f) -> decltype (Bind (value, f))
    {
        return Bind (value, f);
    }

    template<typename MV>
    auto Do (const MV& value)
    {
        return value;
    }

    template<typename MV, typename FHead, typename... FArgs>
    auto Do (const MV& value, const FHead& fHead, const FArgs&... fArgs)
    {
        return Do (Bind (value, fHead), fArgs...);
    }

    // Implementations
    template<typename T>
    struct InstanceMonad<boost::optional<T>>
    {
        template<typename F>
        using BindResult_t = std::result_of_t<F (T)>;

        template<typename F>
        static BindResult_t<F> Bind (const boost::optional<T>& value, const F& f)
        {
            if (!value)
                return {};

            return f (*value);
        }
    };
}
}