monad.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 <optional>
#include "typelist.h"
#include "applicative.h"
#include "either.h"
 
namespace LC
{
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<std::optional<T>>
    {
        template<typename F>
        using BindResult_t = std::result_of_t<F (T)>;
 
        template<typename F>
        static BindResult_t<F> Bind (const std::optional<T>& value, const F& f)
        {
            if (!value)
                return {};
 
            return f (*value);
        }
    };
 
    template<typename L, typename R>
    struct InstanceMonad<Either<L, R>>
    {
        template<typename F>
        using BindResult_t = std::result_of_t<F (R)>;
 
        template<typename F>
        static BindResult_t<F> Bind (const Either<L, R>& value, const F& f)
        {
            using R_t = BindResult_t<F>;
 
            if (value.IsLeft ())
                return R_t::Left (value.GetLeft ());
 
            return f (value.GetRight ());
        }
    };
}
}