lazy.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 "monadplus.h"
 
namespace LC
{
namespace Util
{
    template<typename T>
    using Lazy_t = std::function<T ()>;
 
    template<typename T>
    Lazy_t<T> MakeLazy (const T& t)
    {
        return [t] { return t; };
    }
 
    template<typename R, typename F>
    Lazy_t<R> MakeLazyF (const F& l)
    {
        return l;
    }
 
    template<typename T>
    struct InstanceMonadPlus<Lazy_t<T>, std::enable_if_t<IsMonadPlus<T> ()>>
    {
        static Lazy_t<T> Mzero ()
        {
            return [] { return Util::Mzero<T> (); };
        }
 
        static Lazy_t<T> Mplus (const Lazy_t<T>& t1, const Lazy_t<T>& t2)
        {
            return [=]
            {
                const auto rt1 = t1 ();
                return rt1 != Util::Mzero<T> () ? rt1 : t2 ();
            };
        }
    };
}
}