State Monad in C++11

gistfile1.cpp

namespace hask {
    struct Unit { };
    template<typename S>
    struct Stateful {
        template<typename V>
        struct Value
        {
            std::function<std::pair<S,V>(S)> runState;
        };
    };

    template<typename S, typename V>
    typename Stateful<S>::template Value<V> ret(const V& v)
    {
        decltype(ret<S>(v)) rv;
        rv.runState = [=](S s) { return std::make_pair(s, v); };
        return rv;
    }

    template<typename S>
    typename Stateful<S>::template Value<S> get()
    {
        Stateful<S>::Value<S> rv;
        rv.runState = [](S s) { return std::make_pair(s,s); };
        return rv;
    }

    template<typename S>
    typename Stateful<S>::template Value<Unit> put(const S& s)
    {
        Stateful<S>::Value<Unit> rv;
        rv.runState = [=](S ) { return std::make_pair(s, Unit()); };
        return rv;
    }

    template<typename S, typename A, typename R>
    typename Stateful<S>::template Value<R> bind(typename Stateful<S>::template Value<A> a, 
        std::function<typename Stateful<S>::template Value<R> (typename A)> f)
    {
        Stateful<S>::Value<R> rv;
        rv.runState = [=](S init) -> std::pair<S,R> 
        { 
            std::pair<S, A> p = a.runState(init); 
            Stateful<S>::Value<R> act2 = f(p.second); 
            return act2.runState(p.first); 
        };
        return rv;
    }

    template<typename S, typename A, typename R>
    typename Stateful<S>::template Value<R> seq(typename Stateful<S>::template Value<A> a, typename Stateful<S>::template Value<R> r)
    {
        return bind<S,A,R>(a, [=](A) { return r; });
    }

    
    template<typename S, typename V>
    typename Stateful<S>::template Value<V> lift0(std::function<V()> f)
    {
        Stateful<S>::Value<V> rv;
        rv.runState = [=](S s) { return std::make_pair(s, f()); };
        return rv;
    }
}

using namespace hask;
    Stateful<int>::template Value<int> getit = get<int>();
    std::function< decltype(hask::put<int>(0)) (int) > doit = [](int x) { return hask::put(x + 1); };
    Stateful<int>::Value<Unit> incrS = hask::bind<int, int, Unit>(getit, doit);
    std::cout << incrS.runState(7).first << std::endl;