C++11 non-lazy monad

eager_monad.cpp

#include <boost/optional.hpp>
#include <utility>
#include <string>
#include <cassert>

template <class T>
struct maybe
{
    maybe() {}
    maybe(T x) : value(std::move(x)) {}
    explicit operator T() const { return value; }
    
    boost::optional<T> value;
};

template <class T, class F>
auto operator>>(maybe<T> const& x, F f) -> decltype(f(*x.value))
{
    return x.value ? f(*x.value) : decltype(f(*x.value))();
}

maybe<int> f(int x) { return x < 0 ? maybe<int>() : x; }
maybe<std::string> xes(unsigned int n) { return std::string(n, 'x'); }
maybe<unsigned> length(std::string const& s) { return s.size(); }

int main()
{
    assert(*(f(10) >> xes >> length).value == 10);
    assert(!(f(-10) >> xes >> length).value);
    assert(!(f(-10) >> xes).value);
    assert(!f(-10).value);
}