not_a_monad.cpp

boost::optional<int> f1(int a);
boost::optional<int> f2(int b, int c);
boost::optional<int> f3(int d);
boost::optional<int> f4(int e);

boost::optional<int> result = chain(
	[] { return f1(3); },
	[] { return f1(4); }
).then(
	[] (int b, int c) { return f2(b, c); }
).then(
	[] (int d) { return f3(d); }
).then(
	[] (int e) { return f3(e); }
).run();