> the optional (a.k.a. nullable) type construct is an example of a monad Nullable (in the C# sense) does not form a monad, because it does not form a functor,

> the optional (a.k.a. nullable) type construct is an example of a monad Nullable (in the C# sense) does not form a monad, because it does not form a functor, because you cannot write fmap, because it does not nest.