[haskell-beginners] Ralf Laemmel's riddle on surviving without the monad transformation library

Hi, Can someone provide me the solution to the following riddle that Ralf asked in his lecture at http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Dr-Ralf-Lmmel-Advanced... Riddle: define a custom made monad (only involving (->) and Either String) to survive without the monad transformation library. -- Regards, Kashyap