[Haskell-cafe] Code Example and Haskell Patterns

Hi, Hackage is a sizable repository of Haskell code; makes me wonder if there is a way to use it more effectively for pedagogical purposes. For example, I really would like to study State monad, monad transformers, applicative, arrows etc. in action--i.e., in the context of a real application rather than small toy examples commonly found in tutorials. If there isn't such a source, can someone point me to an application whose implementation you would consider being a paragon for said concepts? Best, Ali