[Haskell-cafe] Stack data structure

I am a graduate student in theoretical computer science playing around with haskell for the past year or so. I wrote a Stack Data structure today and would like to get some feedback from more experienced haskellers if possible. I know its a really simple thing to write. But the implementation I found on hackage wasn't what I was looking for. I haven't wrote unit tests yet, but it should behave as expected. Thanks in advance. Kind regards. Daniel