Programming in Haskell -- a second attempt at a storage typeclass

I have enough of a concrete implementation of both a file-based storage module and a postgresql storage module to try to extract a common interface between them. I originally tried this when I only had the file-based storage but that didn't work when I tried to use it for switching to a database. Hopefully it will go better this time now that I'm armed with a better understanding of how both implementations work.