[Haskell-cafe] Grokking zippers

Hello all- I'm trying to understand the categorical guts underlying zippers. In the Haskell wikibook (and other places) I've seen zippers described roughly as the derivatives of functors. However, I haven't been able to find any references that develop this idea rigorously. For instance, what exactly is the "derivative" of a functor at an object, in the direction of some arrow? I'm interested in studying this concept in more depth, but I can't find a definition to start with. Any pointers to good books or papers would be greatly appreciated. :) Thank you all- Nathan Bloomfield