After my colleague explained me about zippers and how one could derive the datatype using differential rules, I had to read about it. So I started reading http://en.wikibooks.org/wiki/Haskell/Zippers#Mechanical_Differentiation This page contains the sentence: *"For a systematic construction, we need to calculate with types. The basics of structural calculations with types are outlined in a separate chapter **Generic Programming*http://en.wikibooks.org/w/index.php?title=Haskell/Generic_Programming&action=edit&redlink=1 * and we will heavily rely on this material"* * * However, the generic programming link does not exist yet :-) So although I now have a rough idea about it, I don't understand the details yet, e.g. notation like [image: \mathit{Node}\,A = \mu X.\,\mathit{NodeF}_A\,X] doesn't make much sense to me yet, although I suspect I can read the mu as a lambda on types? Thanks, Peter * *