Some of you already know, but it seems I forgot to mention this here - my code has moved to a darcs repo, with a little bit of documentation and a README summarizing the issues. See my toolbox for more info: http://www.cs.kent.ac.uk/~cr3/toolbox/haskell/#syb-utils Neil: It turned out to be tricky to recognize nested types at the Data/Typeable level, let alone nested types that really have an infinite set of potential substructure types (which are the ones that break the PlateData optimization). Instead, I just count nesting levels (where nesting means that we encounter the top-level type constructor while exploring its substructure types), and set an arbitrary bound beyond which I assume the nesting to be recursive and fall back to the unoptimized case. You might want to apply something similar to PlateData. Cheers, Claus