maybe in those cases of "convenience" we could use a tool like Data.Derive, and make some name to derive with it like "DubiousData". Then the dubious Data instances wouldn't have to be defined/imported. I wonder if it would be possible to have Derive have a "NotSoDubiousData" that somehow figured out whether dubious types were involved and rejected them..

generating instances of a different class might lead to ambiguities/ overlaps in use, but I like the idea of making deriving smarter to get rid of dummy instances. Let me see if I got your idea correctly. data Something a = Something a (IO a) Currently, we have two choices to get 'Data Something': - use naive deriving, for which we'd need to supply both 'Data a' and 'Data (IO a)', with unwanted consequences for the latter - write the Data instance for Something by hand, skipping the 'IO a' component If deriving was only a little bit smarter, we could write: data Something a = Something a {-# Data: skip #-} (IO a) deriving (Data,Typeable) and the deriving mechanism could give us a modified Data instance for Something that doesn't need 'Data (IO a)'. No need for instances we can't really write, and a clear hint in the source code that the 'IO a' component is treated specially (skipped rather than traversed)! I like the idea, Claus