I would like to propose improving the Data instances for a number of currently completely opaque data types in the containers package, by using virtual constructors. The instance for Data.Map already uses fromList for gfoldl, it just stops there. Extending it to be able to gunfold and mention the name of that constructor would enable generic traversal libraries like uniplate, etc. to work over the contents of the Map, rather than bailing out in fear or crashing at the sight of a mkNoRepType. An example of the changes for Data.Map are highlighted below. instance (Data k, Data a, Ord k) => Data (Map k a) where gfoldl f z m = z fromList `f` toList m toConstr _ = fromListConstr gunfold k z c = case constrIndex c of 1 -> k (z fromList) _ -> error "gunfold" dataTypeOf _ = mapDataType dataCast2 f = gcast2 f fromListConstr :: Constr fromListConstr = mkConstr mapDataType "fromList" [] Prefix mapDataType :: DataType mapDataType = mkDataType "Data.Map.Map" [fromListConstr] I've used this approach for years on my own libraries to great effect. Discussion Period: 3 Weeks (I added a week for ICFP) -Edward