Is any of the existing Generics packages able to work without compiler extensions, that is Haskell 98? I mean, it is ok if the developer of the generic parts of a library needs compiler extensions or extra tools, but the user who calls 'cabal install' shall not need them and the developer, who does not touch generic definitions, should not need them as well. I think this could be done with pragmas in the following way: class C a where f :: a -> [Int] {-# GENERICDEFAULT f {| Unit |} Unit = [] #-} {-# GENERATEDINSTANCEBEGIN Automatically generated - Do not edit! #-} instance C () where f () = [] {-# GENERATEDINSTANCEEND Automatically generated - Do not edit! #-} Now, by running a tool, the part between the GENERATEDINSTANCE pragmas can generated or updated. Everything outside the pragmas can be Haskell 98. It does not only avoid extensions of the building compiler, but also extensions of a programmer's brain. Because for programmers it is often easier to understand a set of instances and see the common patterns, than to understand generic code that builds the instances.