While I understand you all feel uncomfortable with this, I do not think the problem demonstrated by John has anything to do with Generic. I've made a fork here https://github.com/kosmikus/safe-bugtest that shows an (IMHO) similar problem using Show and Read instead of Generic. (And something slightly different could certainly also produced using Enum). If you're deriving Generic, then yes, you gain the functionality of that class, which is to inspect the structure of the type, and then yes, you can in principle construct any value of that type. So deriving Generic for types that should be abstract is always going to be risky. But this is no different than deriving any other class, only that Generic gives you particularly fine-grained access to the internals of a type. Also, at least in my opinion, it is entirely valid to define your own Generic instances. It's more work, and while I haven't used it often so far, I can imagine that there are good use cases. I don't think it's anywhere near as evil as defining your own Typeable instances. Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com