I have a generic object that I want to wrap in various newtypes to better facilitate type checking. For example, newtype Blog = Blog Obj newtype Comment = Comment Obj newtype User = User Obj Unlike Obj itself, whose internal structure is hidden in a library module, the newtype wrappings are purely to facilitate type checking. It is no secret that each is just a wrapper around Obj. It is obvious how to construct the various wrapper objects. It is not so obvious how to extract the Obj they contain in a reasonably generic way however. What I want is a getObj function that works on all of them. Of course this could work if someone using the library wrote an instance for each wrapper object: instance GetObject Blog where getObj (Blog obj) = obj but this is a pain in the neck to write for each newtype. I discovered that Foldable defines a handy toList function that extracts content from generic Foldable structures. So that I could write: toObj :: Foldable thing => thing Obj -> Obj toObj w = head $ toList w Slightly kludgy but it works. Even better, recent versions of GHC will allow you to automatically derive Foldable. Unfortunately, newtype Blog = Blog Obj deriving Foldable returns a kind error. What does work is: newtype BlogF a = Blog a deriving Foldable type Blog = BlogF Obj After having spent close to a day on this, I am a bit baffled that such a seemingly trivial problem seems so hard to do. I am wondering if I am missing something really, really obvious. Any suggestions? Or is there perhaps a more Haskelly way to place type constraints on a more generic type? Kevin