| Now, I am scared. This should be regarded as a bug in generalised | newtype deriving, shouldn't it? I would expect newtype deriving to be | unable to come up with instances that cannot be written by hand. | | I would have expected people out on the streets marching to GHC | headquarters by now; how can you stay so calm? Indeed, it's a known problem: http://hackage.haskell.org/trac/ghc/ticket/1496 It's a serious problem too, because it affects type soundness. But I have been moving slowly on fixing it, because I want a proper fix not a hack. Dimitrios and I have a variety of solutions now, some of which have the flavour of what's suggested below: | > I wonder if there's a potential refinement of the kind system lurking | > here, distinguishing *, types-up-to-iso, from |*|, types-up-to- | > identity. | > That might help us to detect classes for which newtype deriving is | > inappropriate: GADTs get indexed over |*|, not *; classes of *s are | > derivable, but classes of |*|s are not. I certainly don't have a clear | > proposal just now. It looks like an important distinction: recognizing | > it somehow might buy us something. Stay tuned. Simon