deriving for GADTs
I'm pleased to see that GHC 6.12.1 will include deriving for GADTs. However, I discovered this in a quite accidental way. I tried appending
deriving (Eq, Show) to the end of my type declaration, without success, and decided to check whether there was a request in Trac for the feature, when I discovered ticket #3012 (closed). Sure enough, switching to a standalone deriving clause worked when I compiled my program in GHC 6.12.1 RC1.
It would be nice if this feature were more easily discoverable. I think it deserves mention in the release notes. Also, it would be great if the error message for the non-standalone deriving clause (Can't make a derived instance of ... / Constructor ... does not have a Haskell-98 type) suggested the standalone syntax.