"Simon Peyton-Jones"
You are quite right. Indeed GADTs don't interact properly with type classes at all, let alone functional dependencies, I'm afraid. I decided to pause and release before attending to this; it's not trivial to do it right.
I haven't tripped over anything in particular, but I have been helping the Pugs developers[1] with Haskell. Autrijus Tang pointed out that the type examples in the HaskellDemo[2] are easier for new users with GADTs. HaskellDemo has this: data Temp = Cold | Hot data People = Person Name Age But new users find this much easier to understand: data Temp where Cold :: Temp Hot :: Temp data People where Person :: Name -> Age -> People I'd guess GADTs are clearer because type signatures like this put constructors into the mental category of 'just another function'. They also make the difference between constructor and type names very clear. (This is another common newbie confusion.) I doubt pedagogics was an important part of your goal with GADTs, but now several people wish that "deriving Show" worked so that GADTs could be used for everything. [1] http://pugscode.org/ [2] http://www.haskell.org/hawiki/HaskellDemo -- Programming is the Magic Executable Fridge Poetry, | www.ScannedInAvian.com It is machines made of thought, fueled by ideas. | -- Shae Matijs Erisson