What happens when a type adds driving such as:
newtype SupplyT s m a = SupplyT (StateT [s] m a)
deriving (Functor, Monad, MonadTrans, MonadIO)
Two questions:
How does the deriving implement the instance?
Is there a way for me to add my own classes in the deriving? for example
newtype ..... deriving( xyz)
In general you would have to resort to generic programming techniques to obtain your own deriving mechanism. See the generics category on Hackage for libraries: http://hackage.haskell.org/packages/archive/pkg-list.html To find out what you can do with the various generic programming techniques, you can read (shameless plugs): Alexey Rodriguez Yakushev, Johan Jeuring, Patrik Jansson, Alex Gerdes, Oleg Kiselyov, Bruno C. d. S. Oliviera. Comparing Libraries for Generic Programming in Haskell. In Andy Gill, editor, Proceedings of the ACM SIGPLAN Haskell Symposium. An extended version is available as Technical report Utrecht University UU-CS-2008-010, 2008. Ralf Hinze, Johan Jeuring, and Andres Löh. Comparing approaches to generic programming in Haskell. In Roland Backhouse, Jeremy Gibbons, Ralf Hinze, and Johan Jeuring, editors, Lecture notes of the Spring School on Datatype-Generic Programming 2006, LNCS 4719, pages 72 - 149, 2007, Springer-Verlag. -- Johan Jeuring