What you ask is not easy. When you ask for everywhere special you ask to apply special to each node of the tree -- and special requires the MyClass dictionary. But the library code for 'gmapT' and 'everywhere' don't know about MyClass. In particular, to do (gmapT special), poor old gmapT has to find a MyClass dictionary to pass to each call to special -- and it has no way to do that. I bet that you could do what you want by replacing the 'instance MyClass ExampleType1' by mkTs for special, so special uses run-time type dispatch to implement the operations in MyClass. I guess that's what Keeane is suggesting. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of MR K P SCHUPKE | Sent: 02 February 2004 11:48 | To: doc@drjava.de; glasgow-haskell-users@haskell.org | Subject: Re: Generics and type classes | | Because the 'cast' operator used in generics, works on having a concrete | type to cast to. What you need to do is: | | module TypeTest where | | import Data.Generics | | class Data a => MyClass a | | instance MyClass ExampleType1 | instance MyClass ExampleType2 | | special :: ExampleType1 -> ExampleType1 | special = ... | | special2 :: ExampleType2 -> ExampleType2 | special2 = ... | | generic :: MyClass a => a -> a | generic = everywhere (mkT special `extT` special2 ...) | | Regards, | Keean. | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users