Dear all, For a project involving I use some partially defined node (in this case a simple record, in my project state transformers) in which the defined part is common to all nodes, and the custom part is different for each node. They have to become anonymous so I can put them in a list of connections from each node to another. For some reason GHC complains of 'ambigous type variable' in the code below. The thing is, part of the code may be undefined, but since I'm (explicitly) not using that part, why would GHC care? Are there other solutions to this problem? Any pointers or comments appreciated. Thanks. Maarten (This code is just some dummy code that contains the essence of the problem. I posted the complete code with piggy bagged state transformers in active objects on haskell@haskell.org, but that is rather long and this seems to be the correct mailing list). -- data structure with custom and common part data Node cust = Node cust Common deriving (Show,Typeable) -- anonymous data structure to put use in list data AN = forall ar. (Show ar, Typeable ar) => AN ar instance Show AN where show (AN an) = "AN (" ++ show an ++ ")" -- common part data Common = Common Integer deriving (Show,Typeable) data Custom = Custom Integer deriving (Show,Typeable) data Custom2 = Custom2 Integer deriving (Show,Typeable) -- extract common part, ignoring type of custom part getCommon :: forall gc. (Node gc) -> Common getCommon (Node cust com) = com main = do let a = AN (Node (Custom 5) (Common 10)) let b = case a of (AN a') -> getCommon (case (cast a') of Just a'' -> a'') putStrLn $ "ok:" ++ show b