Hi, The library module Dynamic provides instances of the Typeable class for various datatypes, among which tuples of various lengths. These instances are incorrect, as demonstrated by the following snippet: Dynamic> (typeOf (True,False)) == ((\(Dynamic t _) -> t) (toDyn (True,False))) False In other words, the type representation produced via the "typeOf" member is not equal to the type representation produced via the function "toDyn". This is wrong. As a result, casting back and forth to the type Dynamic fails: Dynamic> fromDynamic (toDyn (True,False)) :: Maybe (Bool,Bool) Nothing The error is caused by the nested definition of tup2Tc in the following instance declaration: instance (Typeable a, Typeable b) => Typeable (a,b) where typeOf tu = mkAppTy tup2Tc [typeOf (fst tu), typeOf (snd tu)] where fst :: (a,b) -> a fst = undefined snd :: (a,b) -> b snd = undefined tup2Tc = mkTyCon "," If tup2Tc is made top-level, the error vanishes. Likewise for longer tuples. The reason that TyCons must be defined top-level is that they are shared, so they can be mapped onto Ints and compared efficiently. When defined nested inside the instance declaration, this sharing apparently fails. Regards, Joost Visser PS: I'm using hugs98-Dec2001.