Hi Has anybody done generic unification algorithm (e.g. using Data.Generics)? The idea is to do the unifications in a state monad (like http://portal.acm.org/citation.cfm?id=507648&dl=ACM&coll=portal ), but without having to have two level types. It would be cool to have a data type, e.g. data Var a = Bound a | Unbound data STVar s a = STRef s (Var a) So then I could define my own data type, say person, with a mix of logical variables and constants: data Person :: Person s { ssn :: Int, name :: STVar s String, spouse :: STVar s (Person s) } So ssn would be a constant, while name and spouse would be logical variables. unify would need to be a generic function, bringing together basic functions like: unifyStringString :: String - > String -> ST s () unifyVarString :: STVar s String - > String -> ST s () (for basic types) and genericUnify :: a -> a -> ST s () genericVarActualUnify :: (STVar s a) -> a -> ST s () genericActualVarUnify :: a -> (STVar s a) -> ST s () genericVarVarUnify :: (STVar s a) -> (STVar s a) -> ST s () Cheers John