On 06/10/14 21:24, Michael Sperber wrote:

Would using a different generic-programming framework help me do this without the unsafe stuff?

Yes. For example, using my traverse-with-class library: {-# LANGUAGE TemplateHaskell, MultiParamTypeClasses, FlexibleInstances, ConstraintKinds, UndecidableInstances, ImplicitParams, OverlappingInstances #-} import Data.Generics.Traversable import Data.Generics.Traversable.TH import Data.Proxy data Foo a = Foo a deriveGTraversable ''Foo -- define a class around our generic operation class Bar b where bar :: b -> b -- default case instance Bar a where bar x = x -- Foo-specific case instance Bar (Foo a) where bar x = x -- deep traversal recursiveBar :: (Bar a, GTraversable (Rec Bar) a) => a -> a recursiveBar x = let ?c = Proxy :: Proxy Bar -- traversal with the Bar dictionary in everywhere bar x Roman