import Control.Monad.Cont

cataCps :: (Traversable f) => (f a -> a) -> Fix f -> a
cataCps algebra expr = runCont (recur algebra expr) id

recur :: (Traversable f) => (f a -> a) -> Fix f -> Cont a a
recur algebra (Fix expr) = do
  sub <- sequence $ fmap (recur algebra) expr
  return (algebra sub)