これはどうにかなりませんかねえ ref: http://qiita.com/items/1580

MainRand.hs

recMapM :: forall a b m. (Data a, Data b, Monad m) => (b -> m b) -> a -> m a
recMapM f x = do
  let recurse = gmapM (recMapM f) x
  case cast x of
    Nothing -> recurse
    Just bx -> do
      z <- f bx
      case cast z of
        Nothing -> recurse
        Just az -> return az