haskell lenses on recursive data structures

LensesRecursive.hs

{-# LANGUAGE TemplateHaskell, FlexibleContexts #-}

import Data.Lenses
import Data.Lenses.Template

-- data structures to work on
--  for fields ending with an underscore, special lens accessor functions
--  can be created with "deriveLenses"
data TD = TD { name_ :: String
             , td_ :: TD
             }
        | TNil
        deriving Show

-- template haskell magic to create accessor functions
$( deriveLenses ''TD )

testdata = TD "level 1" $ TD "level 2" $ TD "level 3" $ TNil

setNameOn3 :: TD -> String -> TD
setNameOn3 d = d `update` (td . td . name)

main :: IO ()
main = do 
    print $ setNameOn3 testdata "blub"