lens + maybe

example.hs

data Box   = Box { _value :: Integer }
data World = World { _box :: Maybe Box }

makeLenses ''Box
makeLenses ''World

steps :: StateT World IO ()
steps = do
  box.value %= succ
  
-- Couldn't match type ‘Box’ with ‘Maybe Box’
-- Expected type: (Integer -> f Integer)
--               -> Maybe Box -> f (Maybe Box)
--   Actual type: (Integer -> f Integer) -> Box -> f Box
-- In the second argument of ‘(.)’, namely ‘pose’
-- In the expression: robot . pose