lens-op-ob-atixnonexamples.hs

atIxNonExamples :: IO ()
atIxNonExamples = do
  let bob = User (UserName "bob") 42 Nothing HM.empty
      -- if you were doing this for-real, you would impl and use Data.Default
      defaultGoldItem = Item 0 0

  print "Return the value of Bob's gold, whether he has it or not."
  print $ bob ^. inventory . at "gold" . non defaultGoldItem . value
  -- 0
  print $ bob ^? inventory . at "gold" . _Just . value
  -- Nothing