lens-op-ob-atixexamples.hs

atIxExamples :: IO ()
atIxExamples = do
  -- Yep, you can use apostrophes in var names. Not that you should...
  let bob'sInventory = HM.fromList [ ("gold", Item 99 10)
                                    , ("silver", Item 10 9)
                                    ]
      bob = User (UserName "bob") 42 Nothing bob'sInventory

  print "Printing Bob's gold value"
  print $ bob ^? inventory . at "gold" . _Just . value
  -- Just 99
  print $ bob ^? inventory . ix "gold" . value
  -- Just 99
  print $ bob ^? inventory . at "doesnotexist" . _Just . value
  -- Nothing
  print $ bob ^? inventory . ix "doesnotexist" . value
  -- Nothing


  print "Bob finds a diamond"
  let bobFindsDiamond  = bob & inventory . at "diamond" ?~ (Item 1000 1)
      bobFindsDiamond' = bob & inventory . at "diamond" .~ Just (Item 1000 1)
  print $ bobFindsDiamond ^? inventory . ix "diamond"
  -- Just (Item 1000 1)
  print $ bobFindsDiamond' ^? inventory . ix "diamond"
  -- Just (Item 1000 1)

  print "Bob loses his gold, some points, and is sad"
  let bobLosesGold = bob
        & inventory . at "gold" .~ Nothing
        & score %~ (\sc -> sc - 41)
        & userName .~ UserName "Sad Bob"

  -- Note the differences in `^./^?/at/ix` usage
  print $ bobLosesGold ^? inventory . at "gold"
  -- Just Nothing
  print $ bobLosesGold ^. inventory . at "gold"
  -- Nothing
  print $ bobLosesGold ^? inventory . ix "gold"
  -- Nothing

  -- This won't compile without an instance of Monoid for "Item".
  -- If you implement that instance, and run this, it will assume you
  -- wanted to use that Monoid instance, and return it's mempty for you.
  -- print $ bobLosesGold ^. inventory . ix "gold"

  print $ bobLosesGold ^. score
  -- 1
  print $ bobLosesGold ^. userName
  -- UserName "Sad Bob"