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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

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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

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-- StorageError in a module somewhere
newtype StorageError = StorageError Text deriving (Eq, Show)

-- WebError wraps storage Error
data WebError
  = WebTextError Text
  | WebStorageError StorageError
  deriving (Eq, Show)

-- Let's convert an 'Either StorageError a' to an 'Either WebError a'

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overExamples :: IO ()
overExamples = do
  let fitz = Pet (PetName "Fitz")
  let bob = User (UserName "bob") 0 (Just fitz) HM.empty

  print "Bob scores a point. Way to go, Bob."
  -- These all print bob with a score incremented by 1.
  print $ bob & score %~ (\sc -> sc + 1)
  print $ bob & score %~ (+1)
  print $ over score (+1) bob

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fancySetExamples :: IO ()
fancySetExamples = do
  let bob = User (UserName "bob") 0 Nothing HM.empty

  -- check out this multi-line string, why don't ya?
  print "Bob changes his name to 'Bill'\
        \, updates his score, and now owns Jeff's pet fish,\
        \who is named Fitzgerald."
  print $
    bob

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setExamples :: IO ()
setExamples = do
  let bob = User (UserName "bob") 0 Nothing HM.empty

  print "Bob, with an updated score"
  print $ set score 42 bob
  -- User {_userName = UserName "bob", _userScore = 42, _userPet = Nothing, _userInventory = fromList []}
  print $ (score .~ 42) bob
  -- User {_userName = UserName "bob", _userScore = 42, _userPet = Nothing, _userInventory = fromList []}

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previewExamples :: IO ()
previewExamples = do
  let maybeIntA = Just 1
  -- Have to tell the compiler this was a 'Maybe Int' for it to be printable
      maybeIntB = Nothing :: Maybe Int

  print "maybeIntA"
  print $ maybeIntA ^? _Just
  -- Just 1
  print "maybeIntB"

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composedViewExamples :: IO ()
composedViewExamples = do
  let
    bob = User (UserName "bob") 42 Nothing HM.empty
    fitzgerald = Pet (PetName "Fitzgerald")
    jeff = User (UserName "jeff") 42 (Just fitzgerald) HM.empty

  print "Bob's pet's name is: "
  print $ preview (pet . _Just . petName) bob
  -- Nothing

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viewExamples :: IO ()
viewExamples = do
  let bob = User (UserName "Bob") 42 Nothing HM.empty

  print "Bob's name is: "
  print $ view userName bob
  -- UserName "bob"
  print $ bob ^. userName
  -- UserName "bob"

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-- | A lens from a User to Text.
--
-- Written quite explicitly with getter and setter helper functions to expose
-- Lens's nature.
userName :: Lens' User UserName
userName = lens getter setter
  where
    getter user = _userName user
    setter user newName = user { _userName = newName }