lens-op-ob-moreoverexamples.hs

-- 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'
moreOverExamples :: IO ()
moreOverExamples = do
  let badStorageResponse =
        Left (StorageError "fail!") :: Either StorageError Text
      goodStorageResponse =
        Right "datadata" :: Either StorageError Text

  -- Wrap the error in WebStorageError if it's a `Left StorageError`
  print $ over _Left (\stErr -> WebStorageError stErr) badStorageResponse
  print $ over _Left WebStorageError badStorageResponse
  print $ badStorageResponse & _Left %~ (\stErr -> WebStorageError stErr)
  print $ badStorageResponse & _Left %~ WebStorageError
  -- Left (WebStorageError (StorageError "fail!"))

  -- This passes the 'Right Text' through just fine.
  print $ over _Left WebStorageError goodStorageResponse
  -- Right "datadata"