lens-op-ob-preview-examples.hs

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"
  print $ maybeIntB ^? _Just
  -- Nothing

  let justiceCity = Just 1
      crashCity = Nothing :: Maybe Int

  print "Unwrap this Maybe Int or die trying!"
  print $ justiceCity ^?! _Just
  -- 1
  print "Crash city!"
  -- print $ crashCity ^?! _Just
  -- This will throw an 'empty fold' exception. `^?!` can be useful for
  -- forcing Maybes to unwrap when writing tests.