doThings1 :: IO ()
doThings1 = do
  putStrLn "Doing thing A."
  putStrLn "Doing thing B."

doThings2 :: IO ()
doThings2 = do
  _ <- putStrLn "Doing thing A."
  _ <- putStrLn "Doing thing B."
  pure ()
  
doThings3 :: IO ()
doThings3 =
  putStrLn "Doing thing A."
    >>= (\_ -> putStrLn "Doing thing B.")
    >>= (\_ -> pure ())
    
listOfUserIds :: [Int]
listOfUserIds = [1, 2, 3]

lookupUser :: Int -> IO String
lookupUser userId = pure ("User" ++ show userId)

usernames :: [IO String]
usernames = fmap lookupUser listOfUserIds

printOutUsernames :: IO ()
printOutUsernames = do
  names <- sequence usernames
  print names
  
printOutUsernamesAgain :: IO ()
printOutUsernamesAgain = do
  names <- traverse lookupUser listOfUserIds
  print names
  
listOfListOfNumbers :: [[Int]]
listOfListOfNumbers = [[1, 2], [3, 4]]

sequenceAListOfLists :: IO ()
sequenceAListOfLists = print (sequence listOfListOfNumbers)

main :: IO ()
main = do
  doThings1
  doThings2
  doThings3
  printOutUsernames
  printOutUsernamesAgain
  sequenceAListOfLists