>>> head [1, 2, 3] 1 >>> head [1..] 1 >>> head [] *** Exception: Prelude.head: empty list
>>> last [1, 2, 3] 3 >>> last [1..] * Hangs forever * >>> last [] *** Exception: Prelude.last: empty list
>>> mconcat ["Hello", " ", "Haskell", "!"] "Hello Haskell!"