List index (subscript) operator, starting from 0. It is an instance of
the more general
genericIndex, which takes an index of any
integral type.
WARNING: This function is partial, and should only be used if you are
sure that the indexing will not fail. Otherwise, use
!?.
WARNING: This function takes linear time in the index.
Examples
>>> ['a', 'b', 'c'] !! 0
'a'
>>> ['a', 'b', 'c'] !! 2
'c'
>>> ['a', 'b', 'c'] !! 3
*** Exception: Prelude.!!: index too large
>>> ['a', 'b', 'c'] !! (-1)
*** Exception: Prelude.!!: negative index