!!

(!!) :: [a] -> Int -> a
base Prelude, base Data.List
List index (subscript) operator, starting from 0. It is an instance of the more general Data.List.genericIndex, which takes an index of any integral type.
($!!) :: NFData a => (a -> b) -> a -> b
deepseq Control.DeepSeq
the deep analogue of $!. In the expression f $!! x, x is fully evaluated before the function f is applied to it.