!! package:rio

List index (subscript) operator, starting from 0. It is an instance of the more general genericIndex, which takes an index of any integral type.

xs !! n returns the element of the stream xs at index n. Note that the head of the stream has index 0. Beware: a negative or out-of-bounds index will cause an error.

the deep analogue of $!. In the expression f $!! x, x is fully evaluated before the function f is applied to it.