Like these?

  > let lst = [1,2,3]
  > lst!!0
  1
  > lst!!1
  2
  > let arr = listArray (0,2) [1,2,3]
  > arr!0
  1
  > arr!2
  2

I don't see how it would be particularly helpful to have the arguments to the indexing functions the other way around, but in case you really like the index coming before the list/array, you can always do

  > (!0) arr
  1
  > (!!1) lst
  2