:: Ord a => [a] -> [a] -package:html-entities -package:safe -package:basement -package:containers -package:vector-algorithms -package:lens package:witherable -is:exact

Removes duplicate elements from a list, keeping only the first occurrence. This is asymptotically faster than using nub from Data.List.
>>> ordNub [3,2,1,3,2,1]
[3,2,1]