:: Ord a => [a] -> [a] -package:containers -package:lens -package:incipit-base -package:base -package:basement is:exact -package:mono-traversable package:witherable

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]