:: Ord a => [a] -> [a] -package:focus -package:vector-algorithms -package:incipit-base -package:safe -package:Cabal-syntax -package:extra 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]