One-Liner nub

Nub.hs

import Data.Ord
import Data.List

nubSimple :: [Int] -> [Int]
nubSimple = map head . group . sort

nubStable :: [Int] -> [Int]
nubStable = map fst . sortBy (comparing snd). map head .
              groupBy ((. fst).(==).fst) . sort . flip zip [0..]

Question: Whether the spec of nub requires to maintain the order of elements?
As in module Data.List
nub [1,2,3,48,6,4,3,2] ==> [1,2,3,48,6,4]

By the way, nub :: Eq a => [a] -> [a], not "Ord a"
so sort :: Ord a => [a] -> [a] may not work on type a.