[Haskell-cafe] ordNub

Thomas DuBuisson thomas.dubuisson at gmail.com
Mon Jul 15 05:16:41 CEST 2013

Just so people are aware - five years ago the notion of nubOrd and
nubWith was discussed and a consensus reached on including nubOrd.  I
think Bart got too busy, didn't submit a final patch, and no one with
commit access actually commited any code.


I fully support an efficient nub implementation making its way into
base - it's far past time.  Using Set seems sensible.


On Sun, Jul 14, 2013 at 4:20 AM, Niklas Hambüchen <mail at nh2.me> wrote:
> tldr: nub is abnormally slow, we shouldn't use it, but we do.
> As you might know, Data.List.nub is O(n²). (*)
> As you might not know, almost *all* practical Haskell projects use it,
> and that in places where an Ord instance is given, e.g. happy, Xmonad,
> ghc-mod, Agda, darcs, QuickCheck, yesod, shake, Cabal, haddock, and 600
> more (see https://github.com/nh2/haskell-ordnub).
> I've taken the Ord-based O(n * log n) implementation from yi using a Set:
>   ordNub :: (Ord a) => [a] -> [a]
>   ordNub l = go empty l
>     where
>       go _ []     = []
>       go s (x:xs) = if x `member` s then go s xs
>                                     else x : go (insert x s) xs
> and put benchmarks on
> http://htmlpreview.github.io/?https://github.com/nh2/haskell-ordnub/blob/1f0a2c94a/report.html
> (compare `nub` vs `ordNub`).
> `ordNub` is not only in a different complexity class, but even seems to
> perform better than nub for very small numbers of actually different
> list elements (that's the numbers before the benchmark names).
> (The benchmark also shows some other potential problem: Using a state
> monad to keep the set instead of a function argument can be up to 20
> times slower. Should that happen?)
> What do you think about ordNub?
> I've seen a proposal from 5 years ago about adding a *sort*Nub function
> started by Neil, but it just died.
> (*) The mentioned complexity is for the (very common) worst case, in
> which the number of different elements in the list grows with the list
> (alias you don't have an N element list with always only 5 different
> things inside).
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