[Haskell-cafe] ordNub

[Roman Cheplyaka]

Something like that should definitely be included in Data.List.
Thanks for working on it.

Roman

* Niklas Hambüchen <mail at nh2.me> [2013-07-14 19:20:52+0800]
> 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).
> 
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe