On Mon, Mar 12, 2007 at 08:25:07PM -0700, Stefan O'Rear wrote:
On Tue, Mar 13, 2007 at 03:16:44AM +0000, Neil Mitchell wrote:
Hi,
http://hackage.haskell.org/trac/ghc/ticket/1218
I propose the addition of sortNub and sortNubBy.
Semantically sortNub = sort . nub
Practically, if you are doing both a sort and a nub, this can be implemented as:
sortNub = map head . group . sort
This is O(n log n) [time to sort], rather than O(n^2) [time to nub].
I have seen this around several times, often called snub. People have also used snub to mean other things, snub itself has a meaning, and sortNub is a more accurate name (following the concatMap tradition).
I personally have defined this function at least 25 times, I suspect others have to. I can find 5 identical versions of it on google code [1], and I know google code is lying because it's missing ones it found earlier.
ordNub = flip evalState S.empty . filterM (State . tst) where tst x st = (not $ S.member x st, S.insert x st)
ordNub ls = ordNub' ls S.empty where ordNub' (x:xs) set | S.member x set = ordNub' xs set | otherwise = x : ordNub' xs (S.insert x set) ordNub' [] set = []
O(n log n), doesn't change order. Perhaps also worthy?
not only does it preserve order, but it works on infinite lists and is lazy :) John -- John Meacham - ⑆repetae.net⑆john⑈