Introduction/Direct Translation
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(+faster qsort) 

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Latest revision as of 08:07, 16 August 2009
The quicksort quoted in Introduction isn't the "real" quicksort and doesn't scale for longer lists like the c code does.
http://programming.reddit.com/info/5yutf/comments/
Here are some points to how the "real" quicksort would look in haskell.
Lennart Augustsson has a quicksort entry on his blog which is pure (no unsafe):
http://augustss.blogspot.com/2007/08/quicksortinhaskellquicksortis.html
Another version (uses System.IO.Unsafe), is below.
There is also a "parallel" quicksort at
http://www.macs.hw.ac.uk/~dsg/gph/nofib/
roconnor claims that in haskell the "real" quicksort is really a treesort:
http://programming.reddit.com/info/2h0j2/comments
Unfortunately none of the above "real" quicksorts seems to compile as given, when copy/pasted into ghci. Can someone fix? The "parallel" quicksort gave error "unknown package concurrent" when I ran make in quicksort/gransim.
Has anyone got a functioning "real" quicksort that works on copy/paste?
The program below is working very very slowly. It's probably slowsort... :o)
import Control.Monad (when) import Control.Monad.ST import Data.Array.ST import Data.Array.IArray import Data.Array.MArray qsort :: (IArray a e, Ix i, Enum i, Ord e) => a i e > a i e qsort arr = processArray quickSort arr processArray :: (IArray a e, IArray b e, Ix i) => (forall s. (STArray s) i e > ST s ()) > a i e > b i e processArray f (arr :: a i e) = runST $ do arr' < thaw arr :: ST s (STArray s i e) f arr' unsafeFreeze arr' quickSort :: (MArray a e m, Ix i, Enum i, Ord e) => a i e > m () quickSort arr = qsort' =<< getBounds arr where qsort' (lo, hi)  lo >= hi = return ()  otherwise = do p < readArray arr hi l < mainLoop p lo hi swap l hi qsort' (lo, pred l) qsort' (succ l, hi) mainLoop p l h  l >= h = return l  otherwise = do l' < doTil (\l' b > l' < h && b <= p) succ l h' < doTil (\h' b > h' > l' && b >= p) pred h when (l' < h') $ do swap l' h' mainLoop p l' h' doTil p op ix = do b < readArray arr ix if p ix b then doTil p op (op ix) else return ix swap xi yi = do x < readArray arr xi readArray arr yi >>= writeArray arr xi writeArray arr yi x
This uses various extensions to make the types ridiculously general, but the actual algorithm (quickSort) is plain Haskell.
A more specific/direct translation (neither this nor the C version is polymorphic) is offered by Daniel Fischer, who reports that this version runs within 2x of the C version:
import Data.Array.Base (unsafeRead, unsafeWrite) import Data.Array.ST import Control.Monad.ST myqsort :: STUArray s Int Int > Int > Int > ST s () myqsort a lo hi  lo < hi = do let lscan p h i  i < h = do v < unsafeRead a i if p < v then return i else lscan p h (i+1)  otherwise = return i rscan p l i  l < i = do v < unsafeRead a i if v < p then return i else rscan p l (i1)  otherwise = return i swap i j = do v < unsafeRead a i unsafeRead a j >>= unsafeWrite a i unsafeWrite a j v sloop p l h  l < h = do l1 < lscan p h l h1 < rscan p l1 h if (l1 < h1) then (swap l1 h1 >> sloop p l1 h1) else return l1  otherwise = return l piv < unsafeRead a hi i < sloop piv lo hi swap i hi myqsort a lo (i1) myqsort a (i+1) hi  otherwise = return ()