Introduction/Direct Translation
From HaskellWiki
(add some context for quicksort in haskell) 
(+faster qsort) 

(8 intermediate revisions by 2 users not shown)  
Line 1:  Line 1:  
−  This code seems to have been created because the quicksort quoted in the introduction isn't the "real" quicksort and doesn't scale for longer lists like the c code does. To be fair, this is how the "real" quicksort would look in haskell. 
+  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): 
Lennart Augustsson has a quicksort entry on his blog which is pure (no unsafe): 

Line 5:  Line 5:  
http://augustss.blogspot.com/2007/08/quicksortinhaskellquicksortis.html 
http://augustss.blogspot.com/2007/08/quicksortinhaskellquicksortis.html 

−  Another version (uses System.IO.Unsafe): 
+  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) 

<haskell> 
<haskell> 

Line 13:  Line 13:  
import Data.Array.IArray 
import Data.Array.IArray 

import Data.Array.MArray 
import Data.Array.MArray 

−  import System.IO.Unsafe 

−  qsort :: (IArray a e,Ix i,Enum i,Ord e) => a i e > a i e 
+  qsort :: (IArray a e, Ix i, Enum i, Ord e) => a i e > a i e 
qsort arr = processArray quickSort arr 
qsort arr = processArray quickSort arr 

−  processArray 
+  processArray :: (IArray a e, IArray b e, Ix i) 
−  :: (IArray a e,IArray b e,Ix i) 
+  => (forall s. (STArray s) i e > ST s ()) > a i e > b i e 
−  => (forall s. (STArray s) i e > ST s ()) > a i e > b i e 
+  processArray f (arr :: a i e) = runST $ do 
−  processArray f (arr :: a i e) = runST (do 
+  arr' < thaw arr :: ST s (STArray s i e) 
−  arr' < thaw arr :: ST s (STArray s i e) 
+  f arr' 
−  f arr' 
+  unsafeFreeze arr' 
−  unsafeFreeze arr') 

quickSort :: (MArray a e m, Ix i, Enum i, Ord e) => a i e > m () 
quickSort :: (MArray a e m, Ix i, Enum i, Ord e) => a i e > m () 

−  quickSort arr = case bounds arr of (lo,hi) > qsort lo hi 
+  quickSort arr = qsort' =<< getBounds arr 
−  where qsort lo hi  lo >= hi = return () 
+  where 
−   otherwise = do 
+  qsort' (lo, hi)  lo >= hi = return () 
−  p < readArray arr hi 
+   otherwise = do 
−  l < mainLoop p lo hi 
+  p < readArray arr hi 
−  swap l hi 
+  l < mainLoop p lo hi 
−  qsort lo (pred l) 
+  swap l hi 
−  qsort (succ 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') $ 

−  swap l' h' 

−  mainLoop p l' h' 

−  
−  doTil pred op ix = do 

−  b < readArray arr ix 

−  if pred ix b then doTil pred op (op ix) else return ix 

−  swap xi yi = do 
+  mainLoop p l h  l >= h = return l 
−  x < readArray arr xi 
+   otherwise = do 
−  readArray arr yi >>= writeArray arr xi 
+  l' < doTil (\l' b > l' < h && b <= p) succ l 
−  writeArray arr yi x 
+  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 

</haskell> 
</haskell> 

This uses various extensions to make the types ridiculously general, but the actual algorithm (quickSort) is plain Haskell. 
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 [http://www.haskell.org/pipermail/haskellcafe/2009August/065269.html Daniel Fischer], who reports that this version runs within 2x of the C version: 

+  
+  <haskell> 

+  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 () 

+  </haskell> 
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 ()