Difference between revisions of "Introduction/Direct Translation"
(+faster qsort) 

(6 intermediate revisions by 2 users not shown)  
Line 1:  Line 1:  
−  The quicksort quoted in [[Introduction]] isn't the "real" quicksort and doesn't scale for longer lists like the c code does. 
+  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 

−  (copy/paste didn't compile for me I'm afraid. 

⚫  
⚫  
+  There is also a "parallel" quicksort at 

−  Can someone fix either of these programs to actually run? I think this would be a great way to learn a lot about some of the less "intro" aspects of haskell. 

+  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 17:  Line 27:  
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) 
−  +  => (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' 

−  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 = 
+  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') $ 

−  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 

−  +  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 

</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 ()