Revision as of 19:40, 22 October 2007
The quicksort quoted in Introduction isn't the "real" quicksort and doesn't scale for longer lists like the c code does. To be fair, 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):
(copy/paste didn't compile for me I'm afraid.
Another version (uses System.IO.Unsafe), is below. Doesn't run in ghc either, I'm afraid.
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.
import Control.Monad (when) import Control.Monad.ST import Data.Array.ST import Data.Array.IArray 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 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 = case bounds arr of (lo,hi) -> qsort lo hi 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 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.