# Introduction/Direct Translation

### From HaskellWiki

< Introduction(Difference between revisions)

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− | 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): | ||

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http://augustss.blogspot.com/2007/08/quicksort-in-haskell-quicksort-is.html | http://augustss.blogspot.com/2007/08/quicksort-in-haskell-quicksort-is.html | ||

− | + | Another version (uses System.IO.Unsafe), is below. | |

− | + | ||

− | Another version (uses System.IO.Unsafe), is below | + | |

− | Can someone fix | + | Unfortunately neither version seems to compile as given, when copy/pasted into ghci. Can someone fix? |

<haskell> | <haskell> |

## Revision as of 19:43, 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.

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/quicksort-in-haskell-quicksort-is.html

Another version (uses System.IO.Unsafe), is below.

Unfortunately neither version seems to compile as given, when copy/pasted into ghci. Can someone fix?

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.