edit distance algorithm in Haskell - performance tuning

Question

I\'m trying to implement the levenshtein distance (or edit distance) in Haskell, but its performance decreases rapidly when the string lenght increases.

I\'m still q

Answer 1

This version is much quicker than those memorised versions, but still I would love to have it even quicker. Works fine with 100's character long strings. I was written with other distances in mind(change the init function and cost) , and use classical dynamic programming array trick. The long line could be converted into a separate function with top 'do', but I like this way.

import Data.Array.IO
import System.IO.Unsafe

editDistance = dist ini med

dist :: (Int -> Int -> Int) -> (a -> a -> Int ) -> [a] -> [a] -> Int
dist i f a b  = unsafePerformIO $ distM i f a b

-- easy to create other distances 
ini i 0 = i
ini 0 j = j
ini _ _ = 0
med a b = if a == b then 0 else 2


distM :: (Int -> Int -> Int) -> (a -> a -> Int) -> [a] -> [a] -> IO Int
distM ini f a b = do
        let la = length a
        let lb = length b

        arr <- newListArray ((0,0),(la,lb)) [ini i j | i<- [0..la], j<-[0..lb]] :: IO (IOArray (Int,Int) Int)

-- all on one line
        mapM_ (\(i,j) -> readArray arr (i-1,j-1) >>= \ld -> readArray arr (i-1,j) >>= \l -> readArray arr (i,j-1) >>= \d-> writeArray arr (i,j) $ minimum [l+1,d+1, ld + (f (a !! (i-1) ) (b !! (j-1))) ] ) [(i,j)| i<-[1..la], j<-[1..lb]]

        readArray arr (la,lb)