Simple Haskell implementation of the soundex algorithm. difference :: String -> String -> Int yields "phonetic distance" between words as an integer in range [0..4].

Soundex.hs

module Soundex
    where

import Data.Maybe (catMaybes)
import GHC.Exts (the)
import Data.List (group)
import Data.Function (on)

classes = [("bfpv",'1'),
           ("cgjkqsxzß",'2'),
           ("dt",'3'),
           ("l",'4'),
           ("mn",'5'),
           ("r",'6'),
           ("aeiouäöüy",'0')]

classOf :: Char -> Maybe Char
classOf c = classOf' c classes
    where
      classOf' _ []       = Nothing
      classOf' c ((cs,cls):clss)
         | c `notElem` cs = classOf' c clss
         | otherwise      = Just cls

encode :: String -> String
encode (c:cs) = c : '-' : take' 3 (encode' cs)
    where
      encode'    = filter (/='0') . map the . group . catMaybes . map classOf
      take' n xs = take n (xs ++ repeat '0')

difference :: String -> String -> Int
difference w1 w2 = length . filter (uncurry (/=)) $ (zip `on` encode) w1 w2