Kaprekar's constant

kaprekar.hs

import Data.List

toDigits x
 | x > 9 = x `mod` 10 : toDigits (x `div` 10)
 | otherwise = [x]

sumFold xs = foldl (\a x -> a*10 + x) 0 xs

kaprekar :: Int -> Int -> Int
kaprekar x n = let
    digs = sort $ toDigits x
    v = sumFold (reverse digs) - sumFold digs
  in
    if v == 6174 then n else kaprekar v (n+1)

main = putStrLn $ show (kaprekar 9715 0)