Sample implementation of KataPotter (http://codingdojo.org/cgi-bin/wiki.pl?KataPotter) in Haskell (FYI, it's my first program in Haskell, so it might not be all that idiomatic)

katapotter.hs

import List
import Array
import System (getArgs)

discounts = listArray (1,5) ([1.0, 0.95, 0.9, 0.8, 0.75]::[Float])
bookPrice = 8.0::Float

grouped :: Ord a => [a] -> [Int]
grouped = sort . (map length) . group . sort

variations :: [Int] -> [[Int]]
variations = takeWhile (not . null) . iterate (sort . next) 
  where next (1:x:xs)  = (x+1):xs
        next (x:x':xs) = (x-1):(x'+1):xs
        next _         = []

groupedPrice :: [Int] -> Float
groupedPrice = minimum . map (variationPrice) . variations
  where variationPrice = sum . map (priceOfSet) . toSets
        priceOfSet   s = bookPrice * (fromIntegral s) * (discounts ! s)
        toSets      [] = []
        toSets      xs = replicate (head xs) (length xs) ++ rest
          where   rest = toSets $ map (subtract (head xs)) $ tail xs

orderPrice :: Ord a => [a] -> Float
orderPrice = groupedPrice . grouped

main = do
  args <- getArgs
  print $ argPrice args
  where argPrice = groupedPrice . sort . map read