Haskell Quiz/Posix Pangrams/Solution Burton

This is a naive solution - it works by taking the sortest subsequence of the original list which is a pangram then trying to replace words in it with shorter, unused ones. The pangrams it produces aren't very short.

module Main
    where
import IO
import Char
import List

{--
*Main> main
pangram:  admin bc ex fg lp sh tr zcat yacc write who vi uux unlink uniq join
wordcount: 16, total chars: 52, repeated chars: 41
--}

makePangram :: [String] -> String -> [String] -> Int -> IO String
makePangram [] pg  rejects pass | isPangram pg = return pg
                                | pass == 0    = makePangram rejects pg [] 1
                                | otherwise    = return ("ran out of words! " ++ pg)
 
makePangram (w:ws) pg rejects pass | isPangram pg      = return pg
                                   | hasNew w (nub pg) = handleDupes
                                   | otherwise         = makePangram ws pg rejects pass
     where handleDupes | not (hasDupes (filter isAlpha w) (nub pg)) 
                         || pass == 1 = makePangram ws (pg ++ " " ++ w) rejects pass
                       | otherwise = makePangram ws pg (w:rejects) pass

hasNew :: String -> String -> Bool
hasNew  x []    = True
hasNew [] _     = False
hasNew (x:xs) w | not $ isAlpha x = hasNew xs w
                | x `notElem` w   = True 
                | otherwise       = hasNew xs w 

hasDupes :: String -> String -> Bool
hasDupes _ [] = False
hasDupes [] _ = False
hasDupes (x:xs) w = x `elem` w || hasDupes xs w

numDupes :: String -> Int
numDupes s = numDupes' s 0
             where numDupes' [] c = c
                   numDupes' (x:xs) c = if x `elem` xs then numDupes' xs (c+1) else numDupes' xs c

isPangram :: String -> Bool
isPangram = (==26) . length . nub . filter isAlpha

stats :: String -> String
stats s = "wordcount: " 
          ++ (show (length (words s))) 
          ++ ", total chars: " 
          ++ (show (length (filter isAlpha s))) 
          ++ ", repeated chars: " ++ (show (numDupes s))

main = do cs <- readFile "posix-utils.txt"
          pg <- makePangram (lines cs) "" [] 0
          putStrLn ("pangram: " ++ pg)
          putStrLn (stats pg)