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     27 <article>
     28 <h2>Haskell mode</h2>
     29 <form><textarea id="code" name="code">
     30 module UniquePerms (
     31     uniquePerms
     32     )
     33 where
     34 
     35 -- | Find all unique permutations of a list where there might be duplicates.
     36 uniquePerms :: (Eq a) => [a] -> [[a]]
     37 uniquePerms = permBag . makeBag
     38 
     39 -- | An unordered collection where duplicate values are allowed,
     40 -- but represented with a single value and a count.
     41 type Bag a = [(a, Int)]
     42 
     43 makeBag :: (Eq a) => [a] -> Bag a
     44 makeBag [] = []
     45 makeBag (a:as) = mix a $ makeBag as
     46   where
     47     mix a []                        = [(a,1)]
     48     mix a (bn@(b,n):bs) | a == b    = (b,n+1):bs
     49                         | otherwise = bn : mix a bs
     50 
     51 permBag :: Bag a -> [[a]]
     52 permBag [] = [[]]
     53 permBag bs = concatMap (\(f,cs) -> map (f:) $ permBag cs) . oneOfEach $ bs
     54   where
     55     oneOfEach [] = []
     56     oneOfEach (an@(a,n):bs) =
     57         let bs' = if n == 1 then bs else (a,n-1):bs
     58         in (a,bs') : mapSnd (an:) (oneOfEach bs)
     59     
     60     apSnd f (a,b) = (a, f b)
     61     mapSnd = map . apSnd
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