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<article> <h2>Haskell mode</h2> <form><textarea id=“code” name=“code”> module UniquePerms (

uniquePerms
)

where

– | Find all unique permutations of a list where there might be duplicates.

uniquePerms

(Eq a) => [a] -> [[a]]

uniquePerms = permBag . makeBag

– | An unordered collection where duplicate values are allowed, – but represented with a single value and a count. type Bag a = [(a, Int)]

makeBag

(Eq a) => [a] -> Bag a

makeBag [] = [] makeBag (a:as) = mix a $ makeBag as

where
  mix a []                        = [(a,1)]
  mix a (bn@(b,n):bs) | a == b    = (b,n+1):bs
                      | otherwise = bn : mix a bs
permBag

Bag a -> [[a]]

permBag [] = [[]] permBag bs = concatMap ((f,cs) -> map (f:) $ permBag cs) . oneOfEach $ bs

where
  oneOfEach [] = []
  oneOfEach (an@(a,n):bs) =
      let bs' = if n == 1 then bs else (a,n-1):bs
      in (a,bs') : mapSnd (an:) (oneOfEach bs)

  apSnd f (a,b) = (a, f b)
  mapSnd = map . apSnd

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