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- <h1>CodeMirror 2: Haskell mode</h1>
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- 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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