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<html>
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<head>
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<meta charset="utf-8">
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<title>CodeMirror: Haskell mode</title>
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<h1>CodeMirror: Haskell mode</h1>
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<form><textarea id="code" name="code">
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module UniquePerms (
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uniquePerms
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)
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where
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-- | Find all unique permutations of a list where there might be duplicates.
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uniquePerms :: (Eq a) => [a] -> [[a]]
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uniquePerms = permBag . makeBag
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-- | An unordered collection where duplicate values are allowed,
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-- but represented with a single value and a count.
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type Bag a = [(a, Int)]
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makeBag :: (Eq a) => [a] -> Bag a
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makeBag [] = []
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makeBag (a:as) = mix a $ makeBag as
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where
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mix a [] = [(a,1)]
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mix a (bn@(b,n):bs) | a == b = (b,n+1):bs
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| otherwise = bn : mix a bs
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permBag :: Bag a -> [[a]]
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permBag [] = [[]]
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permBag bs = concatMap (\(f,cs) -> map (f:) $ permBag cs) . oneOfEach $ bs
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where
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oneOfEach [] = []
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oneOfEach (an@(a,n):bs) =
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let bs' = if n == 1 then bs else (a,n-1):bs
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in (a,bs') : mapSnd (an:) (oneOfEach bs)
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apSnd f (a,b) = (a, f b)
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mapSnd = map . apSnd
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<p><strong>MIME types defined:</strong> <code>text/x-haskell</code>.</p>
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