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<!doctype html>

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<title>CodeMirror: Haskell mode</title>

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<a href="http://codemirror.net"><h1>CodeMirror</h1><img id=logo src="../../doc/logo.png"></a>

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<ul>

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<li><a href="../../doc/manual.html">Manual</a>

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</ul>

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<ul>

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<li><a href="../index.html">Language modes</a>

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<li><a class=active href="#">Haskell</a>

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</ul>

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</div>

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<h2>Haskell mode</h2>

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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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</textarea></form>

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var editor = CodeMirror.fromTextArea(document.getElementById("code"), {

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lineNumbers: true,

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matchBrackets: true,

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theme: "elegant"

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});

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</script>

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<p><strong>MIME types defined:</strong> <code>text/x-haskell</code>.</p>

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</article>

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