scripts/CodeMirror/mode/haskell/index.html (view raw)
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28<h2>Haskell mode</h2>
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30module UniquePerms (
31 uniquePerms
32 )
33where
34
35-- | Find all unique permutations of a list where there might be duplicates.
36uniquePerms :: (Eq a) => [a] -> [[a]]
37uniquePerms = permBag . makeBag
38
39-- | An unordered collection where duplicate values are allowed,
40-- but represented with a single value and a count.
41type Bag a = [(a, Int)]
42
43makeBag :: (Eq a) => [a] -> Bag a
44makeBag [] = []
45makeBag (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
51permBag :: Bag a -> [[a]]
52permBag [] = [[]]
53permBag 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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