Created
February 17, 2010 16:35
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unionAll :: (Ord a) => [[a]] -> [a] | |
unionAll xs = | |
let | |
xs' = filter (not.null) xs -- | filter empty lists, if any | |
in | |
if null xs' -- | if nothing is left after filtering | |
then [] -- | return the empty list | |
else -- | otherwise | |
let | |
next = minimum (map head xs') -- | find the minimum of head elements | |
-- | they are minimum values in their respective lists | |
-- | so this is the minimum of every list | |
xs'' = map (dropWhile (next==)) xs' -- | remove that value from every list | |
-- | (no need to filter this time, since lists are ordered) | |
in next : unionAll xs'' -- | recursively call unionAll on xs'' |
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