Fun with HaskellAt JAOO 2006, I attended a session with Erik Meijer on Haskell. The session gave a very quick introduction to Haskell as a very nice functional language, and the enthusiasm of Erik Meijer on the subject only made it more interesting.
So, I've been messing around with Haskell for a while, without producing anything worthwhile - I'm still trying to figure out how to put the language to good use. However, I found a list of Haskell quizzes, derived from Ruby Quiz. Some of these have not yet been solved, so I sat down and began working on Splitting the Loot. After some hours, I suddenly discovered that I have a working solution - somewhat to my surprise.
Anyways, here it is - as it's one of my first Haskell programs, I'm sure there's lots of room for improvement. What it does is basically to find all possible combinations of a list of numbers, and then it tries to combine these combinations in order to find a valid solution:
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divide :: Integer -> [Integer] -> [(Integer, [Integer])]
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divide persons vals
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| toInteger (length vals) <persons = error "Not enough values"
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| not $ mod (sum vals) persons == 0 = error "Values cannot be divided equally"
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| otherwise = divide'' persons vals (div (sum vals) persons) [] 1
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divide'' persons vals maxval res cnt
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| cnt> persons = res
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| otherwise =
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let p = permut vals maxval
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nx = findE p maxval
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in divide'' persons (deleteFirstsBy (==) vals nx) maxval ((cnt, nx):res) (cnt + 1)
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findE [] c = error "Unable to find fitting element"
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findE xs c
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| sum (head xs) == c = head xs
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| otherwise = findE (tail xs) c
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uniq [] sorted = reverse sorted
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uniq xs [] = uniq (tail xs) [head xs]
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uniq xs sorted
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| head xs == head sorted = uniq (tail xs) sorted
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| otherwise = uniq (tail xs) (head xs:sorted)
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permut :: [Integer] -> Integer -> [[Integer]]
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permut nums maxval = permut' nums [] 0 maxval
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permut' :: [Integer] -> [[Integer]] -> Int -> Integer -> [[Integer]]
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permut' nums [] _ maxval = permut' nums (map (\x -> [x]) nums) 0 maxval
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permut' nums xs cnt maxval
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| cnt + 2> length nums = uniq (sort xs) []
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| otherwise = permut'
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nums
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(xs ++ uniq
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(sort $
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foldr (++) []
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[map (\ ys -> appendIfNecessary x ys nums maxval) xs | x <- nums]
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)
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[])
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(cnt + 1)
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maxval
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appendIfNecessary x xs old maxval
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| elem x (deleteFirstsBy (==) old xs) = if (sum xs) + x> maxval
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then xs
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else x:xs
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| otherwise = xs
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