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Tech Life of Recht » Fun with Haskell

 Fun with Haskell

  • January 19th, 2007
  • 2:02 pm

At 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:

[code]
divide :: Integer -> [Integer] -> [(Integer, [Integer])]
divide persons vals
| toInteger (length vals) < persons = error "Not enough values" | not $ mod (sum vals) persons == 0 = error "Values cannot be divided equally" | otherwise = divide'' persons vals (div (sum vals) persons) [] 1 divide'' persons vals maxval res cnt | cnt > persons = res
| otherwise =
let p = permut vals maxval
nx = findE p maxval
in divide” persons (deleteFirstsBy (==) vals nx) maxval ((cnt, nx):res) (cnt + 1)

findE [] c = error “Unable to find fitting element”
findE xs c
| sum (head xs) == c = head xs
| otherwise = findE (tail xs) c

uniq [] sorted = reverse sorted
uniq xs [] = uniq (tail xs) [head xs]
uniq xs sorted
| head xs == head sorted = uniq (tail xs) sorted
| otherwise = uniq (tail xs) (head xs:sorted)

permut :: [Integer] -> Integer -> [[Integer]]
permut nums maxval = permut’ nums [] 0 maxval

permut’ :: [Integer] -> [[Integer]] -> Int -> Integer -> [[Integer]]
permut’ nums [] _ maxval = permut’ nums (map (\x -> [x]) nums) 0 maxval
permut’ nums xs cnt maxval
| cnt + 2 > length nums = uniq (sort xs) []
| otherwise = permut’
nums
(xs ++ uniq
(sort $
foldr (++) []
[map (\ ys -> appendIfNecessary x ys nums maxval) xs | x <- nums] ) []) (cnt + 1) maxval appendIfNecessary x xs old maxval | elem x (deleteFirstsBy (==) old xs) = if (sum xs) + x > maxval
then xs
else x:xs
| otherwise = xs
[/code]

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