Tech Life of Recht

Tech Life of Recht » archive for January, 2007

Ant task for GWT

January 31st, 2007
5:40 pm

I’ve made a new version of the Ant task for GWT, version 1.1.2. The only change is that when a compile error occurs, a BuildException is thrown and the Ant task fails. Before, an error was just printed, but the build continued. Thanks to Britt Piehler for reporting the bug.

More on Haskell

January 23rd, 2007
5:39 pm

Ok, so I’ve managed to write an actual application in Haskell. It’s actually a rewrite of a C program (which I didn’t write myself). Even though it’s a pretty small program (it’s a wrapper for qmail-local), it’s definitely been interesting. I might even go as far as to say thay I understand monads – although I’m not going to attempt to explain them here.
It’s a little uncertain if the program got any better by rewriting it. I pretty much suck at C (way too much time spent in Java), so at least I avoid the segfaults. The Haskell program came in at 89 lines, the C program at 126, and the Haskell program actually has a bunch of extra features.
Anyways, I’m beginning to like Haskell and the way it works. If only the reference api had more examples…

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:

PLAIN TEXT

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

GWT Ant example

January 10th, 2007
11:49 pm

Due to popular demand, I've packed a small example distribution together for GWT Ant and XDoclet. Download it, unpack it, and run ant. It's also an Eclipse project, so you can import it directly in Eclipse.