October 2010 - Beginners

Word8, Word32, ByteString, Int, etc. conversions.
by David McBride 12 Oct '10

12 Oct '10

I'm writing a pcap sniffer, and I have IP addresses from two different libraries that are equivalent, but typed differently. Data.IP.IPv4 Data.ByteString.Internal.ByteString and Network.Info.IPv4 = Network.Info.IPv4 !GHC.Word.Word32 Both are probably identical, but I cannot for the life of me figure out how to get from one to the other. Bytestring has the ability to take arrays of Word8, how do I split Word32 into Word8's? I have also had this problem in the past with Word8's, Octets and Chars. Beyond this specific problem, what is a good strategy for figuring out how to do these conversions in the future? Sometimes I find a function somewhere, but I have never found a general way to deal with it.

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Reading Multiple Files and Iterate Function Application
by Lorenzo Isella 12 Oct '10

12 Oct '10

Dear All, Another I/O question. Let us say that you are given a list of files file1.dat, file2.dat...file10.dat and so on (i.e. every file is indexed by a number and every file is a single column where every entry is a string without spaces). In the snippet below I read file1.dat, convert it to a list and then print out its length. Now, how can I iterate the process on file1.dat, file2.dat and file3.dat and store the lengths in a list? I would like to map the file reading and following operations on the list [1,2.3], but that is giving me a headache. It is relatively easy to create the file name filename="file"++(show i)++".dat" , for i=1,2,3 but it the the iteration part that is giving me troubles. Any suggestion is appreciated. Cheers Lorenzo -- ##################################################################### import Data.Ord import Data.List main :: IO () main = do list_t <- readFile "file1.dat" let list = lines list_t let b = length list putStrLn "length of list is, " print b

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how to read file with locking
by Joey Hess 12 Oct '10

12 Oct '10

The function below always returns "", rather than the file's contents. _Real World Haskell_ touches on how laziness can make this problimatic, without really giving a solution, other than throwing in a putStr to force evaluation, which can't be done here. How can I make hGetContents strict, to ensure the file contents are really read before it gets closed? readFile' file = do f <- openFile file ReadMode -- locking will go here s <- hGetContents f hClose f return s Also, I noticed that opening a file and locking it involves a very verbose seeming dance. (It's 2 lines of code in most other languages.) Does this indicate that few people bother with file locking in Haskell and so it still has these rough edges, or that there's a better way to do it that I have not found yet? openLocked file = do handle <- openFile file ReadMode lockfd <- handleToFd handle -- closes handle waitToSetLock lockfd (ReadLock, AbsoluteSeek, 0, 0) handle' <- fdToHandle lockfd return handle' -- see shy jo

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Memory tuning
by Thorsten Hater 11 Oct '10

11 Oct '10

Hello everyone, I'm trying to reduce the memory footprint (and possibly execution time, too) of a small program I wrote. It's from the project Euler context, produces the correct result, but it is unsatifiing as indicated by a profiling run: 2,633,675,888 bytes allocated in the heap 1,221,377,976 bytes copied during GC 27,240,696 bytes maximum residency (22 sample(s)) 1,576,200 bytes maximum slop 80 MB total memory in use (1 MB lost due to fragmentation) Generation 0: 5002 collections, 0 parallel, 1.38s, 1.36s elapsed Generation 1: 22 collections, 0 parallel, 0.62s, 0.64s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 1.76s ( 1.80s elapsed) GC time 1.99s ( 2.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 3.76s ( 3.81s elapsed) %GC time 53.1% (52.6% elapsed) Alloc rate 1,494,074,809 bytes per MUT second Productivity 46.9% of total user, 46.3% of total elapsed Not only is the actual memory consumption quite high, but the GC/computation ratio is almost 1:1. I assume that there is something to be done regarding strictness, but a series of experiments with BangPatterns return no yields at all (using foldr instead of foldl' produces stack overflow) Here is my program: -- project euler p75.hs import qualified Data.Map as Map import Data.List -- strict foldl circum :: Int -> Int -> Int circum a b = 2*a*(a+b) count :: Int -> (Map.Map Int Int) -> Int -> (Map.Map Int Int) count lmt mp c = foldl' step mp $ takeWhile (<=lmt) [k*c | k <- [1..] ] where step m x = Map.insertWith' (+) x 1 m solve :: Int -> Int solve l = Map.size $ Map.filter (== 1) primitive where lmt = floor $ sqrt $ (fromIntegral l)/2 cand = [c | m <- [2..lmt], n <- [1..m], odd (m+n), 1 == gcd m n, let c = circum m n, l >= c] primitive = foldl' (count l) (Map.empty) cand main = do print $ solve 1500000 Any advice and or tips welcome, as I'm quite new to Haskell. Best regards.

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Longest palindrome
by Lyndon Maydwell 11 Oct '10

11 Oct '10

Hi Café. I've been looking at implementing the following algorithm in haskell. Take the maximum length palindrome from the list of palindromes generated from the original list by: Taking each element as a starting point, then expanding either side while the string is still a palindrome. I could do this numerically after generating an array from a list, but I was wondering if there was a more idiomatic way to go about it.

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Problem in Gtk2Hs/Glade
by Tom Hobbs 11 Oct '10

11 Oct '10

Hi all, I playing around with simple cell automaton (Conway's Game Of Life). I've got working code which prints everything out nicely to the terminal, I'm trying to wrap everything in a nice GUI using Glade and Gtk2Hs. In order to create my world, I have a function defined as; makeWorld :: Int -> [(Int,Int)] Which accepts an Int, representing the size of the world, then then outs the world itself - note the world is always assumed to be square. The list contains pairs such that the first element of the pair is the index (such as for an array) and the second is the value at that position. For right or wrong, that's how my (working) Game Of Life has been implemented. The problem comes in the Gtk2Hs code... main = do initGUI Just xml <- xmlNew "gameOfLife.glade" window <- xmlGetWidget xml castToWindow "mainWindow" onDestroy window mainQuit quitButton <- xmlGetWidget xml castToButton "quitButton" onClicked quitButton $ do widgetDestroy window setWorldSize <- xmlGetWidget xml castToButton "setWorldSize" sizeBox <- xmlGetWidget xml castToEntry "worldSize" onClicked setWorldSize $ do canvas <- xmlGetWidget xml castToDrawingArea "world" let size = (read (get sizeBox entryText) :: Int) cellworld <- makeWorld size printWorld cellworld widgetShowAll window mainGUI The above function does not compile under GHC because; gameOfLife.hs:27:20: Couldn't match expected type `String' against inferred type `IO String' In the first argument of `read', namely `(get sizeBox entryText)' In the expression: (read (get sizeBox entryText) :: Int) In the definition of `size': size = (read (get sizeBox entryText) :: Int) gameOfLife.hs:28:2: Couldn't match expected type `[(Int, Int)]' against inferred type `IO (Int, Int)' In a stmt of a 'do' expression: cellworld <- makeWorld size In the second argument of `($)', namely `do { canvas <- xmlGetWidget xml castToDrawingArea "world"; let size = (read (get sizeBox entryText) :: Int); cellworld <- makeWorld size; printWorld cellworld }' In a stmt of a 'do' expression: onClicked setWorldSize $ do { canvas <- xmlGetWidget xml castToDrawingArea "world"; let size = (read (get sizeBox entryText) :: Int); cellworld <- makeWorld size; printWorld cellworld } It seems to be complaining about the call to my makeWorld function, although I do not understand why that should be. The line number which GHC complains about does correspond to the "cellworld <- makeWorld size" line in my main function. I've not yet worked out how to draw my world in the GUI, the "printWorld" function mentioned above just spills the world out to the termainal for now. Does anyone have any ideas? Many thanks, Tom

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While Condition in Haskell
by Lorenzo Isella 11 Oct '10

11 Oct '10

Dear All, Definitely I am having some troubles in thinking like an haskeller... Please consider the snippet at the end of the email. Its aim is rather simple: given a list and a value i (position of along the list), it defines the "future" as the chunk of the list from i-th element of he list to the end of the list, whereas the "past" is anything in the list that appears before the i-th element. I wrote a function which checks whether lists of length 1,2...(till I reach the end of the list) in the future (always with respect to position i) has already occurred in past. In order to find the length of the longest list in the future which has already been seen in the past, I could select the "true" values in the output of function iter_find list i but this is a waste of CPU: I simply would like a while condition to tell my function to stop checking new sublists as soon as it finds one which has not occurred in the past and I would like a counter (a kind of i++) telling me how many times the process has been iterated. Any suggestion is helpful. Cheers Lorenzo ------------------------------------------------------------------- import Data.Ord import Data.List main :: IO () main = do let list = [4,55,66,77,88,99,12,9,77,88,99,12,-99] let i = 9 let b = iter_find list i putStrLn "b is, " print b is_sublist sublist list = sublist `isInfixOf` list gen_fut_list list i j = take j $ drop (i-1) list gen_past_list list i = take (i-1) list find_in_list list i j = is_sublist (gen_fut_list list i j) (gen_past_list list i) iter_find list i = map (find_in_list list i) [1..n] where n = (length list) - i +1

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reproducing HashTable benchmark
by Tom Doris 10 Oct '10

10 Oct '10

I've been investigating the performance of some word frequency counting code I wrote, and I came across this stackoverflow thread http://stackoverflow.com/questions/3058529/curious-about-the-hashtable-prob… Dons shows simple benchmarks for HashTable and IntMap, but I can't reproduce the "after the fix" performance of the HashTable code, I am very confused as to what version of ghc actually has the fix. In the post Don writes "With GHC 6.10.2, before the fix, inserting 10M ints:" and then "With GHC 6.13, after the fix:", is this a typo, shouldn't it read 6.10.3? Since there is no 6.13 version of ghc yet? If it should be 6.10.3, and I'm running v 6.12.1, why does the HashTable code in Don's post take 22 seconds to run on my machine (compiled ghc --make -O2) and the IntMap version takes only 5.8s? How can one find out what release the fix for ticket #650 was released into? I've tried the release notes but I can't seem to spot it, and google doesn't either. The ticket has "Milestone: 6.12.2" but that is struck-through, though the comments would suggest this made it into r 6.12.2, I can't spot anything in the release notes for that version either. Can someone help clarify which version the fix will be/is in and how one should figure this out in general for other issues? Thanks Tom

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LOLITA
by Luca Ciciriello 07 Oct '10

07 Oct '10

Hi All. Just a question. The LOLITA (Large-scale, Object-based, LinguisticInteractor, Translator and Analyzer) is still maintained? Is an open source project? And if the answer is yes, where can I find the sources? I've spent several time on Google, but I haven't found nothing. thanks for any answer. Luca.

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A few really short beginners questions
by Klaus Gy 04 Oct '10

04 Oct '10

Hi! I have a few questions to improve my knowledge of Haskell. I didn't stumble over these problems while working on specific tasks, I more or less constructed them explicitly to get an adequate understanding of the basic Haskell semantics. 1 Why can't I apply a class directly to all instances of another, existing class? For example why is it possible to write class Test a instance Num a => Test [a] but not with instance Num a => Test a in the last row? 2 Why is the following example not valid? f :: a -> a f '0' = 0 f x = 1 I think the reason lies in the type system but I can't locate it exactly. 3 Why is the following legal [] :: Num a => [a] but not with a self declared class instad of Num (unresolved overloading)? 4 Why does in the declaration f undefined = '0' the expression undefined work apparently in the same way as a wildcard? Thanks, fweth

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