- Beginners - Haskell.org

A problem with a simple csv file parser
by Andy Elvey 16 Oct '10

16 Oct '10

Hi all - I'm just trying to get the simple csv file parser from "Real World Haskell" working. However, I've got a problem. Here's the code - -- A simple csv parser Module parseCSV where import Text.ParserCombinators.Parsec csvFile = endBy line eol line = sepBy cell (char ',') cell = many (noneOf ",\n") eol = char '\n' parseCSV :: String -> Either ParseError [[String]] parseCSV input = parse csvFile "(unknown)" input ( Btw, I'm running Ubuntu 10.04, GHC 6.12.1 ) When I try to run this (using "runhaskell parseCSV.hs test.csv" ) I get the error - parseCSV.hs:4:16: parse error on input `where' This has me really puzzled - as far as I can see, the first line looks identical to a number of similar programs that I've found (while searching for a solution to this). So, if anyone can let me know what is causing this, that'd be great! Many thanks in advance - - Andy

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Newbie performance question
by Jordan Ellis 16 Oct '10

16 Oct '10

I'm trying (really trying!) to get my head around Haskell, but one (among many) significant stumbling blocks for me so far has been trying to figure out how my code is actually going to perform. Here's a contrived example of the sort of thing that confuses me. Let's say I have a function which produces a list of odd integers from 1 to n: f n = filter (odd) [1..n] ...and I have another function that makes a list of all sums of pairs of odd numbers from 1 to n: g n = [a + b | a <- (f n), b <- (f n)] I assume that in most languages, (f n) would get called twice (once for a, and once for b), but what happens in Haskell? Does GHC recognize that, since f is a pure function, it only has to be evaluated once for a given n? If not, would a 'where clause' make any difference? e.g., g n = [a + b | a <- h, b <- h] where h = f n Thanks.

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Flying Dutchman sailing blind
by Jeroen van Maanen 15 Oct '10

15 Oct '10

A few weeks ago I was greatly helped by members of this list to expose an error that was hiding in my, by now quite extensive, Haskell program. I should still thank Daniel Fisher for his lucid explanation of how an overdose of strictness can prevent a try-expression from wrapping an exception. Now that the error is gone, I face a new challenge: when I let my program run for a while (it is a computationally intensive task that never stops by design), after fifteen minutes or so the CPU usage drops from nearly 100% to around 15% and a few minutes later the process dies with the message: "lexau: memory allocation failed (requested 2097152 bytes)" The whole thing is a tail biting snail pit of threads that are communicating through MVars. Every thread runs a tail recursive function. (I read in another post that it is not a good idea to use explicit recursion, but when I compared alternatives the fold variants produced even worse stack/heap problems.) The thread that is likely to cause the problem is an optimizer that tries many possible improvements of a complex data structure and incrementally applies the successful ones. I use a strict foldl style in an attempt to limit the memory used by the optimizer. Frankly, however, I have no idea what eats up so much heap space. Now I have always proudly labeled myself a 'printf programmer', but I am afraid that I am going to need some profiling tool to determine where the problem is. Any suggestions where I should start? Cheers, Jeroen P.S. For an idea of what is living in the snail pit, have a look at: http://lexau.svn.sourceforge.net/viewvc/lexau/branches/totem/src/LExAu/Pipe… http://lexau.svn.sourceforge.net/viewvc/lexau/branches/totem/src/LExAu/Mode… (I know, HistoryTree.hs badly needs to be split up into smaller modules.)

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comma delimited string
by Bryce Verdier 14 Oct '10

14 Oct '10

Hi All, I have a file with a bunch of lines like such. number,number,string,string,string,string How can I split up the lines in Haskell so that each item in the line becomes an element in a list? Or to put it differently, how do I split up a comma delimited string? Thanks in advance, Bryce

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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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