June 2009 - Beginners

xml parsing
by Benedikt Ahrens 18 Jun '09

18 Jun '09

Hello, I want to write a program which among other should read some xml file. The xml tags will mostly be custom ones defined by a dtd. My question is: which parsing library to use? On http://en.wikibooks.org/wiki/Haskell/XML three libraries are mentioned, but a comparison and recommendation of which one to choose is missing. As you might imagine, I would like to use a library which is actively maintained? Has any of these libraries developed into a "standard" library? Thanks in advance for your help. ben

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Cabal not detecting foreign libraries
by aditya siram 17 Jun '09

17 Jun '09

Hi all, I am trying to install vacuum-cairo and leksah. For both these packages cabal complains that "gtk", "gtksourceview", "svgcairo" etc are not found. They are definetely installed on my Ubuntu Hardy Heron box. Where is Cabal looking? thanks ... -deech

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Strange errors in network
by Maciej Piechotka 15 Jun '09

15 Jun '09

I have problems with my nntp library (http://code.haskell.org/nntp/) If I try to list command it seems to fail. Test code: > (c,p) <- connectToHost "news.example.com" Nothing > forGroups c (System.IO.putStrLn . groupName) *** Exception: <socket: 6>: Data.ByteString.hGetLine: end of file Conncection by telnet (according to wireshark): -> SYN <- SYN, ACK -> ACK <- NNTP Helo Message -> ACK Connection by Haskell: -> SYN <- SYN ACK -> ACK <- NNTP Helo Message -> ACK <- FIN ACK (after 10 s) -> ACK If I try to run by hand the commands in ghci I get mixed results (sometimes works sometimes it fails). What's wrong? PS. example.com is an example, non-existing domain. It is specified by a RFC to be an example domain. PPS. As I'm not subscribe the list please CC me

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ghc-6.10.3 IOException?
by Erik de Castro Lopo 14 Jun '09

14 Jun '09

Hi all, I have some code (basically from http://sequence.complete.org/node/257) which has: import Control.Exception (finally, catch, Exception (..), IOException) ... handler (IOException e) | isEOFError e = return () but gch-6.10.3 still says Not in scope: data constructor `IOException' Someone smack me with the cluestick please :-). Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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timer_create: Invalid argument
by Thomas Friedrich 14 Jun '09

14 Jun '09

Hi, I need to check if a couple of files have the same number of lines in them. So, I written a little Haskell program that counts the number of lines in a file. import System.Environment (getArgs) main :: IO () main = do [f] <- getArgs cs <- readFile f print $ length (lines cs) Now, when I invoke the program on my computer, it does what it should do: ~ $ ./count count.hs 7 However, when I run the program on a different computer, I get the following: ~ $ scp count count.hs thomas\@...:~ thomas@...'s password: count 100% 492KB 492.2KB/s 00:00 count.hs 100% 125 0.1KB/s 00:00 ~ $ ssh thomas@... [thomas@... ~] $ ./count count.hs count: timer_create: Invalid argument [thomas@... ~]$ What does this error message mean? And why does it occur? Other programs that I written and compiled on my computer worked just fine on the other. I run Arch Linux and I am using GHC 6.10.3, the other computer is a RedHat 3. Cheers, Thomas

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Testing unpure network code
by Maciej Piechotka 12 Jun '09

12 Jun '09

Hello. I have a problem with testing. I've wrote a client library for NNTP protocol. However I don't know how to test it. I though about spawning a thread for server inside a test but Haskell (ghc and hugs?) uses green blocking threads so forkIO is not an option. I cannot also use POSIX fork as in documentation it is stated that I cannot use IO in argument. I'd not like to create separate process as IPC is not something nice (and there is separation between two parts of test). Testing on real server have also some disadvantages (on real one it is not predictable and setting one just for tests...) What is the correct solution? PS. Please CC me as I'm not subscribe to the list. I will eventually use Gmane but well. I'm in the process of writing NNTP client as no other suited me ;)

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exercise 3.10 in YAHT
by George Huber 09 Jun '09

09 Jun '09

I'm currently working through "Yet Another Haskell Tutorial" by H. Daume and have a question on how to proceed with his exercise 3.10. The exercise question is: Write a program that will repeatedly ask the user for numbers until he types in zero, at which point it will tell him the sum of all the numbers, the product of all the numbers, and, for each number, its factorial. The hint that is given is to write an IO action that reads a number and either returns an empty list (if the number is zero) or recurses itself making a list from the number and the result of the recursive call. Daume presents and example of such a function: askForWords = do putStrLn "Please enter a word:" word <- getLine if word == "" then return [] else do rest <- askForWords return (word : rest) so, based on his example I created the function: getNums = do putStrLn "Enter a number (zero to stop):"; strNum <- getLine let num = read strNum if num == 0 then return [] else do rest <- getNums return (num:rest) loading the file that contains the above function into GHCi (version 6.8.3) gives the following error "The last statement in a 'do' construct must be an expression" being a beginner, I'm at a loss at the cause of the error and how to fix it -- any suggestions? thanks, george

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type classes and multiple implementations
by Michael Hendricks 06 Jun '09

06 Jun '09

I'm writing a tool for which there will be multiple storage formats. The user can choose his favorite storage mechanism through a configuration file. Possible storage mechanisms might include in-memory storage or on-disk storage. To represent this abstraction, I created a Storage type class and two simple instances: class Storage a where open :: String -> IO a data Disk = Disk instance Storage Disk where open file = do putStrLn $ "opening disk: " ++ file return Disk data Memory = Memory instance Storage Memory where open _ = do putStrLn "opening memory" return Memory I can load this code into ghci (version 6.10.1, if that matters) and it behaves as I expected it to: *Main> open "foo" :: IO Disk opening disk: foo *Main> open "foo" :: IO Memory opening memory Eventually a configuration facility will provide a string indicating which storage mechanism the user prefers. This will return a string like "memory" or "disk". Based on that string, I want to return a value whose type belongs to the Storage type class. I thought this would do the job: by_type :: Storage a => String -> String -> IO a by_type "disk" x = open x :: IO Disk by_type "memory" x = open x :: IO Memory by_type _ t = error $ "no such storage type " ++ t but I get the following compile errors: Couldn't match expected type `Disk' against inferred type `Memory' Expected type: IO a Inferred type: IO Memory In the expression: open x :: IO Memory In the definition of `by_type': by_type "memory" x = open x :: IO Memory It seems as though the type variable `a' is binding to Disk which prevents it from later binding to Memory. That makes sense but I don't see a way forward from here. How can I accomplish multiple implementations of the same interface? Are type classes the right way? Thank you for any help. -- Michael

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Clearing Parsec error messages
by Giuliano Vilela 06 Jun '09

06 Jun '09

Hi all, In a Parsec project I used the *fail* parser, wanting to show a message to the user and halt the parsing process. That's okay, but the error message showed included some other "unexpected" and "expecting" messages that did not seem related to the fail. My guess is that Parsec keeps these messages in an internal state, to use them whenever needed. My question is, how can I clear those error messages and only show the string I pass to fail? -- []'s Giuliano Vilela.

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HXT runtime error
by aditya siram 03 Jun '09

03 Jun '09

Hi all, I am trying to use HXT to parse an XML file. However whenever I try and run 'xread', GHCI crashes. The code and output are below. The following code queries isbndb.com ( a book ISBN database ) which responds with XML describing the book. I am using 'xread' from HXT to parse the response into an XMLTree. respHTML'' isbn = do rsp <- simpleHTTP (getRequest $ " http://isbndb.com/api/books.xml?results=details&access_key=" ++ key ++ "&index1=isbn&value1=" ++ isbn :: Request_String) case rsp of Left x -> putStrLn $ "Error " ++ (show x) Right r -> putStrLn $ show $ xread $ rspBody r When I try this respHTML'' in GHCI usin the isbn for "Real World Haskell" , this is what I get: *Main> respHTML'' rwhis Loading package syb ... linking ... done. Loading package base-3.0.3.1 ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.2 ... linking ... done. Loading package unix-2.3.2.0 ... linking ... done. Loading package filepath-1.1.0.2 ... linking ... done. Loading package directory-1.0.0.3 ... linking ... done. Loading package process-1.0.1.1 ... linking ... done. Loading package pretty-1.0.1.0 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package haskell98 ... linking ... done. Loading package parsec-2.1.0.1 ... linking ... done. Loading package network-2.2.1 ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package parallel-1.1.0.1 ... linking ... done. Loading package HTTP-4000.0.6 ... linking ... done. Loading package HaXml-1.13.3 ... linking ... done. Loading package MaybeT-0.1.2 ... linking ... done. Loading package HUnit-1.2.0.3 ... linking ... done. Loading package curl-1.3.5 ... linking ... done. Loading package network-2.2.1.2 ... GHCi runtime linker: fatal error: I found a duplicate definition for symbol my_inet_ntoa whilst processing object file /home/deech/.cabal/lib/network-2.2.1.2/ghc-6.10.3/HSnetwork-2.2.1.2.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. GHCi cannot safely continue in this situation. Exiting now. Sorry. It seems to want to load the 'network' package twice. Is this causing the trouble? thanks , deech

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