- Beginners - Haskell.org

FoldL/R Reducing List
by Waaaggh 22 Jun '09

22 Jun '09

Hi.how can i reduce list with foldl.r ? np (1,1,1,2,3,3,4,5,5) -> (1,2,3,4,5) ??? Azz.

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ghci reports "The last statement in a 'do' construct must be an expression" error
by Nico Rolle 21 Jun '09

21 Jun '09

Hi Heres my code snippet. It reports that my error is in line 9 right after the main definition. All functions that i call work under normal circumstances. Thanks module Benchmark where import ReadCSV import Operators import Data.Time.Clock (diffUTCTime, getCurrentTime) main = do xs <- readCSV "dataconvert/lineitem.tbl" '|' start <- getCurrentTime let pnp = projection [5] xs let snp = selection (\x -> (x!!0) > (Int 17000)) pnp end <- getCurrentTime putStrLn $ show (end `diffUTCTime` start) start2 <- getCurrentTime let pp = pProjection [5] xs let sp = pSelection (\x -> (x!!0) > (Int 17000)) pp end2 <- getCurrentTime putStrLn $ show (end2 `diffUTCTime` start2) return xs

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Modifications inside a Reader?
by Brian Troutwine 18 Jun '09

18 Jun '09

Hello all. I'm writing a UDP echo server, full source given below. The current implementation echoes back the "payload" of every incoming message but I would prefer that only unique payloads be echoed back. To that end I've started in with Data.BloomFilter but am not sure how to update it accordingly. I imagine that Reader is probably the wrong monad to be using though I'm unsure how I might modify my program to use State. Could someone lead me along a bit? Also, any general comments on the style of my program? Thanks, Brian -- import Prelude hiding (putStrLn, catch) import Network.Socket hiding (send, sendTo, recv, recvFrom) import Network.Socket.ByteString import Data.ByteString.Char8 hiding (head) import Control.Monad.Reader import Control.Monad (forever) import Control.Exception (bracket) import Data.BloomFilter import Data.BloomFilter.Easy import Data.BloomFilter.Hash (cheapHashes) data Globals = Globals { socketG :: Socket , bloomF :: Bloom ByteString } type Echo = ReaderT Globals IO run :: Echo () run = forever $ do s <- asks socketG (msg, addr) <- liftIO $ recvFrom s 1024 let [_, _, _, payload] = split ':' msg liftIO $ sendTo s payload addr main :: IO () main = bracket build disconnect loop where disconnect = sClose . socketG loop st = runReaderT run st build :: IO Globals build = do addrinfos <- getAddrInfo (Just (defaultHints {addrFlags = [AI_PASSIVE]})) Nothing (Just "1514") let serveraddr = head addrinfos sock <- socket (addrFamily serveraddr) Datagram defaultProtocol bindSocket sock (addrAddress serveraddr) return $ Globals sock $ emptyB (cheapHashes 3) 1024

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map increases length of list
by Aaron MacDonald 18 Jun '09

18 Jun '09

For some reason, the map function returns a list that has one more element than my input list. My input list is a range defined by [0, 60..359] (should translate into [0,60,120,180,240,300]). The function I'm giving to map is defined this way: ----- degreesToRadians :: Double -> Double degreesToRadians degrees = degrees * (pi / 180) ----- This is how I'm calling map overall: ----- > map degreesToRadians [0,60..359] [0.0,1.0471975511965976,2.0943951023931953,3.141592653589793,4.1887902047863905,5.235987755982989,6.283185307179586 ] ----- As you can hopefully see, there are seven elements instead of six. Getting the length confirms this: ----- > length [0,60..359] 6 > length $ map degreesToRadians [0,60..359] 7 ----- I do not seem to get this behaviour with the length if I either substitute the degreesToRadians function or substitute the [0,60..359] range. P.S. Is there a built-in function to convert degrees to radians and vice-versa?

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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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