- Beginners - Haskell.org

creating a relational tuple type
by AM 17 Oct '11

17 Oct '11

Hello, As an effort to learn Haskell, I am creating a relational algebra engine. To this end, I have been looking at how haskelldb handles its mapping layer. I found this: data RecNil = RecNil deriving (Eq, Ord) data RecCons f a b = RecCons a b deriving (Eq, Ord) It seems to me that the type of "b" is too loose, no? I would like "b" to be of the type RecCons or RecNil as well. The looseness of b prevents me from writing functions like this: data RecCons a b = RecCons a b | RecNil deriving (Eq, Ord) tupleLength :: RecCons a b -> Int tupleLength RecNil = 0 tupleLength (RecCons x xs) = 1 + tupleLength xs -- xs really should be of type RecCons What I would like is a list of arbitrary values rolled into a type such as (RecCons Int (RecCons String RecNil)). I tried a RecCons with existential quantification, but I ended up with a completely loosely-typed list. I would like Haskell's type engine to catch invalid comparisons between record tuples of disparate types. It seems that HDBRec works around this using a variety of typeclasses, but I don't really understand why. Thanks for any advice. Cheers, M

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Fwd: parser error in pattern
by kolli kolli 17 Oct '11

17 Oct '11

---------- Forwarded message ---------- From: kolli kolli <nammukolli04(a)gmail.com> Date: Mon, Oct 17, 2011 at 3:43 PM Subject: parser error in pattern To: haskell-cafe(a)haskell.org hey can anyone tell me what is parser error in pattern??Plz help me out

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GHC as a library error.
by Paulo Pocinho 17 Oct '11

17 Oct '11

I'm trying GHC as a library, as documented in: http://www.haskell.org/haskellwiki/GHC/As_a_library http://www.haskell.org/ghc/docs/7.2.1/html/users_guide/ghc-as-a-library.html However, this code: import GHC import GHC.Paths ( libdir ) import DynFlags ( defaultDynFlags ) main = defaultErrorHandler defaultDynFlags $ do runGhc (Just libdir) $ do dflags <- getSessionDynFlags setSessionDynFlags dflags target <- guessTarget "test_main.hs" Nothing setTargets [target] load LoadAllTargets Produces: Couldn't match expected type `Severity' with actual type `DynFlags.Settings' Expected type: DynFlags.LogAction Actual type: DynFlags.Settings -> DynFlags In the first argument of `defaultErrorHandler', namely `defaultDynFlags' In the expression: defaultErrorHandler defaultDynFlags How can I fix it?

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haskell-src
by jean verdier 17 Oct '11

17 Oct '11

I'm using haskell-src to build a haskell compiler. I've compiled hscolour and haskell-src to java classes and used the result to colorize and report parsing errors in an eclipse editor. I use 1.0.1.3 and it reports unterminated comments on the token before the {- token so it is mildy informative and possibly an error in the implementation. haskell-src serves me well but i would like some better error reporting as "parse error" is quite non informative. I would like to avoid building my own parser and so would like to know what the options are for parsing haskell sources with an improved error reporting. I've no idea how i should share screenshots so i've uploaded to the first site i've found: http://imageshack.us/photo/my-images/853/scr1g.png/ http://imageshack.us/photo/my-images/706/scr2a.png/

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how to increase the stack size
by kolli kolli 16 Oct '11

16 Oct '11

when I am running the program in my terminal on ubuntu its showing me GHC stack-space overflow: current limit is 536870912 bytes. Use the `-K<size>' option to increase it. how can i increase the stack size????Plz help me out....

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wxHaskell: Make textCtrl logger scroll one line at a time
by Paulo Pocinho 16 Oct '11

16 Oct '11

Hello. I want to know how to make the textCtrl widget scroll only one line at a time instead the default one page at a time. The usage is the same as the one given by the examples. Text is appended to a textCtrl that serves as a logging window. However, once the appended line reaches the bottom of the scrolled text, the textCtrl automatically "jumps" one page. There is an attribute in wxWidgets called ScrollLine () that makes it only scroll the number of lines we want [1] but I can't find it in wxHaskell. How can I make text scroll one line instead of jumping a screen at a time?

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Reading pcap file
by mukesh tiwari 16 Oct '11

16 Oct '11

Hello everyone I am writing application which reads pcap file like wireshark in pure haskell but there is some thing missing. I read this file http://www.viste.com/Linux/Server/WireShark/libpcapformat.pdf<http://www.google.com/url?sa=D&q=http://www.viste.com/Linux/Server/WireShar…> and it say that first 24 bytes are global headers , after that every packet contains pcap local header and data. What i am trying to do is , first trying to get the bytes of data in each packet by reading the third field incl_len in local header but my code is not behaving as it suppose . I am not getting the list of parsed packets . My test libcap file is http://wiki.wireshark.org/SampleCaptures?action=AttachFile&do=view&ta...<http://www.google.com/url?sa=D&q=http://wiki.wireshark.org/SampleCaptures%3…> --http://www.viste.com/Linux/Server/WireShark/libpcapformat.pdf<http://www.google.com/url?sa=D&q=http://www.viste.com/Linux/Server/WireShar…> --http://hackage.haskell.org/packages/archive/bytestring/0.9.0.4/doc/<http://www.google.com/url?sa=D&q=http://hackage.haskell.org/packages/archiv…> html/Data-ByteString-Lazy.html import Data.List import qualified Data.ByteString.Lazy as BS import qualified Data.ByteString.Lazy.Char8 as B import Control.Monad import Text.Printf import Data.Word import Data.Char import System.Time import Numeric import System.Environment hexTodec :: BS.ByteString -> Integer hexTodec lst = read $ "0x" ++ ( concatMap ( \x -> showHex x "" ) $ BS.unpack lst ) parseFile :: BS.ByteString -> Bool -> IO [ BS.ByteString ] parseFile xs revflag | BS.null xs = return [] | otherwise = do let ind =if revflag then hexTodec . BS.reverse . BS.take 4 . BS.drop 8 $ xs else hexTodec . BS.take 4 . BS.drop 8 $ xs print ind let ( x , ys ) = BS.splitAt ( fromIntegral ind ) xs --BS.putStrLn $ x tmp <- parseFile ys revflag return $ x : tmp main = do [ file ] <- getArgs contents <- BS.readFile file let ( a , rest ) = BS.splitAt 24 contents --strip global header let revflag = case BS.unpack $ BS.take 4 a of [ 0xd4 , 0xc3 , 0xb2 , 0xa1 ] -> True _ -> False p <- parseFile rest revflag print $ p !! 0 BS.putStr $ p !! 0 Regards Mukesh Tiwari

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Writing an instance of Read?
by Mike Meyer 15 Oct '11

15 Oct '11

Is there a tutorial - or anything at all, for that matter - on writing Read instances? Particularly so that it integrates properly types that incoporate your type and derive Read? Real World Haskell gives one simple example (which doesn't use the first argument), points out one problem with the example, then says "many people find it easier to use Parsec." And that's the best such I could find. Thanks, <mike -- Mike Meyer <mwm(a)mired.org> http://www.mired.org/ Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

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help me with eroor
by kolli kolli 15 Oct '11

15 Oct '11

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 *showing error: <interactive>:1:32: Not in scope: `main' *

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Re: [Haskell-beginners] help me with eroor
by kolli kolli 14 Oct '11

14 Oct '11

namratha@ubuntu:~/Desktop$ runghc Main.hs <interactive>:1:32: Not in scope: `main' On Fri, Oct 14, 2011 at 2:45 PM, kolli kolli <nammukolli04(a)gmail.com> wrote: > I am using ubuntu..i am running it in the terminal window.... > > > On Fri, Oct 14, 2011 at 2:36 PM, Daniel Fischer < > daniel.is.fischer(a)googlemail.com> wrote: > >> On Friday 14 October 2011, 22:26:45, you wrote: >> > i gave >> > module Main(csvFile, line) 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 >> > >> > its giving me the same error >> >> Did you try to run main from the ghci prompt? >> The file loads without problems here. >> >> >

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