November 2009 - Beginners

Double Trouble
by Philip Scott 10 Nov '09

10 Nov '09

Hi Folks, I'd just like to say thank you very much for your patient answers to my questions so far; it has been a real help on my Haskell adventure. Once I am become a Haskell god like y'all I will endeavour to repay the debt. My current puzzle is doubles. I've extensively scoured the net and found various mentions of the same problem, but as far as I can see no answers; which I can't quite believe as it must be something people do all the time. I need to interface my Haskell program with an existing C++ one over a TCP socket. I need to feed the C++ program doubles in standard 64-bit network order IEEE 765-1985 format, but if I serialize a double using Data.Binary I get something which has altogeather too many bytes (I read somewhere it is an int and a long for the exponent and mantissa). Any advice (or pointers to old threads, it isn't very easy to search the list archives) will be compensated for by a credit note for milk and cookies next time you are in the Cambridge, UK area :) - Philip

2 2

turning a value into an expression
by John Moore 10 Nov '09

10 Nov '09

Hi, How do I turn a value into an expression I want to do for e.g. 8 - 1 turn it into (subtract (Val8) (Val1) Any ideas J

3 2

Simple haskell problem ! Help please
by Denis Firsov 09 Nov '09

09 Nov '09

Hi ! I am beginner in Haskell and have problems with this problem: compress :: Eq a => [a] -> [(a, Int)] If you have string "AAABCCC" it transforms it to : {A, 3} {B,1} {C,3} Could you help me with it ? Thank you in advance !

3 2

Installing packages in Ubuntu
by Glurk 08 Nov '09

08 Nov '09

Hi, I'm a beginner to both Cabal and Linux. I'm a bit confused as to whether I need to be using Cabal when installng packages. I'm running Ubuntu 9.10, and from the Synaptic Package Manager, I can select various Haskell packages to install - will this install the package correctly, or is there something I need to do in Cabal as well ? Thanks ! :)

4 3

Re: Installing packages in Ubuntu
by Nathan M. Holden 08 Nov '09

08 Nov '09

I can't say that you NEED to be using Cabal. I have it, and I'm also an Ubuntu newbie. You can download the bootstrap installer, you'll need to install (via runghc Setup configure, runghc Setup build, runghc Install) the most recent HTML package, and parsec 2.x to get it to run. Then just add an alias (or, I did) so you don't have to write a super-long command to run Cabal. Running Kubuntu 9.10 On Sunday 08 November 2009 08:28:24 am beginners-request(a)haskell.org wrote: > Message: 6 > Date: Sat, 7 Nov 2009 22:05:34 +0000 (UTC) > From: Glurk <streborg(a)hotmail.com> > Subject: [Haskell-beginners] Installing packages in Ubuntu > To: beginners(a)haskell.org > Message-ID: <loom.20091107T230222-873(a)post.gmane.org> > Content-Type: text/plain; charset=us-ascii > > Hi, > > I'm a beginner to both Cabal and Linux. > I'm a bit confused as to whether I need to be using Cabal when installng > packages. > > I'm running Ubuntu 9.10, and from the Synaptic Package Manager, I can > select various Haskell packages to install - will this install the package > correctly, or is there something I need to do in Cabal as well ? > > Thanks ! :)

1 0

parse String -> Expression
by John Moore 08 Nov '09

08 Nov '09

Hi, I'm trying to find a way to parseRPN (Reverse Polish Numbers) to expressions rather than to just numbers. e.g. I want the answer to be in the form (Multiply (Val 2) (Val 3)) rather than just the answer. Are these anyway near the steps parseRPN :: String->Expression This is a lot more complicated then I thought.!!! First do we have to read in a string is this (IsString) fromString :: String -> a Then this goes on a stack pushStack :: a -> Stack -> Stack (Takes a value and puts in on a stack) Later we pop it off popStack :: Stack -> (a,Stack) -- takes the value of the stack and leaves the stack Do we also have to define taking off the stack such as head(popstack) or fst(popstack) if we do we would probably have one for putting it onto a stack. Do we then turn the value into an Expression.?

3 3

MULTICONF-10 Call for papers
by John Edward 08 Nov '09

08 Nov '09

MULTICONF-10 Call for papers The 2010 multi-conference (MULTICONF-10) (website: http://www.promoteresearch.org) will be held during July 12-14, 2010 in Orlando, Florida, USA. The primary goal of MULTICONF is to promote research and developmental activities in computer science, information technology, control engineering, and related fields. Another goal is to promote the dissemination of research to a multidisciplinary audience and to facilitate communication among researchers, developers, practitioners in different fields. The following conferences are planned to be organized as part of MULTICONF-10. International Conference on Artificial Intelligence and Pattern Recognition (AIPR-10) International Conference on Automation, Robotics and Control Systems (ARCS-10) International Conference on Bioinformatics, Computational Biology, Genomics and Chemoinformatics (BCBGC-10) International Conference on Computer Networks (CN-10) International Conference on Enterprise Information Systems and Web Technologies (EISWT-10) International Conference on High Performance Computing Systems (HPCS-10) International Conference on Information Security and Privacy (ISP-10) International Conference on Image and Video Processing and Computer Vision (IVPCV-10) International Conference on Software Engineering Theory and Practice (SETP-10) International Conference on Theoretical and Mathematical Foundations of Computer Science (TMFCS-10) We invite draft paper submissions. Please see the website http://www.promoteresearch.org for more details. Sincerely John Edward Publicity committee

1 0

Applicative Parsec
by Matthias Guedemann 07 Nov '09

07 Nov '09

Hi, what is the benefit if you use Applicative for Parsec instead of Monad? I wrote a converter from a textfile format to a csv file. In order to get more familiar with Haskell I wrote it using Parsec. At first I had some difficulties and the monadic version seemed easier, but now it works as Applicative version and Parsec seems really straightforward. For me, the Applicative version looks more functional and the monadic version looks more imperative. It seems that Applicative is enough for Parsers (think I remember a citation somewhere), but does it have a real advantage? best regards, Matthias -- __________________________________________________________ ___ __ __ Dipl. Inf. Matthias Guedemann / __\/ _\ /__\ Computer Systems in Engineering / / \ \ /_\ Otto-von-Guericke Universitaet Magdeburg / /___ _\ \//__ Tel.: 0391 / 67-19359 \____/ \__/\__/ __________________________________________________________

4 5

Help for type matching
by Hong Yang 06 Nov '09

06 Nov '09

I have the following code: $$$$$$$$ module Main where import System.Environment (getArgs) import Text.CSV.ByteString import qualified Data.ByteString.Lazy.Char8 as L main = do [args] <- getArgs file <- L.readFile args let result = parseCSV file case result of Nothing -> putStrLn "Error when parsing!" Just contents -> do putStrLn "parsing OK!" -- map_header_records contents $$$$$$$$ which yielded the error as follows: Couldn't match expected type `Data.ByteString.Internal.ByteString' against inferred type `L.ByteString' In the first argument of `parseCSV', namely `file' In the expression: parseCSV file In the definition of `result': result = parseCSV file readFile in the Data.ByteString.Lazy.Char8 module returns ByteString type. Since the module is qualified as L, L.readFile returns L.ByteString. But parseCSV expects a ByteString. Sometimes this is annoying, because it makes type matching difficult (at least for me, a beginner). I really wish Haskell can intelligently treat L.ByteString, M.ByteString, and whatever X.ByteString the same as ByteString, and match them. Can someone tell me how to solve the above problem? Thanks, Hong

1 1

if ands
by Nathan M. Holden 06 Nov '09

06 Nov '09

If you have an if statement like if (a&&b) then fun else fun' and a is false, does GHC actually bother to check b?

7 6