- Beginners - Haskell.org

Showing a value, when there are many approaches?
by Chris Schneider 07 Aug '13

07 Aug '13

Hi, I'm writing a module that contains a data type for a MacAddress. Given that, I can do things like validate and format. But there are several ways you can display a MacAddress: 'aabbccddeeff' or '00:11:22:33:AA:BB' or 'aa:bb:cc:00:11:22'. Different external systems want the mac addresses in different formats, and I'd like to centralize that logic. ** So the design question: I can't just use a `show` function directly to do this, since there are options I need to pass. I could use separate functions that return a string. I could use newtype to wrap each display style of mac address in a type that has a specific `show` instance that spits out the right format. So a `Pretty` and `Lower` and similar types. --- Is there a best-practice here? Am I totally missing a better way to model this idea?

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Enterprise-quality XML library?
by Mike Meyer 07 Aug '13

07 Aug '13

I've been looking for an enterprise-grade XML library. My standard for comparison is Python's lxml library (a wrapper around libxml2 and libxslt). The only thing I've found that's even close is hxt, and it falls a bit short. Specifically, when I feed it invalid XML, I get this error: error: The content of element "school" must match ( "author"* ). Element "author" expected, but element "auther" found. error: Element "auther" not declared in DTD. Whereas from lxml I get: lxml.etree.XMLSyntaxError: No declaration for attribute name of element auther, line 24, column 19 This is shorter and clearer, and identifies the location of the error. It does have better error messages for ill-formed XML. Is there a library with a validating parser that provides better quality error messages? It also seems like mapping an XML Schema into a collection of Haskell types should be a function of the library. Is there on that does that, or is this completely untenable?

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Looking for judges for haskell competition
by Mike Meyer 07 Aug '13

07 Aug '13

In order to foster commercial use of Haskell, FP Complete is running a competition for monthly cash prizes (details https://www.fpcomplete.com/blog/2013/07/competition-announcement) We are looking for judges from the community. People helping beginners would seem to be a good fit for this. If you're interested, could you please drop me a note along with pertinent information? Thanks, Mike

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Why is my GHC bigger than the distribution
by vold 07 Aug '13

07 Aug '13

I've built GHC 7.6.3 from source using the default "perf" settings, and all my programs are about 4 Mb bigger after linking than if I used the pre-built binary. What could be causing this? (x64 Linux with gold linker.)

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Threading (lack of) interleaving
by Chris Schneider 05 Aug '13

05 Aug '13

Hi, I'm working on a program that should launch, spawn two threads, each with a zeromq connection, where those threads ping the zmq channel repeatedly. The main thread just sits and waits. The code is here: https://gist.github.com/cschneid/3862b33c29a803be7848 If I launch one thread, or the other (lines 13 & 14), and comment the other one out, it runs as expected. But if I attempt to launch both, both build themselves, and run one call of themselves, but then it seems that one or the other thread "wins", and the other never gets to run again. Most of the time it's the first one launched that wins, but not always. See the output for an example. Adding explicit yield calls, using forkOS vs forkIO, and moving around of where I make the ZMQ context & socket (into and out of the threads) don't seem to help. What am I missing? How can I get these threads to play nice with each other? Thanks!

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Shorten this code
by Nadav Chernin 01 Aug '13

01 Aug '13

Hi, all Below is my solution to SPOJ->Polybius square<http://www.spoj.com/problems/POLYBIUS> Please try to shorten it: *import Data.Maybe* *d=[1..5]* *f s=unwords$map(\c->fromJust$lookup c((' ',""):('J',"24"):zip(['A'..'I']++['K'..'Z'])[show(x+10*y)|y<-d,x<-d]))s* *main=getLine>>(interact$unlines.map f.lines)* Thanks, Nadav

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Viewing Template Haskell output?
by Lee Short 30 Jul '13

30 Jul '13

I'm working with some Yesod code, and it's complaining about some missing function definitions. I'd really like to see the code that Template Haskell is outputting -- this is hard to debug otherwise. The --ddump-splices output is managable at this early stage, but I can't imagine using it once I add more functionality and the code grows. Is there way to get the template haskell output code?

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listenOn specific IP
by vold 30 Jul '13

30 Jul '13

I'm trying to write a simple server with network. listenOnSource takes a socket number, but tries to bind to every available IP, which isn't what I need. How do I tell it to bind to a specific address?

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Question about Regular expression pattern consuming.
by S. H. Aegis 29 Jul '13

29 Jul '13

Hi. I'm newbie to Haskell. I want to get date, it's type is [[String]]. The problem is that "the string that is used in regular expression pattern" is consumed. ie, disappear. Here is my code. ------------------------------------------------------------ import Text.Regex import Control.Applicative main :: IO() main = do myIn <- readFile "Data.dat" print $ lines <$> intoEachPt myIn intoEachPt :: String -> [String] intoEachPt = splitRegex (mkRegex "20[0-9]{13}AH021") ----------------------------------------------------------- How can I fix this? Data: ....there is many DIGIT.....201306000300001AH02112361640 9....... Output: [[....there is many DIGIT..."],["12361640 9......]....] I hope: [[....there is many DIGIT..."],["201306000300001AH02112361640 9......]....]

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Cannot Show Table
by Guenter Fiebig 26 Jul '13

26 Jul '13

Hello, me, I am a very beginner. I build a GUI with Glade containing an input- and an output table as well as two buttons. My target is to take the user-input from the input table, process it and put the result to the output table. My first problem is to display the content from the input table for control. With " onClicked inButton (putStr "Hello, World!") " haskell works fine without errors. With " onClicked inButton (??? inTable) " I do not find the right command to display within Prelude. Thanks for help, Guenter inButton <- builderGetObject builder castToButton "button1"; outButton <- builderGetObject builder castToButton "button2"; inTable <- builderGetObject builder castToTable "table1"; --onClicked inButton (putStr "Hello, World!"); --0nly for testing onClicked inButton (??? inTable);

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