- Haskell-Cafe - Haskell.org

Good XML-Enumerator Examples
by Michael Xavier 11 Jul '11

11 Jul '11

Hi Cafe. I'm writing a library which will be parsing XML data from a web API. I'm fairly well versed at this point in parsing JSON (aeson being my library of choice). For XML, I like the API of xml-enumerator over hxt because it appears to be nice and simple .The problem is that the example in the documentation isn't really sufficient for me to get a grasp of how to traverse XML and parse elements with multiple child nodes into a record. My first question: Am I barking up the wrong tree using xml-enumerator for anything beyond fairly flat, basic XML? If not, can anyone direct me to some good code samples using xml-enumerator to parse more complicated data? If I am indeed barking up the wrong tree, what would you all suggest? Thanks, -- Michael Xavier http://www.michaelxavier.net LinkedIn <http://www.linkedin.com/pub/michael-xavier/13/b02/a26>

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Re: [Haskell-cafe] External system connections
by Richard Wallace 11 Jul '11

11 Jul '11

On Sun, Jul 10, 2011 at 2:54 PM, Brandon Allbery <allbery.b(a)gmail.com> wrote: > On Sun, Jul 10, 2011 at 17:34, Richard Wallace > <rwallace(a)thewallacepack.net> wrote: >> Rather than a single separate thread that makes requests I was hoping to >> make several soap requests concurrently, rather than have them be made >> serially. I would only want to block other threads making soap request if >> one of them returns a response indicating the token they were using is no >> longer valid. > > Is there one shared token, or one per requesting thread, or some other > arrangement? There should be one shared token. > In either case, separation of concerns makes me think > the token handling belongs on the other end: a thread writes a > request down a Chan, a queue dispatcher thread reads it if there are > available workers and the token is available (with callers blocking > otherwise), dispatcher sends request to available worker. If the > worker finds the token is invalid, it calls back in to the dispatcher > with a token renewal request, which causes anything else that needs > that token to block as above, and itself blocks until a valid token is > returned. Ahhh... I see. That sounds like a much better plan. It also keeps the fact that the token is "mutable" confined to a single thread, the dispatcher thread. One thing I'd have to be careful to handle is iIf multiple worker threads find that the token is now invalid. In that case they would all send the token renewal request to the dispatcher and they would each try and do the re-auth. To avoid that, part of the token renewal request could be the used token. When the dispatcher does the token renewal, it checks if the used token is the same as the current token. If so, it does the token renewal. If not, it dispatches to the SOAP worker thread with the new token. > Another advantage of this is that adding more workers is > done in an obvious place (the queuer thread). > > That said, I still don't know enough details to say if that token > management implementation actually makes sense. I'd still go with the > worker/queuer thread setup, which is I think what you were pointed to > (the pool hackage). > Ok, I still don't see how the Pool package helps. I had assumed the pooled resource would be the token, so the size of the pool would be 1. Now that I rethink it, it seems like you are suggesting the resource is the worker threads. Is that right? When I read what you were talking about above with the dispatcher thread I had thought it would use forkIO to start the worker threads. The worker threads would read from a Chan that the dispatcher wrote to. In that way, the dispatcher doesn't have to worry about worker threads being available. Am I misunderstanding something? Thanks, Rich > -- > brandon s allbery allbery.b(a)gmail.com > wandering unix systems administrator (available) (412) 475-9364 vm/sms >

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NLP libraries and tools?
by Dmitri O.Kondratiev 10 Jul '11

10 Jul '11

Hi, Please advise on NLP libraries similar to Natural Language Toolkit ( www.nltk.org) First of all I need: - tools to construct 'bag of words' ( http://en.wikipedia.org/wiki/Bag_of_words_model) which is a list of words in the article. - tools to prune common words, such as prepositions and conjunctions, as well as extremely rare words, such as the ones with typos. - stemming tools - Naive Bayes classifier - SVM classifier - k-means clustering Thanks!

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unbuffered raw keyboard input on Windows
by José Romildo Malaquias 10 Jul '11

10 Jul '11

Hello. I want to write a Haskell console application (a game) in ghc where the user will interact using the keyboard. I need to read the keys as soon as they are typed and without echoing to the console. Although Haskell provides this capability in the standard libraries (using hSetEcho and hSetBuffering from System.IO), it does not work on Windows due to a bug (http://hackage.haskell.org/trac/ghc/ticket/2189) I have tried installing packages like vty, hscurses, and ncurses, but they did not install on Windows. Which workarounds are available for that? Romildo

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Re: [Haskell-cafe] error with nanocurses-1.5.2
by José Romildo Malaquias 10 Jul '11

10 Jul '11

On Sat, Jul 09, 2011 at 01:59:12PM -0300, José Romildo Malaquias wrote: > Hello. > > When trying to install nanocurses-1.5.2 on my Fedora Linux system, I am > getting the following error: > > $ cabal install nanocurses > Resolving dependencies... > Configuring nanocurses-1.5.2... > Preprocessing library nanocurses-1.5.2... > In file included from Curses.hsc:42:0: > cbits/utils.h:7:20: fatal error: config.h: No such file or directory > compilation terminated. > compiling dist/build/UI/Nanocurses/Curses_hsc_make.c failed (exit code 1) > command was: /usr/bin/gcc -c dist/build/UI/Nanocurses/Curses_hsc_make.c > -o dist/build/UI/Nanocurses/Curses_hsc_make.o -fno-stack-protector > -fno-stack-protector -D__GLASGOW_HASKELL__=700 -Dlinux_BUILD_OS > -Dlinux_HOST_OS -Dx86_64_BUILD_ARCH -Dx86_64_HOST_ARCH -Icbits > -I/usr/lib64/ghc-7.0.2/unix-2.4.2.0/include > -I/usr/lib64/ghc-7.0.2/bytestring-0.9.1.10/include > -I/usr/lib64/ghc-7.0.2/base-4.3.1.0/include > -I/usr/lib64/ghc-7.0.2/include -I/usr/lib64/ghc-7.0.2/include/ > cabal: Error: some packages failed to install: > nanocurses-1.5.2 failed during the building phase. The exception was: > ExitFailure 1 > > It would be nice to have this buf fixed. The attached patch fixes the compilation issues. Basically it changes the build type from simple to configure in the cabal file, and it renames the function getline in cbits/utils.{h,c} to mygetline. If appropriate, please release a new version with the fixes. Romildo

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JSON Querry examples
by Dmitri O.Kondratiev 10 Jul '11

10 Jul '11

Hi, I am trying to search known-in-advance fields in JSON object with unknown structure. It looks like Data.JSON2.Query is most appropriate for this task. Yet, I can not figure out how to use filters this package provides to filter JSON arrays and objects. Are there any end-to-end examples of making queries of unknown JSON objects? Thanks!

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class and instance
by Patrick Browne 10 Jul '11

10 Jul '11

Hi, I am trying to understand the following code. I have written my current (mis-)understanding and questions below. I do not wish to improve the code, it is from a research paper[1] that I am trying to understand. Pat [1] ftp://ftp.geoinfo.tuwien.ac.at/medak/phdmedak.pdf -- A specification. The class Points takes two type variables. -- The functions take variables of type p, a; return a value of type a. class Points p a where getX :: p a -> a getY :: p a -> a -- A parameterized representation -- One constructor takes two elements of type a and produces a Point. data Point a = Pt a a -- An implementation of the class Points on the data type (Point a) -- In Pt b c constructor the variables b and c areboth of type a. -- The type a is left undefined until evaluation instance Points Point a where getX (Pt b c) = b getY (Pt b c) = c -- This runs with any type e.g. Integers -- getX(Pt 1 2) -- :t getX(Pt 1 2) -- getX(Pt 1 2) :: forall t. (Num t) => t My main question is in understanding the relationship between the arguments of the functions getX and getY in the class and in the instance. It seems to me that the constructor Pt 1 2 produces one element of type Point which has two components. How does this square with the class definition of getX which has two arguments? Is there a difference between: getX :: p a -> a and getX :: p -> a -> a This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Haddock and the type inference
by Yves Parès 10 Jul '11

10 Jul '11

Hello Café, As far as I know, there is currently no way to tell haddock to run the type inferer to document functions without explicit type signature. Is that done on purpose (e.g. to force the developpers to write at least the type signatures of the public API, which is a fair reason) or is it a limitation? Is it planned to add that support to haddock?

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Re: [Haskell-cafe] NLP libraries and tools?
by wren ng thornton 10 Jul '11

10 Jul '11

(Psst, the nlp list is <nlp(a)projects.haskell.org> :) On 7/9/11 3:10 AM, Jack Henahan wrote: > On Jul 7, 2011, at 10:53 PM, wren ng thornton wrote: >> I can't help but be a (meta)theorist. But then, I'm of the firm opinion >> that theory must be grounded in actual practice, else it belongs more to >> the realm of theology than science. > > Oof, you're liable to wound my (pure) mathematician's pride with remarks like that, wren. :P How's that now? Pure mathematics is perfectly grounded in the practice of mathematics :) I've no qualms with pure maths. Afterall, mathematics isn't trying to model anything (except itself). The problems I have are when the theory branch of a field loses touch with what the field is trying to do in the first place, and consequently ends up arguing over details which can be neither proven nor disproven. It is this which makes them non-scientific and not particularly helpful for practicing scientists. Linguistics is one of the fields where this has happened, but it's by no means the only one (AI, declarative databases, postmodernism,...) There's nothing wrong with not being science. I'm a big fan of the humanities, mathematics, and philosophy. It's only a problem when non-science is pretending to be science: it derails the scientists and it does a disservice to the non-science itself. Non-science is fine; pseudo-science is the problem. For the same reason, I despise math envy and all the pseudo-math that gets bandied about in social sciences wishing they were economics (or economics wishing it were statistics, or statistics wishing it were mathematics). -- Live well, ~wren

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[ANN] PAM library, Iptables web-interface
by Evgeny Tarasov 10 Jul '11

10 Jul '11

Hello! I'd like to announce my project called Iptadmin. It's a web interface for linux Iptables firewall [1]. It supports just few Iptables options by now. The main feature of Iptadmin is user friendly help messages. Iptadmin works in a daemon mode. It doesn't keep any auxiliary information in a system, firewall configuration is taken from the «iptables-save» script. All changes is made by Iptables calling. Firewall rules are saved by «/etc/init.d/iptables save» script. Several linux distributions such as Debian require additional Iptables configuration. Iptadmin uses PAM [2] authentication. Access is granted for root and for any other user with membership in «iptadmin» system group. Iptadmin is written using Happstack, so it doesn't support https yet. To prevent sending system password by network in plain text, the web interface is listening only on loopback network interface. In order to access Iptadmin remotely user should setup https proxying using external web server. There is also dedicated library for parsing and printing Iptables rules [3]. While working on Iptadmin I've implemented library with subset of PAM api bindings[4]. A code of my project is licensed under BSD3. Sources are uploaded to hackage [3,4,5]. More information can be found on a project homepage[6]. [1] http://www.netfilter.org/ [2] http://www.kernel.org/pub/linux/libs/pam/ [3] http://hackage.haskell.org/package/iptables-helpers-0.3 [4] http://hackage.haskell.org/package/pam-0.1 [5] http://hackage.haskell.org/package/iptadmin-1.0.0 [6] http://iptadmin.confmgr.org -- Best regards, Evgeny Tarasov

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