- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax
by Johan Tibell 11 Jul '11

11 Jul '11

On Mon, Jul 11, 2011 at 3:54 PM, Henning Thielemann <thunderbird(a)henning-thielemann.de> wrote: > Johan Tibell wrote: >> I've found this quite annoying, especially when using CPP to >> conditionally include something in a list, as it might force you to >> reorder the list to make the commas appear correctly when the >> conditional section is enabled/disabled. > > In this case I use the colon like a terminator. > > http://www.haskell.org/haskellwiki/List_notation Sorry, I wasn't being clear. I meant for import/export lists. Johan

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Monad.Reader #19 call for copy -- special issue on parallelism and concurrency
by Brent Yorgey 11 Jul '11

11 Jul '11

Call for Copy: The Monad.Reader - Issue 19 SPECIAL ISSUE on parallelism and concurrency -------------------------------------------- Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! Issue 19 will be a special issue focusing on parallelism and concurrency, construed broadly. The submission deadline for Issue 19 will be: **Tuesday, August 16** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

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Re: [Haskell-cafe] IterIO: How to write use my inumReverse
by Maciej Wos 11 Jul '11

11 Jul '11

Sorry, my previous message got truncated. I was trying to say that many iteratees like iterReverse can be defined nicely using combinators from Control.Applicative. You end up with much cleaner code. Also, iterLines doesn't work as the name would suggest. It only consumes one line from the input stream and returns it inside a singleton list. Instead, iterLines can be defined like this: iterLines :: (Monad m) => Iter L.ByteString m [L.ByteString] iterLines = do el <- tryBI lineI case el of -- no more "full" lines left, return the remaining data Left (e :: SomeException) -> (:[]) <$> dataI -- read one line; add it to the list and read more Right line -> (line:) <$> iterLines enumPure "line1\nline2\nline3" |$ iterLines [Chunk "line1" Empty,Chunk "line2" Empty,Chunk "line3" Empty] However, the above returns the resulting list only after consuming the whole stream, which is something to avoid. In case of iterReverse it is better to read and accumulate characters until \n is found and then return the reversed string. One way to do this is to read one character at a time: reverseLineSlow :: Iter L.ByteString IO L.ByteString reverseLineSlow = iter "" where iter acc = do c <- headI case c of 10 -> return $ L.reverse acc _ -> iter (acc `mappend` L.singleton c) But this will be really slow. Instead, the data should be read one chunk at a time: reverseLine :: Iter L.ByteString IO L.ByteString reverseLine = iter "" where iter acc = do -- read the data from the stream one chunk at a time Chunk c eof <- chunkI -- check if there is any \n, i.e. if we read a whole line let (a,b) = L.break (==10) c if b == "" -- haven't found any \n yet; append all data to the accumulator then iter (acc `mappend` a) -- have found \n else do -- put the data after \n back (while removing \n itself) ungetI $ L.tail b -- return reversed accumulator plus the data up to \n return $ L.reverse $ acc `mappend` a Hope this helps! -- Maciej On Tue, Jul 12, 2011 at 12:47 AM, Maciej Wos <maciej.wos(a)gmail.com> wrote: > Don't forget Applicative instance! > iterReverse = L.reverse <$> lineI > > On Monday, 4 July 2011 at 22:54, dm-list-haskell-cafe(a)scs.stanford.edu > wrote: > > At Mon, 4 Jul 2011 20:36:33 +1000, > John Ky wrote: > > Hi Haskell Cafe, > > enum |$ inumLines .| inumReverse .| inumUnlines .| iter > ... > > iterLines :: (Monad m) => Iter L.ByteString m [L.ByteString] > iterLines = do > line <- lineI > return [[line] > > iterUnlines :: (Monad m) => Iter [L.ByteString] m L.ByteString > iterUnlines = (L.concat . (++ [C.pack "\n"])) `liftM` dataI > > iterReverse :: (Monad m) => Iter [L.ByteString] m [L.ByteString] > iterReverse = do > lines <- dataI > return (map L.reverse lines) > > inumLines = mkInum iterLines > inumUnlines = mkInum iterUnlines > inumReverse = mkInum iterReverse > > It all works fine. > > My question is: Is it possible to rewrite inumReverse to be this: > > iterReverse :: (Monad m) => Iter L.ByteString m L.ByteString > iterReverse = do > line <- dataI > return (L.reverse line) > > inumReverse = mkInum iterReverse > > And still be able to use it in the line: > > enum |$ inumLines .| {-- inumReverse goes in here somehow --} .| > inumUnlines .| iter > > The reason I ask is that the Haskell function reverse has the type [a] -> > [a], > not [[a]] -> [[a]]. > > I thought perhaps the alternative inumReverse is cleaner than the original > as > it behaves more similarly to Haskell's own reverse function. > > I'm not sure what you are trying to achieve. If you want an iter that > works on L.ByteStrings, then you can say: > > iterReverse :: (Monad m) => Iter L.ByteString m L.ByteString > iterReverse = do > line <- lineI > return (L.reverse line) > > In that case you don't need inumLines and inumUnlines. If, however, > you want the type to be [L.ByteString], and you would rather do this > one line at a time, instead of calling map, then you could do > something like the following: > > iterReverse :: (Monad m) => Iter [L.ByteString] m [L.ByteString] > iterReverse = do > line <- headI > return [L.reverse line] > > But the code you have above should also work, so it all depends on > what you are trying to achieve. > > David > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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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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