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Grammar hacker in Haskell
by sharadha Raghunathan 22 Jul '11

22 Jul '11

Hi all, I am new to Haskell, I need to write a grammar hacker in Haskell; that would find the first set of grammar, follow set of grammar, determine if it is in LL or not and generate a LR table. Can someone help me with the same? Any reply would be much appreciated. -- Sharadha Raghunathan

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Introspection
by Patrick Browne 22 Jul '11

22 Jul '11

Is it possible to access type information and perform either of the following in a Haskell program? 1) Find the type/class of a variable e.g. a type predicate: is y of-type A, or is y of-Class A 2) Assert the type of a variable e.g. if y.length > 100 then y is of type big. Regards, Pat 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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Haskell Weekly News: Issue 191
by Daniel Santa Cruz 22 Jul '11

22 Jul '11

Welcome to issue 191 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of July 10 to 16, 2011. [1] http://haskell.org/ You can find the HTML (and mobile) version of the issue at: http://contemplatecode.blogspot.com/2011/07/haskell-weekly-news-issue-191.h… Announcements Brent Yorgey put out a call for articles for the next Monad.Reader, which will be focused on parallelism and concurrency. [2] http://goo.gl/Qh6na Quotes of the Week * PenguinOfDoom: Being enlightened gentlemen, we split all programming languages into two groups, sucks and doesn't-suck and put all of them into the first group. * cmccann: beta reducing the chainsaws can be tricky * kmc: edwardk measures API lifetime in hours * _Ray_: the difference in mathematical/CS knowledge of the average haskeller, and the average PHPer, is likely similar to that of the average PHPer and a banana * quicksilver: [on LYAH] by the time I read it I knew everything it was trying to teach, so I just thought "Yay! Cute caterpillar". * edwardk: shapr is the only person i know of who just decides to drive across the country with a car full of machetes * copumpkin is now known as contrapumpkin. contrapumpkin is now known as yoda. yoda is now known as coyoda. + Eduard_Munteanu: Yonoda + coedwardk: i keep reading coyoda as coyoneda + monochrom: I read it as toyota + Eduard_Munteanu: Mmm, the monad is strong in this one... + monochrom: rumours actually say that it should be toyoda but the japaneses want it easier on americans, so toyota Top Reddit Stories * The Haskell School of Music - new book being written by Paul Hudak Domain: plucky.cs.yale.edu, Score: 66, Comments: 9 On Reddit: [3] http://goo.gl/9Jjrd Original: [4] http://goo.gl/r54xG * Confusion between (.) and ($) operators Domain: self.haskell, Score: 41, Comments: 27 On Reddit: [5] http://goo.gl/eFqRj Original: [6] http://goo.gl/eFqRj * Fitter, happier, more productive UTF-8 decoding Domain: serpentine.com, Score: 35, Comments: 6 On Reddit: [7] http://goo.gl/o2REx Original: [8] http://goo.gl/dKB04 * Static Single Assignment for Functional Programmers Domain: wingolog.org, Score: 30, Comments: On Reddit: [9] http://goo.gl/13sSP Original: [10] http://goo.gl/90MEH * [GSoC] Text/UTF-8: Initial results Domain: jaspervdj.be, Score: 29, Comments: 14 On Reddit: [11] http://goo.gl/nJ4pJ Original: [12] http://goo.gl/uPkBl * Data Driven Programming in haskell - live screencast. What are your tips/suggestions? Domain: entirelysubjective.com, Score: 29, Comments: 17 On Reddit: [13] http://goo.gl/kIiX8 Original: [14] http://goo.gl/28k6p * Ur/Web is hiring: web metaprogramming with class Domain: haskell.org, Score: 28, Comments: 11 On Reddit: [15] http://goo.gl/nWf5S Original: [16] http://goo.gl/l9HhI * Monads in C++. Bartosz Milewski's Programming Cafe Domain: bartoszmilewski.wordpress.com, Score: 23, Comments: 0 On Reddit: [17] http://goo.gl/IJnVa Original: [18] http://goo.gl/Bq4Lq * The Typeable class is changing Domain: haskell.org, Score: 22, Comments: 2 On Reddit: [19] http://goo.gl/x1InO Original: [20] http://goo.gl/E2jVK * Reverse Engineering of Compiled Haskell Domain: self.haskell, Score: 18, Comments: 22 On Reddit: [21] http://goo.gl/GCrfN Original: [22] http://goo.gl/GCrfN Top StackOverflow Questions * Haskell - How to avoid typing the same context over and over again? votes: 18, answers: 3 Read on SO: [23] http://goo.gl/0h1qF * Haskell iteratee: simple worked example of stripping trailing whitespace votes: 14, answers: 1 Read on SO: [24] http://goo.gl/bYujx * Parametrised data type in Scala votes: 12, answers: 2 Read on SO: [25] http://goo.gl/4NyZw * How to organize Haskell modules with instances: stick to data type vs type class? votes: 10, answers: 1 Read on SO: [26] http://goo.gl/Tivoh * On improving Haskell's performance compared to C in fibonacci micro-benchmark votes: 10, answers: 3 Read on SO: [27] http://goo.gl/q5O8Q * Constructing a tuple (or list) from already-existing objects - what is the cost? votes: 9, answers: 2 Read on SO: [28] http://goo.gl/UuKft * Writing A Function Polymorphic In A Type Family votes: 9, answers: 1 Read on SO: [29] http://goo.gl/vPpoH * Haskell Lazy ByteString + read/write progress function votes: 9, answers: 1 Read on SO: [30] http://goo.gl/LfNEN About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18844 3. http://www.reddit.com/r/haskell/comments/in1my/the_haskell_school_of_music_… 4. http://plucky.cs.yale.edu/cs431/reading.htm 5. http://www.reddit.com/r/haskell/comments/ipfok/confusion_between_and_operat… 6. http://www.reddit.com/r/haskell/comments/ipfok/confusion_between_and_operat… 7. http://www.reddit.com/r/haskell/comments/im82t/fitter_happier_more_producti… 8. http://www.serpentine.com/blog/2011/07/11/fitter-happier-more-productive-ut… 9. http://www.reddit.com/r/haskell/comments/ip2de/static_single_assignment_for… 10. http://wingolog.org/archives/2011/07/12/static-single-assignment-for-functi… 11. http://www.reddit.com/r/haskell/comments/ilif5/gsoc_textutf8_initial_result… 12. http://jaspervdj.be/posts/2011-07-10-text-utf8-initial-results.html 13. http://www.reddit.com/r/haskell/comments/insp1/data_driven_programming_in_h… 14. http://entirelysubjective.com/programming/data-driven-programming-haskell-1/ 15. http://www.reddit.com/r/haskell/comments/ips56/urweb_is_hiring_web_metaprog… 16. http://www.haskell.org/pipermail/haskell-cafe/2011-July/093994.html 17. http://www.reddit.com/r/haskell/comments/imzaj/monads_in_c_bartosz_milewski… 18. http://bartoszmilewski.wordpress.com/2011/07/11/monads-in-c/ 19. http://www.reddit.com/r/haskell/comments/imm0j/the_typeable_class_is_changi… 20. http://www.haskell.org/pipermail/haskell-cafe/2011-July/093947.html 21. http://www.reddit.com/r/haskell/comments/iqz0l/reverse_engineering_of_compi… 22. http://www.reddit.com/r/haskell/comments/iqz0l/reverse_engineering_of_compi… 23. http://stackoverflow.com/questions/6706248/haskell-how-to-avoid-typing-the-… 24. http://stackoverflow.com/questions/6643495/haskell-iteratee-simple-worked-e… 25. http://stackoverflow.com/questions/6694269/parametrised-data-type-in-scala 26. http://stackoverflow.com/questions/6641105/how-to-organize-haskell-modules-… 27. http://stackoverflow.com/questions/6716315/on-improving-haskells-performanc… 28. http://stackoverflow.com/questions/6655902/constructing-a-tuple-or-list-fro… 29. http://stackoverflow.com/questions/6663547/writing-a-function-polymorphic-i… 30. http://stackoverflow.com/questions/6668716/haskell-lazy-bytestring-read-wri…

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Network.CGI strangeness
by Alexander Solla 22 Jul '11

22 Jul '11

Hi Everybody, I'm having some strange issues with Network.CGI's pathInfo action, or else some other strangeness with pattern matching. My "mainCGI" CGI action is: mainCGI :: CGI CGIResult mainCGI = pathInfo >>= outputSlotTHtml . packagesPage outputSlotTHtml is irrelevant to my problem. packagesPage generates a SlotT IO Html -- Slot semantics are essentially irrelevant to my problem. But packagesPage calls a function called pathCrossRef which is having some problems: packagesPage :: WebPath -> SlotT IO Html packagesPage wp = do infos <- getPackageInfos . pathCrossRef $ wp ... pathCrossRef :: String -> String pathCrossRef "/working/" = "/development/code/haskell/working/" pathCrossRef _ = "/development/code/haskell/packages/" The intention is that when the cgi program is called with "/working/" as an argument, the pathCrossRef should point at "/development/.../working/", and when called with "/packages/" (or anything else) as an argument, pathCrossRef should point to "/development/.../packages/". But it is not working! When I visit http://localhost/working/, I end up using the "packages" cross ref. I have even printed the cgi argument to the Apache log, and the argument is "/working/". If I get rid of the wildcard pattern, I get a "missing pattern" error when I visit http://localhost/working/. Can anybody help? Thanks, Alex

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ANN: boomerang and web-routes-boomerang
by Jeremy Shaw 22 Jul '11

22 Jul '11

Hello, I am pleased to announce the release of two new libraries: boomerang and web-routes-boomerang. boomerang is a library for general purpose, invertible parsing and pretty printing. It provides combinators which allow you to specify a grammar once and automatically extract a parser and pretty-printer from it. This library was refactored, with permission, from the Zwaluw library created by Sjoerd Visscher and Martijn van Steenbergen. hackage: http://hackage.haskell.org/package/boomerang quick intro: http://hackage.haskell.org/packages/archive/boomerang/1.0.0/doc/html/Text-B… web-route-boomerang provides glue code for using boomerang with web-routes. web-routes is a framework-independent, type-safe, url routing library. hackage: http://hackage.haskell.org/package/web-routes-boomerang tutorial: http://happstack.com/docs/crashcourse/WebRoutes.html#web-routes-boomerang - jeremy p.s. boomerang is similar in purpose to the inveritble-syntax library: http://hackage.haskell.org/package/invertible-syntax The libraries take different approaches and are both worth considering.

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Using SAX
by Larry Lee 22 Jul '11

22 Jul '11

I'm new to Haskell and am trying to use the SAX module to parse an XML file. I'm using the following code: module FPHParser where import Data.Text as Text import Data.Maybe import Text.XML.LibXML.SAX as SAX import System.IO f :: IO (Parser IO) f = do let g = do putStrLn ("g called.") return True let h msg = do putStrLn (Text.unpack msg) return False p <- SAX.newParserIO (Just (Text.pack "test.xml")) SAX.setCallback p SAX.reportError h SAX.setCallback p SAX.parsedBeginDocument g SAX.parseComplete p return p and am getting the following error when I parse a file with the following contents: <?xml version="1.0"?> <html/> [This is the error] "Extra content at the end of the document" This error seems to be coming from the LibSAX library, which is a C library. But I think that my error is in my code, as the file is valid XML. Any help would be greatly appreciated!

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Make Show instance
by Александр 22 Jul '11

22 Jul '11

Hello, I have binary tree, with every leaf tuple - (k,v): data Tree k v = EmptyTree | Node (k, v) (Tree k v) (Tree k v) How can i make Show Instance for (Tree Int Int) ? I try: instance Show (Tree k v) where show EmptyTree = show "Empty" show (Node (Int, Int) left right) = .. But get error: Not in scope: data constructor `Int' I have a function: fillTree :: Int -> Tree Int Int -> Tree Int Int fillTree 1000000 tree = tree fillTree x tree = let a = treeInsert (x, x) EmptyTree in fillTree (x + 1) a If i execute it: No instance for (Show (Tree Int Int)) arising from a use of `print' How can i make Show instance for my Tree? Thank you.

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Helsinki Haskellers?
by Dean Herington 21 Jul '11

21 Jul '11

Hi, Cafe, My wife, a biologist, will attend a conference outside Helsinki in mid-September. I've decided to accompany her on the trip to Finland. Are there any Haskellers in the area who might be interested in an informal meetup? I'd also welcome any suggestions on what to see, do, eat, etc. We'll have 3-4 days after her conference. We especially enjoy nature (hiking, biking, boating), music (folk/ethnic, jazz, classical), and dance (performance and participatory). And I love Haskell, of course. As my inquiry is only weakly Haskell-related, I suggest that replies be sent to me rather than the list. Thanks. Dean (from North Carolina)

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pointer equality
by Nikhil A. Patil 21 Jul '11

21 Jul '11

Hi, Is there any way of getting the following code to immediately return True without performing the element-by-element comparison? Essentially this boils down to checking whether pointers are equal before comparing the contents. > main = print $ f == f > where f = [1..10^9] Thanks!! nikhil

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haskell zlip read position
by rahul 21 Jul '11

21 Jul '11

Hi Guys, I am trying to parse a binary stream with the format for one entry [headers, zlib compressed content] , with multiple entries. I can use the Zlib library to get the content for the first entry after the headers, but I cannot find a way to get the offset to start parsing for the second entry. Is there a way I can get this information out of ZLib? or is there a better approach to doing this? Any pointers would be very much appreciated. Regards, Rahul

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