- Haskell-Cafe - Haskell.org

Convert Either to Tree - Occurs check
by André Batista Martins 22 Oct '10

22 Oct '10

hello, i want convert "Either" to a tree. Example: Either ( Either 1 2 ) ( Either 3 4) ---- > Branch ( Branch (Leafl 1) (Leafr2) ) ( Branch (Leafl 3) (Leafr4)) ) Code: data TreeE a b = Empty |Leafl a | Leafr b | Branch (TreeE a b ) (TreeE a b) deriving Show f3 (Right b) (Branch l r) =case( isRight(b) || isLeft(b) )of true -> Branch l (f3 b r) false -> Branch l (Leafl b) f3 (Left b) (Branch l r) = case( isRight(b) || isLeft(b) )of true -> Branch (f3 b l) r false -> Branch (Leafl b) r Error: Occurs check: cannot construct the infinite type: b = Either a b Expected type: Either a b -> TreeE t t1 -> TreeE t t1 Inferred type: Either a (Either a b) -> TreeE t t1 -> t2 In the second argument of `Branch', namely `(f3 b r)' In the expression: Branch l (f3 b r) I don't understand why this happen... Can anyone help me?

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Haskell Weekly News: Issue 155 - October 20, 2010
by Daniel Santa Cruz 22 Oct '10

22 Oct '10

Welcome to issue 155 of the HWN, a newsletter covering developments in the [1]Haskell community in the week of October 10 - 16. This time around we again have 87 posts to HackageDB. Instead of posting the individual packages, we get to see and celebrate the 43 people behind these efforts. Want to keep a close eye on the haskellers that lurk twitter? Don Steward made [2]a twitter list of tweeting haskellers! Let him know if you'd like to be added to the list. There were a total of 24 new stories posted to the Haskell Reddit channel, 27 new questions taged with Haskell in StackOverflow, and 408 messages posted to Haskell-Cafe. So, what was hot last week? Announcements Gregory Crosswhite is pleased to [3]announce the release of a family of packages for type-level natural numbers. He also [4]announced tagged-list, a package which provides fixed-length lists that are tagged with a phantom type-level natural number corresponding to the length. Janis Voigtlander [5]announced that it is time to collect contributions for the 19th edition of the Haskell Communities & Activities Report. The submission deadline is November 1, 2010. Alexander Solla [6]announced his new "Facts" library. The Facts hierarchy is meant to contain commonly used, relatively static facts about the "real world." Kevin Jardine [7]announced polyToMonoid: a library that supplies two very general polyvariadic functions that can map their arguments into any monoid you specify. Simon Hengel [8]announced a new version of DocTest. DocTest now uses Haddock for parsing of comments. Interesting Threads on Haskell-Cafe Michael Snoyman [9]reported that Haskellers.com has become popular a lot falter that he anticipated. Read up on what changes are planned for Haskellers.com. Michael is very interested in some help with running the site. Jacek Generowicz [10]asked how to deal with dynamic dispatch on extensible sets of types. Uwe Schmidt [11]replied to a question about why HXT uses arrows as opposed to using monads. Good comments followed. Jason Dusek [12]asked if there is a way to write a Haskell data structure that is necessarily only one or two or seventeen items long; but that is nonetheless statically guaranteed to be of finite length? Twenty-three messages followed. Simon Thompson [13]annouced the availability of books for review for the Journal of Functonial Programming. Andrew Copping [14]summarized a report produced by Google Szwitzerland on taking a Python system and rewriting bits of it in Haskell, some of which is now in production use. Ben Franksen [15]ranted about the current "Haskell Blurb" on the first paragraph of haskell.org. Quite the read to end the week :) Top Reddit Stories * Using Haskell’s ‘newtype’ in C Domain: blog.nelhage.com Score: 37, Comments: 5 On Reddit: http://www.reddit.com/r/haskell/comments/dptzh/using_haskells_newtype_in_c/ Original: http://blog.nelhage.com/2010/10/using-haskells-newtype-in-c/ * The Haskell theme: consistent visual branding for Haskell Domain: haskell.org Score: 34, Comments: 11 On Reddit: http://www.reddit.com/r/haskell/comments/dqiej/the_haskell_theme_consistent… Original: http://www.haskell.org/pipermail/haskell-cafe/2010-October/084781.html * Haskellers: Survey results and new site features Domain: haskellers.com Score: 22, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/dr37b/haskellers_survey_results_an… Original: http://www.haskellers.com/news/1/ * My Experience Learning Haskell Domain: blog.virtucal.com Score: 21, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/drgcz/my_experience_learning_haske… Original: http://blog.virtucal.com/cyclical/2010/10/14/my-experience-learning-haskell… * Invertible monads for exception handling and memory allocations Domain: docs.yesodweb.com Score: 21, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/drkdj/invertible_monads_for_except… Original: http://docs.yesodweb.com/blog/invertible-monads-exceptions-allocations/ * Accelerating Haskell Array Codes with Multicore GPUs Domain: justtesting.org Score: 19, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/dq1yo/accelerating_haskell_array_c… Original: http://justtesting.org/accelerating-haskell-array-codes-with-multico * Unicode text performance improvements Domain: serpentine.com Score: 17, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/drinz/unicode_text_performance_imp… Original: http://www.serpentine.com/blog/2010/10/15/unicode-text-performance-improvem… * Like a coding challenge? Check out Code Quarterly! Domain: serpentine.com Score: 16, Comments: 9 On Reddit: http://www.reddit.com/r/haskell/comments/dpjgv/like_a_coding_challenge_chec… Original: http://www.serpentine.com/blog/2010/10/11/like-a-coding-challenge-check-out… * C9 Lecture: Ralf Lämmel on Monads Domain: channel9.msdn.com Score: 16, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/dq614/c9_lecture_ralf_lämmel_on_mo… Original: http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Dr-Ralf-Lmmel-AFP-The… * Take the Haskellers.com survey Domain: spreadsheets.google.com Score: 15, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/dq7xj/take_the_haskellerscom_surve… Original: https://spreadsheets.google.com/viewform?formkey=dG8tcVhBdzRSS1lkdVV6U281em… Top StackOverflow Questions * When is memoization automatic in GHC Haskell? Question (votes: 14, answers: 3) http://stackoverflow.com/questions/3951012/when-is-memoization-automatic-in… * Why is my code using monadic lists from the List package so slow? Question (votes: 12, answers: 3) http://stackoverflow.com/questions/3916444/why-is-my-code-using-monadic-lis… * Haskell: lift vs liftIO Question (votes: 8, answers: 2) http://stackoverflow.com/questions/3921237/haskell-lift-vs-liftio * Can anyone suggest a good constraint library for Haskell? Question (votes: 8, answers: 2) http://stackoverflow.com/questions/3926656/can-anyone-suggest-a-good-constr… * Library function to compose a function with itself n times Question (votes: 6, answers: 4) http://stackoverflow.com/questions/3911060/library-function-to-compose-a-fu… * Evaluation of a small math type language that supports one variable Question (votes: 5, answers: 1) http://stackoverflow.com/questions/3906231/evaluation-of-a-small-math-type-… * What's the difference of a “class” in Haskell and in an “abstract class” in OO language? Question (votes: 5, answers: 4) http://stackoverflow.com/questions/3938779/whats-the-difference-of-a-class-… * Haskell and State Question (votes: 5, answers: 4) http://stackoverflow.com/questions/3944170/haskell-and-state * Can Haskell guard on abstract data types in a polymorphic function? Question (votes: 5, answers: 3) http://stackoverflow.com/questions/3944756/can-haskell-guard-on-abstract-da… Quotes of the Week * pumpkin: sounds like a lot of work! it's easier for me to just bitch about it * OlegKiselyov: Haskell is inconsistent in yet another way. * monochrom: just add #! /usr/bin/runghc to your haskell file. then type errors occur at runtime only. * IIcke: [haskell] is like wearing a corset, at the beginning is annoying, but then your tits look bigger and the world becomes a better place Contributors to HackageDB Who contributed, and how many uploads did they make? MichaelSnoyman (14), BryanOSullivan (11), GregoryCrosswhite (5), AndyStewart (5), GregoryCollins (4), JosePedroMagalhaes (3), StephenBlackheath (2), SjoerdVisscher (2), PaulTalaga (2), MichaelZuo (2), DanielDiaz (2), ConalElliott (2), AristidBreitkreuz (2), AlexanderSolla (2), YuriyIskra (1), VivianMcPhail (1), TristanSkudlik (1), TonyMorris (1), StephenTetley (1), SimonHengel (1), ReinierLamers (1), OlegBelozeorov (1), NeilMitchell (1), MaxBolingbroke (1), MatthiasBartsch (1), MatthewSottile (1), MasatakeDaimon (1), MarkSnyder (1), ManuelChakravarty (1), LeePike (1), KiYungAhn (1), KevinJardine (1), JustinBailey (1), IlyaPortnov (1), GeoffHulette (1), EugeneKirpichov (1), DonaldStewart (1), DanielSeidel (1), DanielSchoepe (1), CarlHowells (1), BenFranksen (1), ArieMiddelkoop (1), AlexSuraci (1) About the Haskell Weekly News New editions are posted to [16]the Haskell mailing list as well as to [17]the Haskell Sequence and [18]Planet Haskell. [19]RSS is also available, and headlines appear on [20]haskell.org. To help create new editions of this newsletter, please send stories to dstcruz * at * gmail * dot * com. The code used to produce this version of the newsletter is not yet publicly available, as it is a complete hack designed to get things started again. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://twitter.com/#!/donsbot/haskellers 3. http://groups.google.com/group/haskell-cafe/browse_thread/thread/c007154763… 4. http://groups.google.com/group/haskell-cafe/browse_thread/thread/ff95d6c78d… 5. http://groups.google.com/group/haskell-cafe/msg/d9d227b0f38bcf51 6. http://groups.google.com/group/haskell-cafe/browse_thread/thread/fc86bd03d0… 7. http://groups.google.com/group/haskell-cafe/browse_thread/thread/4ba5741441… 8. http://groups.google.com/group/haskell-cafe/browse_thread/thread/7982f0ad3b… 9. http://groups.google.com/group/haskell-cafe/browse_thread/thread/2440a1683f… 10. http://groups.google.com/group/haskell-cafe/browse_thread/thread/f0a05dd5b5… 11. http://groups.google.com/group/haskell-cafe/browse_thread/thread/38185e7ba6… 12. http://groups.google.com/group/haskell-cafe/browse_thread/thread/7ba4b3b038… 13. http://www.cs.kent.ac.uk/people/staff/sjt/JFP/available.html 14. http://groups.google.com/group/haskell-cafe/browse_thread/thread/81707fea2b… 15. http://groups.google.com/group/haskell-cafe/browse_thread/thread/a7c7b569dd… 16. http://www.haskell.org/mailman/listinfo/haskell 17. http://sequence.complete.org/ 18. http://planet.haskell.org/ 19. http://sequence.complete.org/node/feed 20. http://haskell.org/

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[ANNOUNCE] text 0.10.0.0 - fast Unicode text handling
by Bryan O'Sullivan 22 Oct '10

22 Oct '10

[Blog copy of the announcement here<http://www.serpentine.com/blog/2010/10/22/text-0-10-0-0-is-here/> .] I just pushed it to bitbucket <http://bitbucket.org/bos/text> and github<http://github.com/bos/text>, and you can install it from the text site on Hackage<http://hackage.haskell.org/package/text> in the usual way: cabal update cabal install text What's in this release? - New functions for reading integers and floating point numbers<http://hackage.haskell.org/packages/archive/text/0.10.0.0/doc/html/Data-Tex…>, an oft-requested feature. They're fast, too: they range from parity with their bytestring counterparts, to up to 4 times faster. You can expect to parse 3 to 4 million Int values per second out of a text file, or up 2 million Double values per second. They're also easy to use, give error messages, and come in strict and lazy variants. - UTF-8 decoding and encoding are now very fast<http://www.serpentine.com/blog/2010/10/15/unicode-text-performance-improvem…>. They're up to 9x faster than they were, and close to the performance of pure C UTF-8 decoding and encoding. - The Eq and Ord instances are also now very fast, up to 5x faster than before<http://www.serpentine.com/blog/2010/10/19/a-brief-tale-of-faster-equality/>. They're now faster than the bytestring instances. - Several other common functions received drive-by performance improvements too. - Better protection against rare crashes on really huge volumes of data.

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suspected problem with network.curl version 1.3.5
by Michael Litchard 22 Oct '10

22 Oct '10

Here's my code, I'm pretty sure I am doing this right. The problem seems to be with method_POST. I tried to duplicate manually, but I'm not sure I used command line curl correctly. Take a look at the output below. > import Network.Curl > import System (getArgs) > import Text.Regex.Posix > -- | Standard options used for all requests. Uncomment the @CurlVerbose@ > -- option for lots of info on STDOUT. > opts = [ CurlCookieJar "cookies" , CurlVerbose True ] > -- | Additional options to simulate submitting the login form. > loginOptions user pass = > CurlPostFields [ "login=" ++ user, "password=" ++ pass ] : method_POST > main = withCurlDo $ do > -- Get username and password from command line arguments (will cause > -- pattern match failure if incorrect number of args provided). > [user, pass] <- getArgs > -- Initialize curl instance. > curl <- initialize > setopts curl opts > -- POST request to login. > r <- do_curl_ curl "https://github.com/login" (loginOptions user pass) > :: IO CurlResponse > if respCurlCode r /= CurlOK || respStatus r /= 200 > then error $ "Failed to log in: " > ++ show (respCurlCode r) ++ " -- " ++ respStatusLine r > else do > -- GET request to fetch account page. > r <- do_curl_ curl ("https://github.com/session") method_GET > :: IO CurlResponse > if respCurlCode r /= CurlOK || respStatus r /= 200 > then error $ "Failed to retrieve account page: " > ++ show (respCurlCode r) ++ " -- " ++ respStatusLine r > else putStrLn $ extractToken $ respBody r > -- | Extracts the token from GitHub account HTML page. > extractToken body = head' "GitHub token not found" xs > where > head' msg l = if null l then error msg else head l > (_,_,_,xs) = body =~ "github\\.token (.+)" > :: (String, String, String,[String]) * About to connect() to github.com port 443 (#0) * Trying 207.97.227.239... * connected * Connected to github.com (207.97.227.239) port 443 (#0) * found 142 certificates in /etc/ssl/certs/ca-certificates.crt * server certificate verification SKIPPED * common name: *.github.com (matched) * server certificate expiration date OK * server certificate activation date OK * certificate public key: RSA * certificate version: #3 * subject: O=*.github.com,OU=Domain Control Validated,CN=*.github.com * start date: Fri, 11 Dec 2009 05:02:36 GMT * expire date: Thu, 11 Dec 2014 05:02:36 GMT * issuer: C=US,ST=Arizona,L=Scottsdale,O=GoDaddy.com\, Inc.,OU=http://certificates.godaddy.com/repository,CN=Go Daddy Secure Certification Authority,serialNumber=07969287 * compression: NULL * cipher: AES-128-CBC * MAC: SHA1 > GET /login HTTP/1.1 Host: github.com Accept: */* <snip> See, the code set a GET, where I thought I was doing a POST here is my attempt to do a POST manually, it failed but I'm not sure it's right. curl --data "authenticity_token=BigAssToken&login=UserName&password=Password&commit=Log+in" https://github.com/login > login_out anyway, could someone try to replicate this problem?

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Regression test utility suggestions?
by Peter Schmitz 22 Oct '10

22 Oct '10

I am seeking suggestions for a regression test utility or framework to use while developing in Haskell (in a MS Windows environment). I am developing a Haskell application that parses an input text file, and outputs some information about what was parsed. The application is currently in a GUI, but the parsing/analysis code does not require any user interaction, so it would be simple to implement a command-line based version (that could be invoked by a regression test utility -- http://en.wikipedia.org/wiki/Regression_testing) that takes as args: the input and output file pathnames. As I develop, I have a growing set of input files that are used to test various aspects of the grammar. I am seeking a regression test utility that I could point at a dir of test input files, and it would process each input file using my app, creating a corresponding output file. The utility would then compare each output file with a known good output file, and let me somehow review the results (i.e., the differences), and for each test file (or all of them) allow me to decide one of: -- there were no diffs; all is well -- the diff shows the output changed as I expected, due to coding enhancements or fixes; take the new output file and use it as the new good output file -- oops, unexpected diffs; I need to fix my code and then re-run the utility afterwards I have reviewed http://www.haskell.org/haskellwiki/Introduction_to_QuickCheck and http://hackage.haskell.org/packages/archive/pkg-list.html#cat:testing but I did not see anything that seems to fit. I'm not sure whether (or how) QuickCheck would lend itself to this type of testing. I don't want to generate any test cases; I have a manually created collection of test cases that I want to use (which is all I am interested in, at this point). In the past I have coded something like this using shell or Perl scripts, etc. I am wondering if there is a commonly used utility available for this purpose that you like. (I use a MS Windows environment, but I have a Bash and various gnu shell utils installed.) (It does not matter to me whether it is written in Haskell, although I suppose that would make it more enjoyable to hack on. :) Thanks in advance very much for any suggestions. -- Peter

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playing around with network.curl - redub
by Michael Litchard 21 Oct '10

21 Oct '10

My current problem is how to pass around the cookie jar. I need to gather cookies, while establishing a session. Could someone provide an example? I've messed about with CurlCookieFile and CurlCookieJar, to no avail. I'll provide my failed attempt if needed. I'm using https, if it makes a difference.

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HELP
by Yaadallah Khan 21 Oct '10

21 Oct '10

I am Studying for an exam, and i have just come accross the following 3 questions, i am not familiar with the functions, therefore i would appreciate any help. !! i have already created painting 2 & 3 all i need is the 3 functions, for the three different tasks as shown below. the functions are the showPic, sizeRaw and isPic. questions are: 1) – Define showPic :: Canvas -> IO () for displaying pictures on theterminal. Examples: Cw2010> showPic (painting 2)+---+---+---+---+---+| a | n | e | x | a |+---+---+---+---+---+| m | p | l | e | t |+---+---+---+---+---+| e | x | t | i | s |+---+---+---+---+---+| s | h | o | w | n |+---+---+---+---+---+ Cw2010> showPic (painting 3)+---+---+---+---+---+---+---+| A | n | o | t | h | e | r |+---+---+---+---+---+---+---+| | t | e | s | t | ! | |+---+---+---+---+---+---+---+ 2) – Furthermore, define sizeRaw :: Canvas -> (Int, Int) for findingout the size of a raw picture as demonstrated here.Cw2010> sizeRaw ["anexa","mplet","extis","shown"](5,4) 3) – Finally, define isPic :: Canvas -> Bool for checking a particularnecessary condition which items of type Canvas must satisfy (full orraw). The condition you should check for is whether all the ‘rows’ ofthe input have the same length. (It may be assumed that the inputof isPic is of type Canvas.) Examples: Cw2010> isPic (painting 3)TrueCw2010> isPic (painting 4)False Suggestion. I found it useful in my implementation to define and usehere the auxiliary function isEqual :: [Int] -> Bool; example: Cw2008> isEqual [8,8,8,8]TrueCw2008> isEqual [8,8,4,8]FalseYou may wish to define and use isEqual in your implementation ofisPic too.

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Haskellers.com skills list moderation?
by Daniel Peebles 21 Oct '10

21 Oct '10

Hi all, Might it be worthwhile to take the elected "superusers" on haskellers.comand let them police the skills list? It's become rather messy, with overly broad terms like "Mathematics" in it, as well as overly specific ones like "Other languages I know: C# .NET, XSLT, Microsoft SQL Server, XML, SQL, CSS, C, C++, Java, HTML, Visual Basic Script, Pascal, Rexx, Basic and assembler". Dan

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Should Yesod.Mail be a separate package?
by Michael Snoyman 21 Oct '10

21 Oct '10

Hey all, I wrote a simple email module in Yesod[1] that handles such things as multipart messages and Base64 encoding. It's still missing some features (multipart/alternative, for instance), but it can be useful for throwing together emails. It's currently part of the yesod package, but I'm going to be moving it to a separate package to free me up to make breaking API changes more frequently. As of right now, I'm just going to move it into the yesod-auth package (also being split off from the main yesod package), and therefore it will still have all the dependencies on Yesod. My question is whether people would find this package useful outside the scope of Yesod. There are no dependencies from this module onto any Yesod-specific stuff, so this separation could easily be done. I just don't feel like adding another package to maintain if no one is interested. So if anyone wants this offered up as a separate package, and/or has any API suggestions, please let me know. Michael [1] http://hackage.haskell.org/packages/archive/yesod/0.5.4/doc/html/Yesod-Mail…

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Is it possible to "easily" connect Haskell to JavaScript/JavaFX in the browser and use a browser as a Windows GUI? :)
by Dimitry Golubovsky 21 Oct '10

21 Oct '10

Hi, Atze Dijkstra wrote: [skip] >but to provide a proper interface to XML/HTML DOM (see http://www.haskell.org/haskellwiki/Haskell_in_web_browser) or a GUI abstraction around it (perhaps a subset of wxHaskell?) >will take more work & time. I hope this can be done as part of a studentproject, or maybe picked up during a Hackathon. I think a big portion of this task might be done using this package: http://hackage.haskell.org/package/webidl or its newer (not published on Hackage, but recommended) version split into two (you need both): http://code.haskell.org/yc2js/webidl/ -- lexer (FFI based), parser, and a test program.* http://code.haskell.org/yc2js/webidlsyn/ -- just AST and pretty-printer I must admit: I haven't touched this project for more than a year, and have little plans to do so: it is up for grabs by anyone interested. But if it can parse something like this: http://es-operating-system.googlecode.com/svn/trunk/include/w3c/html5.idl or http://es-operating-system.googlecode.com/svn/trunk/include/w3c/svg.idl it's likely still intact. Back in those days it could. A WebIDL converter helps derive type signatures for DOM methods and attributes (just like HSFFIG does this for C function prototypes). Have fun ;) and don't hesitate to ask questions. PS I'm glad that the idea lives on. With more GHC flrxibility on backends than it was in 2006-2007, and with faster Javascript engines in browsers, this can get a second life... ------------------------------ * the whole repo has to be downloaded: darcs get http://code.haskell.org/yc2js/ -- Dimitry Golubovsky Anywhere on the Web

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