July 2009 - Haskell-Cafe

Haskell Zippers on Wikibooks: teasing! :)
by Peter Verswyvelen 18 Jul '09

18 Jul '09

After my colleague explained me about zippers and how one could derive the datatype using differential rules, I had to read about it. So I started reading http://en.wikibooks.org/wiki/Haskell/Zippers#Mechanical_Differentiation This page contains the sentence: *"For a systematic construction, we need to calculate with types. The basics of structural calculations with types are outlined in a separate chapter **Generic Programming*<http://en.wikibooks.org/w/index.php?title=Haskell/Generic_Programming&actio…> * and we will heavily rely on this material"* * * However, the generic programming link does not exist yet :-) So although I now have a rough idea about it, I don't understand the details yet, e.g. notation like [image: \mathit{Node}\,A = \mu X.\,\mathit{NodeF}_A\,X] doesn't make much sense to me yet, although I suspect I can read the mu as a lambda on types? Thanks, Peter * *

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uncommon IMO problem - toilet management
by Henning Thielemann 18 Jul '09

18 Jul '09

In the last two days I was invigilator at the International Mathematic Olympics 2009 in Bremen, Germany. There we got a problem different from the official math problems. :-) Eventually I solved it using Haskell. Read the detailed description at http://hackage.haskell.org/package/toilet-0.0.1 I think the problem is simple enough to be used in education of programming. (I also think it would have been better to avoid monads and use lazy list processing.)

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Haskell Weekly News: Issue 126 - July 18, 2009
by Brent Yorgey 18 Jul '09

18 Jul '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090718 Issue 126 - July 18, 2009 --------------------------------------------------------------------------- Welcome to issue 126 of HWN, a newsletter covering developments in the [1]Haskell community. [2]Hac phi is next weekend! With almost 30 people already registered, it looks like we're going to have a fantastic time hacking in Philadelphia. It's still not too late to [3]register! Announcements GHC 6.10.4. Ian Lynagh [4]announced a new patchlevel release of GHC, 6.10.4. This version has very few changes over 6.10.3, but fixes some bugs that could be critical for a few users. See the [5]release notes for details. shelltestrunner 0.6 released. Simon Michael [6]announced the first release of [7]shelltestrunner, a small tool for testing any command-line program by running it through "shell" tests defined with a simple file format. generator 0.5.1. Yair Chuchem [8]announced the release of the [9]generator package, which implements an alternative list monad transformer, a list class, and related functions. GLURaw 1.0.0.0. Sven Panne [10]announced a new [11]GLURaw package, containing full support for all GLU functionality and similar in spirit to the OpenGLRaw package: it is a 1:1 mapping of the C interface, no libraries or headers are needed at build time, and the GLU API entries are resolved dynamically at runtime. OpenGLRaw 1.0.1.0. Sven Panne [12]announced a new version of the [13]OpenGLRaw package, which adds support for a number of OpenGL extensions. ObjectName 1.0.0.0. Sven Panne [14]announced a (tiny) new package, [15]ObjectName, which contains a class corresponding to the general notion of explicitly handled identifiers for API objects, e.g. a texture object name in OpenGL or a buffer object name in OpenAL. StateVar 1.0.0.0. Sven Panne [16]announced the [17]StateVar package, which further modularizes the OpenGL/OpenAL packages. It implements state variables, which are references in the IO monad, like IORefs or parts of the OpenGL state. data-ordlist-0.0.1 and NumberSieves-0.0. Leon Smith [18]announced the release of two new packages: [19]Data.OrdList offers a convenient way for efficiently dealing with lists that you happen to know are ordered, and includes operations such as union, merge, exclusive union, intersection, and difference. [20]NumberSieves includes the Sieve of O'Neill, from "The Geniune Sieve of Eratosthenes" by Melissa O'Neill, which offers an incremental primality sieve based on priority queues. Also included are two array-based generalizations of the Sieve of Eratosthenes: one for factoring a large quantity of small numbers, and another for calculating the phi function for a large quantity of small numbers. graphviz-2999.0.0.0. Ivan Lazar Miljenovic [21]announced a new release of the [22]graphviz package for Haskell, which provides bindings to the [23]GraphViz suite of tools. The biggest and most important change in this release is that all 152 attributes utilised/supported by GraphViz are now specified and supported. uncommon IMO problem - toilet management. Henning Thielemann [24]announced a [25]Haskell package for managing toilet use at the International Mathematical Olympiad. darcs 2.3 beta 4. Petr Rockai [26]announced another darcs 2.3 beta release, which features better Windows support. If you're on Windows, you should be able to install it with 'cabal install darcs-beta' -- give it a try! Google Summer of Code Progress updates from participants in the 2008 [27]Google Summer of Code. space profiling. Gergely Patai has been working on a [28]heap profile manager. fast darcs. Petr Rockai put out another [29]another darcs 2.3 beta release, and made a [30]bunch of other progress including getting darcs up and running on win32, working on hashed-storage, and optimizing 'darcs show contents'. Discussion is closing a class this easy? Conor McBride [31]asked for feedback on some code intended to effectively create a closed type class. laziness blowup exercise. Thomas Hartman [32]challenged readers to squash a memory leak. Blog noise [33]Haskell news from the [34]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Gergely Patai: [35]Introducing the heap profile manager. * FP Lunch: [36]Folding Statistics. * Petr Rockai: [37]soc progress 8. * Greg Bacon: [38]Monadic takeWhile. * Petr Rockai: [39]darcs 2.3 beta 4. * David Amos: [40]Counting symmetries using transversals. * Magnus Therning: [41]XML prettifier in Haskell. * Petr Rockai: [42]soc progress 7. Quotes of the Week * Berengal: For me, understanding the basics/reasoning behind haskell's type system was just a minute meditating on the phrase "what's the square root of hello?" * bitwize: The oleg is to functional studliness as the farad is to capacitance: a hopelessly large base unit. * maartenm: euclidate: to promote a conjecture to an axiom just for the sake of simplicity * RobertGreaye: Some suggest the original English remained in Britain when the North American colonies were founded; others claim it was brought to the Americas by the British settlers, leaving a pale imitation back in Britain. The truth is much stranger: the original English was actually smuggled out of Britain to the West Indies in a wardrobe belonging to General Sir Ralph Abercromby, where it ended up on the island of Trinidad after Sir Ralph took possession of that territory in the name of the British Crown. About the Haskell Weekly News New editions are posted to [43]the Haskell mailing list as well as to [44]the Haskell Sequence and [45]Planet Haskell. [46]RSS is also available, and headlines appear on [47]haskell.org. To help create new editions of this newsletter, please see the information on [48]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [49]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://haskell.org/haskellwiki/Hac_%CF%86 3. http://haskell.org/haskellwiki/Hac_%CF%86/Register 4. http://www.haskell.org//pipermail/haskell/2009-July/021508.html 5. http://haskell.org/ghc/docs/6.10.4/html/users_guide/release-6-10-4.html 6. http://article.gmane.org/gmane.comp.lang.haskell.general/17367 7. http://hackage.haskell.org/package/shelltestrunner 8. http://article.gmane.org/gmane.comp.lang.haskell.general/17366 9. http://hackage.haskell.org/package/generator 10. http://article.gmane.org/gmane.comp.lang.haskell.general/17364 11. http://hackage.haskell.org/package/GLURaw 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17363 13. http://hackage.haskell.org/package/OpenGLRaw 14. http://article.gmane.org/gmane.comp.lang.haskell.general/17362 15. http://hackage.haskell.org/package/ObjectName 16. http://article.gmane.org/gmane.comp.lang.haskell.general/17361 17. http://hackage.haskell.org/package/StateVar 18. http://article.gmane.org/gmane.comp.lang.haskell.general/17355 19. http://hackage.haskell.org/package/data-ordlist 20. http://hackage.haskell.org/package/NumberSieves 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61494 22. http://hackage.haskell.org/package/graphviz 23. http://www.graphviz.org/ 24. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61448 25. http://hackage.haskell.org/package/toilet-0.0.1 26. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61302 27. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 28. http://just-bottom.blogspot.com/2009/07/introducing-heap-profile-manager.ht… 29. http://web.mornfall.net/blog/darcs_2.3_beta_4.html 30. http://web.mornfall.net/blog/soc_progress_8.html 31. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/61456 32. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/61283 33. http://planet.haskell.org/ 34. http://haskell.org/haskellwiki/Blog_articles 35. http://just-bottom.blogspot.com/2009/07/introducing-heap-profile-manager.ht… 36. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=232 37. http://web.mornfall.net/blog/soc_progress_8.html 38. http://feedproxy.google.com/~r/gbacon/~3/Q-Y-KapB8iI/monadic-takewhile.html 39. http://web.mornfall.net/blog/darcs_2.3_beta_4.html 40. http://haskellformaths.blogspot.com/2009/07/counting-symmetries-using-trans… 41. http://therning.org/magnus/archives/695 42. http://web.mornfall.net/blog/soc_progress_7.html 43. http://www.haskell.org/mailman/listinfo/haskell 44. http://sequence.complete.org/ 45. http://planet.haskell.org/ 46. http://sequence.complete.org/node/feed 47. http://haskell.org/ 48. http://haskell.org/haskellwiki/HWN 49. http://code.haskell.org/~byorgey/code/hwn/

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cabal version issue
by Vasili I. Galchin 18 Jul '09

18 Jul '09

Hello, vigalchin@ubuntu:~/FTP/Haskell/06052009.Swish-0.2.1$ cabal --version cabal-install version 0.5.1 using version 1.4.0.1 of the Cabal library vigalchin@ubuntu:~/FTP/Haskell/Swish-0.2.1$ cabal configure --user --prefix=$HOME Warning: swish.cabal: A package using section syntax should require "Cabal-Version: >= 1.2" or equivalent. Warning: swish.cabal: A package using section syntax should require "Cabal-Version: >= 1.2" or equivalent. What is the "Cabal-Version" in the latter? Why I am getting this warning? Thanks, Vasili

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Importance of identity element in finger trees?
by Daniel Peebles 18 Jul '09

18 Jul '09

Hi all, I was looking at finger trees and the tricks for getting priority queues out of them seemed a little hackish, with a distinguished infinity element or maxBound. But it seems (although I have not yet tried it) like in many cases the monoid's identity element wouldn't be necessary (a bit like the difference between fold* and fold*1). Could a finger tree be applied to an arbitrary semigroup? Or is the identity more fundamental than it looks? Thanks, Dan

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catchSTM and asynchronous exceptions
by Peter Robinson 18 Jul '09

18 Jul '09

I couldn't find any information on whether catchSTM catches asynchronous exceptions so I tried to run the following: import Control.Concurrent.STM import Control.Concurrent import Control.Exception import Prelude hiding (catch) test = do tid <- myThreadId forkIO (threadDelay 5000000 >> throwTo tid (AssertionFailed "Exception in forked thread!")) (atomically $ retry `catchSTM` stmHandler) `catch` (ioHandler tid) where stmHandler (e::SomeException) = throw $ AssertionFailed ("Caught Exc. in STM; Rethrowing exception: "++ show e) ioHandler tid (e::SomeException) = print (tid,"Caught Exception in IO: ",e) Yielding the following output: # (ThreadId 6942,"Caught Exception in IO: ",Exception in forked thread!) Apparently the exception is caught by "catch" and not by "catchSTM". So the point is that catchSTM is only meant to be used for non-async exceptions, right? Regards, Peter

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Plot data reconstruction reading pixel colors from image files
by Cetin Sert 18 Jul '09

18 Jul '09

Hi, How can I open and read colors of specific pixels of an image file in Haskell? Which packages, functions do you recommend? You can have a look at the quoted conversation for an idea on what I would like to automate. Best Regards, Cetin Sert > Hi dear *^o^*, > > Here are the points from Figure 37 > > [SEE ATTACHED PNG] > > Cisse > Goni > Kodougou > Nouna > normals > log10s > normals > log10s > normals > log10s > normals > log10s > 0.7999999999999998 > 0.5249999999999999 > 0.4249999999999998 > 0.375 > 0.41249999999999964 > 0.4249999999999998 > 0.8500000000000001 > 1.575 > 1.5999999999999999 > 1.325 > 1.0499999999999998 6.30957344480193 > 3.3496543915782757 > 2.6607250597988084 > 2.371373705661655 > 2.5852348395621885 > 2.6607250597988084 > 7.07945784384138 > 37.583740428844415 > 39.81071705534971 > 21.134890398366466 > 11.220184543019629 -0.5625 > -0.3250000000000002 > 0.16249999999999964 > 0.3374999999999999 > 0.5 > 0.8500000000000001 > 1.4874999999999998 > 2.1375 > 2.2625 > 1.875 > 1.0 0.27384196342643613 > 0.4731512589614803 > 1.4537843856076607 > 2.1752040340195222 > 3.1622776601683795 > 7.07945784384138 > 30.725573652674456 > 137.24609610075626 > 183.02061063110568 > 74.98942093324558 > 10.0 0.7250000000000001 > 0.34999999999999964 > 0.3374999999999999 > 0.5 > 0.625 > 0.6374999999999997 > 0.7624999999999997 > 1.2249999999999999 > 1.5125 > 1.2125 > 0.625 5.308844442309884 > 2.2387211385683377 > 2.1752040340195222 > 3.1622776601683795 > 4.216965034285822 > 4.340102636447436 > 5.787619883491203 > 16.788040181225597 > 32.546178349804585 > 16.31172909227838 > 4.216965034285822 0.3374999999999999 > 0.13749999999999973 > 0.375 > 0.6499999999999999 > 0.5749999999999997 > 0.47499999999999964 > 0.9874999999999998 > 1.825 > 2.025 > 1.4749999999999999 > 0.7624999999999997 2.1752040340195222 > 1.372460961007561 > 2.371373705661655 > 4.46683592150963 > 3.7583740428844394 > 2.9853826189179573 > 9.716279515771058 > 66.83439175686145 > 105.92537251772886 > 29.853826189179586 > 5.787619883491203 > Top to Bottom in the table is Left to Right in the figure. > > > Lots of love, > CS ;-) > > P/S: Sorry it took a bit longer than expected but it was lots of fun! > > ------------------------------------------------------------------- > > > > module Main where > > import Control.Monad > > f x = 3 - (x / 80) -- 80: number of pixels > d x = x - 2 -- pixel offset > > cisse, goni, kodou, nouna :: [Double] > cisse = [178,200,208,212,209,208,174,116,114,136,158] > goni = [287,268,229,215,202,174,123,71 ,61 ,92 ,162] > kodou = [184,214,215,202,192,191,181,144,121,145,192] > nouna = [215,231,212,190,196,204,163,96 ,80 ,124,181] > > disp :: (String, [Double]) → IO () > disp (town,pixels) = do > putStrLn $ town > putStrLn $ ">normals" > mapM_ print $ points > putStrLn $ ">log10s" > mapM_ print $ log10s > putStrLn $ "-------------------" > where > points = map (f . d) pixels > log10s = map (10 **) points > > main :: IO () > main = do > mapM_ disp [("Cisse", cisse),("Goni", goni),("Kodougou", kodou),("Nouna", > nouna)] > > > > -------------------- > > Cisse > >normals > 0.7999999999999998 > 0.5249999999999999 > 0.4249999999999998 > 0.375 > 0.41249999999999964 > 0.4249999999999998 > 0.8500000000000001 > 1.575 > 1.5999999999999999 > 1.325 > 1.0499999999999998 > >log10s > 6.30957344480193 > 3.3496543915782757 > 2.6607250597988084 > 2.371373705661655 > 2.5852348395621885 > 2.6607250597988084 > 7.07945784384138 > 37.583740428844415 > 39.81071705534971 > 21.134890398366466 > 11.220184543019629 > ------------------- > Goni > >normals > -0.5625 > -0.3250000000000002 > 0.16249999999999964 > 0.3374999999999999 > 0.5 > 0.8500000000000001 > 1.4874999999999998 > 2.1375 > 2.2625 > 1.875 > 1.0 > >log10s > 0.27384196342643613 > 0.4731512589614803 > 1.4537843856076607 > 2.1752040340195222 > 3.1622776601683795 > 7.07945784384138 > 30.725573652674456 > 137.24609610075626 > 183.02061063110568 > 74.98942093324558 > 10.0 > ------------------- > Kodougou > >normals > 0.7250000000000001 > 0.34999999999999964 > 0.3374999999999999 > 0.5 > 0.625 > 0.6374999999999997 > 0.7624999999999997 > 1.2249999999999999 > 1.5125 > 1.2125 > 0.625 > >log10s > 5.308844442309884 > 2.2387211385683377 > 2.1752040340195222 > 3.1622776601683795 > 4.216965034285822 > 4.340102636447436 > 5.787619883491203 > 16.788040181225597 > 32.546178349804585 > 16.31172909227838 > 4.216965034285822 > ------------------- > Nouna > >normals > 0.3374999999999999 > 0.13749999999999973 > 0.375 > 0.6499999999999999 > 0.5749999999999997 > 0.47499999999999964 > 0.9874999999999998 > 1.825 > 2.025 > 1.4749999999999999 > 0.7624999999999997 > >log10s > 2.1752040340195222 > 1.372460961007561 > 2.371373705661655 > 4.46683592150963 > 3.7583740428844394 > 2.9853826189179573 > 9.716279515771058 > 66.83439175686145 > 105.92537251772886 > 29.853826189179586 > 5.787619883491203 > ------------------- >

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Re: [Haskell-cafe] is closing a class this easy?
by Conor McBride 18 Jul '09

18 Jul '09

On 18 Jul 2009, at 01:43, Lennart Augustsson wrote: > As far as I know it works. It's an old Oleg trick. Then it probably does work. > The only drawback is that error messages may refer to Private. As I found out when probing its security. "No instance for Moo.Private" shows up. I guess that's what happens when you hide stuff: you get told what stuff's being hidden. In some situations, that's insecure, but here it's ok. Cheers (and have fun in China, Lennart!) Conor

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Re: [Haskell] ANN: data-ordlist-0.0.1 and NumberSieves-0.0
by Henning Thielemann 17 Jul '09

17 Jul '09

Leon Smith schrieb: > Two new packages have been uploaded to Hackage, one that implements bag > (multiset) and set operations on ordered lists, and another that offers > three different number theoretic sieves. > > http://hackage.haskell.org/package/data-ordlist > > Data.OrdList offers many of the same kinds of operations as Data.Set, > although Data.Set is likely to often be a better choice. However, > this library is not intended to be used as an abstract datatype for sets > and multisets, rather it is intended to be a convenient way for > efficiently dealing with lists that you happen to know are ordered. You could also implement that in terms of Map a Int.

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ANNOUNCE: first Grapefruit release
by Wolfgang Jeltsch 17 Jul '09

17 Jul '09

Dear friends of Haskell and Functional Reactive Programming, its my pleasure to announce the first official release of Grapefruit, a library for Functional Reactive Programming (FRP) with a focus on user interfaces. With Grapefruit, you can implement reactive and interactive systems in a declarative style. User interfaces are described as networks of communicating widgets and windows. Communication is done via different kinds of signals which describe temporal behavior. Grapefruit consists of five packages with several interesting features: grapefruit-frp: Functional Reactive Programming core * The FRP implementation uses a hybrid push/pull-based approach. * Signals can be memoized by binding them to variables – like ordinary data structures. * Merging of event streams combines simultaneous events properly. * Signals cannot behave differently by starting them at different times. The type system is used to enforce proper aging of signals. grapefruit-records: A record system for Functional Reactive Programming * Input records can specify fields as being required or optional. * Uninteresting output fields can be ignored. The set of interesting fields is specified by pattern matching. grapefruit-ui: Declarative user interface programming * The interface for UI programming is platform-independent and can be implemented on top of different UI toolkits. * A concrete implementation can be selected at compile time or runtime. grapefruit-ui-gtk: GTK+-based implementation of the general UI interface * Gtk2Hs is used internally. grapefruit-examples: examples demonstrating features of Grapefruit * Circular communication and switching are demonstrated. Support for the following is planned for the future: * signals with incremental updates * user interfaces with changing structure * more kinds of UI components * graphical animations Further information about Grapefruit is available on its wiki page at <http://haskell.org/haskellwiki/Grapefruit>. If you have questions, applause or criticism, please get in touch with me. Wolfgang Jeltsch Principal Grapefruit developer

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