March 2010 - Haskell-Cafe

Books for "advanced" Haskell
by Günther Schmidt 08 Mar '10

08 Mar '10

Hi all, there seems to be a huge number of things that monads can be used for. And there are lots of papers, blog posts, etc. describing that, some more or less accessible. Apart from monads there are of course also Applicative Functors, Monoids, Arrows and what have you. But in short the Monad thingy seems to be the most "powerful" one of them all. Is there a book that specializes on Monads? A Haskell-Monad book? Günther

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Haskell Weekly News: Issue 152 - March 08, 2010
by jfredett＠gmail.com 08 Mar '10

08 Mar '10

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20100308 Issue 152 - March 08, 2010 --------------------------------------------------------------------------- Welcome to issue 152 of HWN, a newsletter covering developments in the [1]Haskell community. Hello Haskellers, this week brings updates to some standards (like Parsec), as well as some new tunes to the Haskell songbook. WAI and Yesod were released, giving some new options for Haskell-based web applications, Don Stewart made some improvements to his Genetic Optimizer system to include options from GHC's new LLVM backend, and I made some serious progress on the new HWN software. Speaking of the new software, there have been numerous reports of warts and other unfortunate things with the current editions of the HWN. Fear not, these warts are well known and being worked on intently. Unfortunately, it's somewhat difficult to fix them given the current publication software, however, in the new-and-improved version that (hopefully) should be entering use soon, these warts will be removed. So, without further interuption about the stylistic content, Haskellers, your Haskell Weekly News! Announcements Bluetile 0.3.1. Jan Vornberger [2]announced Bluetile, an adaptation of xmonad for the masses. Bluetile aims to make tiling window managers accessible by drawing on standard window manager conventions and idioms. ED: This is not technically an announcement, but it's very cool and everyone should check it out! yesod 0.0.0 (web framework). Michael Snoyman [3]announced the release of yesod, a web framework based on WAI. wai-0.0.0 (Web Application Interface). Michael Snoyman [4]announced the first release of wai, a library implementing the WAI protocol. iteratee-parsec 0.0.2. Maciej Piechotka [5]announced iteratee-parsec, a library which allows for parsec parsers in the IterateeG monad. Parsec 3.1.0. Derek Elkins [6]announced a new release of Parsec, containing many improvments due (primarily) to Antoine Latter. Progression-0.3 (supporting benchmarking in Haskell). Neil Brown [7]announced a new release of Progression. Progression is a utility built on top of Criterion for recording benchmarks for several version of your code. Ghent Functional Programming Group: First Meeting on April 1, 2010. Jeroen Janssen [8]announced the first meeting of the Ghent Functional Programming group, see the post for location and time details. PhD position in Functional Programming at Chalmers (deadline 2010-04-07). Patrik Jansson [9]announced an opening for a PhD student at Chalmers, working in Functional Programming and DSELs. VSTTE 2010: Verified Software -- Third Call for Papers. Gudmund Grov [10]announced a third CFP for VSTTE 2010, the Third International Conference on Verified Software: Theories, Tools, and Experiments. barchart-0.1.1. Sebastian Fischer [11]announced barchart, a command line program with an associated library for generating bar charts. Call for Participation: CHR Summer School. Jon Sneyers [12]announced a call for participation in the CHR Summer School on Programming and Reasoning with Rules and Constraints. Discussion Free theorem - what do we need it for. Maciej Piechotka [13]asked about Free Theorems and what they were good for. Has anybody translated Douglas Hofstadter's Scientific American articles introducting Scheme to a general audience into Haskell? Benjamin L. Russell [14]asked if anyone had considered translating Douglas Hofstadter's SciAm articles introducting Scheme to Haskell. Evolving Faster Haskell Programs (now with LLVM!). Don Stewart [15]our resident speed demon is at it again, harnessing the new LLVM backend of GHC and the power of Natural Selection to make his programs go even faster. Blog noise [16]Haskell news from the [17]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Neil Brown: [18]Synchronous Channels using MVars. * "FP Lunch": [19]Causal Semantics of (A)FRP. * Darcs: [20]darcs weekly news #56. * Dan Piponi (sigfpe): [21]Products, Limits and Parametric Polymorphism. * Bryn Keller: [22]Real World Haskell. * Isaac Dupree: [23]Next Project, Move doc-parsing to from GHC to Haddock. * Isaac Dupree: [24]Summer of Code Wrap-Up.. * Isaac Dupree: [25]Status update. * Isaac Dupree: [26]platform progress. * Isaac Dupree: [27]another boring update, & question :-). * Isaac Dupree: [28]How To Navigate Your Code:. * Michael Snoyman: [29]Persistence. * Neil Brown: [30]Choice over Events using STM. * Michael Snoyman: [31]Tweedle Beetle Battle. * Galois, Inc: [32]The Semantics of Asynchronous Exceptions. * Bryan O'Sullivan: [33]What's in a parser? Attoparsec rewired (2/2). * Bryan O'Sullivan: [34]What's in a parsing library? (1/2). * Neil Brown: [35]Progression 0.3: Bar Charts and Normalisation. Quotes of the Week * edwardk: heh, i'm amused the ParsecT that Antoine Latter put together is in the form of a right kan extension ;) * Philippa: <TR1N1TY> I'm selling botz and backdoors <Philippa> I'm selling bans * samlee (via reddit): haha those ghc options are longer than actual program About the Haskell Weekly News New editions are posted to [36]the Haskell mailing list as well as to [37]the Haskell Sequence and [38]Planet Haskell. [39]RSS is also available, and headlines appear on [40]haskell.org. To help create new editions of this newsletter, please see the information on [41]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [42]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://www.bluetile.org/ 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/71314 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/71184 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/71144 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/71122 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/71041 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70981 9. http://article.gmane.org/gmane.comp.lang.haskell.general/17842 10. http://article.gmane.org/gmane.comp.lang.haskell.general/17841 11. http://article.gmane.org/gmane.comp.lang.haskell.general/17831 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17825 13. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/71117 14. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/71022 15. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/70967 16. http://planet.haskell.org/ 17. http://haskell.org/haskellwiki/Blog_articles 18. http://chplib.wordpress.com/2010/03/08/synchronous-channels-using-mvars/ 19. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=369 20. http://blog.darcs.net/2010/03/darcs-weekly-news-56.html 21. http://blog.sigfpe.com/2010/03/products-limits-and-parametric.html 22. http://www.xoltar.org/?p=36 23. http://haddock2009.wordpress.com/2009/07/21/next-project-move-doc-parsing-t… 24. http://haddock2009.wordpress.com/2009/08/26/summer-of-code-wrap-up/ 25. http://haddock2009.wordpress.com/2009/05/12/status-update/ 26. http://haddock2009.wordpress.com/2009/05/20/platform-progress/ 27. http://haddock2009.wordpress.com/2009/05/26/another-boring-update-question/ 28. http://haddock2009.wordpress.com/2009/06/23/how-to-navigate-your-code/ 29. http://www.snoyman.com/blog/entry/persistence/ 30. http://chplib.wordpress.com/2010/03/04/choice-over-events-using-stm/ 31. http://www.snoyman.com/blog/entry/tweedle-beetle-battle/ 32. http://www.galois.com/blog/2010/03/04/the-semantics-of-asynchronous-excepti… 33. http://www.serpentine.com/blog/2010/03/03/whats-in-a-parser-attoparsec-rewi… 34. http://www.serpentine.com/blog/2010/03/03/whats-in-a-parsing-library-1/ 35. http://chplib.wordpress.com/2010/03/02/progression-0-3-bar-charts-and-norma… 36. http://www.haskell.org/mailman/listinfo/haskell 37. http://sequence.complete.org/ 38. http://planet.haskell.org/ 39. http://sequence.complete.org/node/feed 40. http://haskell.org/ 41. http://haskell.org/haskellwiki/HWN 42. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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Re: [Haskell] Recursive definition of fibonacci with Data.Vector
by Henning Thielemann 08 Mar '10

08 Mar '10

On Sun, 7 Mar 2010, Edgar Z. Alvarenga wrote: > Hello, > > why I can't define a recursive vector using Data.Vector, like in > the example: > > import qualified Data.Vector as V > > let fib = 0 `V.cons` (1 `V.cons` V.zipWith (+) fib (V.tail v)) Since I liked to have both element-wise lazy construction and packed data, I created a data-structure for that purpose: http://code.haskell.org/storablevector/Data/StorableVector/Cursor.hs However, those vectors/arrays are finite, but could be extended to a chunky infinite structure.

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ghc leaking memory?
by Joachim Breitner 08 Mar '10

08 Mar '10

Hi, in Debian, we are having some problems building large libraries (such as agda, highlighting-kate, xmonad-contrib) on weaker architectures (sparc, armel, s390) which compile with -fvia-C, especially when building the profiling libraries. In the case of highlighting-kate, there were really issues with the code: A huge list of words which ghc seemed to try to inline caused it consuming a lot of memory. But in the case of agda, it even fails with relatively small modules: https://buildd.debian.org/fetch.cgi?pkg=agda&arch=sparc&ver=2.2.6-3&stamp=1… Now, looking at the memory gauge on my computer while compiling such a program with Cabal (which compiles all modules in one ghc call), it looks like it is steadily increasing. I’d expect ghc to free a lot of memory when it is done with a module, but this does not seem to be the case. Is this a bug (i.e. memory leak) in ghc or expected behavior? Is there a way to make Cabal build each module on its own? Are there any fixes or work-arounds? Greetings, Joachim -- Joachim "nomeata" Breitner Debian Developer nomeata(a)debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata(a)joachim-breitner.de | http://people.debian.org/~nomeata

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how to write a function to send a string to a tuple
by Pradeep Wickramanayake 08 Mar '10

08 Mar '10

Hi, Im having problems with sending a string to a tuple. My string contains integers and strings The whole string stay as ("test,dfdf",3,"dfsf") sortList2 :: String -> String sortList2 (x:xs) | x == ',' = "" | otherwise = [x] ++ sortList2 xs The above function separating each words from the string Now I need to put them to a tuple putList :: String -> (Int, String, String, Int, Int) putList (x:xs) |xs /="" = sortList2 ++ putList xs I wrote something like above. But I really have no clue how to continue. Can someone help me. It's a big help

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Haskell platform for GHC 6.12.1?
by Peter Verswyvelen 08 Mar '10

08 Mar '10

Using GHC 6.12.1 on Windows currently is hard, since one must compile the latest version of cabal-install, which is a nightmare to do for a typical windows user (install mingw, msys, utils like wget, download correct package from hackage, compile them in correct order, etc etc) What's the status of the Haskell platform for the latest and greatest Glasgow Haskell Compiler?

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Re: [Haskell] Recursive definition of fibonacci with Data.Vector
by Don Stewart 07 Mar '10

07 Mar '10

edgar: > Hello, > > why I can't define a recursive vector using Data.Vector, like in > the example: > > import qualified Data.Vector as V > > let fib = 0 `V.cons` (1 `V.cons` V.zipWith (+) fib (V.tail v)) > There's a typo: fib = 0 `V.cons` (1 `V.cons` V.zipWith (+) fib (V.tail fib)) Which let's it typecheck. But I don't think this is a sensible use of vectors. In fact, infinite vectors make no sense, as far as I can tell -- these are fundamentally bounded structures. GHC even optimizes it to: fib = fib (literally!). $ time ./A A: <<loop>> -- Don

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GHC's parallel garbage collector -- what am I doing wrong?
by Michael Lesniak 07 Mar '10

07 Mar '10

Hello haskell-cafe, Sorry for this long post, but I can't think of a way to describe and explain the problem in a shorter way. I've (again) a very strange behaviour with the parallel GC and would be glad if someone could either reproduce (and explain) it or provide a solution. A similar but unrelated problem has been described in [1]. EXAMPLE CODE The following demonstration program, which is a much smaller and single-threaded version of my real problem behaves as my real program. It does some number crunching by calculating pi to a definable precision: > -- File Pi.hs > -- you need the numbers package from hackage. > module Main where > import Data.Number.CReal > import System.Environment > import GHC.Conc > > main = do > digits <- (read . head) `fmap` getArgs :: IO Int > calcPi digits > > calcPi digits = showCReal (fromEnum digits) pi `pseq` return () Compile it with ghc --make -threaded -O2 Pi.hs -o pi BENCHMARKS On my two-core machine I get the following quite strange and unpredictable results: * Using one thread: $ for i in `seq 1 5`;do time pi 5000 +RTS -N1;done real 0m1.441s user 0m1.390s sys 0m0.020s real 0m1.449s user 0m1.390s sys 0m0.000s real 0m1.399s user 0m1.370s sys 0m0.010s real 0m1.401s user 0m1.380s sys 0m0.000s real 0m1.404s user 0m1.380s sys 0m0.000s * Using two threads, hence the parallel GC is used: for i in `seq 1 5`;do time pi 5000 +RTS -N2;done real 0m2.540s user 0m2.490s sys 0m0.010s real 0m1.527s user 0m1.530s sys 0m0.010s real 0m1.966s user 0m1.900s sys 0m0.010s real 0m5.670s user 0m5.620s sys 0m0.010s real 0m2.966s user 0m2.910s sys 0m0.020s * Using two threads, but disabling the parallel GC: for i in `seq 1 5`;do time pi 5000 +RTS -N2 -qg;done real 0m1.383s user 0m1.380s sys 0m0.010s real 0m1.420s user 0m1.360s sys 0m0.010s real 0m1.406s user 0m1.360s sys 0m0.010s real 0m1.421s user 0m1.380s sys 0m0.000s real 0m1.360s user 0m1.360s sys 0m0.000s THREADSCOPE I've additionally attached the threadscope profile of a really bad run, started with $ time pi 5000 +RTS -N2 -ls real 0m15.594s user 0m15.490s sys 0m0.010s as file pi.pdf FURTHER INFORMATION/QUESTION Just disabling the parallel GC leads to very bad performance in my original code, which forks threads with forkIO and does a lot of communications. Hence, using -qg is not a real option for me. Do I have overlooked some cruical aspect of this problem? If you've read this far, thank you for reading ... this far ;-) Cheers, Michael [1] http://osdir.com/ml/haskell-cafe@haskell.org/2010-02/msg00850.html -- Dipl.-Inf. Michael C. Lesniak University of Kassel Programming Languages / Methodologies Research Group Department of Computer Science and Electrical Engineering Wilhelmshöher Allee 73 34121 Kassel Phone: +49-(0)561-804-6269

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GPL answers from the SFLC (WAS: Re: ANN: hakyll-0.1)
by Tom Tobin 07 Mar '10

07 Mar '10

After politely pestering them again, I finally heard back from the Software Freedom Law Center regarding our GPL questions (quoted below). I exchanged several emails to clarify the particular issues; in short, the answers are "No", "No", "N/A", and "N/A". The SFLC holds that a library that depends on a GPL'd library must in turn be GPL'd, even if the library is only distributed as source and not in binary form. They offered to draft some sort of explicit response if we'd find it useful. Maybe it would be useful if Cabal had some sort of licensing check command that could be run on a .cabal file, and warn an author if any libraries it depends on (directly or indirectly) are GPL'd but the .cabal itself does not have the license set to GPL. On Fri, Dec 11, 2009 at 10:21 PM, Tom Tobin <korpios(a)korpios.com> wrote: > I'd like to get these questions out to the SFLC so we can satisfy our > curiosity; at the moment, here's what I'd be asking: > > Background: X is a library distributed under the terms of the GPL. Y > is another library which calls external functions in the API of X, and > requires X to compile. X and Y have different authors. > > 1) Can the author of Y legally distribute the *source* of Y under a > non-GPL license, such as the 3-clause BSD license or the MIT license? > > 2) If the answer to 1 is "no", is there *any* circumstance under which > the author of Y can distribute the source of Y under a non-GPL > license? > > 3) If the answer to 1 is "yes", what specifically would trigger the > redistribution of a work in this scenario under the GPL? Is it the > distribution of X+Y *together* (whether in source or binary form)? > > 4) If the answer to 1 is "yes", does this mean that a "BSD-licensed" > library does not necessarily mean that closed-source software can be > distributed which is based upon such a library (if it so happens that > the library in turn depends on a copylefted library)? > > By the way, apologies to the author of Hakyll — I'm sure this isn't > what you had in mind when you announced your library! I'm just hoping > that we can figure out what our obligations are based upon the GPL, > since I'm not so sure myself anymore. >

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cabal install darcs fails on Mac OS X Snow Leopard
by George Colpitts 07 Mar '10

07 Mar '10

I'm unable to install darcs using cabal. It seems to be getting an error configuring haskeline.Can anybody help? Thanks cabal -V cabal-install version 0.6.4 using version 1.6.0.3 of the Cabal library bash-3.2$ ghc -V The Glorious Glasgow Haskell Compilation System, version 6.10.4 bash-3.2$ cabal install darcs Resolving dependencies... [1 of 1] Compiling Main ( /var/folders/Gj/Gj20S2zmFESpNdOqFfUlq++++TI/-Tmp-/haskeline-0.6.2.26747/haskeline-0.6.2.2/Setup.hs, /var/folders/Gj/Gj20S2zmFESpNdOqFfUlq++++TI/-Tmp-/haskeline-0.6.2.26747/haskeline-0.6.2.2/dist/setup/Main.o ) Linking /var/folders/Gj/Gj20S2zmFESpNdOqFfUlq++++TI/-Tmp-/haskeline-0.6.2.26747/haskeline-0.6.2.2/dist/setup/setup ... Configuring haskeline-0.6.2.2... checking whether to use -liconv... using -liconv. *Preprocessing library haskeline-0.6.2.2...* */usr/bin/hsc2hs: line 16: unexpected EOF while looking for matching `"'* */usr/bin/hsc2hs: line 17: syntax error: unexpected end of file* Configuring zlib-0.5.2.0... Preprocessing library zlib-0.5.2.0... /usr/bin/hsc2hs: line 16: unexpected EOF while looking for matching `"' /usr/bin/hsc2hs: line 17: syntax error: unexpected end of file cabal: Error: some packages failed to install: darcs-2.4 depends on zlib-0.5.2.0 which failed to install. hashed-storage-0.4.7 depends on zlib-0.5.2.0 which failed to install. haskeline-0.6.2.2 failed during the building phase. The exception was: exit: ExitFailure 2 zlib-0.5.2.0 failed during the building phase. The exception was: exit: ExitFailure 2 When I installed ghc on Snow Leopard I followed the instructions at http://www.haskell.org/haskellwiki/Mac_OS_X

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