April 2012 - Haskell-Cafe

Haskell Weekly News: Issue 222
by Daniel Santa Cruz 11 Apr '12

11 Apr '12

Welcome to issue 222 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 1 to 7, 2012. Quotes of the Week * tgeeky_: "What I cannot create, I do not understand." -- Richard Feynman, on his blackboard when he died in 1988. * elliott: ... [a] is more of a control structure than a data structure. * Cale: Basically, we've known how to implement first class functions efficiently for 20 or 30 years now, and we've known about their importance to abstraction in programming since before the advent of electronic computers. There's no excuse to still be writing new programs in languages without them. * edwardk: When someone throws an exception at you, duck. * luite: I think it uses levenshtein distance (he invented this distance because nobody could spell his name correctly) * edwardk: 'hey ghc devs can you fix this thing that might or might not be a bug so we can break a huge pile of invariants in your compiler to put go faster stripes on something nobody uses? k thx' Top Reddit Stories * Tfoo, my simple Five in a Row game. Online, with server-sent events, deployed to Heroku, open source. Domain: tfoo.herokuapp.com, Score: 49, Comments: 18 On Reddit: [1] http://goo.gl/Klevh Original: [2] http://goo.gl/PtjKk * Interacting with inner-functions in GHCi [StackOverflow] Domain: stackoverflow.com, Score: 48, Comments: 12 On Reddit: [3] http://goo.gl/23l0G Original: [4] http://goo.gl/2H5kI * ANN: acme-http. 221,693.0 req/s on the PONG benchmark. Domain: groups.google.com, Score: 42, Comments: 4 On Reddit: [5] http://goo.gl/XROlu Original: [6] http://goo.gl/PuOt3 * Recent haskell-mode changes screencast Domain: youtube.com, Score: 39, Comments: 15 On Reddit: [7] http://goo.gl/JyX0s Original: [8] http://goo.gl/VWLIF * An Haskell tutorial. What do you think? Domain: yannesposito.com, Score: 39, Comments: 47 On Reddit: [9] http://goo.gl/eLiQT Original: [10] http://goo.gl/yBDnQ * Galois' Open-Source code available on github Domain: corp.galois.com, Score: 38, Comments: 3 On Reddit: [11] http://goo.gl/xoz4K Original: [12] http://goo.gl/OhlaA * Wadler’s Law Revisited Domain: comonad.com, Score: 27, Comments: 9 On Reddit: [13] http://goo.gl/8p9As Original: [14] http://goo.gl/7iB53 * With Linux supporting x32 ABI, should GHC follow suit? Domain: self.haskell, Score: 23, Comments: 6 On Reddit: [15] http://goo.gl/Nkuob Original: [16] http://goo.gl/Nkuob * cabal-meta: transcending to dependency heaven Domain: yesodweb.com, Score: 22, Comments: 27 On Reddit: [17] http://goo.gl/nsVhn Original: [18] http://goo.gl/U2cDK * [Haskell-cafe] A Modest Records Proposal Domain: haskell.org, Score: 21, Comments: 7 On Reddit: [19] http://goo.gl/MvbRq Original: [20] http://goo.gl/RgaRU Top StackOverflow Questions * What is the best way to test and interact with inner functions defined inside a toplevel function? votes: 44, answers: 1 Read on SO: [21] http://goo.gl/2H5kI * How can a Windows service application be written in Haskell? votes: 15, answers: 1 Read on SO: [22] http://goo.gl/kz8TA * What does ':..' mean in Haskell? votes: 12, answers: 2 Read on SO: [23] http://goo.gl/Whi8a * Is the whole Map copied when a new binding is inserted? votes: 10, answers: 3 Read on SO: [24] http://goo.gl/OabLu * Haskell functions left-to-right votes: 10, answers: 1 Read on SO: [25] http://goo.gl/rhCm3 * How can I avoid writing boilerplate code for functions performing pattern matching? votes: 9, answers: 3 Read on SO: [26] http://goo.gl/6cvXJ * Controlling how test data is generated in QuickCheck votes: 8, answers: 1 Read on SO: [27] http://goo.gl/BNSug * Can I get warnings about overly-restrictive type signatures? votes: 8, answers: 1 Read on SO: [28] http://goo.gl/X7PkN * Slowdown when using parallel strategies in Haskell votes: 8, answers: 1 Read on SO: [29] http://goo.gl/ghV3C * Dealing with boilerplate in Haskell votes: 7, answers: 2 Read on SO: [30] http://goo.gl/Um4Fm Until next time, Daniel Santa Cruz References 1. http://tfoo.herokuapp.com/ 2. http://www.reddit.com/r/haskell/comments/rx20z/tfoo_my_simple_five_in_a_row… 3. http://stackoverflow.com/questions/9987739/what-is-the-best-way-to-test-and… 4. http://www.reddit.com/r/haskell/comments/rr7se/interacting_with_innerfuncti… 5. https://groups.google.com/group/happs/browse_thread/thread/9b078d23b59a7374 6. http://www.reddit.com/r/haskell/comments/ro77b/ann_acmehttp_2216930_reqs_on… 7. http://www.youtube.com/watch?v=E6xIjl06Lr4 8. http://www.reddit.com/r/haskell/comments/rtctu/recent_haskellmode_changes_s… 9. http://yannesposito.com/Scratch/en/blog/Haskell-the-Hard-Way/ 10. http://www.reddit.com/r/haskell/comments/rw61r/an_haskell_tutorial_what_do_… 11. http://corp.galois.com/blog/2012/4/4/galois-open-source-projects-on-github.… 12. http://www.reddit.com/r/haskell/comments/rt8m3/galois_opensource_code_avail… 13. http://comonad.com/reader/2012/wadlers-law-revisited/ 14. http://www.reddit.com/r/haskell/comments/rpyhr/wadlers_law_revisited/ 15. http://www.reddit.com/r/haskell/comments/rpbcx/with_linux_supporting_x32_ab… 16. http://www.reddit.com/r/haskell/comments/rpbcx/with_linux_supporting_x32_ab… 17. http://www.yesodweb.com/blog/2012/04/cabal-meta 18. http://www.reddit.com/r/haskell/comments/rurn4/cabalmeta_transcending_to_de… 19. http://www.haskell.org/pipermail/haskell-cafe/2012-April/100527.html 20. http://www.reddit.com/r/haskell/comments/rnwgn/haskellcafe_a_modest_records… 21. http://stackoverflow.com/questions/9987739/what-is-the-best-way-to-test-and… 22. http://stackoverflow.com/questions/10037654/how-can-a-windows-service-appli… 23. http://stackoverflow.com/questions/9996740/what-does-mean-in-haskell 24. http://stackoverflow.com/questions/10002956/is-the-whole-map-copied-when-a-… 25. http://stackoverflow.com/questions/10003796/haskell-functions-left-to-right 26. http://stackoverflow.com/questions/10024455/how-can-i-avoid-writing-boilerp… 27. http://stackoverflow.com/questions/9977734/controlling-how-test-data-is-gen… 28. http://stackoverflow.com/questions/9986508/can-i-get-warnings-about-overly-… 29. http://stackoverflow.com/questions/10009361/slowdown-when-using-parallel-st… 30. http://stackoverflow.com/questions/9969638/dealing-with-boilerplate-in-hask…

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Nice article on FP and concurrency from a born-again C++ programmer
by Obscaenvs 11 Apr '12

11 Apr '12

http://fpcomplete.com/the-downfall-of-imperative-programming/ /F

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Memory leak from close fields in conduit
by Michael Snoyman 11 Apr '12

11 Apr '12

Hi all, Hideyuki Tanaka alerted me[1] to a memory leak in conduit. Long story short: it appears that Pipe composition leads to collection of a large number of `return ()` actions for unnecessary memory cleanup. We came up with a possible solution: a Finalize type[2]. In both of our testing, this eliminates the memory leak in question. So now I have a few questions: 1. Does anyone have a different idea on how to approach this problem? 2. Are there any refinements to be applied to the solution? 3. Technically, this is an API breakage, but (a) conduit 0.4 was released very recently, (b) this is a fairly important change, and (c) the real-world breakage from this is very minimal. Will anyone call foul if I release such a change as 0.4.1? Just to stress 3c: I compiled all of Yesod against this modified version of conduit, and only http-conduit had to be tweaked at all. Michael [1] https://github.com/snoyberg/conduit/issues/40 [2] https://github.com/snoyberg/conduit/commit/ebc7a68e0f1f0c1b80c0105916b00060…

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Installing REPA
by Dominic Steinitz 11 Apr '12

11 Apr '12

Hi, I'm trying to install REPA but getting the following. Do I just install base? Or is it more complicated than that? Thanks, Dominic. Tests-MacBook-Pro:PDE Test$ cabal install repa Resolving dependencies... cabal: cannot configure repa-2.2.0.1. It requires base ==4.4.* For the dependency on base ==4.4.* there are these packages: base-4.4.0.0 and base-4.4.1.0. However none of them are available. base-4.4.0.0 was excluded because of the top level dependency base -any base-4.4.1.0 was excluded because of the top level dependency base -any Tests-MacBook-Pro:PDE Test$ cabal --version cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library Tests-MacBook-Pro:PDE Test$ Tests-MacBook-Pro:PDE Test$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.3

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building ghc on arch linux ARM?
by . 11 Apr '12

11 Apr '12

Hi Cafe, I hope this is the right place to ask this kind of stuff. I would like to try ghc on a panda board (armv7l) with arch linux. There is apparently no pre-built package, so I was trying the instructions to build, from here: <http://hackage.haskell.org/trac/ghc/wiki/Building/Porting>. However, I still seem to need a ghc and ghc-pkg installed: I am getting this error message: .... checking for tar... /bin/tar checking for gpatch... no checking for patch... /usr/bin/patch checking for dtrace... no checking for HsColour... no checking for xmllint... no configure: WARNING: cannot find xmllint in your PATH, you will not be able to validate your documentation checking for xsltproc... no configure: WARNING: cannot find xsltproc in your PATH, you will not be able to build the HTML documentation checking for dblatex... no configure: WARNING: cannot find dblatex in your PATH, you will not be able to build the PDF and PS documentation checking for ghc-pkg matching ... configure: error: Cannot find matching ghc-pkg --------- Does anyone know if it is possible to build ghc without ghc on this platform? I was using the tarball sources for ghc 7.4.1. Thanks, Christian

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Reconstructing a tree from a list of its paths (to leaves)
by Arnaud Bailly 11 Apr '12

11 Apr '12

Hello, I am manipulating labeled multiway trees, some kind of "lightweight" XML notation. One thing I would like to be able to do is manipulating a tree as a list of (Path, Value). Generating such a list is easy but I am a little bit surprised to find it harder to reconstruct a tree, given such a list assuming some sensible properties (list is ordered, for example). I got the intuition this has already been tackled in one way or another in a functional setting in Haskell (I code in Java but using mostly functional constructs), but don't know where to look. Thanks for your pointers and help, Regards, Arnaud

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XMPP library recommendations
by John Goerzen 10 Apr '12

10 Apr '12

Hi folks, I'm looking for suggestions for XMPP libraries. So far I have found three on Hackage. Thanks to the authors of these - I think a lot of good can come from XMPP in Haskell. However, all of them appear to be minimally maintained, if at all: * XMPP - only one upload, over 2 years ago. Does not support TLS (required by many modern servers). Does not support SASL (also required by many modern servers). After finding the options on a test server to disable the requirements for TLS and SASL, it worked. * network-protocol-XMPP - uploaded recently to fix 7.4 build issue, not much other recent activity. Refuses to connect to a test account with "Received empty challenge" (this does not appear to be an accurate error message, but rather a bug in XML parsing). A bit more low-level than I'm looking for. * haskell-xmpp - only one upload, over 1 year ago. Connects and works. I like the design. Is hard-coded to write debug messages to stdout, making it a bit of a problem for programs that use stdout (though a one-liner mod to the code would fix that). Says it's "alpha" and "work in progress". What experience do people have with these? Any recommendations? Thanks, -- John

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ANN: Selenium WebDriver client for Haskell
by Adam Curtis 10 Apr '12

10 Apr '12

The first release of the webdriver package has been uploaded to Hackage. Selenium is a test suite that allows you to automate web browsers on a variety of platforms. The webdriver package acts as a client library that speaks Selenium's WebDriver protocol, using a simple monadic interface. This alpha release has received a month of testing through my own usage, and has most of the capabilities of the official Selenium clients, with more to come. Future plans include utilizing the http-conduit and attoparsec-conduit packages for memory efficient JSON parsing, and using the reflection package to safely handle multiple versions of the WebDriver protocol through implicit configurations. For more information, see http://hackage.haskell.org/package/webdriver

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Mixing Unboxed Mutable Vectors and Parsers
by Myles C. Maxfield 10 Apr '12

10 Apr '12

CC: Maintainers of STMonadTrans, Vector, and JuicyPixels Hello, I am writing a Haskell Attoparsec parser which will modify 2-d arrays of small values (Word8, Int8, etc.). My first idea was to simply parse all the deltas, and later apply them to the input list. However, I can't do that because the value of the deltas depend on the value they're modifying. My first pass at this program used a function of the form: p :: [[Word8]] -> Parser [[Word8]] This approach works, however, the program uses far too much memory. Some quick testing shows that lists of Word8s are ~52.6x larger than unboxed vectors of Word8s, and boxed vectors of Word8s are ~7.5x larger than unboxed vectors of Word8s. A better approach would be to use Data.Vector.Unboxed.Mutable and do the mutations inline with the parser. Because mutable vectors require a monad in PrimMonad to do the mutations inside of, I'd have to use a monad transformer to combine Parser and something in PrimMonad. Attoparsec doesn't support being used as a monad transformer, so I can't say something like p :: (PrimMonad m, UM.Unbox a) => M.MVector (PrimState m) (UM.MVector (PrimState m) a) -> ParserT m () I can't use Parsec (instead of Attoparsec) because I require streaming semantics -- eventually I'm going to hook this up to Data.Conduit and parse directly from the net. There is STT (in the package STMonadTrans), however, so I could potentially make the function result in STT Parser (). However, STT doesn't work with Data.Vector.Mutable or Data.Vector.Unboxed.Mutable, because STT isn't a member of the PrimMonad class (as far as I can tell). STT itself doesn't define unboxed mutable vectors (only boxed mutable vectors), but I feel that giving up unboxing isn't really an option because of the memory footprint. As a general observation, it seems silly to have two different mutable vector implementations, one for STT and the other for PrimMonad. So here are my questions: 1. Is it possible to implement PrimMonad with STT? I looked around for a little while, but couldn't find anything that did this. 2. Otherwise, is it reasonable to try to implement unboxed mutable vectors in STT? I feel this is probably going down the wrong path. 3. Are there any parsers that support streaming semantics and being used as a monad transformer? This would require rewriting my whole program to use this new parser, but if that's what I have to do, then so be it. Thanks, Myles C. Maxfield

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Call for Participation: Tests and Proofs (TAP 2012) in Prague
by Achim D. Brucker 10 Apr '12

10 Apr '12

Apologies for duplicates. ======================================================================== CALL FOR PARTICIPATION TESTS and PROOFS 2012 (TOOLS EUROPE 2012) 6th International Conference on Tests & Proofs May 31 - June 1, 2012, Prague, Czech Republic http://lifc.univ-fcomte.fr/tap2012/ Co-located with: ICMT 2012, SC 2012, MSEPT 2012 as part of TOOLS 2012: http://toolseurope2012.fit.cvut.cz/ ======================================================================== The TAP conference is devoted to the convergence of proofs and tests. It combines ideas from both sides for the advancement of software quality. Keynote Speakers ================ * Andreas Kuehlmann <http://www.eecs.berkeley.edu/%7Ekuehl/>, Coverity The Technology and Psychology of Testing Your Code as You Develop It (abstract <http://lifc.univ-fcomte.fr/tap2012/abstract_talk_Kuehlmann.pdf>) * Corina Pasareanu <http://ti.arc.nasa.gov/profile/pcorina/>, NASA Ames Research Center Combining Model Checking and Symbolic Execution for Software Testing (abstract <http://lifc.univ-fcomte.fr/tap2012/abstract_talk_Pasareanu.pdf>) * Mehdi Jazayeri, University of Lugano. Software Composition: Why, what, and how Registration: ============= Early registration, at a reduced price, will be open until 25 April 2012. http://toolseurope2012.fit.cvut.cz/index.php/registration.html Organization: ============= Conference Chair Bertrand Meyer, ETH Zurich, Eiffel Software, and ITMO Program Chairs Achim D. Brucker, SAP Research, Germany Jacques Julliand, University of Franche-Comté Local Organization Pavel Tvrdik, CTU Prague Michal Valenta, CTU Prague Jindra Vojikova, CTU Prague Jan Chrastina, CTU Prague Program: ======== Thursday 31th May, 09:00-10:30: Invited Talk * Andreas Kuehlmann, Coverity: The Technology and Psychology of Testing Your Code as You Develop It Thursday, 11:00-13:00: Paper session Model-Based Testing * Malte Lochau, Ina Schaefer, Jochen Kamischke and Sascha Lity. Incremental Model-based Testing of Delta-oriented Software Product Lines * Hernan Ponce De Leon, Stefan Haar and Delphine Longuet. Conformance Relations for Labeled Event Structures * Joseph Kiniry, Daniel M. Zimmerman and Ralph Hyland. Testing Library Specifications by Verifying Conformance Tests * Chedor Sebastien, Thierry J?ron and Morvan Christophe. Test generation from recursive tiles systems Thursday, 14:30-15:30: Paper session Scenario and UML-Based Testing * Nadia Creignou, Uwe Egly and Martina Seidl. A Framework for the Specification of Random SAT and QSAT Formulas * Jens Brüning, Martin Gogolla, Lars Hamann and Mirco Kuhlmann. Evaluating and Debugging OCL Expressions in UML Models * Uwe Egly, Sebastian Gabmeyer, Martina Seidl, Hans Tompits, .... Towards Scenario-Based Testing of UML Diagrams Thursday, 16:00-17:00: Tutorial * Nikolay Kosmatov, Nicky Williams. Automated Structural Testing with PathCrawler Friday 1st June, 09:00-10:30: Invited Talk of SC * Mehdi Jazayeri, University of Lugano. Software Composition: Why, what, and how Friday 11:00-13:00: Invited Talk of TAP * Corina Pasareanu. NASA. Combining Model Checking and Symbolic Execution for Software Testing Friday, 14:30-15:30: Paper session Test and Model-checking * Martin Sulzmann and Axel Zechner Constructive Finite Trace Analysis with Linear Temporal Logic * Alessandro Armando, Roberto Carbone, Giancarlo Pellegrino, Alessio Merlo and Davide Balzarotti. From Model-checking to Automated Testing of Security Protocols: Bridging the Gap Friday, 16:00-17:30: Paper session Test of Complex Data Structures * Valerio Senni and Fabio Fioravanti. Generation of test data structures using Constraint Logic Programming * Valeria Bengolea, Nazareno Aguirre, Darko Marinov and Marcelo Frias. Coverage Criteria on RepOK to Reduce Bounded Exhaustive Test Suites * Matthieu Carlier, Catherine Dubois and Arnaud Gotlieb. A first step in the design of a formally verified constraint-based testing tool: FocalTest -- Dr. Achim D. Brucker, SAP Research, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch

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