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Haskell Weekly News: Issue 199
by Daniel Santa Cruz 15 Sep '11

15 Sep '11

Welcome to issue 199 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of September 4 to 10, 2011. Announcements Johannes Waldmann open the invitation to the 6th Haskell in Leipzig workshop, on October 7, 2011. It "will present an absolutely thrilling mixture of tutorials and talks, with special enphasis on parallel programming." [1] http://goo.gl/u0PwF New and Updated Projects * Hs2lib (Phyx; v-0.5.5) A preprocesor and library which allow you to create dynamic libs from arbitrary annotated Haskell programs with one click. It also allows you to use the generated lib in C, C++, and C# just by including the generated header files. [2] http://goo.gl/xCSXc * Netwire (Ertugrul Soeylemez; 1.2.0) Lots of new features! [3] http://goo.gl/VCwb1 * network-address (Sebastian Nowicki; 0.2.0) Provides data structures and textual representation of network addresses (IPv4, IPv6, subnets). [4] http://goo.gl/DBRI6 * Webrexp (Vincent Berthoux; 1.1) A web scraping command line utility. [5] http://goo.gl/OlovQ * eddie (Mike Meyer; 0.5) Is a tool for applying haskell filters to text files via the unix command line. [6] http://goo.gl/NPc0b * cabal-ghci (Etienne Laurin; 0.1) Package to ease the development of projects using cabal. [7] http://goo.gl/fiDKZ Quotes of the Week * gosu: Type parameters are covariant. This is not sound, and that does not matter. * turtlesalltheway: if haskell is so great why cant it improve itself * mm_freak: bosses are generally quite strong, static and lazy * cmccann: newtype Natural = N { denature :: Integer } Top Reddit Stories * Constraint Kinds for GHC Domain: blog.omega-prime.co.uk, Score: 115, Comments: 12 On Reddit: [8] http://goo.gl/bCXgf Original: [9] http://goo.gl/7aWzn * Yesod :: The Limitations of Haskell Domain: yesodweb.com, Score: 108, Comments: 42 On Reddit: [10] http://goo.gl/5q7wW Original: [11] http://goo.gl/gisIh * Frege: haskell like language on JVM (tries to adhere to haskell 2010 where possible) Domain: code.google.com, Score: 58, Comments: 58 On Reddit: [12] http://goo.gl/6KWEg Original: [13] http://goo.gl/56AkX * Sharing in Haskell – How the let and lambda constructs give a precise way to control sharing Domain: neilmitchell.blogspot.com, Score: 34, Comments: 18 On Reddit: [14] http://goo.gl/Uf1vq Original: [15] http://goo.gl/4Hmfu * Let's play a game : How to implement functions from their type signatures Domain: blog.ezyang.com, Score: 24, Comments: 13 On Reddit: [16] http://goo.gl/T2FiD Original: [17] http://goo.gl/SmbYO * Impredicativity + injectivity + type case analysis = inconsistency (Russell paradox) :: Oleg Domain: okmij.org, Score: 23, Comments: 10 On Reddit: [18] http://goo.gl/zoThI Original: [19] http://goo.gl/jFr5U * Functional Pearls Domain: haskell.org, Score: 23, Comments: 1 On Reddit: [20] http://goo.gl/RVyng Original: [21] http://goo.gl/1A7t0 * An EventSource broker written in Haskell Domain: mathias-biilmann.net, Score: 22, Comments: 0 On Reddit: [22] http://goo.gl/7yywu Original: [23] http://goo.gl/Y5mGj * Haskell For Kids: Week 4 Domain: cdsmith.wordpress.com, Score: 22, Comments: 2 On Reddit: [24] http://goo.gl/KwlqD Original: [25] http://goo.gl/xlNh2 * apfelmus - Vault - a persistent store for values of arbitrary types Domain: apfelmus.nfshost.com, Score: 19, Comments: 2 On Reddit: [26] http://goo.gl/VAE4n Original: [27] http://goo.gl/vvQ7A * Tau is available in Haskell, you can safely avoid Pi Domain: hackage.haskell.org, Score: 19, Comments: 26 On Reddit: [28] http://goo.gl/u5GN8 Original: [29] http://goo.gl/VnrMh * Zipper from any Traversable :: Oleg Domain: okmij.org, Score: 18, Comments: 2 On Reddit: [30] http://goo.gl/xi4ZF Original: [31] http://goo.gl/nqZ3T Top StackOverflow Questions * More on generic Scala functions votes: 20, answers: 4 Read on SO: [32] http://goo.gl/DGH14 * Why is a “type class” called “type class”? votes: 20, answers: 2 Read on SO: [33] http://goo.gl/YbVXX * Is there a Haskell idiom for updating a nested data structure? votes: 14, answers: 2 Read on SO: [34] http://goo.gl/jKEjv * How to tell if a list is infinite? votes: 14, answers: 4 Read on SO: [35] http://goo.gl/fc3e6 * Select random element from a set, faster than linear time (Haskell) votes: 9, answers: 5 Read on SO: [36] http://goo.gl/IRJf3 * GHC not optimising modules other than the main module votes: 9, answers: 0 Read on SO: [37] http://goo.gl/8Ephm * A chess board representation in Haskell votes: 8, answers: 4 Read on SO: [38] http://goo.gl/iSM7d * How to cap memory usage of Haskell threads votes: 7, answers: 2 Read on SO: [39] http://goo.gl/dvLnE * Is this Haskell type inference in action, or something else? votes: 7, answers: 2 Read on SO: [40] http://goo.gl/hTaK7 * Assymmetry in the bind function votes: 6, answers: 7 Read on SO: [41] http://goo.gl/KqMTn 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://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92287 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92182 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92225 4. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18916 5. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18929 6. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18930 7. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92279 8. http://blog.omega-prime.co.uk/?p=127 9. http://www.reddit.com/r/haskell/comments/kbf14/constraint_kinds_for_ghc/ 10. http://www.yesodweb.com/blog/2011/09/limitations-of-haskell 11. http://www.reddit.com/r/haskell/comments/k4lc4/yesod_the_limitations_of_has… 12. http://code.google.com/p/frege/ 13. http://www.reddit.com/r/haskell/comments/kadx6/frege_haskell_like_language_… 14. http://neilmitchell.blogspot.com/2011/09/sharing-in-haskell.html 15. http://www.reddit.com/r/haskell/comments/k4bq9/sharing_in_haskell_how_the_l… 16. http://blog.ezyang.com/2011/09/lets-play-a-game/ 17. http://www.reddit.com/r/haskell/comments/k5okh/lets_play_a_game_how_to_impl… 18. http://okmij.org/ftp/Haskell/impredicativity-bites.html 19. http://www.reddit.com/r/haskell/comments/k5bdb/impredicativity_injectivity_… 20. http://www.haskell.org/haskellwiki/Research_papers/Functional_pearls 21. http://www.reddit.com/r/haskell/comments/kb3w5/functional_pearls/ 22. http://mathias-biilmann.net/posts/2011/9/a-haskell-eventsource-broker 23. http://www.reddit.com/r/haskell/comments/k6fva/an_eventsource_broker_writte… 24. http://cdsmith.wordpress.com/2011/09/10/haskell-for-kids-week-4/ 25. http://www.reddit.com/r/haskell/comments/kbqn0/haskell_for_kids_week_4/ 26. http://apfelmus.nfshost.com/blog/2011/09/04-vault.html 27. http://www.reddit.com/r/haskell/comments/k4l6c/apfelmus_vault_a_persistent_… 28. http://hackage.haskell.org/package/tau 29. http://www.reddit.com/r/haskell/comments/kazfn/tau_is_available_in_haskell_… 30. http://okmij.org/ftp/continuations/zipper.html#traversable 31. http://www.reddit.com/r/haskell/comments/k5bci/zipper_from_any_traversable_… 32. http://stackoverflow.com/questions/7337582/more-on-generic-scala-functions 33. http://stackoverflow.com/questions/7360024/why-is-a-type-class-called-type-… 34. http://stackoverflow.com/questions/7365425/is-there-a-haskell-idiom-for-upd… 35. http://stackoverflow.com/questions/7371730/how-to-tell-if-a-list-is-infinite 36. http://stackoverflow.com/questions/7349080/select-random-element-from-a-set… 37. http://stackoverflow.com/questions/7367070/ghc-not-optimising-modules-other… 38. http://stackoverflow.com/questions/7309554/a-chess-board-representation-in-… 39. http://stackoverflow.com/questions/7297654/how-to-cap-memory-usage-of-haske… 40. http://stackoverflow.com/questions/7342222/is-this-haskell-type-inference-i… 41. http://stackoverflow.com/questions/7319424/assymmetry-in-the-bind-function

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cabal-install to produce a dependency tree
by Ozgur Akgun 15 Sep '11

15 Sep '11

Dear Cafe, What would be the easiest way of generating the following output, given a package name optionally with additional constraints? $- foo "X > 3" "X > 3" depends on A-2.2, B-1.0, C-1.2 A-2.2 depends on D-1.2.3 ... I assume cabal-install internally does this anyway while creating the installation plan, so is there an easy way to get this info out of cabal install? Thanks, Ozgur

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Undefined symbol error coming from shared, dynamic library.
by David Banas 15 Sep '11

15 Sep '11

Hi all, I'm trying to build a shared, dynamic library for use with a C program. I'm getting an `undefined symbol' error, when I try to run that C program, and was hoping that the last line in the output, below, might mean something to someone. I include the entire output of a `make rebuild' command, below, hoping that, maybe, I just have my command line options a little wrong. Thanks! -db dbanas@dbanas-eeepc:~/prj/haskell/AMIParse/trunk$ make rebuild make clean make[1]: Entering directory `/home/dbanas/prj/haskell/AMIParse/trunk' rm -f *.hi *.o *.out ami_test *.so make[1]: Leaving directory `/home/dbanas/prj/haskell/AMIParse/trunk' make all make[1]: Entering directory `/home/dbanas/prj/haskell/AMIParse/trunk' gcc -I/usr/lib/ghc-6.12.3/include/ -g -fPIC -c -o ami_test.o ami_test.c gcc -rdynamic -o ami_test ami_test.o -ldl ghc -c ApplicativeParsec.hs -cpp -package parsec-3.1.1 -package dsp -dynamic -fPIC ghc -c AMIParse.hs -cpp -package parsec-3.1.1 -package dsp -dynamic -fPIC ghc -c ExmplUsrModel.hs -cpp -package parsec-3.1.1 -package dsp -dynamic -fPIC ghc -c AMIModel.hs -cpp -package parsec-3.1.1 -package dsp -dynamic -fPIC gcc -I/usr/lib/ghc-6.12.3/include/ -g -fPIC -c -o ami_model.o ami_model.c rm -f libami.so ghc -o libami.so -shared -dynamic -package parsec-3.1.1 -package dsp AMIParse.o AMIModel.o ami_model.o AMIModel_stub.o ApplicativeParsec.o ExmplUsrModel.o make[1]: Leaving directory `/home/dbanas/prj/haskell/AMIParse/trunk' dbanas@dbanas-eeepc:~/prj/haskell/AMIParse/trunk$ ./ami_test test.ami /usr/lib/ghc-6.12.3/ghc-prim-0.2.0.0/libHSghc-prim-0.2.0.0-ghc6.12.3.so: undefined symbol: stg_newByteArrayzh

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Final CFP: PEPM12 - Workshop on Partial Evaluation and Program Manipulation
by oleg＠okmij.org 14 Sep '11

14 Sep '11

Final Call For Papers Paper submission deadline: Mon, October 10, 2011, 23:59, GMT ACM SIGPLAN 2012 Workshop on Partial Evaluation and Program Manipulation January 23-24, 2012. Philadelphia, Pennsylvania, USA (co-located with POPL'12) http://www.program-transformation.org/PEPM12 The PEPM Symposium/Workshop series aims to bring together researchers and practitioners working in the broad area of program transformation, which spans from refactoring, partial evaluation, supercompilation, fusion and other metaprogramming to model-driven development, program analyses including termination, inductive programming, program generation and applications of machine learning and probabilistic search. PEPM focuses on techniques, supporting theory, tools, and applications of the analysis and manipulation of programs. Each technique or tool of program manipulation should have a clear, although perhaps informal, statement of desired properties, along with an argument how these properties could be achieved. Topics of interest for PEPM'12 include, but are not limited to: - Program and model manipulation techniques such as: supercompilation, partial evaluation, fusion, on-the-fly program adaptation, active libraries, program inversion, slicing, symbolic execution, refactoring, decompilation, and obfuscation. - Program analysis techniques that are used to drive program/model manipulation such as: abstract interpretation, termination checking, binding-time analysis, constraint solving, type systems, automated testing and test case generation. - Techniques that treat programs/models as data objects including metaprogramming, generative programming, embedded domain-specific languages, program synthesis by sketching and inductive programming, staged computation, and model-driven program generation and transformation. - Application of the above techniques including case studies of program manipulation in real-world (industrial, open-source) projects and software development processes, descriptions of robust tools capable of effectively handling realistic applications, benchmarking. Examples of application domains include legacy program understanding and transformation, DSL implementations, visual languages and end-user programming, scientific computing, middleware frameworks and infrastructure needed for distributed and web-based applications, resource-limited computation, and security. To maintain the dynamic and interactive nature of PEPM, we will continue the category of `short papers' for tool demonstrations and for presentations of exciting if not fully polished research, and of interesting academic, industrial and open-source applications that are new or unfamiliar. Student attendants with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses and other support. All accepted papers, short papers included, will appear in formal proceedings published by ACM Press and will be included in the ACM Digital Library. Selected papers may later on be invited for a journal special issue dedicated to PEPM'12. Submission Categories and Guidelines Authors are strongly encouraged to consult the advice for authoring research papers and tool papers before submitting. The PC Chairs welcome any inquiries about the authoring advice. Regular research papers must not exceed 10 pages in ACM Proceedings style. Short papers are up to 4 pages in ACM Proceedings style. Authors of tool demonstration proposals are expected to present a live demonstration of the described tool at the workshop (tool papers should include an additional appendix of up to 6 extra pages giving the outline, screenshots, examples, etc. to indicate the content of the proposed live demo at the workshop). Important Dates - Paper submission: Mon, October 10, 2011, 23:59, GMT - Author notification: Tue, November 8, 2011 - Workshop: Mon-Tue, January 23-24, 2012 Invited Speakers - Markus Pueschel (ETH Zurich, Switzerland) - Martin Berger (University of Sussex, UK) Program Chairs - Oleg Kiselyov (Monterey, CA, USA) - Simon Thompson (University of Kent, UK) Program Committee Members - Emilie Balland (INRIA, France) - Ewen Denney (NASA Ames Research Center, USA) - Martin Erwig (Oregon State University, USA) - Sebastian Fischer (National Institute of Informatics, Japan) - Lidia Fuentes (Universidad de Malaga, Spain) - John Gallagher (Roskilde University, Denmark and IMDEA Software, Spain) - Dave Herman (Mozilla Research, USA) - Stefan Holdermans (Vector Fabrics, the Netherlands) - Christian Kaestner (University of Marburg, Germany) - Emanuel Kitzelmann (International Computer Science Institute, USA) - Andrei Klimov (Keldysh Institute of Applied Mathematics, Russian Academy of Sciences) - Shin-Cheng Mu (Academia Sinica, Taiwan) - Alberto Pardo (Universidad de la Repu'blica, Uruguay) - Kostis Sagonas (Uppsala University, Sweden and National Technical University of Athens, Greece) - Anthony M. Sloane (Macquarie University, Australia) - Armando Solar-Lezama (MIT, USA) - Aaron Stump (The University of Iowa, USA) - Kohei Suenaga (University of Kyoto, Japan) - Eric Van Wyk (University of Minnesota, USA) - Kwangkeun Yi (Seoul National University, Korea)

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Haskell 101 on classes .... duh ..... :^)
by Vasili I. Galchin 14 Sep '11

14 Sep '11

Hello, I am trying to model multigraphs ....but getting errors with ghci and can't figure out why.... I have a serious blind spot .... {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} module Bonzo where class Graph arrow node where source :: arrow -> node target :: arrow -> node data Arrow = Arrow (Int, Int) instance Graph Arrow Int where source Arrow = fst Arrow target Arrow = snd Arrow ~ ~ {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} module Bonzo where class Graph arrow node where source :: arrow -> node target :: arrow -> node data Arrow = Arrow (Int, Int) instance Graph Arrow Int where source Arrow = fst Arrow target Arrow = snd Arrow ~ ~ ghci> :load junk1.hs [1 of 1] Compiling Bonzo ( junk1.hs, interpreted ) junk1.hs:19:12: Constructor `Arrow' should have 1 argument, but has been given 0 In the pattern: Arrow In the definition of `source': source Arrow = fst Arrow In the instance declaration for `Graph Arrow Int' junk1.hs:21:12: Constructor `Arrow' should have 1 argument, but has been given 0 In the pattern: Arrow In the definition of `target': target Arrow = snd Arrow In the instance declaration for `Graph Arrow Int' Regards, Vasili

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Question about type families
by Grigory Sarnitskiy 14 Sep '11

14 Sep '11

Is there a way to make the following code working? {-# LANGUAGE TypeFamilies #-} data family Foo a data instance (Num a) => Foo a = A a deriving Show data instance (Fractional a) => Foo a = B a deriving Show I want to have different constructors for 'Foo a' depending on a class of 'a'. Note also, that in the example above I also meant constructor A to be available for (Fractional a) => Foo, since in that case 'a' has Num too. How can I achieve it, maybe not with TypeFamilies? Current error is Conflicting family instance declarations: data instance Foo a -- Defined at 1.hs:7:33-35 data instance Foo a -- Defined at 1.hs:5:33-35

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Can't resolve class dependences (making Boolean class)
by Grigory Sarnitskiy 13 Sep '11

13 Sep '11

Hello! I'm trying to have a class for booleans called Boolean (the methods are not complete): class MyEq a where (===) :: (Boolean b) => a -> a -> b class (MyEq a) => Boolean a where (/\) :: a -> a -> a instance MyEq Bool where x === y = x==y instance Boolean Bool where (/\) = (&&) However, to make Bool an instance of Boolean I need to make it an instance of MyEq first, which I can't, because to define === I need Bool to be in Boolean. Indeed the code above give the error: Could not deduce (b ~ Bool) from the context (Boolean b) bound by the type signature for === :: Boolean b => Bool -> Bool -> b at 1.hs:8:5-18 `b' is a rigid type variable bound by the type signature for === :: Boolean b => Bool -> Bool -> b at 1.hs:8:5 In the expression: x == y In an equation for `===': x === y = x == y In the instance declaration for `MyEq Bool' Failed, modules loaded: none. How can I overcome the issue?

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extending and reusing cmdargs option specs ?
by Simon Michael 13 Sep '11

13 Sep '11

Hi Neil, I just spent a day converting hledger from getopt to cmdargs. cmdargs feels more high level and featureful and nicer. And yet... I haven't reduced the line count that much - nothing like your HLint 3:1 ratio. And, I may have made things worse for myself in the reuse/avoiding boilerplate department: I'm not sure how to reuse a cmdargs options data structure, extending it with a few more options. Using getopt I was able to do this without repeating myself (as long as I defined the full set of Opt constructors in one place.) I've looked at the more advanced cmdargs api, but don't see a way yet - would you have any ideas ? I want this because I have multiple executables (hledger, hledger-vty, hledger-web etc.) which should share most (but not all) options. Also, I'd like to move a generic subset of report options, without the ui- specific ones, into hledger-lib for use by all apps. Also, the hledger executable has multiple commands, so I'd like to define a mode for each, but not have to redeclare all the same options for each mode - I didn't see how to do that. As always, thanks a lot for cmdargs! -Simon

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Can't link OpenCL on Windows
by Jason Dagit 13 Sep '11

13 Sep '11

Hello, I'm trying to get the OpenCLRaw bindings to a point where I can use them on windows. I've forked the the OpenCLRaw repo on github so I can make modifications as needed. My branch is here: https://github.com/dagit/OpenCLRaw I've been mostly working out of my "FunPtr" branch. The issue I'm having is this: I installed AMD's OpenCL SDK, converted their Visual Studio specific .lib file to a file that gcc can handle (.a file), but ghc can't seem to link with it. I get undefined symbols for everything I use in the OpenCL API. I was able to build a "trivial" C program and link it using the .a file that I generated and gcc from mingw (not from the Haskell install). I'm using the latest windows release of the Haskell platform. These are the steps I used for generating the .a file: http://forums.amd.com/forum/messageview.cfm?catid=390&threadid=138890 I used the commands in the example script (e.g., gendef and dlltool). I've tried to use 32bit everything as much as possible, as I know that GHC will want everything to be 32bit, so I don't think it's a 32bit vs. 64bit issue. Does anyone know if there is something different about invoking gcc under ghc instead of the gcc that I get from mingw? I've also played with the ghc command line (I used cabal-dev --verbose=3 to inspect the command line) and I'm still unable to massage it into a working state. Any help would be appreciated! Thanks, Jason

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Haskell Platform Older Releases for Linux
by Luis Cabellos 12 Sep '11

12 Sep '11

Hi, I search the Haskell Platform webpage for older releases, I found it but maybe it's worth to put a table of older releases as tar.gz file with their ghc dependency, like (versions totally make up): HP 2011.2.0.0 source, March 2011. -> ghc 13.1 HP 2010.2.0.0 source, July 2010. -> ghc 3.14 HP 2010.1.0.0 source, March 2010. -> ghc 616 HP 2009.2.0.2 source, July 2009. -> ghc 1 HP 2009.2.0.1 source, June 2009. -> ghc 0 HP 2009.2.0 source, May 2009. -> ghc -1 Thanks, Luis

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