- Haskell-Cafe - Haskell.org

Haskell Logo write-in candidate
by Warren Harris 22 Mar '09

22 Mar '09

After spending a bit of time trying to decide how to vote, I ended up deciding that my favorite would be a hybrid of several of the designs (#9 & #49 FalconNL, and #50 George Pollard). It's probably too late to include this in the voting, but here it is nonetheless: (My primary criterion is "would I wear this t-shirt?") :-) Warren

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Cabal properties with conditionals
by Manlio Perillo 22 Mar '09

22 Mar '09

Hi. Assuming this configuration fragment: library xxx cc-options: -Wall if flag(HAVE_URANDOM) cc-options: -DHAVE_URANDOM In case the HAVE_URANDOM flag is defined, what will be the value of the used cc-options? 1) -DHAHE_URANDOM 2) -Wall -DHAHE_URANDOM Thanks Manlio

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New Cabal FAQ
by Duncan Coutts 22 Mar '09

22 Mar '09

Hi all, I should have done this ages ago but there's now a Cabal FAQ on the Cabal website: http://haskell.org/cabal/FAQ.html It's not linked in yet, I'm looking for feedback and patches. The Cabal website is now maintained in darcs so it's easy to send in contributions: darcs get http://haskell.org/cabal/ For new pages like the FAQ I've been using markdown via pandoc. If appropriate we can migrate old pages to use that too. The main Cabal home page needs some love. We should have the cabal-install download directly on the front page. We should have quick intro tutorials for using the cabal command line program to install packages. There's lots of good material in the user guide but it is not well sign-posted. Duncan

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ANNOUNCE: Haddock 2.4.2
by David Waern 22 Mar '09

22 Mar '09

-------------------------------------------- -- Haddock 2.4.2 -------------------------------------------- A new version of Haddock, the Haskell documentation tool, is out. This is a bug fix release only, and it's the same version that will ship with GHC 6.10.2, unless any important problems are discovered before the GHC release. Because the .haddock file format has changed, links to previously installed documentation will not work when generating documentation using this version. Please use the bug tracker to submit bug reports or feature requests. -------------------------------------------- -- Changes in version 2.4.2 -------------------------------------------- * Support for GHC 6.10.2 * Haddock no longer crashes on Template Haskell modules (#68) (only with GHC 6.10.2 or above) * Fix problem with Template Haskell-generated declarations disappearing (#59) * Generate two anchors for each link for compatibility between IE and Opera (#45) * Improved error messages * Show re-exports from external packages again (GHC ticket #2746) * Store hidden modules in .haddock files again (needed by the haddock library) * Avoid processing boot modules * Pragmas may exist between document comments and declarations * Do not indicate that a constructor argument is unboxed * Fix problem with non-working links to ghc-prim * Allow referring to a specific section within a module in a module link (#65) * Fixes to the Hoogle backend * Improvements to the haddock library * Many other fixes (including #67, #69, #58, #57) -------------------------------------------- -- Links -------------------------------------------- Homepage: http://www.haskell.org/haddock Hackage page: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haddock-2.4.2 Bugtracker and wiki: http://trac.haskell.org/haddock Mailing list: haddock(a)projects.haskell.org Code repository: http://code.haskell.org/haddock -------------------------------------------- -- Contributors -------------------------------------------- The following people contributed patches to this release: Joachim Breitner Roman Cheplyaka Ian Lynagh Neil Mitchell Simon Peyton-Jones Thomas Schilling David Waern -------------------------------------------- -- Get Involved -------------------------------------------- We welcome new contributors. To get involved, start by grabbing the code: http://code.haskell.org/haddock Then take a look at the bug and feature tracker for things to work on: http://trac.haskell.org/haddock

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[ANN] random-stream 0.1.1
by Manlio Perillo 22 Mar '09

22 Mar '09

I'm pleased to announce a new version of random-stream package. In this version I have rewritten the internal API. Now the package exposes a new System.Random.URandom module, with the function: urandom :: Int -> IO S.ByteString This function is an interface to the system pseudo random numbers generator. The API of the module System.Random.Stream is left unchanged, but it now uses the urandom function, internally. System.Random.Stream provides a pure interface over urandom, using a lazy ByteString of random bytes. Since it provides an infinite stream of random data, things should be ok. I have also added support to Windows (older versions may not be supported, however [1]). The package should be available on Hackage. Mercurial repository is at: http://hg.mperillo.ath.cx/haskell/random-stream/ Example usage: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2738 [1] It is possible to use OpenSSL, as a fallback. Just set the HAVE_SSL flag, during package configuration. The OpenSSL DLL should be placed where the linker can find it. I tried to test this, but without success (but I did not put much effort in it). Regards Manlio Perillo

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Data.Binary, Data.Text and errors
by Alexander Dunlap 22 Mar '09

22 Mar '09

Hi all, I have noticed that in both Data.Binary and Data.Text (which is still experimental, but still), the "decode" functions can be undefined (i.e. bottom) if they encounter malformed input. What is the preferred way to use these functions in a safe way? For example, if one writes data to a disk using Data.Binary and wants to read it back in at a later date, how can one ensure that it is valid so that Data.Binary does not hit an error? Or do you just have to catch the exception in the IO Monad? Alex

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[GSOC] About "Implementing Maps using generalized tries" project idea
by Gautam Sewani 22 Mar '09

22 Mar '09

Hi, I was looking at the list of Haskell GSOC 2009 project ideas, and I came across the one in which we have to implement maps using generalized tries. However, I also found this as one of the GSOC 2008 projects. I want to know if this project is still open, or the required work has been done last year. If it's open, what exactly needs to be done further. Regards Gautam

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Re: [Haskell-cafe] Ease of Haskell development on OS X?
by Thomas Davie 22 Mar '09

22 Mar '09

On 20 Mar 2009, at 16:56, Mark Spezzano wrote: > Hi, > > I’ve been thinking of changing over to an iMac from my crappy old PC > running Windows Vista. > > Question: Does the iMac have good support for Haskell development? As good as, if not better than other platforms I've found, you get none of the weird problems with libraries that aren't designed to run on windows, and more choice of editors than on linux. > Question: What environment setups do people commonly use (e.g. > Eclipse Xcode etc)? I use SubEthaEdit and a Terminal window, I know many people that use emacs/vi, and one or two that use TextMate. > Question: Are there any caveats I should be aware of before changing > systems (i.e. unpleasant surprises). Not particularly – it's mostly like running linux, but without the headaches when things break, and with more choice of software. > I want to be able to use the machine for Haskell OpenGL programming. Other than chose the graphics card carefully, an iMac will do you very well. Hope that helps. Bob

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Re: [Haskell-beginners] laziness and optimization
by Adrian Neumann 21 Mar '09

21 Mar '09

You should not rely on the compiler to spot such things. As far as I know GHC doesn't do automatic caching (in many cases that would hurt performance, I think). Have a look at http://haskell.org/haskellwiki/ Memoization perhaps. Am 21.03.2009 um 14:02 schrieb Michael Mossey: > I understand a bit about the concept of "lazy evaluation." I think > of that as saying an imperative language would always make one > evaluation, whereas Haskell might make 0 evaluations. I have > another similar situation where an imperative language would make N > evaluations of the same expression, and I would like Haskell to > make only 1. > > This is the situation: the graphical score editor displays > "LayoutItems." A LayoutItem can be a single displayed entity, like > a round notehead, or it can be composed of several entities. > > A common situation in my code is the need to determine the size and > shape of a LayoutItem. For a fundamental item, this can be looked > up in a table or read from the font properties. For a composite > item, some computation is required: the code must determine the > positions of each sub-item and compute the bounds of a shape > containing all of them. > > It's this latter computation, finding the bounds of a composite > item, which might come up multiple times. Consider that I ask for > the bounds of a composite-composite item (a composite item composed > of composite items). It will run the computation associated with > each composite sub-item, even though it is very likely I already > make that computation when I first constructed and placed that sub- > item. > > In an imperative language, one might cache values for later lookup. > This raises the problem of keeping the cache current to the current > state. > > So I'm wondering to what extent the haskell compiler recognizes > computations it's done before. In a purely functional language this > should be pretty easy, right? If it sees the same expression, it > knows it will have the same value. That's my understanding, so far. > > Thanks, > Mike > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

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Re: [Haskell-beginners] appropriateness of haskell for GUIs
by Adrian Neumann 21 Mar '09

21 Mar '09

Am 21.03.2009 um 13:30 schrieb Michael Mossey: > > > Thomas Davie wrote: >> On 21 Mar 2009, at 00:16, Michael P Mossey wrote: >>> Hello, I'm totally new to Haskell. I'm thinking of using it for a >>> personal project, which is a gui-based musical score editor. >> The rough situation of GUI programming on Haskell is that it works >> just as well as in any imperative programming language. This is >> rather disappointing, simply because so many other things are >> massively easier in Haskell, and this isn't true of GUI >> programming (yet). > > Hi Bob, > > I can imagine that GUI programming is no easier (yet). It is > inherently very "stateful." GUI's have modes, such as which screens > are displayed, which dialogs are displayed, which options within > those dialogs are valid given the other state of the program, etc. > When I write GUIs, I often diagram them as state machines to get a > handle on what's going on. > > So, I'm not familiar with GUI programming on Haskell, but would you > say the statefulness of GUIs (in their typical implementations) is > the reason they are no easier on Haskell? > > I strongly prefer to use qtHaskell because I'm familiar with Qt, > and Qt is extremely capable. For example, it can draw text and > shapes with antialiasing, which will be great for a music score > editor. Music scores have lots of small shapes to fit on the > screen, and antialiasing will provide ease of reading. I don't know > how much of Qt is implemented in qtHaskell, or whether the latest > version of Qt (4.4) is implemented. > > Thanks, > Mike The main problem is, as far as I know, the complete lack of any usable GUI designer. You have to type everything yourself. That's very annoying. It's a lot easier in other languages because your tools take away the cumbersome twiddling with widgets. However, I haven't googled. Maybe the situation has changed since I last looked. Regards, Adrian

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Haskell Weekly News: Issue 110 - March 21, 2009
by Brent Yorgey 21 Mar '09

21 Mar '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090321 Issue 110 - March 21, 2009 --------------------------------------------------------------------------- Welcome to issue 110 of HWN, a newsletter covering developments in the [1]Haskell community. [2]Facebook apps with Happstack, [3]Sudoku with Cryptol, what next? Tic-tac-toe with darcs? Anyway, lots of neat stuff this week, including new releases of [4]GHC, [5]jhc, and the [6]Monad.Reader, some [7]fun [8]visualizations, and more. Also, students: apply to work on a [9]Haskell project for the Google Summer of Code! Announcements GHC 6.10.2 Release Candidate 1. Ian Lynagh [10]announced the [11]first release candidate for GHC 6.10.2. Please test as much as possible; bugs are much cheaper if we find them before the release! jhc 0.6.0 Haskell Compiler. John Meacham [12]announced the release of [13]jhc 0.6.0. Safe Lazy IO in Haskell. Nicolas Pouillard [14]announced the [15]safe-lazy-io package that provides special types and combinators for performing safe lazy I/O. game-tree - a library for searching game trees. Colin Paul Adams [16]announced [17]game-tree 0.1.0.0, which provides a class for dynamic game trees, and purely functional algorithms for searching them. random-shuffle package. Manlio Perillo [18]announced the availability of the [19]random-shuffle package, which is based on [20]Oleg's description. random-stream package. Manlio Perillo [21]announced the [22]random-stream package, which provides a portable interface for the operating system source of pseudo random data. Supported sources are Unix /dev/urandom, Win32 CryptGenRandom and OpenSSL pseudo random numbers generator. language-python. Bernie Pope [23]announced the [24]language-python package, which provides a parser (and lexer) for Python, written in Haskell. Currently it only supports version 3 of Python (the most recent version), but it will support version 2 in the future. Google Summer of Code. Malcolm Wallace [25]announced that haskell.org has once again been accepted as a mentoring organisation for the 2009 Google Summer of Code. Student applications open on Monday (23rd March) at 1900 UTC, for a period of 12 days (until Fri 3rd April, also at 1900 UTC). Students applicants are encouraged to interact with the community via mailing lists, prior, during, and after the submission of their ideas for projects. Because (sadly) the darcs community did not get accepted as a separate organisation this year, haskell.org will be willing to accept proposals relating to darcs. regex-tdfa-1.1.0. ChrisK [26]announced the release of [27]regex-tdfa-1.1.0. This version is a small performance update to the old regex-tdfa-1.0.0 version. Previously all text (e.g. ByteString) being search was converted to String and sent through a single engine; the new version uses a type class and SPECIALIZE pragmas to avoid converting to String. This should make adding support for searching other Char containers easy to do. Haskell on your system? Information wanted!. Don Stewart [28]announced that haskell.org now features links to wiki pages explaining how to obtain Haskell on windows, mac osx and linux and bsd. If you're a distro maintainer for these systems, please consider adding relevant pointers to the pages, so that users of these systems can find all the info they need. libffi 0.1 released. Remi Turk [29]announced the release of [30]libffi 0.1, bindings to the C library libffi, allowing C functions to be called whose types are not known before run-time. Haskell Logo Voting has started!. Eelco Lempsink [31]announced that voting has begun to choose the new Haskell logo. All subscribed to haskell-cafe should have received a ballot; if you are not directly subscribed, you can still send ballot requests until the end of the competition (March 24, 12:00 UTC). Make sure the message contains 'haskell logo voting ballot request' in the subject. A long discussion of what color to paint the bike shed and why this particular bike shed will not do for storing bikes ensued. The Monad.Reader (13). Wouter Swierstra [32]announced that a new issue of [33]The Monad.Reader, a quarterly magazine about functional programming, is now available. Issue 13 consists of the following four articles: "Rapid Prototyping in TEX" by Stephen Hicks; "The Typeclassopedia" by Brent Yorgey; a Real World Haskell book review by Chris Eidhof and Eelco Lempsink; and "Calculating Monads with Category Theory" by Derek Elkins. dzen-utils 0.1. Felipe Lessa [34]announced the release of [35]dzen-utils 0.1, which contains various utilities for creating dzen input strings in a type-safe way using some combinators, including the ability to apply colors locally (instead of applying for everything beyond some point). It can also emulate dbar and gdbar, do automatic padding, and more. Discussion transformers versus mtl. Ganesh Sittampalam began a [36]discussion on the relative status of the 'transformers' and 'mtl' packages. least fixed points above something. Jens Blanck [37]asked about a function to compute fixed points starting from a seed value (as opposed to computing the least defined fixed point). Type equality proof. Martijn van Steenbergen [38]requested feedback on a proposed module collecting utilities for working with type equality proofs. What unsafeInterleaveIO is unsafe. Yusaku Hashimoto began a [39]discussion by asking why unsafeInterleaveIO is considered unsafe, or under what circumstances its use can be considered safe. Jobs How do students learn Haskell? Postgraduate project at University of Kent. S.J.Thompson [40]announced that funding is available for a [41]postgraduate project to study how students learn Haskell, based on the wealth of data collected through the instrumented version of the [42]Helium system for Haskell. The project will be supervised by Simon Thompson and Sally Fincher, in collaboration with Jurriaan Hage, Utrecht University. Blog noise [43]Haskell news from the [44]blogosphere. * Neil Mitchell: [45]Concise Generic Queries. Neil compares solving a query problem with several different generic programming libraries. * Jeff Heard: [46]Preview: Data Waves. A neat new type of data visualization, built using Hieroglyph. * >>> necrobious: [47]A fun example of Haskell's newtype. * GHC / OpenSPARC Project: [48]Peak issue rate is 18.64 Gig instrs/sec. * happstack.com: [49]Jeremy Shaw creates first Facebook App with Happstack. * GHC / OpenSPARC Project: [50]Peak performance. * Galois, Inc: [51]Solving Sudoku Using Cryptol. * Xmonad: [52]xmonad on ubuntu. A tutorial on getting started with xmonad on Ubuntu. * Don Stewart (dons): [53]Visualising the Haskell Universe. Pretty dependency graphs of ten thousand Haskell modules! * >>> Sean Chapel: [54]Haskell. * >>> Dean Berris: [55]The Haskell Experiment: HaskellDB, HTTP, and Monads. * Holumbus: [56]Hayoo! Update. The Hayoo! package index has been updated to include everything currently on Hackage! * >>> John Wiegley: [57]Journey into Haskell, Part 1. John ventures down the rabbit hole. * Bjorn Buckwalter: [58]Blogging with Pandoc, literate Haskell, and a bug. * Bjorn Buckwalter: [59]Extended sessions with the Haskell Curl bindings. * Brent Yorgey: [60]Monad.Reader #13 is out!. * Manuel M T Chakravarty: [61]Final version of "GPU Kernels as Data-Parallel Array Computations in Haskell".. * Xmonad: [62]Visualising xmonad. * Darcs: [63]darcs weekly news #21. * mightybyte: [64]Transactional Integrity Problem. * >>> Dean Berris: [65]The Haskell Experiment: Learning a New Programming Language. * >>> John Wiegley: [66]Hello Haskell, Goodbye Lisp. Quotes of the Week * ray: three dimensional zippers make my scalp hurt when i get my hair caught in them * dolio: [regarding a paypal spam message on #haskell] Take that, Harrop! Does OCaml have illegal cracking utilities? * lament: I think I speak for everyone in this channel when I say haskell is absolutely horrible and nobody would ever want to use it * MiguelMitrofanov: The first glimpse of this [logo] vote scared me so much that I've closed the page, stopped the browser, and shut my computer down. * osfameron: <ImInYourMonad> can I store gtk2hs-Buttons in a datastructure? <osfameron> ImInYourMonad: I think you have to sew them on with gtk2hs-Thread * chrisdone: I think you mean Peyton `Simon` Jones. About the Haskell Weekly News New editions are posted to [67]the Haskell mailing list as well as to [68]the Haskell Sequence and [69]Planet Haskell. [70]RSS is also available, and headlines appear on [71]haskell.org. To help create new editions of this newsletter, please see the information on [72]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [73]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://blog.happstack.com/2009/03/18/jeremy-shaw-creates-first-facebook-app… 3. http://www.galois.com/blog/2009/03/18/solving-sudoku-using-cryptol/ 4. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/16506 5. http://www.haskell.org//pipermail/haskell/2009-March/021116.html 6. http://www.haskell.org/haskellwiki/The_Monad.Reader 7. http://vis.renci.org/jeff/2009/03/20/preview-data-waves/ 8. http://donsbot.wordpress.com/2009/03/16/visualising-the-haskell-universe/ 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55109 10. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/16506 11. http://www.haskell.org/ghc/dist/6.10.2-rc1/ 12. http://www.haskell.org//pipermail/haskell/2009-March/021116.html 13. http://repetae.net/computer/jhc/ 14. http://article.gmane.org/gmane.comp.lang.haskell.general/16977 15. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/safe%2Dlazy%2Dio 16. http://article.gmane.org/gmane.comp.lang.haskell.general/16976 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/game%2Dtree 18. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55159 19. http://haskell.mperillo.ath.cx/random-shuffle-0.0.2.tar.gz 20. http://okmij.org/ftp/Haskell/perfect-shuffle.txt 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55143 22. http://haskell.mperillo.ath.cx/random-stream-0.0.1.tar.gz 23. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55116 24. http://projects.haskell.org/language-python/ 25. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55109 26. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55073 27. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex%2Dtdfa 28. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55036 29. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54959 30. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/libffi 31. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54958 32. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54880 33. http://www.haskell.org/haskellwiki/The_Monad.Reader 34. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54835 35. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dzen-utils 36. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/10807 37. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/55132 38. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/54946 39. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/54840 40. http://article.gmane.org/gmane.comp.lang.haskell.general/16967 41. http://www.cs.kent.ac.uk/research/pg/ 42. http://www.cs.uu.nl/wiki/Helium 43. http://planet.haskell.org/ 44. http://haskell.org/haskellwiki/Blog_articles 45. http://neilmitchell.blogspot.com/2009/03/concise-generic-queries.html 46. http://vis.renci.org/jeff/2009/03/20/preview-data-waves/ 47. http://necrobious.blogspot.com/2009/03/fun-example-of-haskells-newtype.html 48. http://ghcsparc.blogspot.com/2009/03/peak-issue-rate-is-1864-gig-instrssec.… 49. http://blog.happstack.com/2009/03/18/jeremy-shaw-creates-first-facebook-app… 50. http://ghcsparc.blogspot.com/2009/03/peak-performance.html 51. http://www.galois.com/blog/2009/03/18/solving-sudoku-using-cryptol/ 52. http://xmonad.wordpress.com/2009/03/18/xmonad-on-ubuntu/ 53. http://donsbot.wordpress.com/2009/03/16/visualising-the-haskell-universe/ 54. http://seanchapel.blogspot.com/2009/03/haskell.html 55. http://www.deanberris.com/mental-blabberings/2009/3/17/the-haskell-experime… 56. http://holumbus.fh-wedel.de/blog/?p=19 57. http://www.newartisans.com/2009/03/journey-into-haskell-part-1.html 58. http://flygdynamikern.blogspot.com/2009/03/blogging-with-pandoc-literate-ha… 59. http://flygdynamikern.blogspot.com/2009/03/extended-sessions-with-haskell-c… 60. http://byorgey.wordpress.com/2009/03/16/monadreader-13-is-out/ 61. http://justtesting.org/post/86905420 62. http://xmonad.wordpress.com/2009/03/16/visualising-xmonad/ 63. http://blog.darcs.net/2009/03/darcs-weekly-news-21.html 64. http://softwaresimply.blogspot.com/2008/02/transactional-integrity-problem.… 65. http://www.deanberris.com/mental-blabberings/2009/3/14/the-haskell-experime… 66. http://www.newartisans.com/2009/03/hello-haskell-goodbye-lisp.html 67. http://www.haskell.org/mailman/listinfo/haskell 68. http://sequence.complete.org/ 69. http://planet.haskell.org/ 70. http://sequence.complete.org/node/feed 71. http://haskell.org/ 72. http://haskell.org/haskellwiki/HWN 73. http://code.haskell.org/~byorgey/code/hwn/

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Numeric literals
by Lauri Oksanen 21 Mar '09

21 Mar '09

Hi, Is there some way to tell ghc, how to interpret numeric literals? I would like it to interpret 1 as 1 :: Integer not 1 as fromInteger (1 :: Integer) I have been playing with the following (rather ugly) code. {-# OPTIONS -XFunctionalDependencies -XMultiParamTypeClasses -XTypeSynonymInstances #-} module Test where import Prelude(Integer, Double) import qualified Prelude as P default(Integer, Double) type Z = Integer type R = Double class Plus a b c | a b -> c where (+) :: a -> b -> c instance Plus Z Z Z where j + k = j P.+ k instance Plus R R R where x + y = x P.+ y instance Plus R Z R where x + j = x P.+ P.fromInteger j instance Plus Z R R where j + x = P.fromInteger j P.+ x x = (1 :: Z) + (1.0 :: R) --y = 1 + 1.0 The commented line can't be compiled (because of ambiguous types, I think). - Lauri

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random shuffle and random list partition
by Manlio Perillo 21 Mar '09

21 Mar '09

Hi. For my Netflix Prize project I have implemented two reusable modules. The first module implements a random shuffle on immutable lists. It uses http://okmij.org/ftp/Haskell/perfect-shuffle.txt, with an additional "wrapper" function, having a more friendly interface. The second module implements a function used to partition a list into n sublists of random length. I have pasted the modules here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2483 http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2485 If someone is interested (and if Oleg give me permission), I can release them as a package on Hackage. I need to improve documentation, however. In future I can add an implementation of the random shuffle algorithm on mutable arrays in the ST monad. Manlio

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least fixed points above something
by Jens Blanck 21 Mar '09

21 Mar '09

Hi, I found myself writing the following leastFixedPoint :: (Eq a) => (a -> a) -> a -> a leastFixedPoint f x = fst . head . dropWhile (uncurry (/=)) $ zip l (tail l) where l = iterate f x and was a bit surprised that I couldn't get any matches on hoogle for the type above. The closest one is fix :: (a -> a) -> a but that sort of assumes that we're starting the fixed point search from the bottom element (undefined). Anyway, is there a nicer way of doing the above? Jens

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Grokking zippers
by Nathan Bloomfield 21 Mar '09

21 Mar '09

Hello all- I'm trying to understand the categorical guts underlying zippers. In the Haskell wikibook (and other places) I've seen zippers described roughly as the derivatives of functors. However, I haven't been able to find any references that develop this idea rigorously. For instance, what exactly is the "derivative" of a functor at an object, in the direction of some arrow? I'm interested in studying this concept in more depth, but I can't find a definition to start with. Any pointers to good books or papers would be greatly appreciated. :) Thank you all- Nathan Bloomfield

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Re: [Haskell-cafe] Re: categories and monoids
by Kalman Noel 21 Mar '09

21 Mar '09

Wolfgang Jeltsch schrieb: > Okay. Well, a monoid with many objects isn’t a monoid anymore since a monoid > has only one object. It’s the same as with: “A ring is a field whose > multiplication has no inverse.” One usually knows what is meant with this but > it’s actually wrong. Wrong for two reasons: First, because the multiplication > of a field has an inverse. Second, because the multiplication of a ring is > not forced to have no inverse but may have one. “A ring is like a field, but without a multiplicative inverse” is, in my eyes, an acceptable formulation. We just have to agree that “without” here refers to the definition, rather than to the definitum.

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what does "atomically#" mean?
by Daryoush Mehrtash 21 Mar '09

21 Mar '09

Any idea was the atomically# mean in the following code? atomically :: STM a -> IO a atomically (STM m) = IO (\s -> (*atomically# *m) s ) Code is from GHC.Conc module http://www.haskell.org/ghc/docs/6.6/html/libraries/base/GHC-Conc.html daryoush

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Re: [Haskell-cafe] Ease of Haskell development on OS X?
by Jason Dusek 21 Mar '09

21 Mar '09

2009/03/20 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > Question: Does the iMac have good support for Haskell > development? Yes. I generally use the Mac binary of GHC and then install C libs with MacPorts. > Question: What environment setups do people commonly use (e.g. > Eclipse Xcode etc)? I use screen and vim. In particular, I use MacVim, which provides both a vim console application and a Mac native GUI. > Question: Are there any caveats I should be aware of before > changing systems (i.e. unpleasant surprises). Mac laptops get very warm. You'll have a powerful urge to buy an iPhone and pimp Macs to your friends. There is really no 'home' or 'end' key. To move back and forth on a line in the Terminal application, use fn-shift-leftarrow and fn-shift-rightarrow (annoyed me for days). To move back and forth on a line in grapical application -- for example, the search bar in Firefox -- use cmd-leftarrow and cmd-rightarrow. MacPorts is not as robust as comparable systems on other operating systems: Gentoo's Portage and FreeBSD's Ports. It corrupts its registry and terrible things happen. The remedy is usually a total reinstall of all packages. This has happened to me only once on one Mac in a year and a half of administering several Macs; but it's not something I've ever dealt with on other source-based systems. -- Jason Dusek

1 0

Re: [Haskell-cafe] Ease of Haskell development on OS X?
by Cristiano Paris 21 Mar '09

21 Mar '09

2009/3/20 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > Hi, > > I’ve been thinking of changing over to an iMac from my crappy old PC running > Windows Vista. > > Question: Does the iMac have good support for Haskell development? > > Question: What environment setups do people commonly use (e.g. Eclipse Xcode > etc)? > > Question: Are there any caveats I should be aware of before changing systems > (i.e. unpleasant surprises). > > I want to be able to use the machine for Haskell OpenGL programming. Take a look at this (rather long) thread on Haskell-cafè: http://www.haskell.org/pipermail/haskell-cafe/2009-February/056545.html Cristiano

3 2

Editing Haskell wiki pages
by Colin Paul Adams 21 Mar '09

21 Mar '09

How am I supposed to edit a page on the Haskell wiki? If I click on an "Edit this page" link, then Firefox prompts me to choose a tool to open an application/x-external-editor. When i just choose the default (emacs) I get the following opened in emacs: [Process] Type=Edit text Engine=MediaWiki Script=http://www.haskell.org/haskellwiki/ Server=http://www.haskell.org Path=/haskellwiki Special namespace=Special [File] Extension=wiki URL=http://www.haskell.org/haskellwiki/?title=Applications_and_libraries/Ga… which isn't much use (and further, it's write-protected!). -- Colin Adams Preston Lancashire

3 8

Ease of Haskell development on OS X?
by Mark Spezzano 20 Mar '09

20 Mar '09

Hi, I’ve been thinking of changing over to an iMac from my crappy old PC running Windows Vista. Question: Does the iMac have good support for Haskell development? Question: What environment setups do people commonly use (e.g. Eclipse Xcode etc)? Question: Are there any caveats I should be aware of before changing systems (i.e. unpleasant surprises). I want to be able to use the machine for Haskell OpenGL programming. Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.20/2012 - Release Date: 19/03/2009 12:26 PM

1 0

Forcing evaluation when seq or strictness annotation are not possible [was: guess on stack-overflows - thunks build-up and tail recursion]
by Gü?nther Schmidt 20 Mar '09

20 Mar '09

Hi all, here I am again with my all time favorite unsolved problem: stack overflows. The advice I have received so far from the Haskell community (this list and #haskell) was to use strictness annotation or seq in most cases. And indeed it did help. It certainly helped when I used a data structure that I designed myself and a filling routine that I also wrote myself, thanks guys (and girls possibly, I mean I don't know). But sometimes neither of the 2 helps. Especially when you are using a brilliantly designed data structure such as IxSet, from happstack, originally HAppS. Since it's someone else code it's too late to add strictness annotation. Or the "core" insert that you have to use in your filling routine is lazy. In such cases you're just out of a luck. I have read about alternatives to the above mentioned techniques, CPS, Codensity-Monad and this http://wwwtcs.inf.tu-dresden.de/~voigt/mpc08.pdf in Blogs, Wikis and Papers, also on this mailing list a post called "stupid question #852". Well when I say alternatives I mean I *perceived* these techniques to be *meant* as alternatives, as a way to control evaluation order, or *force* evaluation. And this is my question: Am I barking up the wrong tree, is the purpose of these techniques actually not to force evaluation order? Günther

1 0

[ANN] random-shuffle package
by Manlio Perillo 20 Mar '09

20 Mar '09

I'm pleased to announce the availability of random-shuffle package. The cabalized package is available at http://haskell.mperillo.ath.cx/random-shuffle-0.0.2.tar.gz The code is based on http://okmij.org/ftp/Haskell/perfect-shuffle.txt Thanks to Oleg Kiselyov for sharing this! I have made some changes to Oleg code: - changed coding style to what seems to be a more common Haskell coding style - Used Int, instead of Integer, for the index type - Forced some strict evaluation. It was a trial and error effort (I'm not an expert), but results are very impressive. Example program and timings/memory usage, are on: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2600 NOTE: I don't plan to upload the package to Hackage, for the near future. I'm thinking to add some other functions that operate on lists, like Python random module (random.choice, random.sample), and the partitioning function I have posted the other day (although I'm not sure this is a generic useful function). However, in this case, the package name should be changed. I'm not sure it is a good idea to release a package that implements only one function (but I may be wrong). Feedbacks (on current random-shuffle and future additions) are welcomed. Regards Manlio Perillo

7 10

Using cabal to build an exportable library?
by Justin Bailey 20 Mar '09

20 Mar '09

I've been following this instructions at http://www.haskell.org/haskellwiki/Calling_Haskell_from_C to build a Haskell library which I can call from a C program. I'd like to use cabal to do the build in the future. Are there any examples showing how to do it?

2 1

server directory layout for cabal-install?
by Johannes Waldmann 20 Mar '09

20 Mar '09

Hello. I wonder what is the required layout of directories and files on a server that supplies packages for cabal-install. I figure 00-index.tar.gz contains the *.cabal files. (The cabal files have to be on the server as well, unpacked?) Each $dir/$foo.cabal (in the archive) contains a "Version:$ver" line, and then $dir/$foo-$ver.tar.gz must be on the server? Are there additional constraints on the $dir part? What is the syntax of the .cabal/config file? The standard entry is repos: hackage.haskell.org:http://hackage.haskell.org/packages/archive , where the thing between "repos:" and ":http" is just a symbolic name, and I can add more lines like that? Thanks, J.W.

2 3

Query on list comprehension
by Melanie_Green 20 Mar '09

20 Mar '09

What are the limitations of list comprehension. I want to use listcomprehension to output the pattern below. So a mixture of a's and newline characters. The part im stuck at is creating arguments in the listcomprehension to stop at some point then execute next loop. Is it even possible to create the pattern below purely from list comprehension.Thankyou in advance. a aa aaa -- View this message in context: http://www.nabble.com/Query-on-list-comprehension-tp22573574p22573574.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

6 7

Haskell Logo Voting has started!
by Eelco Lempsink 20 Mar '09

20 Mar '09

Hi there! I updated a couple of logo versions and ungrouped and regrouped the (former) number 31. Other than that, there was nothing standing in the way of the voting to begin imho, so I started up the competition. By now, I suppose everybody should have received their ballot. If you think you should have received it but didn't, please report it, I can resend the invitation. Also, for people not directly subscribed to the haskell-cafe mailing list, you can still send ballot requests until the end of the competition (March 24, 12:00 UTC). Make sure the message contains 'haskell logo voting ballot request' (e.g. in the subject). Depending on the winner of this voting round we can decide whether we need to continue with variations. Jared Updike already offered to donate a bit of time to help create several variations. But for now, good luck with sorting those options! :) -- Regards, Eelco Lempsink

19 28

What unsafeInterleaveIO is unsafe
by Yusaku Hashimoto 20 Mar '09

20 Mar '09

Hello, I was studying about what unsafeInterleaveIO is.I understood unsafeInterleaveIO takes an IO action, and delays it. But I couldn't find any reason why unsafeInterleaveIO is unsafe. I have already read an example in http://www.haskell.org/pipermail/haskell-cafe/2009-March/057101.html says lazy IO may break purity, but I think real matter in this example are wrong use of seq. did I misread?

13 35

Why do functions not inherit class when renamed?
by Duane Johnson 20 Mar '09

20 Mar '09

I'm not sure I understand what is going on here, but it seems to me that the following assignment of "mult = (*)" causes "mult" to lose out on some of the polymorphic behavior of (*). Is that right? If so, why? > Prelude Data.Function Data.List Data.Char> let sizeMult = (*) `on` > length > Prelude Data.Function Data.List Data.Char> sizeMult "Duane" "Johnson" > 35 > Prelude Data.Function Data.List Data.Char> let mult = (*) > Prelude Data.Function Data.List Data.Char> let sizeMult2 = mult `on` > length > > <interactive>:1:26: > Couldn't match expected type `Integer' against inferred type `Int' > In the second argument of `on', namely `length' > In the expression: mult `on` length > In the definition of `sizeMult2': sizeMult2 = mult `on` length > Prelude Data.Function Data.List Data.Char> Thanks, Duane Johnson http://blog.inquirylabs.com/

3 2

curl w/ ghc-win32
by Sigbjorn Finne 20 Mar '09

20 Mar '09

Hi, there's a steady stream of people reporting that getting started with curl on Windows is cumbersome and tricky to get right. A legitimate complaint! :-) To hopefully help out a bit in that regard, I've put together some notes covering the steps reqd + made available a pre-packaged curl binary dist -- http://haskell.forkio.com/Home/curl-win32 enjoy.. :) --sigbjorn

1 0

[ANN] random-stream package
by Manlio Perillo 20 Mar '09

20 Mar '09

Hi. I'm pleased to announce the availability of my random-stream package. The cabalized package is available at: http://haskell.mperillo.ath.cx/random-stream-0.0.1.tar.gz Note that I have not uploaded it on Hackage, and I do not plan to upload it in the near future, at least until I will repute the package mature enough. From package description: Portable interface for the operating system source of pseudo random data. Supported sources are Unix /dev/urandom, Win32 CryptGenRandom and OpenSSL pseudo random numbers generator. This package is based on idea from os.urandom implementation, in CPython. The idea is to view the system pseudo random generator as a stream of infinite random bytes. So, I have used a lazy bytestring, and unsafePerformIO/unsafeInterleaveIO. The underlying data is: newtype Stream = Stream L.ByteString and the constructor *is exposed*. The package configuration *must* be done using a flag to specify the "source" to use. As an example: runghc Setup.hs configure -O2 -fHAVE_URANDOM runghc Setup.hs configure -O2 -fHAVE_SSL runghc Setup.hs configure -O2 -fHAVE_WIN32_CRYPT If the flag is not specified, the compilation will fail. Note that Windows it not yet supported. The stream generator implements tha RandomGen interface. Some notes: 1) The system *pseudo* random generator is used. As an example, on Linux, /dev/random produces "true" random numbers, but it may block if there is not enough entropy in the system. More details on http://en.wikipedia.org/wiki//dev/random 2) When reading data, the lazy bytestring chunk size is used. The default chunk size, however, is fine when reading from a regular file, but may not be the best choice when reading pseudo random numbers. For SSL (and Windows) support, the chunk size can be easily changed; but for Unix support this seems to not be possible, unless I reimplement hGetContents. The Data.ByteString.Lazy module have an hGetContentsN function. Unfortunately it is not exposed. I find this annoying; is it possible to export the *N functions from Data.ByteString.Lazy.Internal? 3) I have tested the package on Debian Linux Etch, using GHC 6.8.2. Feedback will be appreciate. I not sure about some details. Usage example: module Main where import System.Random import System.Random.Stream gen = mkStream l :: [Int] l = randoms gen :: [Int] main = do print l Manlio Perillo

4 7

Hoogling to no avail ...
by Galchin, Vasili 20 Mar '09

20 Mar '09

Hello, I need sleep briefly(1/2 seconds) in code that I have written. I tried searching for "sleep" and "wait" in Hoogle and either didn't get a hit or got something inappropriate. ?? Regards, Vasili

2 1

Summer Of Code 2009 project idea
by Csaba Hruska 20 Mar '09

20 Mar '09

Hi! I'd like to participate in gsoc2009. My project idea is to complete my initial render backend ( http://www.haskell.org/haskellwiki/LambdaCubeEngine) what can be useful for (possibe more) haskell projects. I'd like to extend it to support physics (reusing hpysics source code), etc. Current features: + easy (from user view) model loading and rendering + definig materials (texture layers, blending setup, shaders, etc) via readable material scripts + fast rendering via vertex buffers Some planned features: + skeletal animation (in progress) + physics (hpysics) integration + efficient terrain rendering + some mesh modifier combinators etc.. Do you think is this useful? Or should i think out a more relevant idea? Cheers, Csaba Hruska

3 4

Re: [Haskell-cafe] Re: Haskell Logo Voting has started!
by Wolfgang Jeltsch 20 Mar '09

20 Mar '09

Am Mittwoch, 18. März 2009 13:31 schrieben Sie: > On Wed, Mar 18, 2009 at 04:36, Wolfgang Jeltsch wrote: > > Am Mittwoch, 18. März 2009 10:03 schrieb Benjamin L.Russell: > >> Just go through the list, choose your top favorite, and assign rank 1 > >> to it; > > > > Is rank 1 the best or the worst? > > The condorcet info page makes it clear that higher is better. > > http://www.cs.cornell.edu/w8/~andru/civs/rp.html So assigning rank 1 to my favorite, as Benjamin suggested, might not be the best idea. :-( I hope, everyone who voted did it the right way. Best wishes, Wolfgang

8 7

Proof of duality theorem of fold?
by R J 20 Mar '09

20 Mar '09

I'm trying to prove the following duality theorem of fold for finite lists: foldr f e xs = foldl (flip f) e (reverse xs) where reverse :: [a] -> [a] reverse [] = [] reverse (x:xs) = reverse xs ++ [x] flip :: (a -> b -> c) -> b -> a -> c flip f y x = f x y foldr :: (a -> b -> b) -> b -> [a] -> b foldr f e [] = e foldr f e (x : xs) = f x (foldr f e xs) foldl :: (b -> a -> b) -> b -> [a] -> b foldl f e [] = e foldl f e (x : xs) = foldl f (f e x) xs I'm stuck on the induction case. Can someone help? Here's what I've got so far: Proof: Case _|_. Left-side reduction: foldr f e _|_ = {case exhaustion of "foldr"} _|_ Right-side reduction: foldl (flip f) e (reverse _|_) = {case exhaustion of "reverse"} foldl (flip f) e _|_ = {case exhaustion of "foldl"} _|_ Case []. Left-side reduction: foldr f e [] = {first equation of "foldr"} e Right-side reduction: foldl (flip f) e (reverse []) = {first equation of "reverse"} foldl (flip f) e [] = {first equation of "foldl"} e Case (x : xs). Left-side reduction: foldr f e (x : xs) = {second equation of "foldr"} f x (foldr f e xs) = {induction hypothesis: foldr f e xs = foldl (flip f) e (reverse xs)} f x (foldl (flip f) e (reverse xs)) Right-side reduction: foldl (flip f) e (reverse (x : xs)) = {second equation of "reverse"} foldl (flip f) e (reverse xs ++ [x]) _________________________________________________________________ Hotmail® is up to 70% faster. Now good news travels really fast. http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_HM_70faster_032009

4 5

Text bug in Hieroglyph 1.2
by Danny Chan 20 Mar '09

20 Mar '09

Since I haven't found out how to report bugs for this package, I am posting this here. Changing the color of Text elements in a Hieroglyph image like this: text {attributes = plain {strokeRGBA = opaque blue}} does not work correctly. Instead, the color used for the text is the one of the previously drawn element. With the little knowledge I have about cairo, it seems that the bug is in the renderPrimitive function in Cairo.hs where the order of Cairo.setSourceRGBA and Cairo.showText is wrong. Something like this seems to work: renderPrimitive _ s0 (Text str (Point ox oy) state) = do applyAttributeDelta s0 state Cairo.moveTo ox oy let (sr,sg,sb,sa) = colourToTuple . strokeRGBA $ state when (outlined state) $ Cairo.setSourceRGBA sr sg sb sa Cairo.showText str when (clipped state) $ Cairo.clip return state By the way, for the complete newbie that I am, is there a standard way to report bugs for a package from Hackage? Danny

3 4

Proposal for associated type synonyms in Template Haskell
by Thomas van Noort 19 Mar '09

19 Mar '09

Hello, Recently, we released a library on Hackage for generic rewriting (package "rewriting" if you are curious). The user of the library is expected to define type class instances to enable rewriting on his or her own datatypes. As these instances follow the datatype declarations closely, we tried to generate the instances using Template Haskell. Unfortunately, associated type synonyms are not yet supported by TH. After a presentation at the WGP'08, Simon encouraged us to write a proposal about adding associated type synonyms to TH, so that it can be added to GHC. So, here is our proposal. The TH AST must allow 1) kind declarations of associated type synonyms in class declarations and 2) their definitions in instance declarations. For example, class Foo a where type Bar a :: * instance Foo Int where type Bar Int = String The TH library defines a datatype Dec which contains a constructor for class declarations and instance declarations: data Dec = ... | ClassD Cxt Name [Name] [FunDep] [Dec] | InstanceD Cxt Type [Dec] ... 1) Associated type synonym kind declarations We suggest to add a constructor to the Dec type: ... | AssocTySynKindD Name [Name] (Maybe Kind) ... assocTySynKindD :: Name -> [Name] -> Maybe KindQ -> DecQ The first field is the name of the associated type synonym, the second field is a list of type variables, and the third field is an optional kind. Since kinds are not yet defined in TH, we have to add some kind of kind definition (pun intended): data Kind = StarK | ArrowK Kind Kind type KindQ = Q Kind starK :: KindQ arrowK :: KindQ -> KindQ -> KindQ We explicitly choose not to reuse the Type type to define kinds (i.e., type Kind = Type as in GHC) since we think a separation between the two worlds is much clearer to the users of TH. 2) Associated type synonym definitions We suggest to add another constructor to the Dec type: ... | AssocTySynD Name [Type] Type ... assocTySynD :: Name -> [TypeQ] -> TypeQ -> DecQ The first field is the name of the type synonym, the second field is a list of type arguments, and the third field is the body of the type synonym. We would like to hear your comments to this proposal. Regards, Thomas

5 5

doubts about runGetState in the binary package
by Manlio Perillo 19 Mar '09

19 Mar '09

Hi. I have some doubts about the runGetState function in the binary package. The signature is: runGetState :: Get a -> LBS -> Int64 -> (a, LBS, Int64) however the Int64 "input parameter" is not documented. What value should I pass? How will be used? Thanks Manlio Perillo

2 2

alex documentation
by j.romildo＠gmail.com 19 Mar '09

19 Mar '09

Hello. I need the documentation of alex (a lexical analyser generator in Haskell) in PDF format, so I used the Makefile in the doc directory of the distribution of alex to get it. But unfortunatly, the documentation includes some code listings that are too wide and does not fit in the default page width used by the tools generating the PDF file. So, it would be nice if someone could rearrange the listings int the documentation source, so that it fits better in a printed page. José Romildo PS: I am going to use alex in the Compiler Construction classes I am teaching.

1 0

Re[2]: [Haskell] ANNOUNCE: jhc 0.6.0 Haskell Compiler
by Bulat Ziganshin 19 Mar '09

19 Mar '09

Hello sylvain, Thursday, March 19, 2009, 2:16:06 AM, you wrote: > Something is wrong: how can Haskell be faster than C? 1. you measure efficiency of libs, not compilers 2. don't forget about -O2/-O3 -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

1 0

Announce: language-python
by Bernie Pope 19 Mar '09

19 Mar '09

Language-python provides a parser (and lexer) for Python written in Haskell. Currently it only supports version 3 of Python (the most recent version), but it will support version 2 in the future. The parser is implemented using Happy, and the lexer is implemented using Alex. Web page: http://projects.haskell.org/language-python/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/language-python Darcs: darcs get http://code.haskell.org/language-python/ Example: $ ghci Prelude> :m Language.Python.Version3.Parser Prelude Language.Python.Version3.Parser> parseModule "def inc(x): return x + 1\n" [] Right (Module [Fun {fun_name = Ident "inc", fun_args = [Param {param_name = Ident "x", param_annotation = Nothing, param_default = Nothing}], fun_result_annotation = Nothing, fun_body = [Return {return_expr = Just (BinaryOp {operator = Plus, left_op_arg = Var (Ident "x"), right_op_arg = Int 1})}]}]) Prelude Language.Python.Version3.Parser> :m Language.Python.Version3.Lexer Prelude Language.Python.Version3.Lexer> lexOneToken "def inc(x): return x + 1\n" [] Right (Def (Sloc {sloc_filename = "", sloc_row = 1, sloc_column = 1})," inc(x): return x + 1\n") This is the first release of the package (version 0.1.1) and there is still much to be improved, in particular: - Unicode support is incomplete. - Source locations are sub-optimal, and will eventually become source spans. - The pretty printer needs polish. - The parser only supports version 3 of Python. Support for Version 2 is a major goal, and should be straightforward. - Only minimal performance tuning and correctness testing has been performed. Version 0.1.1 is not intended for production use. Cheers, Bernie.

2 1

What *not* to use Haskell for
by Dave Tapley 19 Mar '09

19 Mar '09

Hi everyone So I should clarify I'm not a troll and do "see the Haskell light". But one thing I can never answer when preaching to others is "what does Haskell not do well?" Usually I'll avoid then question and explain that it is a 'complete' language and we do have more than enough libraries to make it useful and productive. But I'd be keen to know if people have any anecdotes, ideally ones which can subsequently be twisted into an argument for Haskell ;) Cheers, Dave

39 82

Tweaking the garbage collector for realtime usage
by Peter Verswyvelen 19 Mar '09

19 Mar '09

The GHC documentation lists a lot of tweaks that can be done to the garbage collector. However, Haskell spin-offs like Timber <http://www.timber-lang.org/> implement their own incremental garbage collector that is better suitable for real-time usage. Did someone already fiddle with GHC's gc flags so it works better for real-time applications, like games and simulations?

3 2

Crash in GHCI - what is the correct behavior here?
by Zachary Turner 19 Mar '09

19 Mar '09

Basically just learning haskell, I would have posted this in the beginners list but since it involves a segfault of GHCI, I figured it might be worth posting here. I was trying to get a good understanding of local variable scoping issues, so I tried the following: f :: (Num a) => a -> a f x = let p = x*x in let p = x*p in p I have some background in ML, which led me to believe that what should happen here is that the function would return x^3. Instead, GHCI just completely terminates, I guess with a segfault. What's the "correct" behavior here? Should it even compile? I understand that you can't redefine the same symbol twice in the same scope, so I tried this specifically to see what would happen if you defined the same variable again in a nested scope. I thought it would just shadow the original declaration, while still using the original p to calculate the value of the new p. I don't think the problem is the re-declaration of the same symbol in a nested scope (although if someone could clarify that would be nice), but rather the fact that I've attempted to use the previous declaration of p in defining the new declaration of p. Would it be a safe assumption that a bug report should be submitted over this?

3 4

Type equality proof
by Martijn van Steenbergen 19 Mar '09

19 Mar '09

Olá café, With the recent generic programming work and influences from type-dependent languages such as Agda, the following data type seems to come up often: > data (a :=: a') where > Refl :: a :=: a Everyone who needs it writes their own version; I'd like to compile a package with this data type and related utilities, if such a package doesn't exist already (I couldn't find one). Below is what I have so far; I'd much appreciate it if you added your ideas of what else the package should contain. > {-# LANGUAGE GADTs #-} > {-# LANGUAGE TypeOperators #-} > > module Eq where > > data (a :=: a') where > Refl :: a :=: a > > class Eq1 f where > eq1 :: f a -> f a' -> Maybe (a :=: a') > > class Eq2 f where > eq2 :: f a b -> f a' b' -> Maybe (a :=: a', b :=: b') > > class Eq3 f where > eq3 :: f a b c -> f a' b' c' -> Maybe (a :=: a', b :=: b', c :=: c') Thank you, Martijn.

10 17

Fw: hw do i solve this problems:
by THANDO NTUANE 19 Mar '09

19 Mar '09

----- Forwarded Message ---- From: THANDO NTUANE <thandodarl85(a)yahoo.com> To: Haskell-Cafe(a)haskell.org Sent: Thursday, March 19, 2009 10:22:40 AM Subject: hw do i solve this problems: kindly help me.. im stack but managed to solve exercise 1 and 2.

4 3

Memoizing partially evaluated computations.
by Sebastiaan Visser 19 Mar '09

19 Mar '09

Suppose I have a list of IO computations that depends on a few very time consuming pure operations. The pure operations are not dependent on the real world: > computation :: [IO Int] > computation = [ > smallIOfunc timeConsumingPureOperation0 > , smallIOfunc timeConsumingPureOperation1 > , smallIOfunc timeConsumingPureOperation2 > , smallIOfunc timeConsumingPureOperation3 > ] > where smallIOfunc a = print a >> return a In my main function I would like to repeatedly print the values > main = forever $ > sequence_ (map (>>=print) computation) When I do this, all the time consuming operations will be reevaluated every run of the main loop. Is there a any (simple or smart) way to prevent the garbage collector from cleaning up the fully evaluated thunks inside my computation? As if it were something like this: > computation :: [IO Int] > computation = [smallIOfunc 42, smallIOfunc 34385, smallIOfunc 3, smallIOfunc 55] Of course I could plugin some kind of Int memoizer inside my computation, but I do not really have the control to change things `deep' inside the code. I want to have some form of snapshot of a list of partially evaluated IO computations... Any suggestions? Tanks, Sebastiaan

4 6

Haskell Style - Pattern Matching with case vs. function declarations
by Tom.Amundsen 19 Mar '09

19 Mar '09

Hi All, I am new to Haskell. I just started reading "Real World Haskell" a few days ago, so I apologize for being such a noob. But, I am curious why I see a lot of code where people do pattern matching via multiple function declarations instead of using the case ... of ... construct? For example: [code] foldl' _ zero [] = zero foldl' step zero (x:xs) = let new = step zero x in new `seq` foldl' step new xs [/code] instead of this, which I prefer: [code] foldl' f acc xs = case xs of [] -> acc (x:xs) -> let new = f acc x in new `seq` foldl' f new xs [/code] -- View this message in context: http://www.nabble.com/Haskell-Style---Pattern-Matching-with-case-vs.-functi… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 3

hw do i solve this problems:
by THANDO NTUANE 19 Mar '09

19 Mar '09

kindly help me.. im stack but managed to solve exercise 1 and 2.

1 0

Any way to catch segmentation fault?
by Nick Rudnick 19 Mar '09

19 Mar '09

Hi all, doing some work with a regex library I stumbled over some segmentation faults due to illegal byte combinatations. Looking for a way to get this caught some way, I failed in finding any place at GHC (6.10.1) or Hackage libraries where this is covered -- a quick check with HUnit revealed it to be crashing by this phenomenon. So I would like to ask about the state of this issue, o Is there principally no way to (generally) catch segmentation faults?? (This would be hard to believe, as the described kind of noise is to be expected at production systems, especially with user generated content.) o Are segmentation faults «prohibited» in Haskell so the advice is just to change the used library and forget the whole stuff?? o Did I oversee the generic mechanism for catching of segmentation faults?? (If yes, do you please give me a hint??) o If no, is there a workaround which might be practicable?? Thanks a lot in advance, Dorinho

2 1