- Haskell-Cafe - Haskell.org

API for reading a big binary file
by Ranjan Bagchi 22 Dec '06

22 Dec '06

Hi -- I'm still learning to use ghc effectively, and I'm trying to use it for more-or-less mundane programming tasks, hopefully in a way which is a lot more elegant than I'd do in Java or Perl. I've got a big [around a gigabyte] binary file, filled with identical binary structures (imagine a C process writing structs). I'd like to process/analyze them efficiently. In C or even Java, i'd memory map the file and extract the data I need. Is there a fast way to do this using ghc? I can extract fields by using a ByteString, but I may not be using it fast enough: I've had to write my own routines to extract ints, longs and doubles. The other option is to write the loading code largely in C, but I'm not clear how to write a data type which maps well to a C struct. Any help / examples would really be appreciated. Thanks, Ranjan

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cabal running multiple preprocessors on one file - suggestion
by Marc Weber 22 Dec '06

22 Dec '06

Am I right that the way to do this now is create your own preprocessor run on files havirg your extionsion? What about naming the source files eg Module.hs.cpp.di.gc.chs ? This would mean first run c2hs, then greencard thin DrIft then cpp and put the results in Module.hs.cpp.di.gc.chs 1) Module.hs.cpp.di.gc 2)` Module.hs.cpp Module.hs pro : you see immediately which preprocessors are to be invoked you don't need write your own preprocessor handler.. con : There are to much possibilities to use doesFileExist? so the way to get the source file is using getDirectoryContents and filter by regex \.hs.* \.lhs.* matches and take longest filename (So that 1) will be perefered over 2) You need a customizable editor to set filetype by regular expression rather by (.gc .hs .lhs .chs ..) What do you think, would this be an enhancement? I'd like to implement this and provide a darcs patch to cabal 1.1.7 Any ideas/ comments on this? Marc

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Haskell Side Effect
by Ashley Yakeley 22 Dec '06

22 Dec '06

Since learning Haskell, I can now count in Spanish! See: "one" in Spanish, "two" in Spanish, "three" in Spanish, "four" in Spanish.. -- Ashley Yakeley

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Timer with Haskell GUI application
by asilovy 21 Dec '06

21 Dec '06

Hi all, I'm still trying to figure out why, in the Graphics.HGL library, is the timer not treated as the other events (mouse clics, key pressing... ? For instance see : http://haskell.org/ghc/docs/latest/html/libraries/HGL/Graphics-HGL-Window.h… As I understand, the timer in a HGL GUI application must be treated by waiting for a tick to occur (rather than doing useful things and being warned with maybeGetWindowEvent if one occurs). This sounds stupid to me and gives rise to non responding situations. Im a misunderstanding something ? Is it a weakness of HGL and does it force me to change all my approach and turn to another library ? Any help would be appreciated. Thanks Alain -- View this message in context: http://www.nabble.com/Timer-with-Haskell-GUI-application-tf2864634.html#a80… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Shrinking the Prelude: The "categorical" approach
by Imam Tashdid ul Alam 21 Dec '06

21 Dec '06

hi guys, I was just wondering if anyone is interested is a "quasi"-project of rewriting the Prelude (only shrinking it as measured by the total number of names imported, read along...) the idea is (just to be substantially different in approach) to develop an "alternate history" of Haskell. take map for example, and fmap, I don't think they should be named different (fmap is ugly, not suggestive, and conceptually the same). mplus could be renamed (++) (they are conceptually the same, mzero looks odd, name it empty, or simply zero) although I think concat should be replaced by msum (or the other way around preferably :) ? msum is a bad name) I am somewhat confused by join. concat seems to match join's type but I don't think the ideas coincide. and so on. in particular, we would want: * no backwards compatibility. the Prelude as is it, is good enough. we are defining a new module. * clean categorical hierarchy of type classes (category theorists wanted!) promoting uniformity. * cleaner names. foldl1 means nothing. absolutely nothing. what's the 1 for? * our Prelude only contains typeclasses, datatypes, and functions of utmost conceptual (and semantic, such as error, undefined and seq) importance, everything else goes to other modules. our Prelude is not going to be a place for "convenient" declarations. * the suffix method of naming (liftM2) is considered messy. we would prefer a seperate module (promoting qualified import). this is a fun project. we will not rewrite the Prelude, we'll merely rename it. I would suggest the name TheOtherPrelude to make our intentions clear. the conveniences (like concatMap) goes to TheOtherPrelude.Extension. I suggest we do it on the wiki. anyone interested just open a page with your name of choice. I am not doing it only because there's no point doing it if no one's interested. cheers, Imam __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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multi parameter type classes for NP problems
by Joshua Ball 21 Dec '06

21 Dec '06

Hi all, For my own study, I've been playing around with various NP complete problems. Previuosly I was doing so in Java, but because I want to learn Haskell, I'm trying to port the algorithms. In Java, I had an abstract class called AbstractNPProblem which looked like this: public abstract class AbstractNPProblem implements NPProblem { public abstract boolean validates(Certificate c); public abstract List<Certificate> certificates(); public boolean decide() { for (Certificate c : certificates()) { if (validates(c)) { return true; } } return false; } } This has one problem, however: it is slightly dynamically typed. Anybody that implements the verify method must cast the object c to a particular type (could be a bitmask, a list of integers, a subgraph, etc.) I'd like to get rid of this problem in porting to Haskell. Here is how I am trying to solve the problem, using multi-parameter type classes. class NPProblem inst cert where validates :: cert -> inst -> Bool certificates :: inst -> [cert] decide :: inst -> Bool decide i = any (\x -> x `validates` i) $ certificates i Unfortunately, ghc throws the following type error: NPProblem.hs:5:45 Could not deduce (NPProblem inst a) from the context (NPProblem inst cert) arising from use of `certificates' at NPProblem.hs:5:45-58 Possible fix: add (NPProblem inst a) to the class or instance method `decide' In the second argument of `($)', namely `certificates i' In the expression: (any (\ x -> x `validates` i)) $ (certificates i) In the definition of `decide': decide i = (any (\ x -> x `validates` i)) $ (certificates i) Could somebody explain what is wrong with my intuitive approach? Also, is multi parameter type classes the way to go, or is there a better way?

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Origins of (x:xs)?
by Toby Hutton 21 Dec '06

21 Dec '06

Hi, This may have been asked before, sorry if so. I've wondered where the convention of pattern matching a list to (x:xs) came from? I've read a couple of old papers recently which let me believe it may have started back in the '70s with Miranda and its ilk. Does anyone know why (x:xs)? Is xs meant to be a synonym for 'excess'? Yours curiously, Toby.

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YAHT "bug"
by Johan Grönqvist 21 Dec '06

21 Dec '06

Hi, As noted on e.g. http://paste.lisp.org/display/30002, there seems to be a misprint in YAHT. The code on page 124 for law3a does not use g, and uses y without introducing it. The page says this was corrected in the wikibook, but it seems not to have been fixed in the darcs repository at haskell.org. I found no mentioning on the YAHT page of ways to change this, so sending a mail to this list seemed simplest to me. Thanks for all good tutorials! Johan

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#haskell irc channel reaches 300 users
by dons＠cse.unsw.edu.au 21 Dec '06

21 Dec '06

A small announcement :) 5 years after its inception, under the guiding hand of Shae Erisson (aka shapr), the #haskell IRC channel[1] on freenode has finally reached 300 users! We reached 200 users in January, 250 users for the first time in August, and didn't expect hit to reach 300 for a while. Then last night: 12:50 lambdabot> Maximum users seen in #haskell: 305 This puts the channel at around position 15 of the 4000 freenode channels. For comparision, a sample of the state of the other language communities: #perl 484 #php 474 #c 381 #c++ 379 #ruby 368 > #haskell 305 #python 303 #lisp 214 #java 213 #javascript 156 #scheme 112 #lua 72 #ocaml 56 #sed 35 #d 33 You can see the growth of the channel over here: http://www.cse.unsw.edu.au/~dons/irc/nick-activity.png If you've not dropped by the channel yet, feel free to come and chat, and toss around some lambdas! :) Cheers, Don 1. http://www.haskell.org/haskellwiki/IRC_channel

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FD problem in GHC 6.6
by Robert 21 Dec '06

21 Dec '06

Fellow Haskellers, I have a package that uses some light typeclass hackery to automaticly build parsing algorithms based on the type of a function. I was recently informed that my package doesn't compile on GHC 6.6 due to the new restrictions on FD resolution; in particular I have instance declarations which fail the coverage condition. I can use undecidable instances to make the package compile again, but I'd prefer not to if I can avoid it. Are there any easy tricks I can use to make this work? I only sort-of understand the new restrictions, so I'm at something of a loss as to how to work around them. The problem is fairly isolated in my code: the affected section begins at line 140 in http://www.eecs.tufts.edu/~rdocki01/shellac/src/System/Console/Shell/Comman… I've reproduced the class and a few of the instance declarations here: class CommandFunction f st | f -> st where parseCommand :: String -> f -> CommandParser st commandSyntax :: f -> [Doc] instance CommandFunction (Sh st ()) st where parseCommand wbc m str = -- list monad do (x,[]) <- runRegex (maybeSpaceBefore (Epsilon (CompleteParse m))) str return x commandSyntax _ = [] instance CommandFunction r st => CommandFunction (Int -> r) st where parseCommand = doParseCommand Nothing intRegex id commandSyntax f = text (show intRegex) : commandSyntax (f undefined) instance CommandFunction r st => CommandFunction (Integer -> r) st where parseCommand = doParseCommand Nothing intRegex id commandSyntax f = text (show intRegex) : commandSyntax (f undefined) -- Robert robdockins(a)fastmail.fm -- http://www.fastmail.fm - IMAP accessible web-mail

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Success with Takusen (was: Type problem with simple takusen code)
by Paul Moore 20 Dec '06

20 Dec '06

On 12/12/06, Paul Moore <p.f.moore(a)gmail.com> wrote: > Should be, but isn't, unfortunately. The Setup.hs file includes some > code defining configPG etc, but there's nothing I can see for the user > control how it works. Maybe something in the cabal infrastructure > handles this, but I've not had time yet to dive into the cabal > documentation on the hope of finding something. Just a brief note to let people on the list know that, with a lot of extremely patient help from Alistair Bayley, I have managed to resolve the issues I was having. The fundamental issue I was hitting was that I was treating GHC as an interpreted system, rather than a compiler/linker. I was therefore not thinking about the errors I was seeing as I would if I hit (say) a C link error. To summarise, the following works fine: To install Takusen, make sure that an Oracle home is in your PATH. You don't need PostgreSQL or sqlite. Your Oracle install can be simply a client build - all you need is oci.dll, not the full OCI development package. runhaskell Setup.hs configure runhaskell Setup.hs build runhaskell Setup.hs install That's it, and it works fine. To build a program, you need to do ghc --make db.hs -o db D:\Oracle\Ora92\bin\oci.dll (put the path to your oci.dll here). This compiles your program, and links it with oci.dll to get the Oracle externals resolved. Runhaskell doesn't seem to work, as it tries to link all the DB backends at runtime, regardless of whether you use them. That really is all there is to it. Arguably, the error messages you get when you mess up bits of this aren't too helpful, but if you've ever seen a C++ compilation fail with compile or link errors, you'll know not to expect much here (or at least you should!) GHC is certainly no worse than that. Once again, thanks to Alistair for his patience. Now I'll go off and reproduce my success on a clean environment just to be sure, and then write up what I've learnt for the Wiki and the Takusen readme. Paul.

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Re: [Haskell] Haskell Weekly News: December 20, 2006
by Arthur van Leeuwen 20 Dec '06

20 Dec '06

On 20-dec-2006, at 2:17, Donald Bruce Stewart wrote: > ---------------------------------------------------------------------- > ----- > Haskell Weekly News > http://sequence.complete.org/hwn/20061220 > Issue 54 - December 20, 2006 > ---------------------------------------------------------------------- > ----- > > Announcements > > Haskell Vim plugin. Arthur van Leeuwen [15]announced a new [16]vim > plugin for Haskell providing some preliminary folding support, easy > insertion of type signatures into programs, and support for > handling > .hi files. > > 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17675 > 16. http://www.cs.uu.nl/~arthurvl/haskell.vba Read that as 'support for automatically ignoring .hi files' (which really helps when editing large programs consisting of multiple files... it makes tab Do The Right Thing). Doei, Arthur. -- /\ / | arthurvl(a)cs.uu.nl | Work like you don't need the money /__\ / | A friend is someone with whom | Love like you have never been hurt / \/__ | you can dare to be yourself | Dance like there's nobody watching

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WANTED: grey line layout boxes in vim and emacs
by dons＠cse.unsw.edu.au 20 Dec '06

20 Dec '06

I'd like some more help from the editors in getting 2d layout right without trying. Here's a mockup of vim with vertical grey bars delimiting layout: http://www.cse.unsw.edu.au/~dons/tmp/haskell+boxes.png Does anyone know how to get this effect in vim (or emacs)? Bonus points if the grey bars are draggable, changing the indenting. More bonus points for box-based navigation. -- Don

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Possible commercial FP/Haskell job
by dons＠cse.unsw.edu.au 20 Dec '06

20 Dec '06

Noticed this in the blargh-o-sphere, some people might be interested: http://www.nnseek.com/e/comp.lang.python/lisp_python_programmers_among_othe… Senior applications developer: Our advanced applications work is currently being done in Common Lisp, but that could change depending on who we find to fill this position. If you are the sort of person who has always wanted an opportunity to prove that Haskell or OCaml is the Best Way to write software, this could be your chance. But CL is currently our first choice. -- Don

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Haskell Weekly News: December 20, 2006
by dons＠cse.unsw.edu.au 20 Dec '06

20 Dec '06

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20061220 Issue 54 - December 20, 2006 --------------------------------------------------------------------------- Welcome to issue 54 of HWN, a weekly newsletter covering developments in the Haskell community. A new release of the Edison data structures library, along with several other new libraries, and some new Haskell articles in the blogspace. Announcements Edison 1.2.1. Robert Dockins [1]announced the 1.2.1 release of [2]Edison. Edison is a famous library of efficient, purely-functional data structures in Haskell. Notable changes from the previous version include: a new sequence implementation based on finger trees; documentation fixes dealing with the licence; added a few methods to EnumSet for wrapping and unwrapping the underlying Word 1. http://article.gmane.org/gmane.comp.lang.haskell.general/14662 2. http://www.eecs.tufts.edu/~rdocki01/ Basic serialisation library using SYB. Stefan O'Rear [3]announced GenericSerialize, [4]a library for serialization using the existing generic-programming framework. It is often advocated that support for serialization should be added to the compiler (e.g. in the form of a deriving(Binary)). With this project Stefan wants to show that the existing infrastructure is sufficient, and has some advantages over a dedicated serialization interface. GenericSerialize supports multiple serialization modes. 3. http://article.gmane.org/gmane.comp.lang.haskell.general/14656 4. http://members.cox.net/stefanor/genericserialize vty 1.0. Stefan O'Rear [5]announced vty 1.0, a simple [6]terminal interface library. It provides: handling of suspend/resume, window resizes, computation of minimal differences, minimizes repaint area, automatically decodes keyboard keys into (key,modifier) tuples, and more! 5. http://article.gmane.org/gmane.comp.lang.haskell.general/14670 6. http://members.cox.net/stefanor/vty Monad.Reader call for copy. Wouter Swierstra [7]reminded us that its still not too late to write something for the next issue of [8]The Monad.Reader! We have a nice issue slowly shaping up, but your contribution is still very welcome. Get in touch with Wouter if you intend to submit something -- the sooner you let him know what you're up to, the better. 7. http://article.gmane.org/gmane.comp.lang.haskell.general/14638 8. http://www.haskell.org/haskellwiki/TheMonadReader Ranged Sets. Paul Johnson [9]announced that Ranged Sets now have a Monoid instance, and singletons (i.e. a range holding a single value), thanks to Jean-Philippe Bernardy. 9. http://article.gmane.org/gmane.comp.lang.haskell.general/14663 CalDims 1.0 (RC 1). schneegloeckchen [10]announced CalDims, a calculator aware of units. Its available from [11]the Haskell wiki. It includes support for user defined basic units and derrived units; user defined functions; work sheets can be modified/saved via shell; (1/3)*3 == 1 (No rounding errors); built-in feature to simplify units and easy unit-conversion. 10. http://article.gmane.org/gmane.comp.lang.haskell.general/14671 11. http://www.haskell.org/haskellwiki/CalDims JoinCabal. Dougal Stanton [12]announced an alternative to mkcabal, for initialising new cabal projects: [13]JoinCabal, available [14]via darcs. JoinCabal will create stub sources files with a license header, and appropriate license for you code, making it easier to set up a valid cabal build system. 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17600 13. http://brokenhut.no-ip.org/~dougal/cgi-bin/darcsweb.cgi?r=JoinCabal;a=summa… 14. http://brokenhut.no-ip.org/~dougal/darcs/joincabal/ Haskell Vim plugin. Arthur van Leeuwen [15]announced a new [16]vim plugin for Haskell providing some preliminary folding support, easy insertion of type signatures into programs, and support for handling .hi files. 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17675 16. http://www.cs.uu.nl/~arthurvl/haskell.vba Haskell' This section covers the [17]Haskell' standardisation process. * [18]Strictly matching monadic let and overloaded Bool 17. http://hackage.haskell.org/trac/haskell-prime 18. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1956/focus=1974 Discussion Automonadization of code. Adam Megacz [19]asked if there any work on automatic translation of code in some tiny imperative language into Haskell code that uses the ST and/or IO monads (or perhaps even pure functional code)? 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17326/focus=17326 What are the points in pointfree style?. Steve Downey [20]wondered about the origin of the term 'points' in pointfree style. 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17560/focus=17560 Rebinding >>= for restricted monads. David Roundy [21]pondered rebinding the >>= syntax in order to provide a more restricted monad. 21. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17646/focus=17646 #haskell reaches 300 users. Don Stewart [22]noticed that the [23]#haskell IRC channel has now reached the 300 user mark, making it one of the top 15 channels on the freenode network. 22. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17705/focus=17705 23. http://haskell.org/haskellwiki/IRC_channel Blog noise [24]Haskell news from the blogosphere. * [25]Do-notation considered harmful * [26]Reddit discussion considered productive * [27]The Dot-Matrix Printhead: a Haskell Toy * [28]The Revised Dot-Matrix Printhead * [29]The monad laws * [30]On documentation * [31]Software requirements do not change * [32]Haskell and the web: some ideas in progress * [33]Using Haskell in Python: haskellembed * [34]Literate programming * [35]The lambda revolution, Episode V, the deb strikes back * [36]On syntax * [37]Syntax: adding Lisp macros * [38]More on syntax * [39]UCBLogo and the Fibonacci sequence * [40]Linguistic comparisons: string reversal * [41]New Software Technology: Blockage On Line * [42]More silver bullets * [43]Preventing NullPointerExceptions, Maybe * [44]Adventures in Haskell: the Parsec magic weapon * [45]Advertising Haskell * [46]Programming Haskell: part 1 * [47]Programming Haskell: part 2 * [48]Programming Haskell: part 3 * [49]Introductory Haskell Programming in the UNIX Environment * [50]On Haskell, Intuition And Expressive Power * [51]Your order is ready sir * [52]It's alive (the French Haskell community) * [53]Maybe monad in Java * [54]Secret Santas in Haskell I: Preliminaries * [55]Secret Santas in Haskell II: Orbits and Lists * [56]Why Closures in Dolphin is a Good Idea * [57]Haskell Stacks : Two Different Ways * [58]Haskell Golf Scores * [59]Languages * [60]Questions on Haskell Style (and Polynomials redux) * [61]A simple RSS aggregator in 50 lines of Haskell using HXT 24. http://planet.haskell.org/ 25. http://syntaxfree.wordpress.com/2006/12/12/do-notation-considered-harmful/ 26. http://syntaxfree.wordpress.com/2006/12/13/reddit-discussion-considered-pro… 27. http://praisecurseandrecurse.blogspot.com/2006/12/dot-matrix-printhead-hask… 28. http://praisecurseandrecurse.blogspot.com/2006/12/revised-dot-matrix-printh… 29. http://cale.yi.org/index.php/The_Monad_Laws 30. http://blog.milkfarmsoft.com/?p=29 31. http://blog.tmorris.net/software-requirements-do-not-change/ 32. http://haskell-web.blogspot.com/2006/12/some-ideas-in-progress_13.html 33. http://www.kuarepoti-dju.net/index.php?p=86 34. http://benjisimon.blogspot.com/2006/12/programs-as-stories.html 35. http://pupeno.com/blog/the-lambda-revolution-episode-v-the-deb-strikes-back 36. http://cgi.cse.unsw.edu.au/~dons/blog/2006/12/14#on-syntax 37. http://nominolo.blogspot.com/2006/12/syntax.html 38. http://www.cse.unsw.edu.au/~dons/blog/2006/12/15#on-syntax-again 39. http://sob.apotheon.org/?p=171 40. http://sob.apotheon.org/?p=170 41. http://cogito.blogthing.com/2006/12/14/new-software-technology-blockage-on-… 42. http://clabs.org/blogki/blogki.cgi?page=/ComputersAndTechnology/AndTheNomin… 43. http://rickyclarkson.blogspot.com/2006/09/using-strong-typing-to-eliminate.… 44. http://jpmoresmau.blogspot.com/2006/12/adventures-in-haskell-parsec-magic.h… 45. http://neilmitchell.blogspot.com/2006/12/advertising-haskell.html 46. http://cgi.cse.unsw.edu.au/~dons/blog/2006/12/16#programming-haskell-intro 47. http://www.cse.unsw.edu.au/~dons/blog/2006/12/17#programming-haskell-part-2 48. http://cgi.cse.unsw.edu.au/~dons/blog/2006/12/18#ph-3 49. http://blog.kfish.org/software/haskell/intro-haskell-unix.html 50. http://www.defmacro.org/ramblings/haskell-productivity.html 51. http://lambda-the-ultimate.org/node/1905#comment-23273 52. http://koweycode.blogspot.com/2006/12/its-alive.html 53. http://blog.tmorris.net/maybe-monad-in-java/ 54. http://mult.ifario.us/articles/2006/12/18/secret-santas-in-haskell-i-prelim… 55. http://mult.ifario.us/articles/2006/12/18/secret-santas-in-haskell-ii-orbit… 56. http://rickyclarkson.blogspot.com/2006/12/why-closures-in-dolphin-is-good-i… 57. http://progexpr.blogspot.com/2006/11/haskell-stacks-two-different-ways.html 58. http://progexpr.blogspot.com/2006/11/haskell-golf-scores.html 59. http://computativestylings.blogspot.com/2006/12/languages.html 60. http://sambangu.blogspot.com/2006/12/questions-on-haskell-style-and 61. http://cale.yi.org/index.php/HRSS Quotes of the Week * Cale: I spent a whole summer being paid to write a nondeterministic search program in the list monad * monochrom: 007 titles for Haskellers: fromForeignPtr withCString; You Only lift Twice; Bottoms Are Forever; The Monad with the Pure Function; For Your Types Only; A view to A seq; Licence to killThread; iterate Never Dies; unsafePerformIO Is Not Enough. * DavidRoundy: ... fun of haskell-cafe is that it's where all the cool people hang out * dons: Coding drunk in Haskell is ok though, its like drinking on a passenger plane flight -- the stewards will prevent you from hurting anyone. However, coding drunk in C is just scary, DUI territory, and should not be allowed. * JulianMorrison: Small startup companies are the expendable sensory and feeding apparatus of the mainstream. * droundy: With a higher-kinded monad and phantom existential witness types, darcs would be very fun... * dons: The alien rulers of the galaxy must surely use a statically typed language with type inference * dibblego: Java programmers prefer referential transparency too, they just call it 'design patterns' or something * tessier: After the last newbies head exploded trying to read everything on that monad link there was a lot of paperwork. We'd like to avoid doing that again. Code Watch Fri Dec 8 21:03:34 PST 2006. Andy Gill. [62]Adding tracing support (HPC) 62. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18263 Wed Dec 13 08:45:38 PST 2006. simonpj. [63]You can now have an MVar with a polymorphic contents 63. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18279 About the Haskell Weekly News Each week, new editions are posted to [64]the Haskell mailing list as well as to [65]the Haskell Sequence and [66]Planet Haskell. [67]RSS is also available, and headlines appear on [68]haskell.org. Headlines are available as [69]PDF. The Haskell Weekly News is also [70]available in Spanish translation. To help create new editions of this newsletter, please see the [71]contributing information. Send stories to dons at cse.unsw.edu.au. The darcs repository is available at darcs get [72]http://www.cse.unsw.edu.au/~dons/code/hwn 64. http://www.haskell.org/mailman/listinfo/haskell 65. http://sequence.complete.org/ 66. http://planet.haskell.org/ 67. http://sequence.complete.org/node/feed 68. http://haskell.org/ 69. http://www.cse.unsw.edu.au/~dons/code/hwn/archives/20061220.pdf 70. http://haskell.org/haskellwiki/HWN/es 71. http://haskell.org/haskellwiki/HWN 72. http://www.cse.unsw.edu.au/~dons/code/hwn

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howto c2hs marshall in out argument by reference
by Marc Weber 20 Dec '06

20 Dec '06

The function takes a pointer to an integer, prints its value and returns the increment. void cl_inc(int *i){ printf("changing int from %d to %d", *i, (++i)); } I've managed it by using this code: {#fun pure cl_inc {alloca `CInt' peek*} -> `()'#} -} and calling it this way: alloca $ \mem -> do poke mem 3; print_int_ptr_value mem then its a matter of reading mem by using peak again. Is there a better way letting {#func do the marshalling to be able to do directly? print $ cl_inc 3 4 Marc

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Data.Generics question
by Grote Collective 20 Dec '06

20 Dec '06

Hello all, I have two datatypes, both representing the same language. However, one is a "flattened" version of the other. For instance: data Flattened = Const Flattened | Sum Flattened Flattened | ... | IntConst Int | RealConst Float | BoolConst Bool | Void ---- data Term = Const Constant | Sum Term Term | ... data Constant = IntConst Int | RealConst Float | BoolConst Bool | Void Now I want to create functions to convert between these two datatypes. This is easy to do but *very* tedious in its most naive approach. However, I think I can use Data.Generics to make a simple implementation of both functions. The question is: can I? I think that this conversion is a simple fold over the structure, changing merely the constructor name (the qualified name, because the unqualified name is the same), but I cannot seem to understand from the documentation how to do this... Thanks in advance, Mark Smith

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Class annotation on datatype ineffective in function
by Reto Kramer 20 Dec '06

20 Dec '06

The code below does not compile unless the "bar" function is annotated with a suitable constraint on the class of the formal parameter. > module Main where > > class (C a) > data (C foo) => XY foo = X foo | Y foo > > bar :: a -> XY a > bar aFoo = X aFoo > > main = return () I get: > $ ghc Test.hs > > Test.hs:8:8: > No instance for (C a) > arising from use of `X' at Test.hs:8:8-10 > Possible fix: add (C a) to the type signature(s) for `foo' > In the expression: X a > In the definition of `foo': foo a = X a As suggested, this works: > bar :: (C a) => a -> XY a Can someone explain to me why the compiler can not infer that "a" (in bar) must be (C a) from the bar result type "XY a" (by way of the "C class" provided for the datatype)? Thanks, - Reto

3 2

On the various faces of Composition
by Jacques Carette 20 Dec '06

20 Dec '06

[As recently seen on LtU - http://lambda-the-ultimate.org/node/1919] An amazing paper http://www.dbmcintyre.pwp.blueyonder.co.uk/index_f/menu_f/j_f/apl95.pdf on "composition" as seen from APL and J. The terminology (using verbs, nouns, adverbs, etc) is _so_ much more evocative than the terminology used for similar ideas here, I hope that some of it can rub off. Also, if anyone thought that "point free" programming was something mostly-recent, mostly-Haskell, they _really_ need to read this paper! Also, I am fairly sure that some of the high-powered operators in J may stress Haskell's type system quite severely, if not to the breaking point. It would be a fascinating exercise to try to reproduce the various examples in that paper in as-idiomatic-as-possible Haskell. While J's syntax is somewhat on the terse side, its power regarding practical mathematics is superb -- especially when it comes to polymorphism. Jacques

1 0

exception output
by Ferenc Wagner 20 Dec '06

20 Dec '06

Hi, the following program main = readFile "nonexistent" >>= putStr is meant to produce an exception. When compiled with GHC 6.4, it outputs the "*** Exception..." string to stderr, which makes me happy. However, when run with runghc6 or runhugs, the message is sent to stdout, which makes me sad. Is this inconsistency intentional? -- Thanks, Feri.

1 0

Re: [Haskell-cafe] Data.Generics question
by Vyacheslav Akhmechet 19 Dec '06

19 Dec '06

Sorry, I sent the email to the wrong place and messed up the code (it's early :)). Take two: transform = everywhere (mkT transform') where transform = (\Term a->Flattened a) `extT` (Contstant a->Flattened a) On 12/19/06, Vyacheslav Akhmechet <coffeemug(a)gmail.com> wrote: > That should be pretty simple. This code probably won't work, but you > can use it as an example to look into this further. > > transform everywhere = (mkT transform') > where transform = (\Term a->Flattened a) > `extT` (Contstant a->Flattened a) > > Something like this should work. Look into the docs for more info. > > On 12/19/06, Grote Collective <grotecollective(a)gmail.com> wrote: > > Hello all, > > > > I have two datatypes, both representing the same language. However, one is a > > "flattened" version of the other. For instance: > > > > data Flattened = > > Const Flattened > > | Sum Flattened Flattened > > | ... > > | IntConst Int > > | RealConst Float > > | BoolConst Bool > > | Void > > > > ---- > > > > data Term = > > Const Constant > > | Sum Term Term > > | ... > > > > data Constant = > > IntConst Int > > | RealConst Float > > | BoolConst Bool > > | Void > > > > > > Now I want to create functions to convert between these two datatypes. This > > is easy to do but *very* tedious in its most naive approach. However, I > > think I can use Data.Generics to make a simple implementation of both > > functions. The question is: can I? I think that this conversion is a simple > > fold over the structure, changing merely the constructor name (the qualified > > name, because the unqualified name is the same), but I cannot seem to > > understand from the documentation how to do this... > > > > Thanks in advance, > > Mark Smith > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > >

1 0

maybeToM
by Joachim Breitner 19 Dec '06

19 Dec '06

Hi, dons mentions in his blog post that Data.Map’s lookup is generalized over the Monads, whereas Prelude.maybe isn’t. Are there good reasons not to do that to Prelude.maybe as well? Alternatively, how about adding this function to Data.Maybe, analogous to maybeToList > maybeToM Nothing = fail "Nothing" > maybeToM (Just x) = return x (Incidentially, this function is the same as maybeToList if run in the List Monad). This way, Prelude.lookup (or any function that returns a Maybe) can be generalized as (maybeToM . lookup). Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: joachimbreitner(a)amessage.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

4 8

Newbie project and a question
by Jian Fan 19 Dec '06

19 Dec '06

Hi all, This is my first post. Inspired by Don's hmp3, I've started my pet project hxine: A Haskell music player using Xine engine. This was long time ago and I just picked this thing up again: darcs get http://lintao.org/repos/hxine To play CD: ./hxine cd:/ ./hxine cd://2- ./hxine cd://-5 to play files Xine understand: ./hxine *.ogg Key bindings: j : next track k : prev track p : pause/unpause d : toggle display mode q : quit Now my question: Yesterday I tried to compile (on Ubuntu Edgy) this again after couple months. First I had to hand install latest hsffig. Then the generated XINE_H.hsc give me some duplicated definitions at compile time and then some un-defined symbols at link time. I deleted those in XINE_H.hsc and everything working now. I'd like to know if this is a hsffig problem. I'd appreciate any comment on design, coding etc. Thanks. Jian

1 0

Haskell ftplugin for vim
by Arthur van Leeuwen 19 Dec '06

19 Dec '06

Hiya folks, I got slightly annoyed at having to tab beyond .hi files when editing haskell code with vim, so I decided to wrap some stuff together in a haskell file type plugin. It has some preliminary (read: very stupid) folding support, and allows you to call out to ghci easily to get type signatures included in your text. Get it at http://www.cs.uu.nl/~arthurvl/haskell.vba Doei, Arthur. -- /\ / | arthurvl(a)cs.uu.nl | Work like you don't need the money /__\ / | A friend is someone with whom | Love like you have never been hurt / \/__ | you can dare to be yourself | Dance like there's nobody watching

1 0

problem with Graphics.HGL
by asilovy 18 Dec '06

18 Dec '06

Hello, I posted this message to Haskell-Libraries but were advised to try Haskell-Cafe... I'm trying some stuffs with the Graphics.HGL library and have a question. I wonder if someone could help me with it. I'm trying to write some Win32 application using openWindowEx. I see it has a parameter to (eventually) define a timer. On the other hand, the maybeGetWindowEvent function is used to get "notified" of different events such as mouse button pressed, key pressed, resizing... I don't understand the special status of the timer. Why is it not an event like the others. It does'nt sound right to me. As I understand it, you have to wait for a timer tick rather than being informed by a timer event. For instance, I've written a small test program that uses the timer to draw some random "stars" on a window. Eventually, when the user presses the mouse button, the window is closed. But, as you can try, if you trigger a lot of events (by, for instance, moving the mouse a lot) BEFORE pressing the mouse button those events, although I don't treat them, will have to be processed with the delays of the timer before the mouse clic can be treated. And so, the window takes a long time to close. I'm I misunderstanding something ? And is there another way to treat the timer ? Thanks for your answer Alain {--------------------------------------------------------------------------------------- Draws randomly colored stars at random places Uses Events from : http://haskell.org/ghc/docs/latest/html/libraries/HGL/Graphics-HGL-Window.h… -} import Graphics.HGL import Hugs.Prelude import System.Random import Graphics.HGL.Win32.Types import Control.Monad import Data.Maybe main = runGraphics myWin myWin = do w <- openWindowEx "TestWindow" Nothing (400,400) DoubleBuffered (Just 100) let loop n = do x0 <- randomRIO (0, 39) y0 <- randomRIO (0, 39) couleur <- randomRIO (0, 0x00FFFFFF) let (x, y) = (x0 * 10, y0 * 10) c = if mod couleur 10 == 0 then couleur else 0 -- 1/10 will be colored drawInWindow w $ withRGB (toRGB (fromInt c)) $ ellipse (x, y) (x + 9, y + 9) e <- maybeGetWindowEvent w -- some Event ? when (isJust e) $ -- yes... case fromJust e of -- which ? Button _ _ _ -> closeWindow w -- mouse button _ -> return () -- something else -> nothing to do getWindowTick w -- wait and... loop (n + 1) -- loop loop 0 -------------------------------------------------------------------------------------- -- View this message in context: http://www.nabble.com/problem-with-Graphics.HGL-tf2834899.html#a7914596 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 1

I'd like start with Haskell, but...
by Waldemar Biernacki 18 Dec '06

18 Dec '06

Hello! I'd like to start programming in Haskell. But as an "industry programmer" I have a hope to use Haskell in my every-day work. Big part of my every-day work are GUI applications (in MS-Windows) working with SQL databases (PostgreSQL on Linux servers). My question: Is there a TRUE possibility to use Haskell for such applications? Is anybody there who have ANY experience in the field? If answer would be positive what GUI+database libraries could be used in such a case? I've seen some GUI libraries web pages, but they seem to be NOT maintenanced any more. cheers Waldemar

11 13

type hackery question
by jeff p 18 Dec '06

18 Dec '06

Hello, The HList paper gives a type cast: class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x and a type equality check: class TypeEq x y b | x y -> b instance TypeEq x x HTrue instance TypeCast HFalse b => TypeEq x y b The type cast succeeds causing its two argument types to be unified, or simply fails if the arguments are not unifiable. The type equality always succeeds "returning" HTrue if the arguments are syntactically equal, and HFalse if the arguments are not syntactically equal; thus the type equality will return false for two arguments which could be unified. If I understand correctly, these two classes do not provide a way to check for unifiability. Is it possible to write a class which checks to see if two given type arguments are unifiable? thanks, Jeff

2 3

greencard and cabal, how to do it right?
by Marc Weber 18 Dec '06

18 Dec '06

I want to have a look at fruit which is using GCJNI, greencard, ... That's why I want to learn how to use greencard with cabal. If this is only a RTFM problem, can you point me to the docs? When using the cabal builtin preprocessor I get the error that --libdir has to be specified. So I tried adding this manually using a custom proprocessor beeing copied mostly from the already existing one: -- setup.hs ----------------------------------------------------------- import Distribution.PackageDescription import Distribution.Simple.Utils (rawSystemVerbose, dieWithLocation) main= do defaultMainWithHooks $ defaultUserHooks { hookedPreProcessors = [ myGreencardPP ] } where myGreencardPP :: (String, BuildInfo -> LocalBuildInfo -> PreProcessor) myGreencardPP = ("gc", ppGreenCard' ["--libdir ../c_lib"] []) -- << --libdir is added here -- copied from Distribution.PreProcess ppGreenCard' :: [String] -> [String] -> BuildInfo -> LocalBuildInfo -> PreProcessor ppGreenCard' args inputArgs bi lbi = maybe (ppNone "greencard") pp (withGreencard lbi) where pp greencard inFile outFile verbose = rawSystemVerbose verbose greencard (args ++ ["-tffi", "-o" ++ outFile, inFile] ++ inputArgs) ppNone :: String -> PreProcessor ppNone name inFile _ _ = dieWithLocation inFile Nothing $ "no " ++ name ++ " preprocessor available" ---------------------------------------------------------------------------- I don't understand why the command does work but not when beeing executed by cabal: The commandline shows the directory content (no Greencard.hs). (1>) Then it executes ./setup -v build which shows the commandline failing (2>) Then it executes this commandline from shell (using bash eval and getting the greencard line by grepping the output from ./setup -v build) The final ls (3>) shows that Greencard.hs is there. I've marked these parts using > and * * I've also replaced the greencard executable by a shell script printing pwd which is the same.. Whats going wrong here? How to do it right? > $ ls; ./setup -v build || echo -e "\n\n ===> build failed \n\n"; eval `./setup -v build 2>&1 | grep /usr/local/bin/greencard` && echo -e "\n\n =====> this works why?" ; ls 1> Greencard.cabal Greencard.gc Greencard_stub_ffi.c Greencard_stub_ffi.h dist greencard_stub_ffi.c greencard_stub_ffi.h setup setup.hi setup.hs Preprocessing executables for greencard-0.0... 2> */usr/local/bin/greencard --libdir ../c_lib -tffi -oGreencard.hs Greencard.gc* unrecognized option `--libdir ../c_lib' Usage: greencard [OPTION]... SOURCE Run Green Card, a foreign function interface preprocessor for Haskell, over SOURCE Green Card home page: http://www.haskell.org/greencard -?, -h --help print out this help message and exit -V --version output version information and exit -v --verbose print more information -d --debug output extra debug information --print-libdir output default library location --libdir=DIR use base library location -i DIRS --include-dir=DIRS Add DIRS (colon separated) to the include search path -s SUFS --suffix=SUFS colon separated list of source file suffixes -t TARGET --target=TARGET generate Haskell code for a particular system (ghc, ffi) -S --safe-code call C `safely' (GHC only) -n --no-inline put C code in a separate file (GHC only) -4 --haskell1.4 generate code compatible with a Haskell 1.4 system -m SCHEME --name-mangling-scheme=SCHEME controls mapping of external names into Haskell ones (std, classic) -D --stub-debug include debugging code in generated code -f DLL --dllname=DLL generate 'foreign imports' to dynamic library DLL (FFI backend only) -C CCONV --callconv=CCONV use specific calling convention (stdcall, ccall) -o FILE --output=FILE write Green Card Haskell output to FILE -c FILE --output-c=FILE write Green Card C output to FILE -p PRE --output-prefix=PRE write Green Card Haskell output to PRE.{hs,c} Building greencard-0.0... /usr/bin/ghc -Idist/build -o dist/build/greencard/greencard --make -hide-all-packages -i -idist/build/autogen -i. -odir dist/build/greencard/greencard-tmp -hidir dist/build/greencard/greencard-tmp -package haskell98-1.0 -package base-2.0 -package QuickCheck-1.0 Greencard.gc -lc_lib -L../c_lib/ ghc-6.5.20060917: no input files Usage: For basic information, try the `--help' option. ===> build failed =====> this works why? 3> Greencard.cabal Greencard.gc *Greencard.hs* Greencard_stub_ffi.c Greencard_stub_ffi.h dist greencard_stub_ffi.c greencard_stub_ffi.h setup setup.hi setup.hs Marc

3 4

Re: A suggestion for the next high profile Haskell project
by Paul Johnson 18 Dec '06

18 Dec '06

Magnus Therning <magnus(a)therning.org> wrote: > When reading this[1] I couldn't help thinking that rewriting GPG is an > excellent opportunity for using Haskell to have an impact on the world. The Fedora project is planning to rewrite the foundations of RPM. I've noticed that the higher level RPM tools are written in Python, and are (1) very slow, (2) single threaded, so window redraws don't happen promptly. This would also be a cool place for Haskell to shine. Paul.

2 1

Re: [Haskell-cafe] Aim Of Haskell
by Neil Bartlett 17 Dec '06

17 Dec '06

I think this hits the nail on the head. To be blunt, the presence of so many academics and scientists in the Haskell community is intimidating to those of us that work "in industry". Our brains are, after all, not as highly trained as yours, and we care about different things than you do. Now I don't mean to say that the academics and scientists should go away! Far from it. Just that it would be great to hear more about the mundane aspects of programming occasionally. Like, how exactly do I read from a relational database with Haskell? Or process an XML file? Or build an event-driven GUI? And crucially, why does Haskell do those things better than Java, or C#, or Ruby? If somebody could write some articles on those subjects, and get them up on popular websites like Digg or Reddit, this would be far more helpful than yet another monad tutorial. The Haskell web server that Simon Peyton-Jones et al described in their paper would be a great example. But where's the download? How do I get a copy to play with? In the "real world", things don't stop with the publication of a paper ;-) I think Haskell has huge potential to improve mainstream programming, if it could only catch on a bit. I don't know how to make that happen, unfortunately (if I did, I would do it, and hopefully get rich in the process). But whatever Haskell needs, it's not getting at the moment. Neil > Hello Kaveh, > > Sunday, December 10, 2006, 6:15:23 PM, you wrote: > >> chosen one. But Haskell seems to be buzz-full research platform. Now again to the top : what is the aim of Haskell project? If it is going to be used in real world applications it needs more attention to real world application developers and their needs. > > you are right - just now Haskell is a huge technology with non-obvious path to learn. there is some work to make Haskell more pragmatic, but it's an chicken-and-egg problem - we have a small number of pragmatic programmers > that use Haskell and therefore it's hard to change Haskell to suit their needs, on the other hand this means that pragmatic programmers can't grok > Haskell > > on the way to make Haskell more pragmatic i especially mention renewal of > Haskell standard to include modern language extensions, modern programming > environments such as WinHugs or BusinessObjects, development of web/db/gui > libraries, and definition of core (standard) libraries set > > one particular thing that we still lack is something like book "Haskell in > real world" > > -- > Best regards, > Bulat mailto:Bulat.Ziganshin@gmail.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

11 12

what are the points in pointsfree?
by Steve Downey 17 Dec '06

17 Dec '06

i'm not naive enough to think they are the composition function, and i've gathered it has something to do with free terms, but beyond that i'm not sure. unless it also has something to do with fix points?

9 10

Composing monads (sort of)
by Mark Wassell 17 Dec '06

17 Dec '06

Hi, I have a set of functions: f1 :: DBRecord -> Maybe Int f2 :: Int -> IO Maybe DBRecord f3 :: DBRecord -> Maybe Int The odd numbered functions are field accessors, accessing a field that might hold an identifier for another record. The even numbered functions are record fetch functions that get records from the database. I want to compose these so that I can navigate structures of joined records in the database. How can I concisely compose these functions without having to write a cascade of case statements such as: case f1 rec1 of Nothing -> return Nothing Just id1 -> do rec2 <- f2 id2 return $ case rec2 of Nothing -> return Nothing Just rec2' -> case f3 rec2' of .... I understand that if I was just dealing with Maybe I could use the fact that Maybe is a monad. I am also not sure if composing the IO and the Maybe will get me what I want (some of the functions only return Maybe Int). Cheers Mark PS Heard this on the 'West Wing' and thought it was appropriate in a way: A coach goes up to a player and asks "Are you ignorant or apathetic?". The player replies "I don't know and I don't care".

4 3

Fwd: Build failure on FC6, ghc66
by Conrad Parker 17 Dec '06

17 Dec '06

Hi Haskellers, I received the following bug report from someone trying to build HOgg on Fedora Core 6 (FC6) with its ghc66 package. The build error is: Could not find module `Data.Map': it is a member of package base, which is hidden I've built HOgg on Debian unstable with its ghc6.6 package without problems. I know that non-base libraries have been separated out in the GHC 6.6 release. Is this problem FC6 specific or a bug in HOgg's cabal setup? any clues? cheers, Conrad. ---------- Forwarded message ---------- From: Ian Malone <ibmalone(a)gmail.com> Date: 11-Dec-2006 06:36 Subject: Re: [annodex-announce] HOgg 0.2.0 Released To: Conrad Parker <conrad(a)metadecks.org> Hi, On 06/12/06, Conrad Parker <conrad(a)metadecks.org> wrote: > HOgg 0.2.0 Released > ------------------- > > The HOgg package provides a commandline tool for manipulating Ogg files, > and a corresponding Haskell library. > > http://snapper.kfish.org/~conrad/software/hogg/ > Can't get it to build on FC6, using their ghc66 package, tried installing hugs98 and haddock as well (although they don't seem to be related to the problem). "Could not find module `Data.Map'" [ian@atlas hogg-0.2.0]$ ./Setup.hs configure Configuring hogg-0.2.0... configure: /usr/bin/ghc-pkg configure: Using install prefix: /usr/local configure: Binaries installed in: /usr/local/bin configure: Libraries installed in: /usr/local/lib/hogg-0.2.0/ghc-6.6 configure: Private binaries installed in: /usr/local/libexec configure: Data files installed in: /usr/local/share/hogg-0.2.0 configure: Using compiler: /usr/bin/ghc configure: Compiler flavor: GHC configure: Compiler version: 6.6 configure: Using package tool: /usr/bin/ghc-pkg configure: Using ar found on system at: /usr/bin/ar configure: Using haddock found on system at: /usr/bin/haddock configure: No pfesetup found configure: Using ranlib found on system at: /usr/bin/ranlib configure: Using runghc found on system at: /usr/bin/runghc configure: Using runhugs found on system at: /usr/bin/runhugs configure: No happy found configure: No alex found configure: Using hsc2hs: /usr/bin/hsc2hs configure: No c2hs found configure: No cpphs found configure: No greencard found [ian@atlas hogg-0.2.0]$ ./Setup.hs build Preprocessing library hogg-0.2.0... Preprocessing executables for hogg-0.2.0... Building hogg-0.2.0... Codec/Container/Ogg/MessageHeaders.hs:17:7: Could not find module `Data.Map': it is a member of package base, which is hidden -- imalone

3 2

Stack, Heap and GHC
by Felix Breuer 17 Dec '06

17 Dec '06

Hello everyone, I have been trying to run a Haskell program of mine that does an extensive computation with very large amounts of data. I compiled the program with ghc --make. When I run it it terminates after some time with the message: Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. The program isn't that well written so the overflow did not surprise me, I expected that it might run out of memory. What did surprise me was the *stack* overflow. I do not use recursion in my program except for a couple of fold operations over very large lists. So I have a number of questions: 1) Which Haskell operations cost space on the stack, which cost space on the heap? I guess this is implementation dependent, so I looked into the GHC manual but did not find an answer. Where can I look these things up? 2) What could be possible sources of a stack overflow? (Apart from a recursive but not tail-recursive function.) 3) I tried using +RTS -K<size> as suggested, but these options do not seem to be passed through if I use --make. How can I use both, these compilation flags and --make? Thanks, Felix

10 11

Re: [Haskel-cafe] Re: what are the points in pointsfree?
by Steve Downey 17 Dec '06

17 Dec '06

I spent time working with cubic mappings. With two critical points, there are Julia sets that consist of infinitely many disconnected components that are still locally connected, corresponding two one critical point escaping and one falling into one of the basins of attraction. Of course that was about 15 years ago, and I haven't really kept up. Too much time keeping up with software. On 12/16/06, Jacques Carette <carette(a)mcmaster.ca> wrote: > > Steve Downey wrote: > > No fair. Although I've a B.S. in Mathematics, I spent most of my time > > in complex analytic dynamical systems, rather than hanging with the > > algebraists. Except for a bit where I did some graph theory. > > Never thought I'd run into a fellow dynamicist on haskell-cafe! I did > my PhD on the links between the geometry and dynamics of Julia sets (co > supervised by the (late) Adrien Douady and John Hubbard). What > 'flavour' of this stuff are you interested in? > > Jacques > > PS: now I have converted to being a computer scientist, which is why I > hang out on haskell-cafe! >

1 0

Well-typed functions with nonexisting signatures [Was: type variable question]
by oleg＠pobox.com 16 Dec '06

16 Dec '06

Nicolas Frisby wrote > The commented out signature is the signature that GHC infers for the > function. When I uncomment that signature, it will no longer type > check. Why does this happen? Even with the forall and the explicit > signatures in the where clause, it chokes. Alas, this is the property of Haskell, even of Haskell98. Half a year ago I even wanted to write a message on the Haskell' list, pointing out the embarrassment. One one hand, writing signatures is highly recommended. On the other hand, there exist, even in Haskell98, well-typed functions for which signatures simply do not exist. The number of such exceptions multiply when we move to higher-rank types (where such anomalies are to be expected). The reason for the anomalies is the difference between the 'internal' language that expresses inferred types and the external type language. > Is there a canonical example example that exhibits this behavior? Yes, it was discussed back in 2003. Here's the canonical form: g::(Show a,Show b) => a->b g = undefined --h :: Show a => b -> a h x = g(h x) Both Hugs and GHC (in the pure Haskell98 mode!) are happy with h. Both compilers infer the type for h, shown in the comment. If you give h the type signature -- the one that is inferred by the compilers themselves -- both compilers complain. This example has been distilled from practical problems (the earliest one was by Levent Erkok reported back on 2000) http://www.mail-archive.com/haskell@haskell.org/msg06754.html The following thread gives more detail and discussion: http://www.haskell.org/pipermail/haskell/2003-May/011730.html The final solution to the problem is in http://www.haskell.org/pipermail/haskell/2003-May/011762.html I have actually managed to give h the type signature. The following message indicates that the problem is actually quite common: http://www.haskell.org/pipermail/haskell-cafe/2003-May/004302.html Here's an example of an anomaly with higher-rank types: http://www.haskell.org/pipermail/haskell-cafe/2004-October/007207.html http://www.haskell.org/pipermail/haskell-cafe/2004-October/007212.html we can define functions Foo and Bar (data constructors are functions) such that we can write > myFoo :: Int -> Foo > myFoo i = Foo (Bar run op) where > run :: Monad m => StateT Int m a -> m a > run prog = do (a, s) <- runStateT prog i > return a > op :: Monad m => StateT Int m Int > op = get but we cannot write the composition of Foo and Bar more directly: > foo run op = Foo (Bar run op) The latter is not accepted: it must have a signature but no signature can ever be given to this function.

2 1

Why is this array use wrong?
by Maurício 16 Dec '06

16 Dec '06

Hi, I'm trying this in ghci: let b = array (1,33) [(i,False) | i <- [1..33]] after :m Data.Array.IArray. It gives me that error message: <interactive>:1:8: No instance for (IArray a Bool) arising from use of `array' at <interactive>:1:8-12 Probable fix: add an instance declaration for (IArray a Bool) In the definition of `b': b = array (1, 33) ([(i, False) | i <- [1 .. 33]]) Why? Thanks, Maurício

2 1

GHC package lang not found
by Aditya Siram 16 Dec '06

16 Dec '06

Hi all, I am trying to install Hare but I cannot build it. GHC complains that package lang is not found. Here is the output: .... editors/GenEditorInterfaces pfeRefactoringCmds > refactorer/PfeRefactoringCmds.hs cd refactorer; rm -f hidir/`uname`/Main.hi; ./myghc--make -o pfe pfe.hs 2>&1 | tee log.txt ghc-6.6: unknown package: lang cd refactorer; rm -f hidir/`uname`/Main.hi; ./myghc--make -o pfe_client pfe_client.hs 2>&1 | tee -a log.txt ghc-6.6: unknown package: lang I've searched the net for lang but I cannot find it. Here is my ghc-pkg list: /usr/local/lib/ghc-6.6/package.conf: Cabal-1.1.6, HUnit-1.1, QuickCheck-1.0, base-2.0, cgi-2006.9.6, fgl-5.2, (ghc-6.6), haskell-src-1.0, haskell98-1.0, html-1.0, mtl-1.0, network-2.0, parsec-2.0, readline-1.0, regex-base-0.71, regex-compat-0.71, regex-posix-0.71, rts-1.0, stm-2.0, template-haskell-2.0, time-1.0, unix-1.0, xhtml-2006.9.13 /home/deech/.ghc/i386-linux-6.6/package.conf: (hat-2.4), (hat-2.5) What am I doing wrong? Thanks... Deech _________________________________________________________________ WIN up to $10,000 in cash or prizes enter the Microsoft Office Live Sweepstakes http://clk.atdmt.com/MRT/go/aub0050001581mrt/direct/01/

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[ANN] JoinCabal makes its brave entry into the world
by Dougal Stanton 16 Dec '06

16 Dec '06

Okay, after a bit of a wait (I believe it was several weeks, in fact) there's an alternative to Don Stewart's mkcabal. JoinCabal is available from here: <http://brokenhut.no-ip.org/~dougal/darcs/joincabal/> or <http://brokenhut.no-ip.org/~dougal/cgi-bin/darcsweb.cgi?r=JoinCabal;a=summa…> It follows on from my post yesterday about the default licences in cabal, in that it'll create a stub source file with a licence header and description relevant to the licence you chose. (Apart from the BSD licences, which are not well documented. I'll get round to it.) If you use the [L]GPL a lot then this'll save you a wee bit of time. In future I also hope to download licences where I can, straight into the LICENSE file. Full usage can be found in the README. I hope someone finds it useful. Cheers, D. -- Dougal Stanton

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Re[2]: [Haskell] Haskell alternatives: functional programming in rich IDEs
by Bulat Ziganshin 16 Dec '06

16 Dec '06

Hello Martin, Friday, December 15, 2006, 3:51:24 PM, you wrote: >>1) F#, is Ocaml dialect integrated in .NET environment >>2) Clean, very Haskell-like language with a commercial IDE, GUI libs and so on >>3) Business Objects, integration of Haskell-like language into Java > 4) Scala, which has a nice eclipse plugin, and I believe there's a > plugin for netbeans as well. is Scala FP language or imperative/OOP language with FP extensions like C#, for example? in other words, how it looks compared to F#, at least? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Building the community
by dons＠cse.unsw.edu.au 16 Dec '06

16 Dec '06

One of the great aspects of programming Haskell is the community. We should aim to have the strongest, friendliest, most productive community we can. We're doing a good job so far, can we do even better? Reading this: "How to Build a User Community" http://headrush.typepad.com/creating_passionate_users/2006/12/how_to_build_… has inspired me to make some suggestions of what *you* can do to help make Haskell the language and community people want to spend time on. * Always encourage newbies to ask questions + I rate us at 8/10 * Encourage newer users--especially those who've been active askers--to start trying to answer questions + we can do more here. If you know the solutoin to a problem, dive in and help out. Canonical, type correct answers preferred! + if you're at the 'intermediate' stage, you can help *now* by contributing to the wiki, solving new user problems, blogging your ideas and code and so on. * Give tips on how to answer questions + Ok. we can put up an article here. Some suggestions: - No questions are bad questions - Code should come with examples of how to run it - Solutions with unsafePerformIO should be discouraged (moreso ;) - Be polite! (we're good at this) * Adopt a near-zero-tolerance "Be Nice" policy when people answer questions + 8/10, we can do even better I think * Teach and encourage the more advanced users (including moderators) how to correct a wrong answer while maintaining the original answerer's dignity. + Pretty good. I encourage people to point out (privately) and encourage grumpy people to be nicer So dive in! Help out new users, collaborate, contribute to the wiki, release your code, and help make Haskell the language people want to use. -- Don

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Aim Of Haskell
by Kaveh Shahbazian 16 Dec '06

16 Dec '06

What is the aim of Haskell? This question may seems too stupid to answer. But I think it needs a re-clarifying attempt. Why? i.e recently there was a project not that critical, but an internal tool for a group. There were three candidates : Haskell, Clean and Erlang. In this company all projects are in C# and this was the first one for using a new approach in development. After all Erlang was chosen, in second place Clean and in 3rd Haskell. Because those two ones has some standards, and Erlang licensing was the chosen one. But Haskell seems to be buzz-full research platform. Now again to the top : what is the aim of Haskell project? If it is going to be used in real world applications it needs more attention to real world application developers and their needs. And if not; and it is just for research, then there could be a global advise : do not use this for doing your tasks, because there is strange variations in syntax, and strange behaviors in components. (Even clean has a simple GUI. Is it that hard to provide a simple GUI like that to be installed by default?) And real applications are not those that really worked in research! Real applications are those that works in real world (That mean-level day to day developing life!). So please do not talk about Lispiry things - that are totally ananymous and virtual. And still there is ambition in licensing GHC and licensing of programs that are developed in that environment. Thanks to all.

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Re: [Haskell] Haskell alternatives: functional programming in rich IDEs
by Jason Dagit 16 Dec '06

16 Dec '06

Moving this to Haskell-Cafe which seems more appropriate given my question. On 12/15/06, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > Hello haskell, > > it was several times mentioned that Haskell doesn't provide rich > environments for productive work, including lack of IDEs, bundled > "real-world" libs and so on. i want to mention alternative environments > that is better in these areas > > 1) F#, is Ocaml dialect integrated in .NET environment > 2) Clean, very Haskell-like language with a commercial IDE, GUI libs and so on > 3) Business Objects, integration of Haskell-like language into Java > environment, http://labs.businessobjects.com/cal Can anyone comment about their experiences with these alternatives? Jason

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Book "Haskell in real world"
by Bulat Ziganshin 15 Dec '06

15 Dec '06

Hello haskell-cafe, first, i should say that i'm real-world programmer with 15 years experience. and my experience taught me that programming language in first place should allow to express complex algorithms easily, factor out any code patterns, and facilitate development of error-proof programs. from several tens of languages that i know, Haskell is best in these areas and therefore i think that it is the best language for professional program development i've written several medium-sized apps and libs in Haskell so i somewhat know how to write real-world programs here, and have written a few pages that may help other haskellers to grok tricky parts of Haskell the note on lambdadot mentioned here don't contain anything new for us - the problems are the same as for any other not-so-popular language. there is even Wadler's paper "Why Haskell will NOT become popular" which said exactly the same: while language itself is great, we lack in areas of libraries, tools/IDEs, and information support (teaching, books, training...); i call all these things "infrastructure". Moreover, for any other language not in "top 5" list, you will found just the same problems Is this means that developers of C# and Java are only smart guys and all other are suckers? No, it's just chicken-and-egg problem - development of rich infrastructure needs large number of developers and gathering large number of interesting developers needs rich infrastructure. the only thing we cam do now is to find "critical points", where small improvements may give significant overall changes and address these problems one of such points may be the book for commercial programmers that will show them how their real programs can be written Haskell and convince them that Haskell really cuts off development cycle and increase software reliability a few pages that i already wrote may be considered as precursors of such book and i will be glad to take part in this work but only if this book will be online-available. not everyone prefers (or even can) order the paper book. it should be allowed to translate book to other languages too next question - how this book should be organized. i think that it should contain two parts: first, we should look which concepts are already described in YAHT and other introductory books, and cover all remaining concepts that will be required for professional programming. second part will describe process of development real program(s) which use all above-mentioned concepts. we can even start with second part just to find which concepts will be required in first part. of course, first part should also contain small examples for each described detail, because it's better one time to look rather than 100 times to hear my a few pages: http://haskell.org/haskellwiki/Modern_array_libraries http://haskell.org/haskellwiki/OOP_vs_type_classes http://haskell.org/haskellwiki/IO_inside http://www.haskell.org/bz/th3.htm -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Draft MissingH Reorg Plan
by John Goerzen 15 Dec '06

15 Dec '06

Please tell me if I should just go away or go to another list here. Thanks again for all the feedback you've sent. I've got the new MissingH website getting started, and I've posted there the draft reorganization, module rename, and package split plan here: http://software.complete.org/missingh/wiki/TransitionPlanning Your comments (and edits! -- must register/login first) are welcome. I have not yet audited the plan for dependency sanity. This could complicate things a few places, but hopefully not too many. -- John

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Re: what are the points in pointsfree?
by Chad Scherrer 15 Dec '06

15 Dec '06

> so pointsfree is a step beyond leaving the domain unspecified. Actually, the domain is specified - a function written as f = g . h has the same domain as h has. > my reading knowledge of haskell at this point far exceeds my ability > to write haskell. but so far, it has seemed to me that functions > written in the pf style are the most reuseable. > > from what you just told me, it's not an artifact of the pf style, but > that maximally reusable functions will be expressible in a pointsfree > style. that those functions embody a pattern of computation, without > concern for the details. > I don't see where reusability is affected either way. Many (all?) functions can be written in either style, and the definitions are equivalent. There are a few advantages I've seen to pf style: 1. Function definitions are shorter, and sometimes clearer. 2. It saves you from having to give points in the domain a name. 3. It can make reasoning about programs simpler. For example, if we know that reverse . reverse == id then anywhere we see reverse . reverse, we can replace it with id, without having to track any other variables. In particular, Richard Bird's book and articles use this to great effect for program transformation and derivation from specification ("Bird-Meertens formalism") The only disadvantage I know of is that it can lead to obfuscation, especially if Haskell hasn't twisted your brain yet (in a good way). -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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On improving libraries: wanted list
by dons＠cse.unsw.edu.au 15 Dec '06

15 Dec '06

With the upcoming hackathon[1], and its timely focus on real world libraries, infrastructure and tools support, I've started a list of truly missing libraries, for particular problem domains. If you have a problem, for which you think there *should* be a Haskell library, please add it to this page: http://haskell.org/haskellwiki/Wanted_libraries Or, if you see a 'missing' entry there, but happen to know of a real library that solves the problem, add it to the list. If you feel an existing library isn't suitable for a task, note this in the 'could be improved section'. A good constructive list here will help focus developer efforts, and someone may just step up with the code! -- Don The future only happens if we write the code. 1. http://haskell.org/haskellwiki/Hac_2007

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Dynamic types through unsafeCoerce
by Alfonso Acosta 15 Dec '06

15 Dec '06

Hi, I've been using Data.Dynamic but the Typeable requirement doesn't go well with FFI declarations (which don't accept type contexts). This is a little example of what I would like to do: data MyDyn = MyDyn myToDyn :: a -> MyDyn myToDyn = unsafeCoerce myFromDyn :: MyDyn -> a myFromDyn = unsafeCoerce -- this is a type I want to transform back and forth from dynamic -- and store into lists etc etc ... data MyType a b = MyType a b -- This it's "abstract" Generic type (it's contructor should be hidden) data MyGenType = MyGenType MyDyn mt2mgt :: MyType a b -> MyGenType mt2mgt = MyGenType.myToDyn mgt2mt :: MyGentype -> MyType a b mgt2mt (MyGenType dyn) = myfromDyn dyn The question is, ¿if only mt2mgt and mgt2mt are used by the user, would the use of unsafeCoerce be dangerous? Thanks in advance, Alfonso Acosta

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Reversing a string of words: C# v Perl V Ruby v Haskell
by Andrew Savige 15 Dec '06

15 Dec '06

Recently, the C# programmers at work have been asking job applicants to write a function to reverse a string of words. For example, given an input string of: " one two three four " the function should produce: "four three two one" That is, the input string reversed word by word, with a single space between each word and with no leading or trailing space. (You may assume that the input string consists only of alphabetic characters, spaces and tabs). A popular C# approach goes something like: private static string reverseWords(string str) { string[] words = Array.FindAll<string>(str.Split( new char[] {' ','\t'}), delegate(string s) { return !String.IsNullOrEmpty(s); }); int i = words.Length - 1; if (i < 0) return String.Empty; StringBuilder sb = new StringBuilder(words[i]); while (--i >= 0) sb.Append(' ').Append(words[i]); return sb.ToString(); } Though the C# programmers seem happy enough with this solution, it's a bit too verbose for my personal taste. For cheap thrills, I'd like to show them some solutions to their little problem in other, er, less verbose languages. In Perl, I have: sub reverseWords { join ' ', reverse split(' ', shift) } In Ruby: def reverseWords(str) str.split(' ').reverse.join(' ') end Finally, and please don't laugh, in Haskell (GHC): reverseWords :: String -> String reverseWords xs = (concat (intersperse " " (reverse (words xs)))) which does appear to work correctly. However, I would like to do justice to Haskell as a language -- and given that my clumsy Haskell is not written in expert style -- I'm hoping that someone on this list might offer a better, faster, or more idiomatic Haskell solution to this little problem. Thanks, /-\ Send instant messages to your online friends http://au.messenger.yahoo.com

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Interoperating with Java
by mark＠ixod.org 15 Dec '06

15 Dec '06

I was wondering, what's the status of the various means of two-way interoperation with Java? Are things actively maintained? According to the WWW, HaskellDirect's (I have Lambada in mind) current version was released Jan 2004 and compiles with 'recent' versions of GHC, such as ghc-6.2, and the latest released JVM Bridge isn't much newer. We were looking at using the Standard Widget Toolkit (SWT) but don't want to rely on driving it from Haskell if the connectors are going to rot away. (We could still use FP, after all - e.g. Scheme's Kawa and Bigloo have good Java interoperation.) -- Mark

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