- Haskell-Cafe - Haskell.org

Google Android
by Galchin, Vasili 25 Sep '08

25 Sep '08

Hello, Do there currently (or in the works) exist FFI bindings for Google's Android API? Kind regards, Vasili

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Am I doing it right?
by Creighton Hogg 25 Sep '08

25 Sep '08

Hi Haskellers, So as another step in my education, I wanted to ask if the following code at least feels 'morally' correct as a find/replace function, replacing each occurrence of the sub-list before with after in the list. unfoldr felt like the most natural higher order function to capture the recursion, but I imagine there are still slicker ways to have done this because I am creating an intermediate list-of-lists. However, doing it this way seems at least moderately speedy given that I was able to read in, find/replace, and write out a 120 MB file in 30 seconds on a fairly dinky linux box. replace :: (Eq a) => [a] -> [a] -> [a] -> [a] replace before after = concat . unfoldr aux where aux [] = Nothing aux str | take l str == before = Just (after,drop l str) | otherwise = Just (take 1 str,drop 1 str) l = length before Thanks, Creighton

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Haskell board game
by Rafael C. de Almeida 25 Sep '08

25 Sep '08

Hello, I'm interested in doing a simple board game on haskell. For that I want to be able to draw stuff like the possible player movements and I want to be able to display very simple animations. I want to know what graphical interface library you suggest to me. I have almost no prior experience with graphical interfaces of any kind, so I rather start with something easy and straightforward. I have no need for great performance or anything like that. I'd like if it runs on windows with much trouble, that is, it'll be easy to package it for windows without requiring the user to install anything besides my game. My first thought was to use GTK's gtkTable, but I'm unsure how easy it is to make it work on windows. Beside that, I'm not sure it would be the easiest API for me to use. []'s Rafael

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need help making sense of the type ...
by Daryoush Mehrtash 25 Sep '08

25 Sep '08

I am having hard time making sense of the types in the following example from the Applicative Programming paper: http://www.cs.nott.ac.uk/~ctm/IdiomLite.pdf ap :: Monad m ⇒ m (a → b ) → m a → m b ap mf mx = do f ← mf x ← mx return (f x ) Using this function we could rewrite sequence as: sequence :: [ IO a ] → IO [ a ] sequence [ ] = return [ ] sequence (c : cs ) =* return (:) 'ap' c *'ap' sequence cs I am specifically confused over the type of "m" in: return (:) 'ap' c "c" is obviously an instance of IO a monad. "return (:)" on the other hand (at least as I would expect it) is an instance of " ->" monad. a) are the above statements correct? b) if so, does it make sense for the "ap" function to have two different instances of the "m"? thanks for you help daryoush

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Injecting Haskell into C
by Roman Cheplyaka 24 Sep '08

24 Sep '08

I have a C function whose behaviour is customized by user-supplied function (think of libc qsort). Typically these user-supplied functions are written in C, but I'd like to use FFI to write them in Haskell. Precisely, I'd like to write high-order function which will generate these functions (e.g. mkCompare :: ... -> (Ptr a -> Ptr a -> Int) for feeding the result to qsort). As I understand, there are two ways to do that. Either Haskell code is called from C, or C code is called for Haskell. So my questions are: 1. Are they both possible? 2. If yes, which is better performance-wise? (C function is performance-critical). If generated function is called many times, how big an overhead is going to be? -- Roman I. Cheplyaka :: http://ro-che.info/ kzm: My program contains a bug. How ungrateful, after all I've done for it.

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Re: [Haskell] ICFP programming contest results
by David Roundy 24 Sep '08

24 Sep '08

On Wed, Sep 24, 2008 at 1:06 AM, Malcolm Wallace <malcolm.wallace(a)cs.york.ac.uk> wrote: > The ICFP programming contest results presentation: > http://video.google.com/videoplay?docid=-4697764813432201693 > > Feel free to pass on this link to any other appropriate forum. Yikes. Haskell did pretty terribly! Anyone have an idea why, given our success in previous contests? David (speaking as someone who was traveling that weekend and didn't have time to compete)

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the way to fail
by Jason Dusek 24 Sep '08

24 Sep '08

Whenever `fail` comes up, there are usually remarks to the effect that it doesn't really belong in the definition of `Monad`. Where does `fail` belong? Could it go in `Arrow`? -- _jsn

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Haskell Weekly News: Issue 86 - September 20, 2008
by Brent Yorgey 24 Sep '08

24 Sep '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080920 Issue 86 - September 20, 2008 --------------------------------------------------------------------------- Welcome to issue 86 of HWN, a newsletter covering developments in the [1]Haskell community. Lots of NEW stuff this week! A new generics library, new versions of Pandoc and darcs, a new website for xmonad, a new GADT/type family inference engine for GHC, a Haskell binding for Qt, and some new, astonishingly elegant ideas from Oleg. Also, here's hoping that everyone has a lot of fun at ICFP! Announcements GHC version control. Simon Peyton-Jones [2]sent out a revised proposal for GHC version control. darcs 2.0.3pre1. Eric Kow [3]announced the first pre-release of [4]darcs 2.0.3, featuring a few major bug fixes and a handful of interesting features. EMGM. Sean Leather [5]announced a release of [6]Extensible and Modular Generics for the Masses (EMGM), a library for generic programming in Haskell using type classes. Pandoc 1.0.0.1. John MacFarlane [7]announced the release of [8]pandoc 1.0.0.1, the swiss army knife of text markup formats. Iteratee-based IO. oleg [9]described a [10]safe, declarative approach to input processing which will be the subject of a talk at DEFUN08 on September 27. MetaHDBC paper. Mads Lindstroem [11]announced a [12]draft version of a paper about the [13]MetaHDBC library, which uses Template Haskell to do type-safe database access. Comments are welcomed, especially about the overall quality of the paper, whether it can be called scientific, and anything Mads could do to improve the paper. qtHaskell 1.1.2. David Harley [14]announced a second preview release of [15]qtHaskell, a set of Haskell bindings for Trolltech's Qt. Discussion Library design question. Andre Nathan [16]asked for advice on designing a simple graph library. The resulting discussion included an analysis of using the State monad versus a more functional approach. A round of golf. Creighton Hogg [17]learns about laziness by [18]making grown men cry. XML (HXML) parsing :: GHC 6.8.3 space leak from 2000. Lev Walkin [19]discovers a nice example of an obscure class of space leaks while writing some XML-processing code, prompting an in-depth analysis by Simon Marlow. Proofs and commercial code. Daryoush Mehrtash [20]asked about automated proof tools and techniques, and their uses in the real world. Blog noise [21]Haskell news from the [22]blogosphere. * Creighton Hogg: [23]Haskell Cafe or: How I learned to stop worrying & love laziness. * Douglas M. Auclair (geophf): [24]Animal as RDR, part II. Doug continues his posts on RDR expert systems. * Ivan Lazar Miljenovic: [25]Getting Real World Haskell Down Under. * Douglas M. Auclair (geophf): [26]Animal: an RDR implementation study. Doug describes "ripple-down rules" expert systems, and illustrates the types needed to encode one in Haskell. * Mark Jason Dominus: [27]data Mu f = In (f (Mu f)). Mark writes about fixpoints of type constructors. * John Goerzen (CosmicRay): [28]Switched from KDE to xmonad. John has taken the plunge to xmonad and seems to like it so far! * Eric Kow (kowey): [29]darcs weekly news #4. Pre-release of darcs 2.0.3; darcs hacking sprint next month; code.haskell.org upgrades to darcs 2; and other news. * Mads Lindstroem: [30]MetaHDBC paper (draft). Mads's first paper ever, on using Template Haskell for type-safe database access. Comments welcome! * Braden Shepherdson: [31]xmonad-light 0.8 Released. * Manuel M T Chakravarty: [32]GHC HEAD just got a new inference engine for GADTs and type families.. * Magnus Therning: [33]Haskell and Time. Magnus describes the solution to a problem with Data.Time. * Dan Piponi (sigfpe): [34]Two Papers and a Presentation. * Xmonad: [35]New xmonad website launched. xmonad has a shiny new website! Quotes of the Week * Botje: GHC 11 will have shootout entries as primitives. * wjt: oh, i see what you're doing. ...no, i don't. *splode* * Benjamin Pierce: [on existential types] I have a term, and it has a type. So there. * bos: come on, real programmers use "(((,) <$>) .) . (<*>)" * quicksilver: #haskell : Sometimes we answer your question, sometimes we lay hideous traps which will devour your soul. It's a risk you take. * harrison: [on computing 1000000!] it is the same as factorial 999999 * 1000000, big deal About the Haskell Weekly News New editions are posted to [36]the Haskell mailing list as well as to [37]the Haskell Sequence and [38]Planet Haskell. [39]RSS is also available, and headlines appear on [40]haskell.org. To help create new editions of this newsletter, please see the information on [41]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [42]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.haskell.org//pipermail/libraries/2008-September/010661.html 3. http://www.haskell.org//pipermail/haskell-cafe/2008-September/047664.html 4. http://www.darcs.net/darcs-2.0.3pre1.tar.gz 5. http://www.haskell.org//pipermail/haskell-cafe/2008-September/047751.html 6. http://www.cs.uu.nl/wiki/GenericProgramming/EMGM 7. http://www.haskell.org//pipermail/haskell-cafe/2008-September/047437.html 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pandoc 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44828 10. http://okmij.org/ftp/Haskell/Iteratee/ 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44713 12. http://lindstroem.files.wordpress.com/2008/09/metahdbc.pdf 13. http://www.haskell.org/haskellwiki/MetaHDBC 14. http://article.gmane.org/gmane.comp.lang.haskell.general/16425 15. http://qthaskell.berlios.de/ 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/44733 17. http://abstractabsurd.blogspot.com/2008/09/haskell-cafe-or-how-i-learned-to… 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/44724 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/44708 20. http://www.haskell.org//pipermail/haskell-cafe/2008-September/047440.html 21. http://planet.haskell.org/ 22. http://haskell.org/haskellwiki/Blog_articles 23. http://abstractabsurd.blogspot.com/2008/09/haskell-cafe-or-how-i-learned-to… 24. http://logicaltypes.blogspot.com/2008/09/animal-as-rdr-part-ii.html 25. http://ivanmiljenovic.wordpress.com/2008/09/19/getting-real-world-haskell-d… 26. http://logicaltypes.blogspot.com/2008/09/animal-rdr-implementation-study.ht… 27. http://blog.plover.com/prog/springschool95-2.html 28. http://changelog.complete.org/posts/756-Switched-from-KDE-to-xmonad.html 29. http://koweycode.blogspot.com/2008/09/darcs-weekly-news-4.html 30. http://lindstroem.wordpress.com/2008/09/18/metahdbc-paper-draft/ 31. http://braincrater.wordpress.com/2008/09/17/xmonad-light-08-released/ 32. http://justtesting.org/post/50500880 33. http://therning.org/magnus/archives/389 34. http://sigfpe.blogspot.com/2008/09/two-papers-and-presentation.html 35. http://xmonad.wordpress.com/2008/09/14/new-xmonad-website-launched/ 36. http://www.haskell.org/mailman/listinfo/haskell 37. http://sequence.complete.org/ 38. http://planet.haskell.org/ 39. http://sequence.complete.org/node/feed 40. http://haskell.org/ 41. http://haskell.org/haskellwiki/HWN 42. http://code.haskell.org/~byorgey/code/hwn/

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A question about parsec implementation
by Xinyu Jiang 24 Sep '08

24 Sep '08

Hi everybody, I am trying to port (a part of) the parsec library to scheme. I have read the paper and the haskell source of parsec, but I still have a problem. May be the problem is from my lack of parser knowledge, but I have to bother you. Scheme does not support lazy evaluation by default, so I simulated laziness when implementing finite lookahead. The result was not what I expected: it slowed down the parser and ate more memory, not speeding it up and reducing its space leak. If finite lookahead is a real gain, it would beat the cost of simulating lazy evaluation, which is not much. After analysis my code, I drew a conclusion (which may be wrong) that if the grammar you write is LL(1), that is, you don't use the "try" combinator, the time complexity of the predictive parser and the backtracking parser will be the same when they both suceed. Because in an LL(1) grammar, any consumed errors will make the whole parser return errors. Since the behaviors of the predictive parser and the backtracking parser are the same on LL(1) grammars, the backtracking parser will only eat more memory than the predictive parser when it fails. Is my conclusion right? Does parsec perform better than backtracking parsers on a pure LL(1) grammar when it suceeds? I am very grateful for your answer, thanks.

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what is the magic hash?
by Jason Dusek 24 Sep '08

24 Sep '08

It is not much covered in the docs. It has something to do with magic triggered by a postfix octothorpe? -- _jsn |...docs.| http://haskell.org/ghc/docs/latest/html/users_guide/flag-reference.html

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