February 2009 - Haskell-Cafe

Re: [Haskell-cafe] Re: [Haskell-beginners] Just how unsafe is unsafe
by Roel van Dijk 06 Feb '09

06 Feb '09

On Fri, Feb 6, 2009 at 5:22 PM, Alberto G. Corona <agocorona(a)gmail.com> wrote: > then Data.List.head Data.Maybe.fromMaybe etc are also unsafe?. Yes, I consider them unsafe. Whenever I see those functions I know that I have to look elsewhere to see if their preconditions hold. I would have preferred that listToMaybe was called head and the existing head called unsafeHead, partialHead or something else of that nature. > unsafe does > not mean "with possible errors". unsafeXXX convert something in the IO monad > into something that is not. So it can potentially contaminate your pure > code. > But Data.List.head applied to a empty list will interrupt the computation > abruptly, so your code using head will either run side effect free or not > run at all. I guess what unsafe should mean is a matter of taste. Personally I find correctness more important that pureness. An unsafe function will crash your program if evaluated when its preconditions do not hold. Whether that is because of impurity (segmentation fault?), a partial pattern match or a direct error "bla" is not that important. It might be important when determining why your program crashed, but the result is still the same. The ByteString library has a module called Data.ByteString.Unsafe with plenty of unsafeXXX functions. The comment for unsafeHead for example states: "A variety of head for non-empty ByteStrings. unsafeHead omits the check for the empty case, so there is an obligation on the programmer to provide a proof that the ByteString is non-empty." That perfectly reflects my own philosophy on this issue.

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Are you using Haskell on the job?
by Kirk Martinez 06 Feb '09

06 Feb '09

Hello, fellow Haskell hackers! I am writing a term paper on Haskell in Business, and while I have gathered a lot of good information on the Internet, I would really like direct feedback from software professionals who have used Haskell in a business setting. I would really appreciate a few minutes of your time to provide insights gained from applying Haskell in the real world. Who knows, this could lead to a greater adoption of Haskell in the business community! Rather than a list of Haskell's technical strengths (purity, laziness, composition, etc.), I want to get a sense of the process leading up to the decision to use Haskell for a given project and the insights gained during and after completion. I am particularly interested in questions related to *business value*: - What were the pros and cons you considered when choosing a language? Why FP? Why Haskell? - What aspects of your problem domain were most critical to making that choice? - How has using Haskell given you a competitive advantage? - How is the software development lifecycle positively/negatively affected by using Haskell as opposed to a different language? - How did you convince management to go with a functional approach? - Was the relative scarcity of Haskell programmers a problem? If so, how was it managed? - Would you choose to use Haskell again for a similar project given what you know now? The best responses will not simply list answers, but also provide background and a bit of narrative on the project and insights gained. Feel free to reply to the list, or just to me personally if you prefer. My email is below. Thank you, Kirk Martinez (Kirk dot Martinez at gmail dot com)

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ANN: diagrams 0.2
by Brent Yorgey 06 Feb '09

06 Feb '09

I am very pleased to announce the 0.2 release of the diagrams package, an embedded domain-specific language for creating simple graphics in a compositional style. This release includes a number of significant new features, including: * support for arbitrary straight and curved paths * more shape primitives, including polygons and rounded rectangles * support for rendering text * PNG, PDF, PS, and SVG output * built-in color support replaced with external dependence on the feature-rich colour library and many more! More information, examples, and installation instructions can be found at http://code.haskell.org/diagrams. More features are planned for future releases, so contributions and suggestions are welcome. (Please note: since diagrams depends on the Cairo library, which has unfortunately not been Cabalized, you cannot install the diagrams library with cabal-install, unless you already have the cairo package installed. See the above website for instructions.) A special thanks to Dougal Stanton for his contributions to the library and help in preparing this release. -Brent

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HGL window update problem
by Rodrigo Queiro 06 Feb '09

06 Feb '09

In this program, the HGL window only updates when I mouseover it. Can someone tell me why this is / how I should avoid it? Thanks! Rodrigo module Main where import Graphics.HGL import Control.Concurrent import Control.Concurrent.MVar import Control.Monad main = do stringMVar <- newEmptyMVar forkIO . runGraphics $ do w <- openWindowEx "HGLTest" Nothing (300, 100) DoubleBuffered Nothing forever $ do string <- takeMVar stringMVar setGraphic w $ text (0, 0) string forever $ do string <- getLine putMVar stringMVar string

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Happstack 0.1 Released!
by Matthew Elder 06 Feb '09

06 Feb '09

Hello Haskell Cafe, I just wanted to inform you that the Happstack project has shipped it's 0.1 release. For those of you who don't know, Happstack is the successor for the HAppS project. More details on the release can be found at http://blog.happstack.com/2009/02/03/happstack-01-released . Thanks for your time, Matthew Elder -- Need somewhere to put your code? http://patch-tag.com Want to build a webapp? http://happstack.com

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Re: [Haskell-cafe] Happstack 0.1 Released!
by Henning Thielemann 06 Feb '09

06 Feb '09

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Re: [Haskell-cafe] about integer and float operations
by Henning Thielemann 06 Feb '09

06 Feb '09

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Re: OpenAL
by Paul Langevin 06 Feb '09

06 Feb '09

> AL lib: alSource.c:2291: alcDestroyContext(): 1 Source(s) NOT deleted Brian, have you tried currentContext $= Nothing before closing the device ? It works for me now : no message from ALSA (and still the sounds) _________________________________________________________________ Téléphonez gratuitement à tous vos proches avec Windows Live Messenger ! Téléchargez-le maintenant ! http://www.windowslive.fr/messenger/1.asp

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Re: Haddock
by David Waern 06 Feb '09

06 Feb '09

Hi everyone, I received this question from Lennart Augustsson (via Simon M) and thought I'd send out an inquiry to the Haskell community in general (Lennart, I hope you don't mind): Lennart writes: > We have some local patches for haddock that extends the <<blah>> > syntax so you can put TeX formulae in the documentation. > It looks like, <<! LaTeX stuff here !>>, but I'd like to extend it so > you can process the string with any command. > > Are you interested in folding this into the main branch? So the question is about extending the Haddock markup language. When modifying the language we should think about the tension between familiarity, presentation features (pictures, math, whatever) and visual portability across different mediums (HTML, ghci, IDE tooltips, etc). And here I should say that Haddock already supports pictures using the << url >> syntax. IMHO, adding <<! LaTeX !>> for TeX math is fine, because: - math in documentation is often useful - if you're going to write math, you need a format, even when the medium is plain text as in ghci. - TeX formulae seem to be relatively widely used and understood. As for running arbitrary commands, I think we are opening up to a lot of unfamiliar syntax. I'd like to hear what everyone thinks about that. There was also a thread about Haddock markup on haskell-cafe@ about a year ago, which originated with the interesting idea of using Markdown (or a Pandoc-extended version of it) instead of the current language: http://www.mail-archive.com/haskell-cafe@haskell.org/msg38054.html I think the original idea there is pretty nice, but let's first focus on the current markup language in order to answer Lennart's question. That thread contains some useful opinions on this matter, also. So, any comments? :) David

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Mutually recursive modules
by Henning Thielemann 06 Feb '09

06 Feb '09

I have written a small overview, how mutually recursive modules are currently supported and how they can be avoided: http://haskell.org/haskellwiki/Mutually_recursive_modules Please add information about other compilers and more ideas on breaking cycles.

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