January 2012 - Haskell-Cafe

STM: "nested atomically" error
by Johan Brinch 24 Jan '12

24 Jan '12

Hi all, I'm seeing the "Control.Concurrent.STM.atomically was nested" error, but I just can't figure out what's happening. I'm not using any unsafe IO (only for debug printing), and the test program is only running one thread. Where is a transaction being nested? What are the scenarios where this error is reported? The behaviour is consistent on GHC 7.4.0 RC and GHC 7.2.2 (stable). That function that's being run can be found here: https://gist.github.com/30b94760abc27b05ec7c And here is the last output from the runtime system: https://gist.github.com/4356ae541895becb4169 Any ideas to track this down will be greatly appreciated! -- Johan Brinch, Dept. of Computer Science, University of Copenhagen

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bindings for libvirt
by Michael Litchard 24 Jan '12

24 Jan '12

Due to the direction things are going at work, I have become interested in Haskell bindings for libvirt. Noticed that this hasn't been done yet. I was wondering if this was due to lack of motivation, or if there were some difficult hurdles with libvirt that make the project cost-prohibitive. If it's the former, I don't see a problem proceeding with exploration. If it's the latter, I'd like to know what the hurdles are.

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Data & newtype differences. Today: strictness
by Yves Parès 24 Jan '12

24 Jan '12

Hello, I had for long thought that data and newtype were equivalent, but then I spotted some differences when it comes to strictness. Those can be summed up as: data Test = Test Int newtype TestN = TestN Int pm (Test _) = 12 -- Strict (pm undefined = undefined) pm2 t = t `seq` 12 -- Strict pmN (TestN _) = 12 -- Non strict (pm undefined = 12) pmN2 t = t `seq` 12 -- Strict When I think about it, pm and pmN are logical, as newtype layers are removed by the compiler and then do not exist at runtime. Some would maybe expected pmN2 to behave just like pmN (as I don't 'seq' the inner Int but the outer TestN), but if you follow the same logic (TestN layer doesn't exist at runtime), then you ask to evaluate the inner Int. This can however be misleading when you use an opaque type, can't it? As you don't know if it's declared using data or newtype... These make me think that pattern matching against a newtype is always lazy (irrefutable). Am I right? Is there some litterature expliciting in a less empiric way than I did the differences like this between data and newtype? I've never come against such documentation through all my learning of Haskell, yet I think it's an important point.

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interest in Irish Haskell User's Group?
by John Lato 24 Jan '12

24 Jan '12

Hello, Some other Haskellers and I have been discussing starting an Irish Haskell User's Group. I think we're close to critical mass, and need just a few more interested people to give it a run. To that end, I'd appreciate hearing from anyone who would be interested in attending or participating. Our initial meeting would likely be in Maynooth as that's where we're based, however we'd like to see involvement across at least the greater Dublin area, and possibly further afield. So, please let me know if you're interested in participating. I'll post further details when we've established an initial meeting. Thanks, John L.

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Haskellers in Delhi, India
by Anupam Jain 24 Jan '12

24 Jan '12

Hi all, Are there any haskellers in Delhi or nearby areas interested in a meetup? I've been dabbling in Haskell for a long time but only recently became interested in creating large "real world" programs with it. It would be interesting to meet folks who already have such experience (or are looking to get such experience). Thanks, Anupam Jain

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Efficient temporary file storage??
by Nick Rudnick 24 Jan '12

24 Jan '12

Dear all, if you want to temporarily store haskell data in a file – do you have a special way to get it done efficiently? In an offline, standalone app, I am continuously reusing data volumes of about 200MB, representing Map like tables of a rather simple structure, key: (Int,Int,Int) value: [((Int,Int),LinkId)] which take quite a good deal of time to produce. Is there a recommendation about how to 'park' such data tables most efficiently in files – any format acceptable, quick loading time is the most desirable thing. Thanks a lot in advance, Nick

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ANNOUNCE: hxournal-0.6.3
by Ian-Woo Kim 24 Jan '12

24 Jan '12

Hi, I am very happy to announce hxournal-0.6.3. (To learn about hxournal, see http://ianwookim.org/hxournal ) Compared with the previous version (0.6.2), it has now a big enhancement in rendering, implementing resizing selected elements and highlighter pen implementation. Now, after selection, you can see nice visual feedback of selection handle and selected strokes. When moving or resizing, it smoothly shows intermediate steps. Enhancement in double buffer rendering also enables smoother scrolling. I think this program is now in a pretty usable state as I use it as my daily notetaking program. Installation must be simple as always. Type "cabal install hxournal" after updating your cabal database. (you need to have gtk2hs installed. ) Last improvement has been done much while I was participating in Hac Boston activity. Thank Hac Boston organizers and participants very much! Thank you for your interest. Enjoy hxournaling! Ian-Woo

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Munich Haskell Meeting
by Heinrich Hördegen 24 Jan '12

24 Jan '12

Dear all, I hope, all of you had a good start into this year. For Munich's Haskellers, it's time for our next get-together. With a number of participants between 12 and 14, our last meetings have been a real success. Let's continue this joyful events. Even if you don't live at Munich, if you can make it one day, we will be happy to receive you there. Check out the dates! So, this year's first get-together will take place next week, Monday, 30. January at Cafe Puck, our usual venue. If you plan to join, please go to the site and click the button: http://www.haskell-munich.de/dates This will help us to reserve tables. With hope of a large attendance, have a nice time, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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Happy holidays Haskell
by R J 24 Jan '12

24 Jan '12

hello Haskell ever since I started this my life has been better than ever http://www.news13open.com

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[C][enums][newbie] What is natural Haskell representation of such enum?
by Данило Глинський 24 Jan '12

24 Jan '12

What is natural Haskell representation of such enum? enum TypeMask { UNIT, GAMEOBJECT, CREATURE_OR_GAMEOBJECT = UNIT | GAMEOBJECT }; More sophisticated question is: and what data structures must be used when converting this naturally one to Haskell? // 1-byte flaged enum enum TypeMask { // ... UNIT = 0x0004, GAMEOBJECT = 0x0008, // ... CREATURE_OR_GAMEOBJECT = UNIT | GAMEOBJECT WORLDOBJECT = UNIT | PLAYER | GAMEOBJECT | DYNAMICOBJECT | CORPSE // ... even more enum combos ... };

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