- Haskell-Cafe - Haskell.org

[ANN] lxc-0.2 and bindings-0.2 – Haskell LXC API bindings
by Nickolay Kudasov 28 Sep '14

28 Sep '14

Greetings, I am pleased to announce the Haskell LXC API bindings. Bindings are represented by 2 cabal packages: lxc and bindings-lxc. LXC is a userspace interface for the Linux kernel containment features. For more information see https://linuxcontainers.org. http://hackage.haskell.org/package/lxc High level Haskell bindings to LXC (Linux containers). Git repository: https://github.com/fizruk/lxc http://hackage.haskell.org/package/bindings-lxc Direct Haskell bindings to LXC (Linux containers) C API. Git repository: https://github.com/fizruk/bindings-lxc === Before installation make sure you have LXC installed on your system with header files and static library. Packages have been tested with Ubuntu 12.04 and 14.04. On Ubuntu 14.04 LTS (Trusty Tahr): $ sudo apt-get install lxc-dev On previous Ubuntu versions (including 12.04 LTS Precise Pangolin) standard repositories do not contain liblxc1 package. You might want to use ppa:ubuntu-lxc/stable repository instead: $ sudo apt-get install software-properties-common python-software-properties $ sudo add-apt-repository ppa:ubuntu-lxc/stable $ sudo apt-get update $ sudo apt-get install lxc-dev === To get the latest stable version of both packages: $ cabal install lxc === While Haddock documentation is absent one can either build haddock documentation locally or view it online on GitHub Pages: lxc documentation: http://fizruk.github.io/lxc/docs/ bindings-lxc documentation: http://fizruk.github.io/bindings-lxc/docs/ See also Usage section of lxc package README file: https://github.com/fizruk/lxc/blob/master/README.md#usage === Feedback of any kind is welcome! Cheers, Nickolay Kudasov

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Debugging with ghci
by Dominick Samperi 28 Sep '14

28 Sep '14

For a long time I've worked under the assumption that a breakpoint can only be used once in a ghci session, because if you run the function (main say) again, it runs to completion, ignoring the breakpoint. Restarting ghci and resetting the break point resolves the problem, but does not explain why a breakpoint cannot be used more than once. How can this behavior be explained? Also, when I stop at a breakpoint :step works as expected, but :trace does not? It simply runs the rest of the function, and using :back subsequently results in the message "not stopped at a breakpoint." Any comments on this behavior? Thanks, Dominick

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Sum and Product do not respect the Monoid laws
by Jason Choy 27 Sep '14

27 Sep '14

The Monoid instances for Sum and Product do not respect the Monoid laws. The instances are: Num a => instance Monoid (Sum a) Num a => instance Monoid (Product a) Float and Double are instances of the Num typeclass, however, floating point addition and multiplication are not associative. Here's an example: > (Sum 1234.567 `mappend` Sum 45.67834) `mappend` Sum 0.0004 Sum {getSum = 1280.2457399999998} > Sum 1234.567 `mappend` (Sum 45.67834 `mappend` Sum 0.0004) Sum {getSum = 1280.24574} Shouldn't these instances be constrained to Integral types? Jason

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Munich Haskell Meeting
by Heinrich Hördegen 27 Sep '14

27 Sep '14

Dear all, our next Haskell meeting in Munich takes place at Cafe Puck at 19h30 on Mon, 29th of September. Please go to http://www.haskell-munich.de/dates and click the button if you plan to join. Everyone is welcome! Heinrich

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foreign ccall export / ghc 7.6.3
by Clint Adams 27 Sep '14

27 Sep '14

Are there examples of "foreign ccall export" that work with GHC 7.6.3?

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SIGVTALRM and Unbound threads
by Lane Seppala 26 Sep '14

26 Sep '14

Howdy, I've banged into this issue of FFI errors stemming from a C library improperly handling system call interrupts caused by the SIGVTALRM/SIGALRMs emitted by the Haskell runtime. Usually the 'proper' solution is to fix the C library—easier said than done for most C quagmires. The most popular workaround, described by Bryan O'Sullivan[1] and several others, is to block the culprit signals from reaching the FFI call. I found another workaround using unbound threads, and it seems to work generally for this problem. However, I'm not satisfied with my understanding about *why* it works given what I know about FFI, bound vs unbound threads, and sys calls. As the simplest example, compiled with the threaded runtime (and neglecting imports): main = void $ sleep 10 will be interrupted, whereas main = void $ runInUnboundThread $ sleep 10 will complete its 10 second sleep. How does running the FFI call on an unbound thread block SIGVTALRM or otherwise avoid the interrupt? My understanding is that these signals *should* reach the FFI process, since they're on the same OS thread the timers were set. Is there any danger to consider before saying that this is a viable workaround? Kindly, Lane [1]: http://www.serpentine.com/blog/2010/09/04/dealing-with-fragile-c-libraries-…

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Re: [Haskell-cafe] What happened to hugs?
by Doug McIlroy 26 Sep '14

26 Sep '14

> We know where the old contents of the hugs documentation and source are, > and are going to get them back online sometime soon. Welcome news! For my purposes, there is no substitute. GHCi's disregard for "default" is a fatal flaw. Doug McIlroy

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Hackage bad gateway?
by Dominick Samperi 26 Sep '14

26 Sep '14

Today (9/25/2014) Hackage is not reachable due to "bad gateway." Is there a status page? Thanks, Dominick

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api-tools questions
by Karsten Gebbert 25 Sep '14

25 Sep '14

Hi All, I have a question concerning the api-tools package. Its not clear from the tests, sources or the tutorial how I can use other date/time formats that the default `utc` type can handle. I'm trying to wrap a JSON API that is not under my control, so I have to adhere to whatever I get back from it. Could anybody with experience with the package point me to some example, relevant bits in the sources or a tip how to do it? I'm planning to create some more documentation around the package to contribute back once I figured out a few more details, because I think its quite a useful abstraction when dealing with (foreign) APIs IMO. Thanks already for any hints, karsten

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Replacement for Ptr () in FFI declaration?
by Howard B. Golden 25 Sep '14

25 Sep '14

Hi, Apparently, before GHC 7.8, it was possible to specify FFI declarations using Ptr (). However, it now gets an error message: This is from com-1.2.3.1 when I try to compile it: ------ message follows ----- [12 of 21] Compiling System.Win32.Com.Automation.TypeLib ( System\Win32\Com\Automation\TypeLib.hs, dist\build\System\Win 32\Com\Automation\TypeLib.o ) System\Win32\Com\Automation\TypeLib.hs:1379:1: Unacceptable argument type in foreign declaration: Ptr () When checking declaration: foreign import stdcall safe "dynamic" prim_System_Win32_Com_Automation_TypeLib_setGuid :: Ptr () -> Ptr () -> Ptr GUID -> IO Int32 ------ end of message ----- What is the proper declaration to use instead of Ptr () ? Thanks. Howard B. Golden

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