January 2012 - Haskell-Cafe

How to terminate the process group of a process created with createProcess?
by André Scholz 12 Jan '12

12 Jan '12

Hello, (on unix) creating a process A which spawns itself a subprocess B and terminating process A before it finishes leaves process B as a process on its own. This is because "terminateProcess" sends the sigterm signal to the process only and not to its process group. Is there a way to terminate the process group of process A? Or alternatively is there a way to get he Posix ProcessId of process A? The following is a minimal example. The process "sleep 60" survives the termination of both "sleep" and of the main program. <code> module Main where import Control.Concurrent import System.Process main = do h@(_, _, _, hProc) <- createProcess (proc "time" ["sleep", "60"]) threadDelay $ 5 * 1000000 terminateProcess hProc waitForProcess hProc </code> Kind regards, André

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ANNOUNCE: quickcheck-instances
by Antoine Latter 12 Jan '12

12 Jan '12

Hello! I'd like to announce the first release of the quickcheck-instances package, which aims to consolidate commonly needed class instances for use with QuickCheck. These instances are appropriate when your tests don't have strong requirements on the nature of the input data - for example, if you are testing a network-protocol parser, the ByteString instance provided in this package might not be what you want to use. Other limitations: * I do not have full coverage of the Haskell Platform - I have been adding instances as I need them. Patches are warmly welcomed, however. * I only have instances of the 'Arbitrary' class, however I would be happy to accept patches for instances of CoArbitrary and other classes that would be useful in quickcheck-based testing. On Hackage: http://hackage.haskell.org/package/quickcheck-instances Ordinarily I frown on orphaned instances in library packages, but the fact that quickcheck properties should never appear in a library Cabal package, most of the downsides of orphans are mitigated. Thanks, Antoine

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Well-Typed are hiring: Haskell consultant
by Andres Löh 12 Jan '12

12 Jan '12

In order to keep up with customer demand, we are looking to hire a Haskell expert to work with us at Well-Typed (http://www.well-typed.com/) as a Haskell consultant. This is an exciting opportunity for someone who is passionate about Haskell and who is keen to improve and promote Haskell in a professional context. The role is quite general and could cover any of the projects and activities that we are involved in as a company. The tasks may involve: * working on the Haskell compilers, libraries and tools; * Haskell application development; * working directly with clients to solve their problems. Well-Typed has a variety of clients. For some we do proprietary Haskell development and consulting. For others, much of the work involves open-source development and cooperating with the rest of the Haskell community: the commercial, open-source and academic users. At the moment, we are running the Parallel GHC Project (http://www.haskell.org/haskellwiki/Parallel_GHC_Project/) It is likely that initial tasks will have some connection with parallel and/or concurrent programming in Haskell. We are also doing quite a bit of GHC maintenance, and some knowledge or interest in compiler internals, operating systems, the foreign language interface, and/or deployment issues would be welcome. Our ideal candidate has excellent knowledge of Haskell, whether from industry, academia, or personal interest. Familiarity with other languages, low-level programming, and good software engineering practices are also useful. Good organisation and ablity to manage your own time, and reliably meet deadlines, is important. You are likely to have a batchelor's degree or higher in computer science or a related field, although this isn't a requirement. Experience of consulting, or running a business, is also a bonus. The position is initially as a contractor for one year with a salary of 150 GBP per day. We offer flexible hours and work from home. Living in England is not required. In the longer term there is the opportunity to become a member of the partnership with a full stake in the business: being involved in business decisions, and fully sharing the risks and rewards. If you are interested, please apply via info(a)well-typed.com. Tell us why you are interested and why you would be a good fit for the job, and attach your CV. Please also indicate when you might be able to start. We are more than happy to answer informal enquiries. Contact Duncan Coutts, Ian Lynagh or Andres Löh (http://www.well-typed.com/who_we_are/) for further information, either by email or IRC. The deadline for applications is Friday 27th January 2012. == About Well-Typed Well-Typed LLP is a Haskell services company, providing consultancy services, writing bespoke applications, and offering commercial training in Haskell and related topics. -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com

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Re: [Haskell-cafe] How to terminate the process group of a process created with createProcess?
by John Lato 12 Jan '12

12 Jan '12

> From: Brandon Allbery <allbery.b(a)gmail.com> > > On Wed, Jan 11, 2012 at 16:26, Andr? Scholz <andre.scholz(a)uni-bremen.de>wrote: > >> (on unix) creating a process A which spawns itself a subprocess B and >> terminating process A before it finishes leaves process B as a process on >> its >> own. This is because "terminateProcess" sends the sigterm signal to the >> process only and not to its process group. >> > > terminateProcess passes on the semantics of kill(2); on SVID-compliant (and > I think POSIX.1-compliant) systems, the negative of the process group > leader's process ID is used to signal the process group. Note that you may > need to arrange for your initial child process to invoke setpgrp() to > insure that the parent program is not part of the process group. System.Process.terminateProcess takes a ProcessHandle as input, which is an opaque type which essentially can only be created via System.Process.createProcess or similar; it's not possible to pass arbitrary pid's to it. However, on Posix systems it is currently possible (ghc-7.2.1) to import System.Process.Internals, which would allow you to either create a new ProcessHandle from an arbitrary pid or retrieve the pid from a ProcessHandle. My answer on SO now shows how to do so. Of course this approach won't be portable, and it may change depending on your compiler version. (link to SO: http://stackoverflow.com/questions/8820903/haskell-how-to-timeout-a-functio… ) John L.

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GHCi with wxhaskell-dev
by Dave Tapley 12 Jan '12

12 Jan '12

I'm working on a dev branch of wxHaskell [1] and one of the new features is that you can use wxHaskell in GHCi (again), but I seem to have broken something in the process: Firstly: wxHaskell works in GHCi insomuch as: You can load the hello world sample [2] in GHCi, do "> main", see the window spawn, close it and be returned to the GHCi prompt. What I'm interested in is the case where you call wxHaskell's start function [3] from GHCi. The result is becoming stuck with no way back to the GHCi prompt without resorting to sending kill signals. I made a screen cast to illustrate my findings [4]: 1. Once you've done "> start $ return ()" you can press ^C all you want, there's no way back to the GHCi prompt. 2. If you send three (less has no effect) kills to the GHCi process it will exit, but it will exit GHCi as well (instead of putting you back at the GHCi prompt). 3. If you do "> start $ return ()" but *do not* try and ^C, you can send a single kill to GHCi and be returned to the GHCi prompt. My questions: 1. Why doesn't GHCi respond to the ^C? 2a. Why does entering ^C from within GHCi then prevent you from getting back to the GHCi prompt? 2b. Why doesn't any other input (i.e. entering anything but ^C) cause this behaviour? 3. If you have sent ^C from GHCi, why do you then have to kill three times to get out of GHCi? Feel free to pull the repo [1] and try it out, you'll need the latest dev release of wxWidgets [5] installed. Thanks! Dave, [1] http://darcsden.com/DukeDave/wxhaskell-dev [2] http://darcsden.com/DukeDave/wxhaskell-dev/browse/samples/wx/HelloWorld.hs [3] http://darcsden.com/DukeDave/wxhaskell-dev/browse/wx/src/Graphics/UI/WX.hs#… [4] http://youtu.be/sEI48ultdS0 [5] http://sourceforge.net/projects/wxwindows/files/2.9.3/

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How to catch all exceptions that could be caught?
by Magicloud Magiclouds 12 Jan '12

12 Jan '12

Hi, With Prelude.catch, I could write "catch () $ \_ -> return Nothing". But with Control.Exception.catch, I must specify a type for the "_". What should I do? PS: In Java document, a function declaration would tell us both the incoming args and outgoing, AND what exceptions would it throw. But it does not seem to exist in Haskell document? And the most awful part is that even when the program failed with error message, I still did not know what exception was this. Take "recv: does not exist (Connection refused)" for example. -- 竹密岂妨流水过山高哪阻野云飞

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A simple telnet client using Conduit
by Erik de Castro Lopo 12 Jan '12

12 Jan '12

Hi all, I've written a simple telnet client using Michael Snoyman's Conduit library and was looking for comments as to whether I'm doing it right. In particular, is my usage of a ResourceT to track a thread a good idea, necessary or waste of time. The code is here: https://gist.github.com/1596792 Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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GHC exceeding command line length limit with split-objs - and a possible fix
by Eugene Kirpichov 11 Jan '12

11 Jan '12

Hi, I'm building gtk2hs on a mac with -fsplit-objs, and during compilation of the largest file, which produces about 7000 split objects, the linking phase fails. I'm assuming that's because the command line length has been exceeded, because other files compile fine, without -fsplit-objs it compiles fine too, and it compiles fine *with* -fsplit-objs on other OS - so perhaps the reason is in mac's tempfile name generation (they're longer than on other platforms) or something. Anyway. I think a nice fix would be to employ gcc's ability to read options from a file - gcc @file - and write overly long option strings into temp files. It'd be fun for me to implement this as my first contribution to ghc; is this a good idea in general? -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Re: [Haskell-cafe] GHC exceeding command line length limit with split-objs - and a possible fix
by John Lato 11 Jan '12

11 Jan '12

I used https://github.com/kennethreitz/osx-gcc-installer/downloads to get a real gcc on Lion. Biggish download, but it worked. I've also seen reports of success by self-compiling gcc, or by installing XCode 4 on top of an existing XCode 3 installation. John L. > From: Eugene Kirpichov <ekirpichov(a)gmail.com> > > Oh well... looks like building ghc won't be easy, as it doesn't build with > llvm-gcc and it's not easy to get a "real" gcc on Lion. But I don't stop > trying :)

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Where is the pairing-with-monoid monad instance?
by Conal Elliott 11 Jan '12

11 Jan '12

Is the standard pair-with-monoid monad instance in some standard place? I see the Applicative instance in Control.Applicative, and the pair-with-monoid Functor instance in Control.Monad.Instances, and the (->) e and Either e monad instances also in Control.Monad.Instances. I'm looking for something like instance Monoid o => Monad ((,) o) where return a = (mempty,a) (o,a) >>= f = (o `mappend` o', b) where (o',b) = f a Where is it hiding from me? Thanks, - Conal

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