April 2011 - Haskell-Cafe

Re: [Haskell-cafe] How to contact OpenGL package maintainer (where is Sven?)
by Joachim Breitner 18 Apr '11

18 Apr '11

[Moving to haskell-cafe, not really library specific any more] Hi, Am Sonntag, den 17.04.2011, 12:39 -0700 schrieb Jason Dagit: > In this specific case, I'll do what I can to clean things up but your > request makes me pause and think that the debian packaging for cabal > packages is not automated enough. As haskell developers it seems a > little odd to me that we need to consider the cost of creating new > packages for the sake of debian. I like debian, so please don't take > that the wrong way :) the observation is correct, but unfortunately hard to change – Debian is large, slow moving, and unlikely to change policies for a corner-case such as Haskell libraries. Also, the policies that make life a bit more harder for us are also the policies that make Debian a good distribution: E.g. manually creating a copyright file describing who all has copyright on the files, and what license the files are under – .cabal has fields for that, but unfortunately, they very often do not reflect the reality of the .hs files. Or the other one, requiring that an upload of a source package is accompanied by a manually built and signed binary package ensures that the maintainer actually checked that the package builds and hopefully has tested it. If you think more about it, the lack of some of the policies on Hackage, e.g. nobody checking that licenses and copyright are correctly specified and actually compatible with dependencies, make Hackage a dangerous source for serious users. I am not saying that this should change, but the cheapness of Hackage uploads has its downsides. Greetings, Joachim -- Joachim Breitner e-Mail: mail(a)joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nomeata(a)joachim-breitner.de

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Killing threads in foreign calls.
by Jason Dusek 18 Apr '11

18 Apr '11

I am building an application that uses Postgres for storage. If a query runs too long, I would like to kill the querying thread, releasing its lock on the connection; if the connection is a in a bad state -- for example, busy -- I would like to clean up the connection. Unfortunately, killing calls in to libpq seems not to work. I have put a minimal example on hpaste: http://hpaste.org/45774/minimal_pg_contention_example If you install libpq with Cabal, you can run it. In the example, the main thread spawns a worker thread that queries Postgres, running "SELECT pg_sleep(10);"; the main thread waits half a second and then tries to kill the worker. Unfortunately, the worker always manages to get as far as printing "complete". In the code, I call `Database.PQ.exec': http://hackage.haskell.org/packages/archive/libpq/0.4.1/doc/html/src/Databa… This in turn calls a `safe' binding, `c_PQexec', to `PQexec' in the C library: http://hackage.haskell.org/packages/archive/libpq/0.4.1/doc/html/src/Databa… There are async interfaces, too; they do not seem to be any more killable then the sync ones. Maybe the problem is that you can't kill a thread while it's in a foreign call? I do not see any documentation to this effect; but I may have missed it. -- Jason Dusek () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

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Template Haskell question
by Alexey Karakulov 17 Apr '11

17 Apr '11

I'm interested if it's possible to use functions from some module without explicitly importing it. In ghci it's done on the fly, like this: Prelude> Data.Map.empty Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. fromList [] But without gchi it seems impossible. I have the file Test.hs: > {-# LANGUAGE TemplateHaskell #-} > module Test where > import Language.Haskell.TH > > x :: ExpQ > x = global $ mkName "Data.Map.empty" When I load it in ghci, all works: $ ghci -XTemplateHaskell Test.hs *Test> $x Loading package pretty-1.0.1.2 ... linking ... done. Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. Loading package template-haskell ... linking ... done. fromList [] But when I try to use it from other module, it fails. File Main.hs: > {-# LANGUAGE TemplateHaskell #-} > module Main where > import Test > > main = do > print $x $ runhaskell Main.hs Main.hs:5:9: Not in scope: `Data.Map.empty' In the result of the splice: $x To see what the splice expanded to, use -ddump-splices In the first argument of `print', namely `$x' In the expression: print ($x) -- All the best, Alexey

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"import" functionality in DSLs
by Nikhil A. Patil 17 Apr '11

17 Apr '11

Hi, I am planning a simple monadic DSL (frankly, calling it a DSL is a bit of a stretch; it's just a somewhat glorified state monad), and I wish to implement some kind of "import" functionality for it. The DSL code looks something like this: > doit :: DSL Term > doit = do (+) <- define_function "+" 2 > n0 <- define_constant "0" > k <- define_constant "k" > -- begin beautiful DSL code > let x = k + n0 > return $ x + x The code above adds identifiers "+", "0", "k" to the DSL monad and conveniently exposes haskell identifiers (+), n0, k for the user to use in code that follows. (Note that these define_* functions make state updates in the underlying state monad.) Needless to say, most functions like doit have very similar define_* calls in the beginning. Thus, I want to implement some kind of import functionality. Ideally, the code would look like this: > module DSLPrelude where > > prelude :: DSL () > prelude = do (+) <- define_function "+" 2 > n0 <- define_constant "0" > k <- define_constant "k" > return () > module Main where > import DSLPrelude > > doit :: DSL Term > doit = do prelude > -- begin beautiful DSL code > let x = k + n0 > return $ x + x ...but of course that won't work since (+), n0, k are not in scope. I can think of two solutions, both of which I dislike: Solution 1: > module DSLPrelude where > > prelude :: DSL (Term -> Term -> Term, Term, Term) > prelude = do (+) <- define_function "+" 2 > n0 <- define_constant "0" > k <- define_constant "k" > return ((+), n0, k) > module Main where > import DSLPrelude > > doit :: DSL Term > doit = do ((+), k, n0) <- prelude > -- begin beautiful DSL code > let x = k + n0 > return $ x + x This is quite unsafe: I have mistakenly swapped k and n0 in doit, without failing typecheck. Solution 2: > module DSLPrelude where > > (+) :: DSL (Term -> Term -> Term) > n0 :: DSL Term > k :: DSL Term > (+) = define_function "+" 2 > n0 = define_constant "0" > k = define_constant "k" > module Main where > import DSLPrelude > > doit :: DSL Term > doit = do (+) <- (+) > n0 <- n0 > k <- k > -- begin beautiful DSL code > let x = k + n0 > return $ x + x ...which works, but still has quite a bit of boilerplate crap. I feel this would be a common problem with a lot of DSLs, so I am curious to know how others solve it. Any pointers and suggestions are most welcome and greatly appreciated. Thanks! nikhil

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Oddness with binary-0.5.0.2 user install
by Antoine Latter 17 Apr '11

17 Apr '11

Hello folks, The docs for binary-0.5.0.2 on Hackage claim that their is a Binary instance for lazy ByteStrings, but there does not seem to be such an instance in my local install of binary-0.5.0.2. In addition, the docs on Hackage claim that there is an Applicative instance for the 'Get' type, which my install is also lacking. I've verified this in ghci and by looking at my own locally generated haddocks. The module structure for the 'binary' package is pretty straightforward - so I don't think I'm missing any imports in ghci. Has anyone seen this before? Antoine

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Haskell Platform 2011.2.0.1 now available
by Don Stewart 17 Apr '11

17 Apr '11

We're pleased to announce the 2011.2.0.1 release of the Haskell Platform: a single, standard Haskell distribution for everyone. Download the Haskell Platform 2011.2.0.1: http://haskell.org/platform/ This release adds support for GHC 7.0.3, and significant improvements for Mac OS X users. The specification, along with installers (including Windows, Mac and Unix installers for a full Haskell environment) are available. The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage. This release ships with GHC 7.0.3. What you get is specified here: http://hackage.haskell.org/platform/contents.html Thanks! -- The Platform Infrastructure Team P.S. a big thanks to Mark Lentczner and Mikhail Glushenkov who built the Mac and Windows installers!

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Announcing AusHac 2011 - The second annual Australian Haskell Hackathon (July 8-10)
by Alex Mason 17 Apr '11

17 Apr '11

Attention all Australasian Haskellers! After last year’s fantastic turnout for the first Australian Haskell Hackathon - AusHac 2010 - we’ve decided to organise another. As it’s rather boring to have a Hackathon with only two people, we encourage and welcome anyone interested in Haskell in Australia, New Zealand and surrounding areas (or further afield if you want!) to come and join us. Last year we had more than 25 Haskellers come from Brisbane, Canberra, Melbourne, Sydney and even two from as far away as New Zealand. AusHac 2011 will be held from Friday July 8th until Sunday July 10th, again at UNSW’s Computer Science building in Sydney. There’s no need to come for the entire weekend, just for whenever you’re able to. Last year, people worked on several projects including: • Accelerate: a high performance library for array computations using various backends (CUDA, LLVM, etc.) being written at UNSW. • Leksah: the Haskell IDE written in Haskell • DDC: the compiler for the Haskell-like language Disciple, which has strict evaluation by default, region, effect and closure typing and other interesting language features. • Hubris: the Haskell/Ruby bridge for calling Haskell from Ruby and vice-versa. • haskell-mpi: bindings to MPI, the message passing interface used frequently on high performance super-computers to allow programs to run on many nodes in parallel with easy communication. • There was even some work on LLVM to produce HTML output showing register allocations (as an aid to those writing new code generators etc.), which has now been included upstream. This work did not use Haskell, but it did happen at AusHac 2010. • And much more! This is a great chance to collaborate with others and get feedback on your work, while having heaps of fun and meeting other Haskellers, to verify they do actually exist! And don’t worry if you can’t think of a project to work on; just come along and pitch in with whatever strikes your fancy! If you’re interested in coming and have a project you’d like to work on, check out the wikipage http://www.haskell.org/haskellwiki/AusHac2011 where you can add your project ideas, or just take a look and see what others will be working on that you can help out with. People of all skill levels are welcome, from complete beginners to Oleg-level sage masters. We’re more than happy to help you learn Haskell if you’re just starting out or help you solve that little niggly bit in your code. If you’re interested in coming along, fill out our registration form http://axman6.wufoo.com/forms/aushac-2011-sign-up/. Registration is required so we know numbers and for you to get access to the university Wifi. Even if you’re not sure, fill it in to express your interest, obligation free. We’d rather know we have room for too many people than not enough. So come along! And if for some reason you don’t enjoy yourself, registration comes with a money-back guarantee! Hope to see you there, -- Alex Mason and Ivan Miljenovic, AusHac organisers P.S: If you have received this email directly, it is because you have shown previous interest in AusHac by signing up via the previous signup. If you do not wish to receive any more email regarding AusHac, please email me (axman6(a)gmail.com) and I'll be sure to take you off the list of interested people.

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Re: [Haskell-cafe] Cairo and Haskell
by Hans van Thiel 16 Apr '11

16 Apr '11

Hello Paulo, << I guess that would be using Cairo so I can have a 2d canvas to draw in and maybe even preview before exporting to PDF. However, I can't find any documentation on Cairo with Haskell or any code examples related to what I want to do. >> With regard to export to pdf and other formats this might be useful: http://muitovar.com/gtk2hs/app1.html (There's a Spanish translation too.) However, this tutorial is a few years old now, and I don't know if it's still up to date for the later Gtk2Hs versions. Best Regards, Hans van Thiel

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64 bit generic link warning on every compile
by Andrew Pennebaker 16 Apr '11

16 Apr '11

GHC 7 compiles fine, but there's an additional warning during linking. $ system_profiler SPSoftwareDataType | grep "System Version" System Version: Mac OS X 10.6.7 (10J869) $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.2 $ cat hello.hs #!/usr/bin/env runhaskell module Main where main :: IO () main = putStrLn "Hello World" $ ghc --make hello.hs [1 of 1] Compiling Main ( hello.hs, hello.o ) Linking hello ... ld: warning: -read_only_relocs cannot be used with x86_64 $ ./hello Hello World Cheers, Andrew Pennebaker www.yellosoft.us

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ANN: bindings to xfconf-4.8.0.0
by John Obbele 15 Apr '11

15 Apr '11

Hi all, I'm very happy to announce my first hackageDB package: xfconf[0], a Haskell binding to the XFCE xfconfd configuration dæmon. It's mainly a pet project I have done to learn Haskell using the FFI and QuickCheck but if someone can find it useful or instructive I would be happy. This package allows easy access to your favorite XFCE desktop settings (wallpaper, colors, police, etc.) and easier synchronisations between the configuration backend and your own Gtk+2 widget properties (a feature missing in gconf but now part of gsettings). A practical use of this binding can be found in hThemes[1], a quick&dirty front-end to switch between pre-defined desktop themes (eg. switching from a light-white theme for daytime to a dark-black one for your nightly hack sessions). [0.a]: http://hackage.haskell.org/package/xfconf [0.b]: http://patch-tag.com/r/obbele/xfconf/home [1]: http://patch-tag.com/r/obbele/hThemes/home Thanks to Andy Stewart and Axel Simon for their help in integrating xfconf to the gtk2hs stack. Any comments on this first work are welcomed ,-) regards, /John

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