March 2016 - Haskell-Cafe

hslua and language-lua need new maintainers
by Ömer Sinan Ağacan 31 Mar '16

31 Mar '16

Hi all, I need new maintainers for my packages hslua[1] and language-lua[2]. None of these packages is super popular so it's not going to be a lot of work. I just need someone to take care of new issues and pull requests. + points if the maintainer is a user of the library :-) I just don't have time and motivation to maintain those as I'm not a user anymore. Some background, lanugage-lua was the first Haskell program I've written (according to git logs, 3 years 6 months ago). I maintained it over the years and used in some projects. Currently Eric Mertens (glguy) is also a maintainer who occasionally sends some patches, but I need someone to take care of the new issues and pull requests. One thing you may want to do is to merge Eric's patch that ports the parser to Happy (currently it's Parsec) and improves the performance significantly. hslua was originally written by Gracjan Polak. I was using it and sending some patches and eventually I became the maintainer, created a Git repo, wrote some blogs posts about it, wrote tests and examples etc. I think the library is in good shape. One thing you may want to do if you take over the maintainership might be to port it to Lua 5.3 and drop the LuaJIT support. Every once in a while someone asks for it, there's even a pull request that is in bad shape that makes it working with 5.3. There's also another feature request for handling memory exhaustion in the interpreter. If you're interested in maintaining any of these please reply to this thread or send me a mail. Thanks. --- [1]: http://hackage.haskell.org/package/hslua [2]: http://hackage.haskell.org/package/language-lua

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ghc 7.10.3 crashes frequently on Windows
by Anakreontas 31 Mar '16

31 Mar '16

I have installed ghc from Haskell Platform version 7.10.3. When I run QuickCheck or HSpec tests interactively, every so often ghc crashes when I reload a file form withing Emacs and haskell mode. This problem manifested in earlier versions as well. Loading a file which does not contain tests never crashes ghc so far. How could I try to identify the cause of the crash?

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cabal: --enable-tests by default
by Andrew Pennebaker 31 Mar '16

31 Mar '16

In the future, could cabal --enable-tests by default when `cabal install`ing local project directories? Other languages' package managers automatically do this, a more intuitive default behavior. -- Cheers, Andrew

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Melbourne Haskell Users Group Meetup, Tonight, 31-03-2016 @ 6:30pm
by Lyndon Maydwell 30 Mar '16

30 Mar '16

Hi All, Tonight we're having our monthly Haskell meetup in the Melbourne CBD. Please come along if you're interested and free! http://www.meetup.com/Melbourne-Haskell-Users-Group/events/228915323/ Hope to see you there :) Regards, - Lyndon

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parsec reorganization
by Rustom Mody 30 Mar '16

30 Mar '16

Ive been trying parsec And I gather that the modules structures are heavily reorganized eg https://kunigami.wordpress.com/2014/01/21/an-introduction-to-the-parsec-lib… starts with import Data.Char (char) which immediately gives Module ‘Data.Char’ does not export ‘char’ Change that to import Text.Parsec.Char (char) and that error goes and more and more arcane ones start Likewise all the stuff here http://jakewheat.github.io/intro_to_parsing/ seems to be wrong paths Is there no tutorial for the current versions? ghc 7.8.4 parsec 3.1.9-2

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[2nd CFP] Haskell Symposium 2016
by Geoffrey Mainland 30 Mar '16

30 Mar '16

======================================================================== ACM SIGPLAN CALL FOR SUBMISSIONS Haskell Symposium 2016 Nara, Japan, 22-23 September 2015, directly after ICFP http://www.haskell.org/haskell-symposium/2016 ======================================================================== ** The Haskell Symposium has an early track this year ** ** See the Submission Timetable for details. ** The ACM SIGPLAN Haskell Symposium 2016 will be co-located with the International Conference on Functional Programming (ICFP 2016) in Vancouver, Canada. The Haskell Symposium aims to present original research on Haskell, discuss practical experience and future development of the language, and to promote other forms of denotative programming. Topics of interest include: * Language Design, with a focus on possible extensions and modifications of Haskell as well as critical discussions of the status quo; * Theory, such as formal semantics of the present language or future extensions, type systems, effects, metatheory, and foundations for program analysis and transformation; * Implementations, including program analysis and transformation, static and dynamic compilation for sequential, parallel, and distributed architectures, memory management, as well as foreign function and component interfaces; * Libraries, that demonstrate new ideas or techniques for functional programming in Haskell; * Tools, such as profilers, tracers, debuggers, preprocessors, and testing tools; * Applications, to scientific and symbolic computing, databases, multimedia, telecommunication, the web, and so forth; * Functional Pearls, being elegant and instructive programming examples; * Experience Reports, to document general practice and experience in education, industry, or other contexts. Papers in the latter three categories need not necessarily report original academic research results. For example, they may instead report reusable programming idioms, elegant ways to approach a problem, or practical experience that will be useful to other users, implementors, or researchers. The key criterion for such a paper is that it makes a contribution from which other Haskellers can benefit. It is not enough simply to describe a standard solution to a standard programming problem, or report on experience where you used Haskell in the standard way and achieved the result you were expecting. More advice is available via the Haskell wiki: (http://wiki.haskell.org/HaskellSymposium/ExperienceReports) Regular papers should explain their research contributions in both general and technical terms, identifying what has been accomplished, explaining why it is significant, and relating it to previous work, and to other languages where appropriate. In addition, we solicit proposals for: * System Demonstrations, based on running software rather than novel research results. These proposals should summarize the system capabilities that would be demonstrated. The proposals will be judged on whether the ensuing session is likely to be important and interesting to the Haskell community at large, whether on grounds academic or industrial, theoretical or practical, technical, social or artistic. Please contact the program chair with any questions about the relevance of a proposal. Travel Support: =============== Student attendees with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses. PAC also offers other support, such as for child-care expenses during the meeting or for travel costs for companions of SIGPLAN members with physical disabilities, as well as for travel from locations outside of North America and Europe. For details on the PAC program, see its web page (http://pac.sigplan.org) Proceedings: ============ Accepted papers will be included in the ACM Digital Library. Authors must grant ACM publication rights upon acceptance (http://authors.acm.org/main.html) Authors are encouraged to publish auxiliary material with their paper (source code, test data, etc.); they retain copyright of auxiliary material. Accepted proposals for system demonstrations will be posted on the symposium website but not formally published in the proceedings. All accepted papers and proposals will be posted on the conference website one week before the meeting. Publication date: The official publication date of accepted papers is the date the proceedings are made available in the ACM Digital Library. This date may be up to two weeks prior to the first day of the conference. The official publication date affects the deadline for any patent filings related to published work. Submission Details: =================== Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines (http://www.acm.org/sigs/sigplan/authorInformation.htm) The text should be in a 9-point font in two columns. The length is restricted to 12 pages, except for "Experience Report" papers, which are restricted to 6 pages. Papers need not fill the page limit---for example, a Functional Pearl may be much shorter than 12 pages. Each paper submission must adhere to SIGPLAN's republication policy, as explained on the web. Demo proposals are limited to 2-page abstracts, in the same ACM format as papers. "Functional Pearls", "Experience Reports", and "Demo Proposals" should be marked as such with those words in the title at time of submission. The paper submission deadline and length limitations are firm. There will be no extensions, and papers violating the length limitations will be summarily rejected. Papers may be submitted at: https://icfp-haskell2016.hotcrp.com/ Submission Timetable: ===================== Early Track Regular Track System Demos ---------------- ------------------- --------------- 1st April Paper Submission 20th May Notification 6th June Abstract Submission 10th June Paper Submission 17th June Resubmission Demo Submission 8th July Notification Notification Notification 31st July Camera ready due Camera ready due Deadlines stated are valid anywhere on earth. The Haskell Symposium uses a two-track submission process so that some papers can gain early feedback. Papers can be submitted to the early track on 1st April. On 20th May, strong papers are accepted outright, and the others will be given their reviews and invited to resubmit. On 17th June, early track papers may be resubmitted and are sent back to the same reviewers. The Haskell Symposium regular track operates as in previous years. Papers accepted via the early and regular tracks are considered of equal value and will not be distinguished in the proceedings. Although all papers may be submitted to the early track, authors of functional pearls and experience reports are particularly encouraged to use this mechanism. The success of these papers depends heavily on the way they are presented, and submitting early will give the program committee a chance to provide feedback and help draw out the key ideas. Program Committee: =================== James Cheney University of Edinburgh Iavor Diatchki Galois David Duke University of Leeds Richard Eisenberg University of Pennsylvania Ken Friis Larsen University of Copenhagen Andy Gill University of Kansas Zhenjiang Hu National Institute of Informatics Ranjit Jhala UC San Diego Yukiyoshi Kameyama University of Tsukuba Geoffrey Mainland (chair) Drexel University Mary Sheeran Chalmers University of Technology David Terei Stanford Niki Vazou UC San Diego Dimitrios Vytiniotis Microsoft Research =====================================================================

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Hspec + QuickCheck passing type
by Martijn Rijkeboer 29 Mar '16

29 Mar '16

Hi, I'm writing the following Hspec + QuickCheck test for some type: context "Type1" $ it "decode inverses encode" $ property $ \x -> (decode . encode) x == Right (x::Type1) However, I have a number of these types, and I could write the same test for every type, but that isn't very DRY. Therefore I would like to put the test in a function and call that function for every type. Something like the following: typeTest name = do context name $ it "decode inverses encode" $ property $ \x -> (decode . encode) x == Right x Unfortunately this doesn't work, since QuickCheck doesn't know which Arbitrary instance to use. How can I pass the type, e.g. x::Type1, to this function so QuickCheck knows what Arbitrary instance to use? Kind regards, Martijn Rijkeboer

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Type equality with "forall"
by Oleg 29 Mar '16

29 Mar '16

There are no problems in the code you have posted so far: it works exactly as it is supposed to. Overlapping instances worked as designed. It seems the problem is not in Haskell but in your expectations of existentials. Existentials are a means of abstraction, that is, deliberately forgetting details. In our case, forgetting all the details about the type. Consider the following code data W where Wrap :: a -> W test1 = case Wrap True of Wrap x -> not x It does not type-check. The variable x in test1 has the type which is different from any other existing type. Therefore, you cannot do anything sensible with it -- in particular, you cannot apply 'not', which works only for Booleans. Can't GHC just see that ``x is Bool'' -- just look at the line above!? No, by using the existential you specifically told GHC to forget all it knows about the type of the Wrapped value. If you want to remember some information about the existential type, you have to do it explicitly. For example, you can use Typeable and cast data W where Wrap :: Typeable a => a -> W test1 = case Wrap True of Wrap x' | Just x <- cast x' -> not x Wrap _ -> error "not a Bool" or build your own Universe (if you know in advance all the types you will be working with) data TypeRep a where TInt :: TypeRep Int TBool :: TypeRep Bool TArr :: TypeRep a -> TypeRep b -> TypeRep (a -> b) data W where Wrap :: TypeRep a -> a -> W test1 :: Bool test1 = case Wrap TBool True of Wrap TBool x -> not x Wrap _ _ -> error "not a Bool" In the latter case, the type signature is required. Basically, you have to write signature for every function that pattern-matches on the real GADTs (sometimes you can get away; but oftentimes you'll get a really cryptic error message instead). The latter pattern, using TypeRep, is the foundation of basically all generic programming out there.

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Munich Haskell Meeting, 2015-03-30 @ 19:30
by Christian Neukirchen 28 Mar '16

28 Mar '16

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Wednesday, March 30 at Cafe Puck at 19h30. For details see here: http://chneukirchen.github.io/haskell-munich.de/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-mar-2016/ Everybody is welcome! -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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build error using hsqml
by Fabien R 27 Mar '16

27 Mar '16

Hello, I'm trying to build a program using HsQml. cabal 1.14.0 fails with this error: Building HsQmlTest-0.0.1... Preprocessing executable 'HsQmlTest' for HsQmlTest-0.0.1... [1 of 1] Compiling Main ( src/Main.hs, dist/build/HsQmlTest/HsQmlTest-tmp/Main.o ) Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package array-0.4.0.0 ... linking ... done. Loading package deepseq-1.3.0.0 ... linking ... done. Loading package containers-0.4.2.1 ... linking ... done. Loading package filepath-1.3.0.0 ... linking ... done. Loading package tagged-0.4.2.1 ... linking ... done. Loading package bytestring-0.9.2.1 ... linking ... done. Loading package text-0.11.2.0 ... linking ... done. Loading package transformers-0.3.0.0 ... linking ... done. <command line>: can't load .so/.DLL for: /home/fabien/.cabal/lib/hsqml-0.3.4.0/ghc-7.4.1/libcbits-hsqml-0.3.4.0.so (libQt5Widgets.so.5: cannot open shared object file: No such file or directory) Loading package hsqml-0.3.4.0 ... Using --extra-lib-dirs to set the path of the library does not improve the situation. Here is the cabal description: name: HsQmlTest version: 0.0.1 cabal-version: >=1.2 build-type: Simple license: AllRightsReserved license-file: "" description: data-dir: "" executable HsQmlTest build-depends: QuickCheck -any, base -any, hsqml -any, text -any main-is: Main.hs buildable: True cpp-options: -DMAIN_FUNCTION=exeMain extra-lib-dirs: /media/travail/debian-sources/qt5/qtbase/lib/ hs-source-dirs: src Any hint ? -- Fabien

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