- Haskell-Cafe - Haskell.org

Announce: keera-three-balance-checker-0.0.1
by Ivan Perez 16 Apr '12

16 Apr '12

Hi everyone, I wrote a small app to keep my Three Mobile PayG balance always under control. I've been using it for a while and it seems to work fine. It's just an icon docked in the system bar that shows the current balance when you move the mouse over it. If you don't know what Three Mobile is, then you don't need this. If you use a different operator and would find this kind of app handy, let me know and we can probably find a way to make it work for you. Code, instructions and screenshots: https://github.com/ivanperez-keera/keera-three-balance-checker My cabal repo: remote-repo: ivanperez-keera:http://ivanperez-keera.github.com/packages/archive If any of you find it useful, I'd be glad to know. If you find any bugs, drop me a line and we'll find out how to fix them together. Cheers, Ivan. PS. Why Announce instead of ANNOUNCE? Because this is not a big deal, it's just a toy.

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Building cabal-install for new ghc build
by Lyndon Maydwell 16 Apr '12

16 Apr '12

Hi Café. I'm building GHC from package ghc-7.4.1-src.tar.bz2 as the binary download was throwing segfaults for me (and apparently a few others). This has worked well and my issues with GHC and GHCi are now resolved. However I have needed to build cabal-install. This can't be done using cabal-install because it isn't installed :) I've found that I had to make several modifications to the bootstrap.sh and cabal-install.cabal files in order to get this to work. I am wondering how the maintainers of the Haskell-platform, etc are dealing with these issues. There seems to be a cabal-install-7.4.1 package on Hackage to address these issues, but I couldn't get it to work for me. Is this actually being used by the community? I realize that I am making trouble for myself by not just using the Haskell-Platform, but there are some packages I want to use that depend on recent versions of base. Thanks!

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ANNOUNCE : Juicy.Pixels 1.2.1
by Vincent Berthoux 16 Apr '12

16 Apr '12

Hello everyone, I'm pleased to announce you the new version of Juicy.Pixels, the 1.2.1, no new major features, here's the changelog : - Removed dependency on the array package. - Updated dependency list to handle latest mtl version. - Helper functions to save files without thinking about it. - Some code snippets in the API documentations GitHub : https://github.com/Twinside/Juicy.Pixels Hackage : http://hackage.haskell.org/package/JuicyPixels Regards Vincent Berthoux

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Conduit Best Practices for leftover data
by Myles C. Maxfield 15 Apr '12

15 Apr '12

Hello, I am interested in the argument to Done, namely, leftover data. More specifically, when implementing a conduit/sink, what should the conduit specify for the (Maybe i) argument to Done in the following scenarios (Please note that these scenarios only make sense if the type of 'i' is something in Monoid): 1) The conduit outputted the last thing that it felt like outputting, and exited willfully. There seem to be two options here - a) the conduit/sink should greedily gather up all the remaining input in the stream and mconcat them, or b) Return the part of the last thing that never got represented in any part of anything outputted. Option b seems to make the most sense here. 2) Something upstream produced Done, so the second argument to NeedInput gets run. This is guaranteed to be run at the boundary of an item, so should it always return Nothing? Instead, should it remember all the input it has consumed for the current (yet-to-be-outputted) element, so it can let Data.Conduit know that, even though the conduit appeared to consume the past few items, it actually didn't (because it needs more input items to make an output)? Remembering this sequence could potentially have disastrous memory usage. On the other hand, It could also greedily gather everything remaining in the stream. 3) The conduit/sink encountered an error mid-item. In general, is there a commonly-accepted way to deal with this? If a conduit fails in the middle of an item, it might not be clear where it should pick up processing, so the conduit probably shouldn't even attempt to continue. It would probably be good to return some notion of where it was in the input when it failed. It could return (Done (???) (Left errcode)) but this requires that everything downstream in the pipeline be aware of Errcode, which is not ideal.I could use MonadError along with PipeM, but this approach completely abandons the part of the stream that has been processed successfully. I'd like to avoid using Exceptions if at all possible. It doesn't seem that a user application even has any way to access leftover data anyway, so perhaps this discussion will only be relevant in a future version of Conduit. At any rate, any feedback you could give me on this issue would be greatly appreciated. Thanks, Myles

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strange GHCi type inference behavior involving map and partially applied functions
by Ting Lei 15 Apr '12

15 Apr '12

Hi All, I found a really strange case where GHC and GHCi behave differently in inferring types. It took me hours to figure this out. The following program {-# LANGUAGE NoMonomorphismRestriction #-} g x i = x ++ show i [a,b] = map g ["X","Y"] will not load without "NoMonomorphismRestriction". With that option, it will load and return the correct types (as expected): *Main> :t [a,b] [a,b] :: Show a => [a -> [Char]] *Main> a 1 "X1" However, if I do the same thing GHCi, the type inferencing seems to have been messed up: *Main> let g x i = x ++ show i *Main> let [a,b] = map g ["X","Y"] *Main> :t [a,b] [a,b] :: [() -> [Char]] *Main> :t map g ["X","Y"] map g ["X","Y"] :: Show a => [a -> [Char]] Note how in the last two instances the terms on the left and right-hand sides of the definition return different types. Also, the correct return type should be a list of unary functions taking an (Show a) as the parameter. Now the result is unusable: *Main> a 1 <interactive>:52:3: No instance for (Num ()) arising from the literal `1' Possible fix: add an instance declaration for (Num ()) In the first argument of `a', namely `1' In the expression: a 1 In an equation for `it': it = a 1 I am using GHCi 7.4.1 under windows. I also tried this under GHC 7.0x Is this a GHCi bug or could anyone please explain why this can of strange behavior happens? If this is a bug, could anyone with an account help file a bug for this? Thanks in advance, Ting

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: notcpp-0.0.1
by Steffen Schuldenzucker 15 Apr '12

15 Apr '12

On 04/13/2012 10:49 PM, Ben Millwood wrote: > I'm pleased to announce my first genuinely original Hackage package: > notcpp-0.0.1! > > http://hackage.haskell.org/package/notcpp > > [...] Why is it > scopeLookup :: String -> Q Exp with n bound to x :: T => @scopeLookup n@ evaluates to an Exp containing @Just x@ , not > scopeLookup :: String -> Q (Maybe Exp) with n bound to x :: T => @scopeLookup n@ evaluates to Just (an Exp containing @x@) ? Shouldn't n's being in scope be a compile time decision? That would also make the "openState: runtime name resolution has its drawbacks :/"[1] a compile time error. -- Steffen [1] http://hackage.haskell.org/packages/archive/notcpp/0.0.1/doc/html/NotCPP-Sc…

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Re: [Haskell-cafe] How helpful is h-99 (and should we complete the missing ones)?
by Daniel Hlynskyi 15 Apr '12

15 Apr '12

> > So here's my question: how useful is h-99 (are they overrated as learning > tools)? I find myself solve most of them in a "from the scratch" fashion > (e.g., no Monad, no Applicative, no Functor aside from List and a few > Maybe). Some of them are paper-worthy, for example the prime problems. I > hope some guru-level Haskeller could do away the missing few, and maybe > dive deeper into the surface to produce more insights (like the knights > travel page or the sieve paper, which are both beautiful). As another new-haskeller I say: Yes & No. "Yes" for necessity of several described algorithms and gained intuition for real programming. "No" for needed ammount of work to understand pitfalls of Haskell when real programming. And of-course, problems and solutions are not annotated with there typical real world aplications, they are not obvious for average beginners. Why would I make Binary tree balanced, when I don't know, what I'll gain (except balanced binary tree)?

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[Announce] wxHaskell 0.90
by Jeremy O'Donoghue 14 Apr '12

14 Apr '12

Hi Haskellers, I am delighted to announce the release of wxHaskell 0.90. This release represents a significant milestone for us as it includes support for wxWidgets 2.9.x. The release is avalable from Hackage and as a darcs repo from http://code.haskell.org/wxhaskell. Build and installation instructions have been updated on the Haskell wiki at http://www.haskell.org/haskellwiki/Wxhaskell. Highlights of the new release: - Builds and runs cleanly on 64 bit platforms (particularly MacOS X Lion). - The wxWidgets C wrapper is now built as a DLL on all platforms. - This is reported to enable meaningful use of wxHaskell in GHCi, at least on OS X and Windows. - It also theoretically allows wxc to be used independently of wxHaskell as the basis of a wxWidgets wrapper for other programming languages. Some D language hackers have expressed an interest in this. - New controls: - Styled Text Control (actually, this is reinstated as it was 'lost' a while back during cabalization) - OpenGL support - PropertyGrid control - Many events added in anticipation of wrapping more controls in the near future. There were many contributors to this release, the most notable being Dave Tapley (with the generous support of Mentics Inc.). Dave was responsible for the refactoring of wxc and PropertyGrid support. Eric Kow put in quite a bit of work with Kenny Frodo, Doaitse Swierstra and Alessandro Vermeulen on MacOS support. There were a couple of contributions from long-time wxHaskell contributor shelarcy, and bug reports, fixes and support from a larger community than I ever realised we had. Support for wxWidgets 2.8.x will continue in a 'maintenance mode' continuing from the wxHaskell 0.13 codeline. If you continue to use the old codeline, please take note of the changes to the procedure to get the correct version for your needs. This is also documented at http://www.haskell.org/haskellwiki/Wxhaskell. Thanks to everyone involved. Best regards Jeremy

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Final CFP: 16th Brazilian Symposium on Programming Languages
by Simon Thompson 14 Apr '12

14 Apr '12

============================================================ CALL FOR PAPERS 16th BRAZILIAN SYMPOSIUM ON PROGRAMMING LANGUAGES Natal, Rio Grande do Norte, Brazil September 24-28, 2012 http://www.cbsoft.dimap.ufrn.br ============================================================ IMPORTANT DATES Paper abstract submission (15 lines): April 27th, 2012 (extended) Full paper submission: May 4th, 2012 (extended) Notification of acceptance: June 4th, 2012 Final papers due: June 29th, 2012 INTRODUCTION The 16th Brazilian Symposium on Programming Languages, SBLP 2012, will be held in Natal, Brazil, on September 24-28, 2012. SBLP provides a venue for researchers and practitioners interested in the fundamental principles and innovations in the design and implementation of programming languages and systems. The symposium will be part of the 3rd Brazilian Conference on Software: Theory and Practice, CBSoft 2012, http://www.cbsoft.dimap.ufrn.br, which will host four traditional, well-established symposia: * XXVI Brazilian Symposium on Software Engineering (SBES) * XVI Brazilian Symposium on Programming Languages (SBLP) * XV Brazilian Symposium on Formal Methods (SBMF) * VI Brazilian Symposium on Components, Software Architecture and Software Reuse (SBCARS) SBLP 2012 invites authors to contribute with technical papers related (but not limited) to: * Program generation and transformation, including domain-specific languages and model-driven development in the context of programming languages. * Programming paradigms and styles, including functional, object-oriented, aspect-oriented, scripting languages, real-time, service-oriented, multithreaded, parallel, and distributed programming. * Formal semantics and theoretical foundations, including denotational, operational, algebraic and categorical. * Program analysis and verification, including type systems, static analysis and abstract interpretation. * Programming language design and implementation, including new programming models, programming language environments, compilation and interpretation techniques. SUBMISSIONS Contributions should be written in Portuguese or English. We solicit papers that should fall into one of two different categories: full papers, with at most 15 pages, or short papers, with at most 5 pages. We encourage the submission of short papers reporting on partial results of on-going master dissertations or doctoral theses. All accepted papers will be published in the conference proceedings distributed in a digital media by the CBSOFT organizers. Full papers submitted in English will be published in a volume of the Lecture Notes in Computer Science (LNCS), by Springer, to be distributed in the conference. For this reason, all papers must be prepared using the LNCS template, available at http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0. As in previous editions, a journal special issue, with selected papers from accepted contributions, is anticipated. Selected papers from 2003 to 2008 editions of SBLP were published in special issues of the Journal of Universal Computer Science, by Springer. The post-proceedings of SBLP from 2009 to 2011, also with selected papers from the conference proceedings, are being edited as special issues of Science of Computer Programming, published by Elsevier. Submissions should be done using SBLP 2012 installation of the EasyChair conference management system at http://www.easychair.org/conferences/?conf=sblp2012. GENERAL CO-CHAIRS NÈlio Cacho, UFRN Gibeon Aquino, UFRN Martin Musicante, UFRN PROGRAMME CHAIRS Francisco Heron de Carvalho Junior, UFC Luis Soares Barbosa, Univ. do Minho PROGRAMME COMMITTEE * Alberto Pardo, Univ. de La Republica * Alex Garcia, IME * Alvaro Freitas Moreira, UFRGS * AndrÈ Rauber Du Bois, UFPel * Andre Santos, UFPE * Carlos Camarao, UFMG * Christiano Braga, UFF * Fernando Castor Filho, UFPE * Fernando Quint„o Pereira, UFMG * Francisco Heron de Carvalho Junior, UFC (co-chair) * Joao Saraiva, Universidade do Minho * Jo„o F. Ferreira, Teesside University * Jonathan Aldrich, Carnegie Mellon Univ. * Jose Luiz Fiadeiro, Univ. of Leicester * Lucilia Figueiredo, UFOP * Luis Soares Barbosa, Univ. do Minho (co-chair) * Manuel AntÛnio Martins, Univ. de Aveiro * Marcelo A. Maia, UFU * Marcello Bonsangue, Leiden Univ/CWI * Marcelo d'Amorim, UFPE * Marco Tulio Valente, UFMG * Mariza A. S. Bigonha, UFMG * Martin A. Musicante, UFRN * Noemi Rodriguez, PUC-Rio * Paulo Borba, UFPE * Peter Mosses, Swansea University * Qiu Zongyang, Peking University * Rafael Dueire Lins, UFPE * Renato Cerqueira, PUC-Rio * Ricardo Massa, UFPE * Roberto S. Bigonha, UFMG * Roberto Ierusalimschy, PUC-Rio * Sandro Rigo, UNICAMP * Sergio Soares, UFPE * Simon Thompson, Univ. of Kent * Varmo Vene, Univ. de Tartu Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: notcpp-0.0.1
by Facundo Domínguez 14 Apr '12

14 Apr '12

Hi, > scopeLookup :: String -> Q Exp The example provided here [1] fails at runtime is no name can be found in scope. Would it make more sense to have scopeLookup :: [String] -> Q Exp and have compilation fail if no name in the list is in scope? By the way, the library brings an interesting approach to conditional compilation. Facundo [1] http://hackage.haskell.org/packages/archive/notcpp/0.0.1/doc/html/NotCPP-Sc…

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