- Haskell-Cafe - Haskell.org

Munich Haskell Meeting
by Heinrich Hördegen 22 Apr '12

22 Apr '12

Dear all, we will meet again for our monthly Haskell Meeting in Munich on the 26th of April, at 19h30 at Cafe Puck. If you plan to join, please go to the site an click the button in order to reserve enough tables: http://www.haskell-munich.de/dates Also, consider joining us for our local Hackathon which will be held on Saturday, the 12th of May. See the details on the site. Until then, I wish all of you a good time, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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Re: [Haskell-cafe] default instance for IsString
by Markus Läll 22 Apr '12

22 Apr '12

Isn't this a special case already? I.e you already can default a string literal to anything you want, the only thing needed is a language pragma (and more obvious docs). The only "problem" is, that it's not part of the Haskell2010 report, and thus not supported in other compilers (or is it?). On Sun, Apr 22, 2012 at 8:51 PM, Johan Tibell <johan.tibell(a)gmail.com> wrote: > On Sun, Apr 22, 2012 at 10:37 AM, Brent Yorgey <byorgey(a)seas.upenn.edu> wrote: >> I do not think this is a bug. Since type classes are open, GHC does >> not do any reasoning of the form "X is the only instance in scope, so >> I will pick that one". Other instances could be added at any time >> (perhaps in other modules). In this particular instance, GHC has no >> reason to choose the Text instance other than the fact that it is the >> only instance in scope -- that is, type inference is not enough to >> determine that the Text instance should be chosen. >> >> However, I do agree that it would be nice to have a mechanism for >> specifying default instances for arbitrary (user-defined) type >> classes. > > Couldn't we make a special case for IsString, like we do for Num, > given it's special syntactic association with OverloadedStrings? > > -- Johan > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users -- Markus Läll

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Re: [Haskell-cafe] default instance for IsString
by Markus Läll 22 Apr '12

22 Apr '12

Hi Greg You *can* have what you want -- enable extended defaulting and set Text as default by > {-# LANGUAGE ExtendedDefaultRules #-} > default (Integer, Double, T.Text) -- keep the default defaults At the moment you only by chance have one instance of NoDefault in scope, which is why it shouldn't work at the moment. (ExtendedDefaultRules is required because the standard defaulting rules apply only when defaulting numeric literals.) On Sun, Apr 22, 2012 at 7:55 AM, Greg Weber <greg(a)gregweber.info> wrote: > This is a better demonstration of the issue. I am going to open a GHC > bug report, as I can't see how this behavior is desirable. > > > {-# LANGUAGE OverloadedStrings #-} > import Data.Text as T > > class NoDefault a where noDefault :: a -> Text > instance NoDefault T.Text where noDefault = id > > main = print (noDefault "Hello!") > > default.hs:7:15: > Ambiguous type variable `a0' in the constraints: > (NoDefault a0) arising from a use of `noDefault' > at default.hs:7:15-23 > (Data.String.IsString a0) arising from the literal `"Hello!"' > at default.hs:7:25-32 > Probable fix: add a type signature that fixes these type variable(s) > In the first argument of `print', namely `(noDefault "Hello!")' > In the expression: print (noDefault "Hello!") > In an equation for `main': main = print (noDefault "Hello!") > > > On Sat, Apr 21, 2012 at 7:51 PM, Greg Weber <greg(a)gregweber.info> wrote: >> my actual use case looks more like this: >> >> {-# LANGUAGE OverloadedStrings #-} >> {-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-} >> >> import Data.Text as T >> >> class ShowT a where >> showT :: a -> String >> >> instance ShowT T.Text where >> showT = show >> >> instance ShowT String where >> showT = show >> >> main = print (showT "Hello!") >> >> Ambiguous type variable `a0' in the constraints: >> (ShowT a0) arising from a use of `showT' at default.hs:16:15-19 >> (Data.String.IsString a0) arising from the literal `"Hello!"' >> >> >> So I actually want to define a default instance for a typeclass I >> define that uses isString instances. >> >> >> >> On Sat, Apr 21, 2012 at 6:24 PM, Daniel Peebles <pumpkingod(a)gmail.com> wrote: >>> I think it'll be hard to do that without putting Text in base, which I'm not >>> sure anyone wants to do. >>> >>> Dan >>> >>> On Sat, Apr 21, 2012 at 8:20 PM, Greg Weber <greg(a)gregweber.info> wrote: >>>> >>>> I would like to default IsString to use the Text instance to avoid >>>> ambiguous type errors. >>>> I see defaulting capability is available for Num. Is there any way to >>>> do this for IsString? >>>> >>>> Thanks, >>>> Greg Weber >>>> >>>> _______________________________________________ >>>> Glasgow-haskell-users mailing list >>>> Glasgow-haskell-users(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >>> >>> > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users -- Markus Läll

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http-enumerator: users?
by Michael Snoyman 22 Apr '12

22 Apr '12

Hi all, I'm wondering if there are still active users out there of http-enumerator. Four months ago I released http-conduit, and since then, my development efforts have gone almost exclusively there. At this point, I'm not longer actively using http-enumerator on any of my projects, and while I've backported any security fixes to http-enumerator, it is otherwise not be updated. I see three possibilities for the future of http-enumerator: 1. Deprecate it in favor of http-conduit 2. Someone else takes over as maintainer 3. Turn it into a frontend for http-conduit, exposing an enumerator interface. This will likely, though not necessarily, result in API breakage I'm curious how others feel on this. If I don't hear anything back on the topic, I'll assume that no one is interested in http-enumerator, and thus will opt for option (1). Michael

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Call for Papers - Haskell Symposium 2012 - six weeks to go
by Janis Voigtländer 21 Apr '12

21 Apr '12

======================================================================== "Haskell 2012" ACM SIGPLAN Haskell Symposium 2012 Copenhagen, Denmark 13th September, 2012 (directly after ICFP) CALL FOR PAPERS http://www.haskell.org/haskell-symposium/2012/ ======================================================================== The ACM SIGPLAN Haskell Symposium 2012 will be co-located with the 2012 International Conference on Functional Programming (ICFP), in Copenhagen, Denmark. The purpose of the Haskell Symposium is to discuss experiences with Haskell and future developments for the language. Topics of interest include, but are not limited to: * 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 treatments of the semantics of the present language or future extensions, type systems, 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; * Tools, in the form of profilers, tracers, debuggers, pre-processors, testing tools, and suchlike; * Applications, using Haskell for scientific and symbolic computing, database, multimedia, telecom and web applications, and so forth; * Functional Pearls, being elegant, instructive examples of using Haskell; * Experience Reports, general practice and experience with Haskell, e.g., in an education or industry context. Papers in the latter three categories need not necessarily report original research results; they may instead, for example, report practical experience that will be useful to others, reusable programming idioms, or elegant new ways of approaching a problem. (Links with more advice appear on the symposium web page.) 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 program! 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 (also for other languages where appropriate). In addition, we solicit proposals for system demonstrations, based on running (perhaps prototype) software rather than necessarily on novel research results. Such short demo proposals should explain why a demonstration would be of interest to the Haskell community. 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 programme, see its web page (http://www.sigplan.org/PAC.htm) Proceedings: ============ There will be formal proceedings published by ACM Press. In addition to printed proceedings, accepted papers will be included in the ACM Digital Library. Authors must transfer copyright to ACM upon acceptance (for government work, to the extent transferable), but retain various rights (http://www.acm.org/publications/policies/copyright_policy) Authors are encouraged to publish auxiliary material with their paper (source code, test data, etc.); they retain copyright of auxiliary material. Accepted demo proposals, assessed for relevance by the PC, will be published on the symposium web page, but not formally published in the proceedings. Submission Details: =================== * Abstract Submission: Thu 31st May 2012 * Submission Deadline: Sun 3rd June 2012, 11:00 am, UTC * Author Notification: Wed 27th June 2012 * Final Papers Due : Tue 10th July 2012 Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines (9pt format, more details appear on the symposium web page). The length is restricted to 12 pages, except for "Experience Report" papers, which are restricted to 6 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 format. "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. Submission is via EasyChair: https://www.easychair.org/conferences/?conf=haskell2012 Programme Committee: ==================== * Amal Ahmed, Northeastern University * Jost Berthold, University of Copenhagen * Nils Anders Danielsson, University of Gothenburg * Iavor Diatchki, Galois Inc. * Jeremy Gibbons, University of Oxford * Jurriaan Hage, Utrecht University * Zhenjiang Hu, National Institute of Informatics Tokyo * Daan Leijen, Microsoft Research * Ben Lippmeier, University of New South Wales * Simon Peyton Jones, Microsoft Research * Colin Runciman, University of York * Eijiro Sumii, Tohoku University * Janis Voigtländer (chair), University of Bonn * Brent Yorgey, University of Pennsylvania Links: ====== * http://www.haskell.org/haskell-symposium (the permanent web page of the Haskell Symposium) * http://www.haskell.org/haskell-symposium/2012 (the 2012 Haskell Symposium web page) * http://www.icfpconference.org/icfp2012 (the ICFP 2012 web page) -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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Problem Installing ad (Automatic Differentiation) Package
by Dominic Steinitz 21 Apr '12

21 Apr '12

Hi, I was trying to install ad (Automatic Differentiation) but got the following error. I guess the answer is to upgrade my ghc (perhaps someone could confirm) but I am using the Haskell Platform and would rather stick with that. I see from http://hackage.haskell.org/platform/ that the next release is due out in May. Will that be early May? in which case I may well wait a week or will it be late May? in which case I might bite the bullet and upgrade my ghc. Thanks, Dominic Building template-haskell-2.7.0.0... [1 of 7] Compiling Language.Haskell.TH.Syntax.Internals ( Language/Haskell/TH/Syntax/Internals.hs, dist/build/Language/Haskell/TH/Syntax/Internals.o ) [2 of 7] Compiling Language.Haskell.TH.Syntax ( Language/Haskell/TH/Syntax.hs, dist/build/Language/Haskell/TH/Syntax.o ) [3 of 7] Compiling Language.Haskell.TH.PprLib ( Language/Haskell/TH/PprLib.hs, dist/build/Language/Haskell/TH/PprLib.o ) Language/Haskell/TH/PprLib.hs:55:10: Illegal instance declaration for `Show Doc' (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.) In the instance declaration for `Show Doc' Updating documentation index /Users/Test/Library/Haskell/doc/index.html cabal: Error: some packages failed to install: ad-1.3.0.1 depends on template-haskell-2.7.0.0 which failed to install. template-haskell-2.7.0.0 failed during the building phase. The exception was: ExitFailure 1 The Glorious Glasgow Haskell Compilation System, version 7.0.3

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Parameterize constraints of existentially quantified types
by Bas van Dijk 21 Apr '12

21 Apr '12

Hi, I just found out that with the new ConstraintKinds extension we can parameterize the constraint of an existentially quantified type: {-# LANGUAGE KindSignatures, ConstraintKinds, ExistentialQuantification #-} import GHC.Exts data Some (c :: * -> Constraint) = forall a. c a => Some a This could be used to define SomeException for example: import Control.Exception (Exception) type SomeException = Some Exception Are there any other use cases? Bas

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[ANN] blaze-html-0.5
by Jasper Van der Jeugt 21 Apr '12

21 Apr '12

Hello all, I've just released a new version of blaze-html on Hackage. It has some backward-incompatible changes, so feel free to contact me if you run into any trouble. Summary of changes: - Split into blaze-markup and blaze-html - Easy creation of custom HTML elements - Very simple HTML tree manipulation For more details, see: http://jaspervdj.be/posts/2012-04-21-blaze-html-0.5.html Cheers, Jasper

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Google Summer of Code - Lock-free data structures
by Florian Hartwig 21 Apr '12

21 Apr '12

Hi everyone, I would like to do the GSoC project outlined in http://hackage.haskell.org/trac/summer-of-code/ticket/1608 One of Haskell's great strengths is its support for all kinds of concurrent and parallel programmming, so I think that the Haskell ecosystem would benefit from having a wider range of efficient concurrent data structures. Also, more selfishly, I remember reading the article in CACM last summer and thinking that the whole idea of updating data structures directly using atomic compare-and-swap was really cool, so I would love to have an excuse to play around with it. Some (initial) ideas for lock-free data structures worth implementing: 1. A lock-free priority queue, e.g. using the approach based on skip-lists explained in [2] 2. Stacks, see [1] and [3] 3. Hash tables [4] but if anyone has other suggestions, I would obviously happy to hear them. GSoC stretches over 13 weeks. I would estimate that implementing a data structure, writing tests, benchmarks, documentation etc. should not take more than 3 weeks (it is supposed to be full-time work, after all), which means that I could implement 4 of them in the time available and still have some slack. About me: My name is Florian Hartwig, I'm a fifth year (Master's) student in Computing Science at the University of Glasgow. I've been using Haskell for a bit more than two years now (both for university courses and my recreational programming) and I'm currently using it for my Master's project, so I won't have to spend any time learning the Haskell basics. I don't have any other plans for the summer that might conflict with the project. I'd be thankful for any thoughts, questions and suggestions. Cheers, Florian [1] http://cacm.acm.org/magazines/2011/3/105308-data-structures-in-the-multicor… [2] http://www.sciencedirect.com/science/article/pii/S0743731504002333 [3] http://dl.acm.org/citation.cfm?id=1007944 [4] http://dl.acm.org/citation.cfm?id=564881

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TLS support for haskell-xmpp
by John Goerzen 20 Apr '12

20 Apr '12

Dmitry & others, Attached is a diff implementing TLS support for haskell-xmpp, and correcting a build failure. The support is rough but it seems to work so far. -- John

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