April 2012 - Haskell-Cafe

No email verification token from trac.haskell.org
by Niklas Broberg 23 Apr '12

23 Apr '12

Hi all, I have problems with my trac for haskell-src-exts. When I log in, I am asked to verify my email address, but no verification token is ever sent to my email address. And before I verify my address, I'm not allowed to do anything on the trac. Help? :-) /Niklas

2 2

ANNOUNCE: hsx-0.10.1, hjscript-0.6.0, hsp-0.7.0
by Niklas Broberg 23 Apr '12

23 Apr '12

Hi all, I'm pleased to announce the release of the latest versions of the libraries in the Haskell Server Pages suite. Haskell Server Pages enables literal XML syntax to be used mixed with Haskell code (like e.g. PHP), in a sane way (unlike said PHP). In principle the functionality could be used together with any Haskell web framework, but is (currently) best integrated in and supported by Happstack. For this release I owe much to Jeremy Shaw, both for inspiration and for actual patches. There are some grand plans in the pipeline for some more extensive changes to the suite, to shape it up to be worthy of the current exciting state-of-the-art of Haskell web development. Stay tuned! Cheers, /Niklas Specific changes in this release: * In the XMLGen class, the following associated types have been renamed to avoid frequent name clashes, and to better match the intention of those types: XML => XMLType, Child => ChildType, Attribute => AttributeType. * Literal CDATA strings are now given an explicit type signature in the post-processed code. This to avoid ambiguities when OverloadedStrings is enabled. * class XMLGenerator m now has EmbedAsChild m () as another premise. * HSX.Transform now exports a method transformExp :: Exp -> Exp, intended to be used with quasi-quotation. * trhsx no longer performs fixity resolution before transforming, allowing the tool to be used on files using operators with (locally) unknown fixity. * hsp and hjscript updated to work with hsx-0.10.

1 0

Data-Type Serialization
by Lyndon Maydwell 23 Apr '12

23 Apr '12

Hi Café. I'm pondering over an old chestnut that I'd forgotten about until recently. I'd like to be able to express schemas for general recursive datatypes (excluding functions and possibly encoding bounds) and have an interactive constructor for that datatype generated automatically. Originally this idea came about as an XML/JS framework for creating advanced form inputs on the client-side, however, I'm interested in how this could be implemented idiomatically in Haskell. I'm guessing that the best way to go about this would be to capture the declaration in template Haskell, having a passthrough for the declaration itself, and also generating a representation that can be used for introspection and the creation of the interactive component. Ideally I'd like to integrate this concept into one of the web-frameworks such as Yesod so that complicated data-types can be constructed interactively on the client-side using (generated) javascript in order to get instantaneous feedback about validity of fields, etc, and avoid page reloads and even ajax communication. The details of the web-side of things would probably be quite messy, but if the Haskell side can be made general enough then this can be considered a separate problem :-) Has anyone had experience with trying to solve this, or a similar issue? What approach did you take? Did you use Template-Haskell, or something else? Thanks!

2 1

Munich Haskell Meeting
by Heinrich Hördegen 23 Apr '12

23 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 --

1 0

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

1 0

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

2 3

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

2 2

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

1 0

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

2 1

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

2 1