February 2011 - Haskell-Cafe

Re: [Haskell-cafe] Haskell for children? Any experience?
by Chris Smith 19 Feb '11

19 Feb '11

Manuel, Wow, that gloss package is really cool, and exactly the sort of thing I was looking for. As I've said before, I don't think I can prevent this from becoming about how to write games eventually. Gloss looks like provides a nice way to approach graphics programming in a simple functional style, with a clean interface consisting entirely of high-level ideas, and which easily switches over to the game interface later. Awesome! Thank you so much for pointing it out. -- Chris Smith

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Haskell for children? Any experience?
by Chris Smith 19 Feb '11

19 Feb '11

So I find myself being asked to plan Haskell programming classes for one hour, once a week, from September through May this coming school year. The students will be ages 11 to 13. I'm wondering if anyone has experience in anything similar that they might share with me. I'm trying to decide if this is feasible, or it I should try to do something different. To be honest, as much as I love Haskell, I tried to push the idea of learning a different language; perhaps Python. So far, the kids will have none of it! This year, I've been teaching a once-a-week exploratory mathematics sort of thing, and we've made heavy use of GHCi... and they now insist on learning Haskell. (By the way, GHCi is truly amazing for exploratory mathematics. We really ought to promote the idea of Haskell for elementary / junior-high level math teachers! It's so easy to just try stuff; and there are so many patterns you can just discover and then say "Huh, why do you think that happens? Can you write it down precisely? ...") -- Chris Smith

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skin info of haskellwiki
by hanjoosten 19 Feb '11

19 Feb '11

Hi, I would like to use a copy of the hawiki skin for http://ampersand.sf.org my projects mediawiki . I like the way the front page of www.haskell.org works. It uses a mediawiki skin, that is customized for the haskellwiki. However, it used to be http://www.haskell.org/haskellwiki/HaskellWiki:Site_software published , but I find broken links. Is there anyone that could help me out? thanks! -- View this message in context: http://haskell.1045720.n5.nabble.com/skin-info-of-haskellwiki-tp3392503p339… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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linear and dependent types
by Vasili I. Galchin 19 Feb '11

19 Feb '11

Hello, Does Haskell currently have support for linear types and dependent types? If so, is it necessary to specify a "pragma" to use and if so, what is the pragma(s)? Kind regards, Vasili

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Re: [Haskell-cafe] linear and dependent types
by oleg＠okmij.org 19 Feb '11

19 Feb '11

There has been extensive discussion of whether Haskell has dependent types, made larger by the fact that the definition of dependently-typed languages is a bit fuzzy. On some accounts, type functions or GADTs are sufficient for calling a language dependently-typed, and Haskell got both features. The situation is clearer with linear types. First of all, Haskell has all facilities for statically ensuring the proper use of resources. The simplest example is enforcing the locking protocol: a lock can be acquired no more than once; only acquired locks may be released, all acquired locks must be released at the end. This simple example is described in Sec 5.2 of http://research.microsoft.com/en-us/um/people/simonpj/papers/assoc-types/fu… That example can be made more complex: a program with two locks choosing the lock to acquire based on the data it reads from a file. We can still statically guarantee that all acquired locks are released and no lock is acquired or released twice -- even if we cannot statically tell which particular lock will be acquired during a program run. Other examples of resource control include session types and regions. There are several Hackage packages with these keywords. Generally, Haskell can easily embed languages with dependent or linear types -- just as easily as Haskell embeds languages with IO, global mutable state, non-determinism. The latter languages go by the name of monads. One can easily embed typed linear lambda-calculus, with Haskell type-checker ensuring that each bound variable is referenced exactly once. http://okmij.org/ftp/tagless-final/course/index.html#linear The linear lambda-calculus turns out not very useful; no wonder Girard had to add exponentials.

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WGP 2011: Call for Papers
by Shin-Cheng Mu 18 Feb '11

18 Feb '11

====================================================================== CALL FOR PAPERS WGP 2011 7th ACM SIGPLAN Workshop on Generic Programming Tokyo, Japan Sunday, September 18th, 2011 http://flolac.iis.sinica.edu.tw/wgp11/ Collocated with the International Conference on Functional Programming (ICFP 2011) ====================================================================== Goals of the workshop --------------------- Generic programming is about making programs more adaptable by making them more general. Generic programs often embody non-traditional kinds of polymorphism; ordinary programs are obtained from them by suitably instantiating their parameters. In contrast with normal programs, the parameters of a generic program are often quite rich in structure; for example they may be other programs, types or type constructors, class hierarchies, or even programming paradigms. Generic programming techniques have always been of interest, both to practitioners and to theoreticians, and, for at least 20 years, generic programming techniques have been a specific focus of research in the functional and object-oriented programming communities. Generic programming has gradually spread to more and more mainstream languages, and today is widely used in industry. This workshop brings together leading researchers and practitioners in generic programming from around the world, and features papers capturing the state of the art in this important area. We welcome contributions on all aspects, theoretical as well as practical, of * generic programming, * programming with (C++) concepts, * meta-programming, * programming with type classes, * programming with modules, * programming with dependent types, * polytypic programming, * adaptive object-oriented programming, * component-based programming, * strategic programming, * aspect-oriented programming, * family polymorphism, * object-oriented generic programming, * and so on. Organizers ---------- Co-Chair Jaakko Järvi, Texas A&M University, USA Co-Chair Shin-Cheng Mu, Academia Sinica, Taiwan Programme Committee ------------------- Dave Abrahams, BoostPro Computing, USA Magne Haveraaen, Universitetet i Bergen, Norway Akimasa Morihata, Tohoku University, Japan Pablo Nogueira, Universidad Politécnica de Madrid, Spain Ulf Norell, Chalmers University of Technology and University of Gothenberg, Sweden Ross Paterson, City University London, UK Rinus Plasmeijer, Radboud University Nijmegen, The Netherlands Sibylle Schupp, Technische Universität Hamburg-Harburg, Germany Andrew Sutton, Kent State University, USA Tarmo Uustalu, Institute of Cybernetics, Estonia Important Information --------------------- We plan to have formal proceedings, published by the ACM. Submission details Deadline for submission: Monday 2011-06-06 Notification of acceptance: Tuesday 2011-07-01 Final submission due: Monday 2011-07-25 Workshop: Sunday 2011-09-18 Authors should submit papers, in postscript or PDF format, formatted for A4 paper, to the WGP11 EasyChair instance by the above deadline. The length should be restricted to 12 pages in standard (two-column, 9pt) ACM format. Accepted papers are published by the ACM and will additionally appear in the ACM digital library. History of the Workshop on Generic Programming ---------------------------------------------- This year: * Tokyo, Japan 2011 (affiliated with ICFP11) Earlier Workshops on Generic Programming have been held in * Baltimore, Maryland, US 2010 (affiliated with ICFP10) * Edinburgh, UK 2009 (affiliated with ICFP09) * Victoria, BC, Canada 2008 (affiliated with ICFP), * Portland 2006 (affiliated with ICFP), * Ponte de Lima 2000 (affiliated with MPC), * Marstrand 1998 (affiliated with MPC). Furthermore, there were a few informal workshops * Utrecht 2005 (informal workshop), * Dagstuhl 2002 (IFIP WG2.1 Working Conference), * Nottingham 2001 (informal workshop), There were also (closely related) DGP workshops in Oxford (June 3-4 2004), and a Spring School on DGP in Nottingham (April 24-27 2006, which had a half-day workshop attached). Additional information: The WGP steering committee consists of J. Gibbons, R. Hinze, P. Jansson, J. Järvi, J. Jeuring, B. Oliveira, S. Schupp, and M. Zalewski

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Status update on {code, trac, projects, planet, community}.haskell.org
by Duncan Coutts 18 Feb '11

18 Feb '11

All, As you will be aware, some of the *.haskell.org websites have been down recently, specifically: code.haskell.org trac.haskell.org projects.haskell.org planet.haskell.org community.haskell.org These are all hosted on the community server. The community server was hacked on the 26th January and we took it offline. The server was running an old version of debian that was no longer supported with security updates. (Ironically, two days previously the infrastructure team had been discussing the fact that nobody seemed to have any time available to do the planned migration to a new host). The hacker replaced sshd which we noticed because the ssh host signature changed and it started prompting for passwords (we use key-based rather than password based logins). MSR kindly allowed Ian to take time off from the GHC 7.0.2 release to work on migrating the services to the new host (that had previously been partially prepared). Thanks to Ian's efforts and some help from other members of the infrastructure team, the migration is now nearly complete. planet.haskell.org should now be working, along with email, project mailing lists and trac instances. We have not yet re-enabled user login accounts, nor re-enabled access to code repositories. The support ticket system is not yet enabled. We will send a further update when these are re-enabled, or procedures for people to re-enable them are finalised. On a more positive note, the new VM that we are using is a lot more powerful than the old one, in particular about 5x more memory. As many of you will have experienced, services on the old server tended to go AWOL which was primarily due to running up against memory limits, causing services to get killed off. In the case of the web server it was mainly due to having to use a very small number of concurrent connections, again to minimise memory use. So all in all we expect the new server to be a good deal more reliable. Duncan (On behalf of the Haskell infrastructure team)

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`cabal install darcs` fail !
by z_axis 18 Feb '11

18 Feb '11

>cabal update >cabal install darcs ...... Installing library in /home/sw2wolf/.cabal/lib/hashed-storage-0.5.5/ghc-6.10.4 Registering hashed-storage-0.5.5... Reading package info from "dist/installed-pkg-config" ... done. Writing new package config file... done. cabal: Error: some packages failed to install: darcs-2.5.1 depends on haskeline-0.6.3.2 which failed to install. haskeline-0.6.3.2 failed during the configure step. The exception was: ExitFailure 1 >uname -a FreeBSD mybsd.zsoft.com 8.1-RELEASE FreeBSD 8.1-RELEASE #1: Wed Sep 8 09:07:54 CST 2010 root@mybsd.zsoft.com:/media/G/usr/obj/media/G/usr/src/sys/MYKERNEL i386 It seems that the cabal is NOT very strong, isnot it ? ----- e^(π.i) + 1 = 0 -- View this message in context: http://haskell.1045720.n5.nabble.com/cabal-install-darcs-fail-tp3390542p339… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell Web Framework Happstack 6 Released
by Jeremy Shaw 17 Feb '11

17 Feb '11

Hello, I am pleased to announce the release of Happstack 6. We fully recommend that all Happstack users migrate to the new release. Happstack is a high-performance Haskell web framework. It has high- level functionality for routing requests, extracting parameters, manipulating responses, serving static file contents, compression responses, and more. It includes an integrated HTTP server and a native RAM-cloud database system known as MACID. This allows you to deploy your web application with out having to configure apache, php, mysql, memcached, etc! Install instructions can be found here: http://happstack.com/download If you are porting existing applications you should check out the Happstack 6 migration guide: http://code.google.com/p/happstack/wiki/Happstack6Migration Porting should be pretty straight-forward. Despite the massive reorganization, the exports from Happstack.Server are largely the same. What's Exciting! ---------------- The focus of this release has been happstack-server. The primary goal was to make happstack-server much easier to learn and to use. This includes: 1. improving the overall organization of the library 2. greatly improving the documentation 3. making it easier to install 4. ensuring that the server provides fast, reliable performance 5. making it easier for new (and old) developers to understand and get started If you have looked at Happstack before and found it too difficult to understand, I highly recommend you take another look. The new organization and documentation make it far easier to understand. More Logical Organiation ------------------------ In prior versions, the Happstack.Server.SimpleHTTP module contained just about everything, with little organization, and no real differentiation between the external API and the library internals. In Happstack 6, SimpleHTTP has been refactored into numerous, smaller modules. The internals have been moved into Happstack.Server.Internal.* so that they are not leaking into the external API. This should make it much easier to find what you are looking for, and prevent you from accidently running across internal things like the WebT monad, which you do not really need to know anything about. Better Documentation -------------------- The API documentation for happstack-server is also much better. All external API functions should now have good haddock documentation with many inline examples. However, learning a new library by studying the API documentation can still be pretty frustrating. So we also have the brand new Happstack Crash Course. http://happstack.com/docs/crashcourse/index.html. The Happstack Crash Course covers a vast majority of the happstack- server API. It covers the API in a logical manner and includes many downloadable, runnable examples. It is intended to be read start to finish, and to also be usable as a reference guide for specific "How do I do XXX" questions. Additionally, the guestbook example has been updated to a cleaner, more modern style of Happstack coding. It also has more comments/ haddock documentation. Better API ---------- During the process of documenting Happstack, many small improvements where made to the API. Once you try to document something stupid you realize how stupid it is and decided it would be good to fix it first ;) For example, the Cookie API now makes it more obvious how to create session cookies vs persistent cookies. And makes it obvious how to expire a cookie. The functions for looking up values in the query string and require body no longer require the use of the RqData monad. You can use the look* functions directly in the ServerPart monad. If you do choose to use RqData, it now has an Applicative instance which can accummulate and report lookup errors. (http://happstack.blogspot.com/2010/10/is-rqdata-monad-still-needed.html ) There are a bunch of other improvements to request data handling documented here, http://happstack.com/docs/crashcourse/RqData.html. Easier Install -------------- In order to make Happstack easier to install we have removed as many dependencies as possible. happstack-data no longer depends on happstack-util, which makes it easier to install happstack-data or happstack-state with out the rest of the framework. HSP and HStringTemplate support has been moved into *optional* happstack-hsp and happstack-hstringtemplate packages. New Features ------------ Happstack also has a number of new features including: - support for Heist templates (http://happstack.com/docs/crashcourse/Templates.html#helloheist ) - support for Hamlet templates - improved environment for extracting query string and form values (http://happstack.blogspot.com/2010/10/is-rqdata-monad-still-needed.html ) - support for recompilation and reloading of templates in a running server (http://happstack.blogspot.com/2010/10/recompile-your-haskell-based-template… ) - directory browsing when using serveDirectory And numerous other improvements including enhancements to IxSet, file locking, and much more. Bigger, Better Version Number ----------------------------- We have also upgraded our version numbers! We have decided to call this release Happstack 6.0 instead of Happstack 0.6. There are two reasons for this: 1. the amount of changes we are making really do represent a new major release 2. it will make it easier to comply with the Haskell package version policy. Now that Happstack 6 is out, we plan to make more frequent releases. The new version scheme will allow us to talk about the next major release (Happstack 7), but still put out changes to Happstack 6 which change the API (aka, Happstack 6.2, etc). GHC 7 - mostly -------------- Happstack mostly works with GHC 7. Due to some compiler bugs, HSP does not work with GHC 7.0.1. These bugs were reported and are fixed in the upcoming GHC 7.0.2 release. If you are not using the optional happstack-hsp package, then GHC 7 is fine. If you are using happstack- hsp, then you will need to use GHC 6.12, a recent version of GHC from darcs, or wait until GHC 7.0.2 comes out. The Future ========== The current future plan looks like this: Happstack 7 ----------- The next major release of Happstack will be Happstack 7. The primary focus of Happstack 7 will be improving happstack-state (also known as MACID). Big features include: - sharding support - a rewrite of IxSet based on kdtrees with much better RAM and CPU performance - performance testing to show how awesome MACID is - much better documentation and examples - better tools for examining and modifying state in running applications There are also a bunch of lower-level API improvements planned which should make it nicer to use happstack-state. Happstack 8 ----------- Happstack 8 will finally feature an enumerator-based HTTP backend. This is a feature we have been hoping to add since January of 2009. http://code.google.com/p/happstack/issues/detail?id=29 Rather than develop a competing HTTP backend, Happstack will put its efforts into improving an existing HTTP backend. We currently have our eye on WAI/warp. However, we will also consider snap and hyena when the time comes. It should be noted that the current lazy I/O based backend is not the relentless nightmare of space leaks that some people might lead you to believe it is. The current happstack-server can easily handle uploads and downloads of large or streaming files with out leaks. But, enumerators *are* nice. How You can Help ================ You are encouraged to get involved in Happstack. The easiest way to get involved is to complain about how horrible Happstack is. If you don't voice your complaints, then we might not know something things to be fixed. If you are looking to make code contributions, please feel free to ask on #happs or the mailing list. There are tasks that are easy for novices, tasks that are worthy of a Ph.D, and everything in between. If you want to work, we can certainly find a way to use your talents! There are also plenty of non-coding related tasks including documentation, graphic design, user surveys, etc. You are also encouraged to blog about your Happstack projects! Thanks! - jeremy shaw

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upgrading mtl1 to mtl2
by Evan Laforge 17 Feb '11

17 Feb '11

I just got started on this because packages are starting to use mtl2. I had hoped it would be simple, and hopefully it is, but... Do I really have to add (Functor m) to the 300 or so functions with (Monad m) on them? Or just not use fmap or applicative? Ow. Not what I would call "small incompatibilities." Is there a more graceful way to get around this? So I thought if it's going to be this much of a hassle I might as well just port to transformers, which I gather is supposed to be the future anyway. But transformers is lacking the classes, and I gather they're in monads-tf and monads-fd. But monads-fd says it's now deprecated because of the existence of mtl-2. So what's the story? Has transformers now turned around and been deprecated in favor of mtl-2? Should I just keep using mtl but port to mtl-2? Or is there some other non-obsolete package for transformers? This page: http://haskell.org/haskellwiki/Monad_Transformers Says that the only benefit of 'transformers' is that it's "haskell 98 and thus more portable," but doesn't that come with the caveat that "only if you don't use classes and do all the lifting manually"?

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