- Haskell-Cafe - Haskell.org

Can you do everything without shared-memory concurrency?
by Bruce Eckel 12 Sep '08

12 Sep '08

As some of you on this list may know, I have struggled to understand concurrency, on and off for many years, but primarily in the C++ and Java domains. As time has passed and experience has stacked up, I have become more convinced that while the world runs in parallel, we think sequentially and so shared-memory concurrency is impossible for programmers to get right -- not only are we unable to think in such a way to solve the problem, the unnatural domain-cutting that happens in shared-memory concurrency always trips you up, especially when the scale increases. I think that the inclusion of threads and locks in Java was just a knee-jerk response to solving the concurrency problem. Indeed, there were subtle threading bugs in the system until Java 5. I personally find the Actor model to be most attractive when talking about threading and objects, but I don't yet know where the limitations of Actors are. However, I keep running across comments where people claim they "must" have shared memory concurrency. It's very hard for me to tell whether this is just because the person knows threads or if there is truth to it. The only semi-specific comment I've heard refers to data parallelism, which I assumed was something like matrix inversion, but when I checked this with an expert, he replied that matrix inversion decomposes very nicely to separate processes without shared memory, so now I'm not clear on what the "data parallelism requires threads" issue refers to. I know that both Haskell and Erlang only allow separated memory spaces with message passing between processes, and they seem to be able to solve a large range of problems -- but are there problems that they cannot solve? I recently listened to an interview with Simon Peyton-Jones where he seemed to suggest that this newsgroup might be a helpful place to answer such questions. Thanks for any insights -- it would be especially useful if I can point to some kind of proof one way or another. -- Bruce Eckel

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mailing list choices?
by Conal Elliott 12 Sep '08

12 Sep '08

I want to set up some kind of mailing list for reactive (which I plan to release soon). The most obvious thing is to set up a mailman-based list on haskell.org, but I wonder -- do people really want to keep using mailman technology? Or something more modern like Yahoo or Google groups. I use my email reader also with Yahoo & Google groups, so my day-to-day experience is the same as with mailman-based lists. For less regular operations like searching archives, I prefer the experience on Yahoo & Google group. And I like some of the other modern features, like personal profiles and polls. I've been wondering if haskellers really like mailman or are just used to it. Maybe people make new mailman lists simply because others did it before them. Any thoughts/preferences? Thanks, - Conal

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Workshop on Generic Programming: Call for Participation (co-located w/ ICFP08)
by Matthew Fluet (ICFP Publicity Chair) 12 Sep '08

12 Sep '08

Dear all, the Workshop on Generic Programming is only a few days away: 20th September 2008 (http://www.regmaster.com/conf/icfp2008.html) ==> Invited talk: The Generic Paradigm ==> Lambert Meertens (Utrecht University) ==> We have reserved 20 minutes for *lightning talks*. If you plan to ==> attend and if you would like to give a short talk (about half-baked, ==> exciting, new stuff) please drop me a short note. Slots will be ==> reserved on a first-come-first-serve basis. Looking forward to seeing you in Victoria, Ralf Hinze ============================================================================ CALL FOR PARTICIPATION Workshop on Generic Programming 2008 Victoria, Canada, 20th September 2008 http://www.comlab.ox.ac.uk/ralf.hinze/wgp2008/cfp.{html,pdf,ps,txt} The Workshop on Generic Programming is sponsored by ACM SIGPLAN and forms part of ICFP 2008. Previous Workshops on Generic Programming have been held in Marstrand (affiliated with MPC), Ponte de Lima (affiliated with MPC), Nottingham (informal workshop), Dagstuhl (IFIP WG2.1 Working Conference), Oxford (informal workshop), Utrecht (informal workshop), and Portland (affiliated with ICFP). ============================================================================ Preliminary program ------------------- 9:00 - 10:00, Session Chair: Ralf Hinze (University of Oxford) Welcome Invited talk: The Generic Paradigm Lambert Meertens (Utrecht University) 10:30 - 12:00, Session Chair: Jeremy Gibbons (University of Oxford) A Functional Model-View-Controller Software Architecture for Command-oriented Programs Alley Stoughton (Kansas State University) A Lightweight Approach to Datatype-Generic Rewriting Thomas van Noort (Radboud University Nijmegen), Alexey Rodriguez, Stefan Holdermans (Utrecht University), Johan Jeuring (Utrecht University and Open University of the Netherlands), Bastiaan Heeren (Open University of the Netherlands) Lightning talks 13:30 - 15:00, Session Chair: Ralf Hinze (University of Oxford) Report from the program chair Ralf Hinze (University of Oxford) Scala for Generic Programmers Bruno C. d. S. Oliveira, Jeremy Gibbons (University of Oxford) A Comparison of C++ Concepts and Haskell Type Classes Jean-Philippe Bernardy, Patrik Jansson, Marcin Zalewski, Sibylle Schupp, Andreas Priesnitz (Chalmers University of Technology and University of Gothenburg) Lightning talks 15:30 - 17:30, Session Chair: Patrik Jansson (Chalmers University of Technology and University of Gothenburg) Polytypic Programming in Coq Wendy Verbruggen, Edsko de Vries, Arthur Hughes (Trinity College Dublin) Bialgebra Views: A Way for Polytypic Programming to Cohabit with Data Abstraction Pablo Nogueira, Juan Jose Moreno-Navarro (Universidad Politecnica de Madrid) Discussion ============================================================================

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darcs hacking sprint, venues confirmed! (25-26 October)
by Eric Y. Kow 12 Sep '08

12 Sep '08

Dear darcs users and Haskell hackers, Earlier I wrote an announcement for the darcs hacking sprint on 25-26 October. Tonight, I am delighted to announce that we have two venues confirmed for the sprint and one serious offer. Venues ------ We plan to host the sprint across three sites: * CONFIRMED: Brighton, UK (University of Brighton) * CONFIRMED: Portland, USA (Galois Connections) * likely: Paris, France (Université Paris Diderot) So if you were waiting to book tickets, this is the time! For more details, please see http://wiki.darcs.net/index.html/Sprints Agenda ------ During this first sprint, we shall be focusing our attention on the day to day performance issues that darcs users commonly face. This is what we are reaching for: 1. Fast network operations. We want to make it very pleasant for users to darcs get a repository and pull some patches to it over http and ssh. Git does this very well, and we plan to learn from them. 2. Cutting memory consumption. We want to profile the heck out of operations like darcs record, darcs convert and darcs whatsnew. What's eating up all the memory? And how can we can cut it down to size? 3. Responsiveness. Sometimes basic darcs commands can take long enough for programmers to lose their train of thought. We want to track down these lost seconds and kill that dreaded context switch. Of course, if you are interested in other areas, then you can work on those instead. Note that if you are new to the darcs code or to Haskell, there will also be a lot interesting jobs for you to get started with. Everyone will have something to hack on, so come join us! Thanks very much to the University of Brighton, Galois connections and University of Paris VII for their generous offers. Hope to see you there, everyone! :-) -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Connecting to wireless network
by Marco Túlio Gontijo e Silva 11 Sep '08

11 Sep '08

Hello, I've made a small program[0] to connect to a wireless network. Comments are welcome. Greetings. 0: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/n-m -- Marco Túlio Gontijo e Silva Página: http://marcotmarcot.iaaeee.org/ Blog: http://marcotmarcot.blogspot.com/ Correio: marcot(a)riseup.net XMPP: marcot(a)jabber.org IRC: marcot(a)irc.freenode.net Telefone: 25151920 Celular: 98116720 Endereço: Rua Turfa, 639/701 Prado 30410-370 Belo Horizonte/MG Brasil

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typeclass question
by Tim Docker 11 Sep '08

11 Sep '08

I have a typeclass related question that I have been puzzling over. In a library I am working on, I have a series of functions for converting values to Renderables: | labelToRenderable :: Label -> Renderable | legendToRenderable :: Legend -> Renderable | axisToRenderable :: Axis v -> Renderable | layoutToRenderable :: Layout x y -> Renderable These names are overloaded for convenience via a typeclass: | class ToRenderable a where | toRenderable :: a -> Renderable | | instance ToRenderable Label where | toRenderable = labelToRenderable | ... But some recent changes mean that Renderable needs to become a type constructor, and the functions now product different types: | labelToRenderable :: Label -> Renderable () | legendToRenderable :: Legend -> Renderable String | axisToRenderable :: Axis v -> Renderable () | layoutToRenderable :: Layout x y a -> Renderable a Is there a nice way to overload a "toRenderable" function for these? Something like this is possible: | class ToRenderable a b where | toRenderable :: a -> Renderable b But the above is, I think, too general for my needs. I don't want to be able to generate Renderables of different type b for a single input type a. Also, MPTC take me out of the world of haskell 98, which I was trying to avoid. Am I missing something simple? Any pointers would be much appreciated. Tim

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Haskell as a scripting language.
by David F. Place 11 Sep '08

11 Sep '08

Hi, All. I needed to make a batch of edits to some input files after a big change in my program. Normally, one would choose some scripting language, but I can't bear to program in them. The nasty thing about using Haskell is that giving regexes as string constants sometime requires two levels of quoting. For instance. (mkRegex "\\\\\\\\") matches \\. To get around that, I put the regexes in the head of a literate program and let the program gobble itself up. Works great! I think I'll always turn to Haskell for my scripting needs now. I put the file in the Haskell Pastebin, if you would like to see it. http://hpaste.org/10249 Cheers, David

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Field names
by Daryoush Mehrtash 11 Sep '08

11 Sep '08

In type section of the Gentle Introduction to Haskell http://www.cs.sfu.ca/CC/SW/Haskell/hugs/tutorial-1.4-html/moretypes.htmlthe… is this example: > data Point = Pt {pointx, pointy :: Float} > abs (Pt {pointx = x, pointy = y}) = sqrt (x*x + y+y) Why is it pointx=x and not x=pointx? -- Daryoush Weblog: http://perlustration.blogspot.com/

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Re: [Haskell-cafe] Re: Field names
by Richard A. O'Keefe 11 Sep '08

11 Sep '08

On 11 Sep 2008, at 3:54 am, Brandon S. Allbery KF8NH wrote: > I think that only counts as the origin of the idea; isn't :-prefixed > infix constructors a ghc-ism? Haskell 98 report, page 10: "An operator symbol starting with a colon is a constructor". (I seem to have four copies of the report on my Mac...)

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STM and FFI
by Mauricio 10 Sep '08

10 Sep '08

Hi, Is it possible to use foreign function interface with STMs? If so, where can I find examples? Thanks, Maurício

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