December 2006 - Haskell-Cafe

Re[2]: [Haskell-cafe] Dealing with incoherent instances
by Bulat Ziganshin 28 Dec '06

28 Dec '06

Hello Vyacheslav, (returning to cafe) Wednesday, December 27, 2006, 6:10:30 PM, you wrote: > Ah, so the moment something is passed through a polymorphic function > its type information is lost... This seems like a bug in the > specification/implementation, no? This is most certainly not the > desired behavior. It seems like the compiler has all the information > it needs but still can't select the right instance. sorry, but i think that it is how type class system should work in this case. look at the page for details > Are there ways to get around this problem? existential types, for example. we need exact specification of your problem. "show me the code" >> are you seen http://haskell.org/haskellwiki/OOP_vs_type_classes ? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: [Haskell] state of HaXml?
by dons＠cse.unsw.edu.au 27 Dec '06

27 Dec '06

nr: > I have some XML things to take care of, and I had hoped to use > a nice functional language with combinators. But I'm having > trouble getting HaXml to do anything useful. Here is a program > that I wrote just to read in the XML and prettyprint it. > But it fails with an error message. The 'expat' tool 'xmlwf' > claims the XML is well formed, and the XML was generated from > gpsbabel, a tool I trust to behave properly. And yet: > > : nr@curlycoat 10429 ; xmlwf 2006-12-27-backup.gpx > : nr@curlycoat 10430 ; $AWD/Main 2006-12-27-backup.gpx > Main: Parse error: unexpected EOF > > Here is Main.hs: > > module Main where > import qualified Text.XML.HaXml as X > import qualified Text.XML.HaXml.Parse as XP > import qualified Text.XML.HaXml.Pretty as XPP > import qualified IO > import qualified System > > load :: String -> IO X.Document > load fn = do handle <- IO.openFile fn IO.ReadMode > contents <- IO.hGetContents handle > IO.hClose handle > return $ XP.xmlParse fn contents > > main = do [xml] <- System.getArgs > d <- load xml > IO.putStrLn $ show $ XPP.document $ d > > > I'm using HaXml version 1.13.2-5 as distributed by Debian, > with GHC 6.6. > > Does anyone have thoughts or suggestions? Is there software I should > prefer to HaXml? (I notice that HaXml does not understand XML Schema, > which I seem to be stuck with...) You might also want to have a peek at HXT, the XML toolbox. That's been used to good effect recently, for a little RSS aggregator: http://cale.yi.org/index.php/HRSS -- Don P.S. Redirected to the -cafe

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Dealing with incoherent instances
by Vyacheslav Akhmechet 27 Dec '06

27 Dec '06

I'm looking at GHC's overlapping instances docs here: http://web.mit.edu/ghc/www/users_guide/type-extensions.html#instance-decls and I've ran into the incoherent instances problem. Basically, I have a "catch all" instance that handles all types in a generic manner using SYB introspection, and then I have type specific instances that specialize behavior for certain types. This works with overlapping instances extension but whenever I take advantage of this functionality from polymorphic functions I run into the incoherent instances. If I enable incoherent instances GHC always picks the general case which seems like the wrong thing to do. What I want it to do is delay comitting to an instance until it's processing a specific invocation of a polymorphic function. All the information is available at compile time but I found no way to do this. Is there a way to get around this problem? Thanks, - Slava.

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by omit singh rathore 27 Dec '06

27 Dec '06

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Lambda Revolution tee shirts
by Paul Johnson 25 Dec '06

25 Dec '06

I was inspired by Don Stewart's Blog posting "The Lambda Revolution" to create a tee shirt design on this theme. Samples are available from CafePress at http://www.cafepress.com/l_revolution , but the designs are freely available. If anyone wants to send them to a more local tee-shirt printer than CafePress then let me know and I will email the SVG files. Paul.

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Getting my feet wet - small browser game
by Joachim Durchholz 24 Dec '06

24 Dec '06

OK, after years of looking and discussing and doing HSoE exercises, I have finally decided that Haskell is far enough into practical usefulness that I should give it a try in a "real" project. The basic idea is a browser game - this touches all the potentially hard issues without miring me too deeply in target platform details. I'd like to lay out my plans here and ask how they are going to work out, and take advice. THE PLAN I'll start with http://haskell.org/haskellwiki/How_to_write_a_Haskell_program and get a toolchain together. I haven't decides which compiler (interpreter?) to choose; I'll probably go for the one that give the least amount of trouble. Next would be library selection. I'm willing to undergo some modest amount of hassle here, since I don't expect all libraries that I need to be mature yet. My preliminary plan is to split the application into a central world simulation process, and satellite processes that accept HTTP requests, feed them into the simulation, read back the results, and generate the response HTML. The interface between simulation and satellite is: * Satellites can read a snapshot of the game data. * Satellites cannot directly write game data. What they can do is to post commands to a blackboard, which are marked as "no more updatable" as soon as the simulation starts executing them. I expect the simulation and the satellites to be separate OS processes, so I'll need a way to marshall command and game data between processes. The simulation will have to store its entire state to disk, too - in theory, it could run forever and never write to disk (and I don't need a database, too), but in practice, I have to plan for the occasional reboot. Since the server will be running Apache for other purposes anyway, and I don't want to force the players to use a URL with a port number, I think I'll set up Apache so that it proxies game-related URLs to the Haskell software. I just hope that Apache doesn't incur much overhead in that mode. I have no idea how to organize software upgrades. The satellites are easy, but how do I make sure that revision N+1 of the simulation can read the marshalled data from revision N? The final software should be efficient. Ideally, the satellites are able to saturate the network card of today's typical cheap rootserver if the simulation is a no-op. I have two data points for a "typical cheap rootserver": * 100 MBit/s Ethernet, 256 MB RAM, 1.2 GHz Celeron (~3 years old) * 1 GBit/s Ethernet, 1 GB RAM, 2.2 GHz Athlon (current) Of course, not needing an RDBMS will give the system a head start efficiency-wise. Comments and suggestions welcome :-) Regards, Jo

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Writing "Haskell For Dummies Or At Least For People Who Feel Like Dummies When They See The Word 'Monad'"
by Kirsten Chevalier 23 Dec '06

23 Dec '06

It's not as if this is the first time that this has been suggested, but some people have suggested that a practical book about Haskell would be a good idea. I agree. Some people have also suggested that the right moment for this hasn't arrived yet, and I see that as a challenge. I'm willing to take the lead in at least thinking about what such a book might look like. I'm potentially about to have some free time for such things, and am still young and foolish enough to think that writing a book would be a good idea. Of course, there are many good Haskell books out there already, but many of them are intended as class textbooks or are aimed at more theoretical-minded people. There's nothing wrong with that, but I think that it would be nice if a friendly, conversational, informal book about Haskell existed, since after all this is such a friendly and informal community. (If there already is a book like this, point it out, but I get the impression there's not.) There's also excellent Haskell documentation available on the web already, but people like to buy books and they like to have an artifact that they can hold in their hands without getting laser printer toner all over themselves. But if I were going to do this, I'd need all the help I could get, so if you're interested in working with me on this, email me off-list and we'll talk. Don't feel like you need to be named "Simon" for this; I don't think you need to be a Haskell guru to contribute to a book like this (I know I'm not one), though it wouldn't hurt. Being interested in good writing and explaining things to a wider audience is more important. And, the more people who are interested in working on this, the more we can all pool our various talents to create something awesome. Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "Everyone's too stupid." -- _Ghost World_

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Followup on writing "Haskell for Dummies..."
by Kirsten Chevalier 23 Dec '06

23 Dec '06

I think I've decided that this isn't the right time for me to take the lead on this book-writing project, and in addition, I've come to think that maybe the things *I'd* like to say about Haskell hacking can't be transmitted so easily in book form. However, if any others would like to organize the writing of one (or several) books based on any of the ideas brought up in the mailing list discussion, they should feel free to step forward and do so, and know that they won't be duplicating work (or at least not my work). Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "You can't learn everything you need to know legally."--John Irving

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"Secret Santas" in Haskell / better data structure?
by Paul Brown 23 Dec '06

23 Dec '06

Cafe Crew -- I posted my simple-but-slow solution to Ruby Quiz problem #2: http://mult.ifario.us/articles/2006/12/22/secret-santas-in-haskell-iii-coll… (Or http://tinyurl.com/y3l2re if your mail client messes up the URL...) It's more than practically fast enough unless you have, say 10^8 friends to do secret Santa with, but the problem just cries out for a well-chosen data structure. (One obvious improvement over the approach I posted is to sort the list first, but maybe there's something even better...) As I'm learning, critiques of syntax and usage welcome. I'll collect the source and post to the Haskell Quiz wiki as well. -- Paul

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partial functions / failure, Maybe and MonadError and good style
by Steve Downey 23 Dec '06

23 Dec '06

Although terse, the subject really says it all. If i've a partial function, like a parser, what is considered good style for a library. The tradeoffs that I can see are that Maybe is a binary operation, while Error can communicate more information in the type of the error case. Is there some way to defer the error handling Monad to the calling context?

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