March 2010 - Haskell-Cafe

new Quake 3 BSP loader and viewer
by Csaba Hruska 16 Mar '10

16 Mar '10

Hi! I did a little experiment in last week with haskell. According this ( http://graphics.cs.brown.edu/games/quake/quake3.html) tutorial, I wrote a quake 3 bsp map loader and viewer from scratch. (It is independent from frag) The code is quiet small (~900 lines with comments) and it is based on binary and vector libraries. You can find the cabalized code here: http://code.google.com/p/lambdacube/source/browse/#svn/trunk/bsploader This was my first meet with vector library, so the code is probably not the most efficient one. Any improvement suggestions are welcome! :) Cheers Csaba PS: read the README file. navigation: arrows + mouse (holding right shift increase cam speed)

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HGL concurrency problems
by Carsten Schultz 16 Mar '10

16 Mar '10

[Cc to the HGL maintainer, I hope that is ok.] Hallo, I am trying to use HGL. My configuration is Mac OS X 10.6.2 (using X11) ghc 6.12.1 HGL 3.2.0.2 (via cabal-install) My program opens a window, draws points and lines, with a considerable amount of calculation between the lines, and then waits for a key stroke before exiting. What happens depends on whether I compile with -threaded or without. With -threaded: The programs runs ok, but after a short time stops updating the window, so that I never see when it has finished. Moving the mouse over the window can trigger updates. Without -threaded: Similar as in the other, but the program nor only stops updating the window, it also stops the calculation and sits idle. Playing with the mouse can again make it run for a short time. I attach the program in case that this might help. I did not try to extract a minimal example, but the program is short. Thanks for any help. I would also like to know if it works on other platforms/versions. Carsten module Main where {- Calculates the convex hull of a set of points in an inefficient way. O(n^3) For each `o' on stdout a red line segment should be drawn -} import qualified Graphics.HGL as HGL import System.Random import System.IO nrOfPoints = 10000 -- CONFIGURE HERE data Point = Point {xc, yc :: Float} randomPoints :: Int -> IO [Point] randomPoints n = sequence $ replicate n randomPoint where randomPoint = do phi <- randomRIO (0, 2*pi) r0 <- randomRIO (0,0.48 ** 2) let r = sqrt r0 return $ Point (r * cos phi + 0.5) (r * sin phi + 0.5) boundarySegments :: [Point] -> [(Point, Point)] boundarySegments ps = filter isBoundary (pairs' ps) where pairs' l = concat [[(a,b), (b,a)] | (a,b) <- pairs l] isBoundary s = all (`leftOf` s) ps leftOf :: Point -> (Point, Point) -> Bool q `leftOf` (p1,p2) = (xc p2-xc p1)*(yc q-yc p1) - (yc p2-yc p1)*(xc q-xc p1) >= 0 pairs :: [a] -> [(a,a)] pairs [] = [] pairs [x] = [] pairs (x:xs) = [(x,x') | x' <- xs] ++ pairs xs main :: IO () main = do HGL.runGraphics $ do points <- randomPoints nrOfPoints redPen <- HGL.createPen HGL.Solid 1 (HGL.RGB 255 0 0) w <- HGL.openWindowEx "hull" Nothing (ww,wh) HGL.Unbuffered Nothing drawInWindow w $ sequence_ $ map pt points drawInWindow w $ (HGL.selectPen redPen >> return ()) sequence_ $ map ((drawInWindow w) . seg) $ boundarySegments points drawInWindow w $ HGL.text (0,0) "done" putStr "\ndone\n" HGL.getKey w HGL.closeWindow w where drawInWindow w a = do putStr "o" >> hFlush stdout HGL.drawInWindow w a ww, wh :: Int ww = 400 * 2 wh = 300 * 2 wwf = fromIntegral ww whf = fromIntegral wh xf, yf :: Point -> Float xf p = xc p * wwf yf p = yc p * whf x, y :: Point -> Int x = round . xf y = round . yf pt :: Point -> HGL.Graphic pt (Point x y) = HGL.ellipse (round (x*wwf-2),(round(y*whf-2))) (round (x*wwf+2),(round(y*whf+2))) seg :: (Point, Point) -> HGL.Graphic seg (u,v) = HGL.line (x u, y u) (x v, y v)

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Second Call for Copy: Monad.Reader Issue 16
by Brent Yorgey 16 Mar '10

16 Mar '10

I already have one or two submissions, but a few more would be great. If you've been thinking about submitting something but weren't sure or haven't gotten around to it yet, now's the time! -------------- Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 16 will be: **Friday, April 16, 2010** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki-page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

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More strange errors when compiling code
by Giuseppe Maggiore 16 Mar '10

16 Mar '10

Hi! I am getting more problems with compilation. The two files where the problem occurs (a bit more of a general case of the sample of a few hours ago) are: ------------------------------------------------- --------------------Methods.hs------------------- ------------------------------------------------- {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} module Methods where import Records import References class Converter a rec_a where convert :: a -> rec_a data Method s a b = Method (s -> a -> (b,s)) (<<-) :: forall rec_a s a b c n . (CNum n, HasField n (b->(c,a)) (rec_a), Converter a rec_a) => (Reference s (rec_a)) -> n -> Method s b c (<<-) r n = Method(\s -> \(x :: b) -> let (v,s') = getter r s :: (rec_a,s) m = v .! n :: (b -> (c,a)) (y,v') = m x :: (c, a) v'' = convert v' :: rec_a ((),s'') = setter r s' v'' in (y,s'')) (.!!) :: Method s a b -> a -> Reference s b (Method m) .!! x = from_constant (Constant (\s -> m s x)) ------------------------------------------------- ---------------------Main.hs--------------------- ------------------------------------------------- {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} --module Main where import Records import References import ReferenceMonad import Methods val = firstLabel incr = nextLabel val instance Converter RecCounter (Counter RecCounter) where convert (RecCounter r) = r type Counter k = (Integer :* (() -> ((),k)) :* EmptyRecord) data RecCounter = RecCounter (Counter RecCounter) mk_test :: Integer -> RecCounter mk_test i = RecCounter( val .= i .* incr .= (\() -> ((), mk_test (i+1))) .* EmptyRecord) RecCounter test' = mk_test 0 res2 :: Reference (Counter RecCounter) Integer res2 = do (this <<- incr :: Method (Counter RecCounter) () ()) .!! () v <- (this <-- val) return v count = fst (getter res2 test') Whenever I try to compile I get the following error: *Methods> :load Main.hs [1 of 5] Compiling Records ( Records.hs, interpreted ) [2 of 5] Compiling References ( References.hs, interpreted ) [3 of 5] Compiling ReferenceMonad ( ReferenceMonad.hs, interpreted ) [4 of 5] Compiling Methods ( Methods.hs, interpreted ) [5 of 5] Compiling Main ( Main.hs, interpreted ) Main.hs:55:11: No instances for (HasField Z (() -> ((), a)) (AddField (() -> ((), RecCounter)) Emp tyRecord), Converter a (Counter RecCounter)) arising from a use of `<<-' at Main.hs:55:11-23 Possible fix: add an instance declaration for (HasField Z (() -> ((), a)) (AddField (() -> ((), RecCounter)) EmptyRecord), Converter a (Counter RecCounter)) In the first argument of `(.!!)', namely `(this <<- incr :: Method (Counter RecCounter) () ())' In a stmt of a 'do' expression: (this <<- incr :: Method (Counter RecCounter) () ()) .!! () In the expression: do (this <<- incr :: Method (Counter RecCounter) () ()) .!! () v <- (this <-- val) return v Failed, modules loaded: Methods, References, Records, ReferenceMonad. *Methods> Any hints? PS: just to give some perspective on the code: I am putting together a system for expressing mutable objects through monads. I could provide the rest of my code in case of need :) -- Giuseppe Maggiore Ph.D. Student (Languages and Games) Microsoft Student Partner Mobile: +393319040031 Office: +390412348444

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Re: [Haskell-cafe] ANNOUNCE: tupleinstances-0.0.1
by Diego Echeverri 16 Mar '10

16 Mar '10

On Tue, Mar 16, 2010 at 11:15 AM, Henning Thielemann <thunderbird(a)henning-thielemann.de> wrote: > > newtype T3 a = T3 (a,a,a) > > which could well be > > data T3 a = T3 a a a > I chose that specific representation just because at the end I want to use normal tuples. I thought this way was easier for "boxing" an "unboxing" > Are these types used for vector computations or what application do you have > in mind? I didn't have any specific application for the library. One day I just wanted to do an fmap with a tuple and somebody in #haskell suggested join (***) Of course this only works for binary tuples. I thought this would be a good excuse to play with template-haskell and give instances to some of the type classes I learnt about in the Typeclassopedia. I found it useful for vector computations. But aggregating with the Foldable instance is also handy. > Then again, is tuple-instance still an appropriate name? "There are only two hard things in Computer Science: cache invalidation and naming things" - Phil Karlton I'm open to suggestions :-) Diego Echeverri

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calling hledger zurihackers
by Simon Michael 16 Mar '10

16 Mar '10

Roman C. has cleverly added hledger to the list of topics for this weekend's ZuriHac. I will be working remotely at least on sunday (PDT), and available to support where possible. You are invited to join us! Some ideas - - add Chart or google charts to the web ui - smarter add/convert commands - separate library package - plugins - a how-to for tracking project finances; a tutorial; screencasts - portability, packaging, installability - code review/design review/planning Even one commit helps! Discussions too. Hope to see you "there". -Simon (sm) #ledger, #zurihac http://hledger.org http://www.haskell.org/haskellwiki/ZuriHac

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ANNOUNCE: direct-plugins-1.0
by Dan Knapp 16 Mar '10

16 Mar '10

For some time I have relied on Dons's Plugins package, which was perhaps something of a mistake as it's kind of messy inside and I don't think it's maintained. With the upgrade to GHC 6.12, it broke for me, and I wasn't able to get it fixed. But I was able to write my own subset of its functionality. I don't do most of the things that Plugins does - in fact I export only a single function, "load" - but I think it might be useful to somebody, so I've decided to release it. direct-plugins: Lightweight replacement for Plugins, specific to GHC http://hackage.haskell.org/package/direct-plugins This is my first post to this list; I hope it's appropriate. Please let me know if not. -- Dan Knapp "An infallible method of conciliating a tiger is to allow oneself to be devoured." (Konrad Adenauer)

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a poorly thought out suggestion for cabal and packages that have lots of instances
by Jeremy Shaw 16 Mar '10

16 Mar '10

Hello, There are a number of libraries like syb-with-class, happstack-data, etc, which provide a new class that potentially has a lot of new instances. For example, let's say I want a 'Data ctx Text' instance. Where should that go? To avoid orphan instances it would need to go in the 'text' package where Text is defined. But it seems unlikely that I am going to convince the authors of text to add additional dependencies on things like syb-with-class, happstack-data, etc, just to satisfy a small number of users. A second place to put the instance would be in the syb-with-class package. They will be orphan instances, but as long as you can't import the Data class without also getting those instances, it doesn't seem like much could go wrong. But now syb-with-class has to depend on all the libraries that it is going to provide instances for. So, I have just shifted the problem from one library to another. A third option is to just define the instance in the library where I want to use it. But now I have opened up the possibility of conflicting orphan instances for the type -- which is no good. A fourth option is to create a new package which contains just the instance. For example: syb-with-class-text This still opens the possibility of conflicting orphan instances. But at least there is a version that is supposedly the 'official' one. The problem is that it seems like this might really clutter up hackage? Also, it is annoying to have to create and maintain and entire new cabal package just for one single class instance. And if i want to install all the instances, it would be annoying to have to do 20 different cabal installs. What might be nicer is if I could have a single project directory that somehow generated all those instances. And if cabal new how to work with them as a family. Something like: cabal upload syb-with-class* cabal install syb-with-class* Obviously, I have not really thought this through all the way. I just know that I am continually wondering what I should do when I want to do something like add a Text instance for syb-with-class? If we were to get really experimental I might suggest that when you install syb-with-class, you don't automatically get all the extra instances by default. But the package system and compiler would know they exist. So if you tried to write code that required the missing instance, it would suggest that you need to install the extra package. This would help reduce instances of people accidentally creating a conflicting instance. But I am sure it makes something else horrible happen instead ;) - jeremy

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Code that doesn't compile - but should :)
by Giuseppe Maggiore 16 Mar '10

16 Mar '10

Hi! Can anyone tell me why this code does not work? I cannot seem to figure why it is broken... {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} data Data k = Pair Integer (() -> k) data RecData = RecData (Data RecData) mk_data x = RecData(Pair x (\() -> mk_data (x+1))) class Converter a f where convert :: a -> f a instance Converter RecData Data where convert (RecData r) = r class Selector s a where select :: s -> a f :: (Selector s (a->f a), Converter a f) => s -> (a->a) f s = let method = select s in (\x -> let res = method x in convert res) -- Giuseppe Maggiore Ph.D. Student (Languages and Games) Microsoft Student Partner Mobile: +393319040031 Office: +390412348444

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ANNOUNCE: feldspar-language-0.2, feldspar-compiler-0.2
by Emil Axelsson 16 Mar '10

16 Mar '10

We are happy to announce the new release of Feldspar and its compiler! http://feldspar.sourceforge.net/ Feldspar is an embedded domain-specific language for digital signal processing. It is developed as a joint project with Ericsson, ELTE university and Chalmers university. This is an intermediate release to make some of our ongoing work available. The most user-visible change from the previous version is that the vector types have got much simpler, leading to much simpler programs. There have also been some substantial changes under the hood: Most notably, the compiler has a completely new structure which allows defining various optimizations as plugins. The examples available are still very limited. We hope to make a new release quite soon with a reworked tutorial and many more examples. / Emil

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