- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Correct way to record state in OpenGL?
by Mark Spezzano 08 Apr '10

08 Apr '10

I sort of understand what people are getting at. My basic question is now, given that I have a bunch of parameters that represent state should I bundle up ALL of these parameters in one data type (DemoState, say) and have ONE IORef variable that references the lot, or have an IORef for each piece of state, separately. What are the disadvantages of each approach? Also, is reading and writing to an IORef expensive computationally? Thanks, Mark On 05/04/2010, at 2:25 AM, Tobias Brandt wrote: > HOpenGL uses Data.StateVar. One can define the following function: > > newStateVar :: a -> IO (StateVar a) > newStateVar val = do > ref <- newIORef val > let readAction = readIORef ref > let writeAction = \newVal -> writeIORef ref newVal > return (makeStateVar readAction writeAction) > > Now one can use StateVar consistent with HOpenGL: > > create: foo <- newStateVar bar > read: get foo > write foo $= baz > > On 4 April 2010 11:03, Mark Spezzano <mark.spezzano(a)chariot.net.au> wrote: > Hi > > What is the correct way to record "custom" state when using OpenGL? > > By this, I refer to, say, properties of a square--say it's x,y coordinates as it moves across the screen. This requires that the program keep track of the object's state as it moves. These coordinates are _not_ part of the OpenGL state machine, so is the correct way to use the State monad? > > If so--or if not so--how would I proceed to keep track of the state of my objects as they move about the screen? Maybe the HOpenGL implementation comes with such state-tracking devices? > > Please post code snippets if necessary. > > Thanks in advance, > > Mark Spezzano > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Seeking advice about monadic traversal functions
by Darryn Reid 08 Apr '10

08 Apr '10

Hi, I hope that this is an appropriate place to post this question; I initially posted to Haskell-beginners since that seemed appropriate to my knowledge level. Brent Yorgey kindly suggested I should post here instead. Just for a little background: I'm an experienced computer scientist and mathematician but am very new to Haskell (I love it, I might add, but have much to learn). I've coded a (fairly) general rewriting traversal - I suspect the approach might be generalisable to all tree-like types, but this doesn't concern me much right now. My purpose is for building theorem provers for a number of logics, separating the overall control mechanism from the specific rules for each logic. The beauty of Haskell here is in being able to abstract the traversal away from the specific reduction rules and languages of the different logics. I have paired down the code here to numbers rather than modal formulas for the sake of clarity and simplicity. My question is two-fold: 1. Does my representation of the traversal seem good or would it be better expressed differently? If so, I'd appreciate the thoughts of more experienced Haskell users. 2. I cannot really see how to easily extend this to a queue-based breadth-first traversal, which would give me fairness. I'm sure others must have a good idea of how to do what I'm doing here except in breadth-first order; I'd appreciate it very much if someone could show me how to make a second breadth-first version. Thanks in advance for any help! Darryn. start of code. ------------------------------------------------------------------------- import Control.Monad import Control.Monad.Identity -- Tableau is a tree containing formulas of type a, in -- Fork (disjunctive) nodes and Single (conjunctive) nodes. -- Paths from root to Nil each represent an interpretation. data Tableau a = Nil -- End of a path | Single a (Tableau a) -- Conjunction | Fork (Tableau a) (Tableau a) -- Disjunction deriving (Eq, Show) -- 'Cut' directs the traversal to move back up the tree rather than into -- the current subtree, while 'Next' directs the traversal to proceed -- into the subtree. data Action = Cut | Next deriving (Eq, Show) -- The type of the rewrite function to perform at each node in -- the traversal of the tree. The Rewriter returns the new -- subtree to replace the existing subtree rooted at the -- traversal node, and a direction controlling the traversal -- to the next node. type Rewriter m a = Tableau a -> m (Tableau a, Action) -- The traversal function, depth-first order. rewrite :: (Monad m) => Rewriter m a -> Tableau a -> m (Tableau a) rewrite rf t = do (t', d) <- rf t rewrite' rf d t' -- Worker function for traversal, really its main body. rewrite' :: (Monad m) => Rewriter m a -> Action -> Tableau a -> m (Tableau a) rewrite' rf Cut t = return t rewrite' rf Next Nil = return Nil rewrite' rf Next (Single x t1) = do t1' <- rewrite rf t1 return (Single x t1') rewrite' rf Next (Fork t1 t2) = do t1' <- rewrite rf t1 t2' <- rewrite rf t2 return (Fork t1' t2') -- Some examples to test the code with: -- ghci> let t0 = (Single 2 (Single 3 Nil)) -- ghci> let t1 = (Single 1 (Fork t0 (Fork (Single 4 Nil) (Single 5 Nil)))) -- ghci> let t2 = (Fork (Single 10 Nil) (Single 11 Nil)) -- ghci> let t3 = (Fork (Single 3 Nil) (Single 3 Nil)) -- Running "test4 t1 t2" produces the expected result; running -- "test4 t1 t3" does not terminate, because this endlessly -- generates new patterns that match the substitution condition: test4 :: (Num t) => Tableau t -> Tableau t -> Tableau t test4 tab tab' = runIdentity (rewrite rf tab) where rf (Single 3 Nil) = return (tab', Next) rf t = return (t, Next) -- Running "test5 t1 t3", however, does terminate because the "Cut" -- prevents the traversal from entering the newly spliced subtrees -- that match the pattern. test5 :: (Num t) => Tableau t -> Tableau t -> Tableau t test5 tab tab' = runIdentity (rewrite rf tab) where rf (Single 3 Nil) = return (tab', Cut) rf t = return (t, Next) ------------------------------------------------------------------------- end of code.

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ANNOUNCE: Hac phi 2010
by Chris Casinghino 08 Apr '10

08 Apr '10

Greetings, I am very pleased to officially announce Hac phi 2010, a Haskell hackathon/get-together to be held May 21-23 at the University of Pennsylvania in Philadelphia. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Last year's Hac phi was a lot of fun, and many people have already expressed interest in coming back this year. I want to stress that everyone is welcome---you do not have to be a Haskell guru to attend! Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1]. Registration, travel, lodging and many other details can be found on the Hac phi wiki [2]. Note that we're in a different space this year and may have to cap attendance, so register early. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in Philadelphia! -The Hac φ team [1] http://haskell.org/haskellwiki/Hac_φ/Register [2] http://haskell.org/haskellwiki/Hac_φ [3] http://haskell.org/haskellwiki/Hac_φ/Talks

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multi-ghc: Managing Multiple GHC Distributions
by Sean Leather 08 Apr '10

08 Apr '10

I created a few tools to help me manage multiple GHC distributions in a Bash shell environment. Perhaps it's useful to others. http://github.com/spl/multi-ghc Feedback welcome. I'd also like to know if something similar exists. Regards, Sean

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Traypoweroff questions
by Luis Cabellos 08 Apr '10

08 Apr '10

Hi, I just put an application into hackage http://hackage.haskell.org/package/traypoweroff. I made this application to allow a tray icon in xmonad to poweroff and reboot my computer. But I dislike two things I made. The first one is the use of */sbin/poweroff* and */sbin/reboot* with system call. I feel than i should be a better way to do the poweroff or the reboot. err <- system "sudo /sbin/reboot" The second issue is, the user need to put into sudoers the permission to execute reboot and poweroff. The use of *sudo* seems a little awkward. I'll like a cleaner installation without force user touch configuration files. myuser ALL=(ALL) NOPASSWD: /sbin/poweroff Thanks for the help, Luis Cabellos p.d: I'll add suspend with pm-suspend command in the next version BTW

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GHC LLVM ARM Cross Compilation
by Nathaniel Neitzke 08 Apr '10

08 Apr '10

I saw the Google Summer of Code project for using LLVM to cross compile for other architectures such as ARM. Professionally I write embedded Linux code that targets ARM processors such as the TI DaVinci DM355 and am very intrigued by the potential use of Haskell for future projects. If I was interested in attempting to add support for LLVM ARM cross-compilation, are there any thoughts on what parts would have to be modified to do so? Or has work already been conducted in this area? ~Nathan

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what are the safety conditions for unsafeIOToST
by Nicolas Frisby 08 Apr '10

08 Apr '10

I haven't been able to find it via Google or Haddock. An old message suggests is was just a matter of exceptions? I only want to use the IO for generating Data.Uniques to pair with STRefs in order to make a map of them. I'm guessing this would be a safe use since it's exception free (... right?). Thanks!

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ANNOUNCE: JSONb-1.0.0
by Jason Dusek 07 Apr '10

07 Apr '10

A new release of JSONb, taking advantage of the latest Attoparsec. http://hackage.haskell.org/package/JSONb-1.0.0 Thanks to Grant Monroe for help with the numerics parser. -- Jason Dusek

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lhaskell.vim: syntax highlighting fix for pragmas and comments in TeX mode
by Tomas Janousek 07 Apr '10

07 Apr '10

Hello cafe :-), you're the maintainer of lhaskell.vim and I'm sending you a patch that fixes the highlighting of curly brackets and anything between them. (Who's supposed to send it to vim maintainer? Me, or cafe?) Regards, -- Tomáš Janoušek, a.k.a. Liskni_si, http://work.lisk.in/

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libraries [was GUI haters]
by gladstein＠gladstein.com 07 Apr '10

07 Apr '10

As a working engineer, one of my greatest frustrations is my inability to use Haskell in the workplace. The unfortunate fact is that my media industry clients use mostly Windows, some Macs, and no linux except for servers. The core system works everywhere, but many contributed libraries don't. GUIs are the big showstopper. One of the reasons Java won out over Common Lisp is that it had huge libraries. Franz's libraries were superb but few in number. One diehard Lisp user converted his lab to Java because "Java gives you everything you want, for free." That languages are distinct from their libraries escapes a lot of people; they see each language as a package. I met a COBOL programmer recently (I'm not making this up) that was looking into Java. He didn't see how people could use it, he said, because it had "thousands of commands". I'll stop whining now.

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