April 2010 - Haskell-Cafe

Announce: hothasktags
by Luke Palmer 09 Apr '10

09 Apr '10

Hi, I'd like to draw attention to a little script I wrote. I tend to use qualified imports and short names like "new" and "filter". This makes hasktags pretty much useless, since it basically just guesses which one to go to. hothasktags is a reimplementation of hasktags that uses haskell-src-exts to analyze the import structure to generate (scoped) tags pointing to the right definition. I'm pretty addicted to it, since it provides the only functionality I miss from visual studio :-). VIm only for now, since I don't know if emacs tags format supports scoped tags. I am aware that it is not perfect -- patches and bug reports welcome. http://hackage.haskell.org/package/hothasktags Luke

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Suitable structure to represents lots of similar lists
by Dan Piponi 09 Apr '10

09 Apr '10

I have a situation where I have a bunch of lists and I'll frequently be making new lists from the old ones by applying map and filter. The map will be applying a function that's effectively the identity on most elements of the list, and filter will be using a function that usually gives True. This means that there is potential for a large amount of sharing between these lists that could be exploited, something that ordinary lists would do badly. Does anyone have a recommendation for a pure functional data structure for this case? (It reminds me a bit of my own antidiagonal type, but that's not well adapted to the highly dynamic situation I'm describing.) -- Dan

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Why haskell.org down again?
by Andy Stewart 09 Apr '10

09 Apr '10

Hi all, haskell.org down again? Hardware not stable? Attack? Can't we avoid haskell.org off? -- Andy

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Nested unsafePerformIO?
by DavidA 09 Apr '10

09 Apr '10

Hi, I am having difficulty debugging a troublesome stack overflow, which I think might be related to calling unsafePerformIO from within the IO monad. So I have the following code: import System.Random import System.IO.Unsafe import Data.Time.Clock timedIterateIO :: Int -> (a -> a) -> a -> IO a timedIterateIO durationSecs f x = do t0 <- getCurrentTime timedIterateIO' t0 x where duration' = fromIntegral durationSecs timedIterateIO' t0 x = do let y = f x t <- getCurrentTime let d = diffUTCTime t t0 if d >= duration' then return y else timedIterateIO' t0 y f x = unsafePerformIO $ do m <- randomRIO (1,2) return (m+x) The idea of the timedIterateIO function is that it should repeatedly apply f to x until a specified duration is up. The use case is for game search, where I want to keep searching until time is up, and then play the best move I've found at that point. So the following works: *Main> timedIterateIO 1 (+1) 0 45580 (1.01 secs, 360357840 bytes) The following also seems to work: *Main> unsafePerformIO $ timedIterateIO 1 f 0 67866 (1.25 secs, 394938540 bytes) Now, in the real use case, I have another function, g let's say. It is similar to f, in that it uses unsafePerformIO internally to get random numbers. It's proven to work under normal circumstances, for example: *Main> (!! 100) $ iterate g x works, and completes in around a second. However, the following doesn't work, failing with a stack overflow: *Main> unsafePerformIO $ timedIterateIO 1 g x Any ideas? Thanks, David

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LLVM study SoC proposal
by Marco Túlio Gontijo e Silva 09 Apr '10

09 Apr '10

Hi. I wrote a(nother) SoC proposal for studying LLVM performance, compared with the Native Code Generator back-end. It's available at: http://www2.dcc.ufmg.br/laboratorios/llp/wiki/doku.php?id=marco_soc2 I know the deadline is now near, but I'd be happy to hear comments about it and update it before 19:00 UTC. Thanks. -- marcot http://wiki.debian.org/MarcoSilva

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HDBC-ODBC and SqlValues
by Tim Docker 08 Apr '10

08 Apr '10

I'm experimenting with haskell and relational databases. I have successfully coupled unixodbc + freetds + hdbc-odbc, and can make trivial queries. However, I'm surprised at the result types: $ ghci GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> c <- Database.HDBC.ODBC.connectODBC "DSN=xxxxx;UID=xxxxx;PWD=xxxx" Loading package syb ... linking ... done. Loading package base-3.0.3.1 ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.2 ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package utf8-string-0.3.5 ... linking ... done. Loading package time-1.1.4 ... linking ... done. Loading package convertible-1.0.9 ... linking ... done. Loading package HDBC-2.2.4 ... linking ... done. Loading package HDBC-odbc-2.2.3.0 ... linking ... done. Prelude> Database.HDBC.quickQuery c "select 1+3" [] [[SqlByteString "4"]] Prelude> Why do I see an SqlByteString returned, rather than a numeric type? Taking a closer look at the HDBC-ODBC implementation it would appear that data will always be returned as SqlByteString, or as SqlNull. Is this the intended behaviour, or just a sign of an incomplete implementation of the ODBC driver? It would certainly seem possible for the ODBC driver to return more specific types. Tim

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Haskell.org re-design
by Christopher Done 08 Apr '10

08 Apr '10

This is a post about re-designing the whole Haskell web site. We got a new logo but didn't really take it any further. For a while there's been talk about a new design for the Haskell web site, and there are loads of web pages about Haskell that don't follow a theme consistent with Haskell.org's, probably because it doesn't really have a proper theme. I'm not a designer so take my suggestion with a grain of salt, but something that showed pictures of the latest events and the feeds we currently have would be nice. The feeds let you know that the community is busy, and pictures tell you that we are human and friendly. Anyway, I came up with something to kick off a discussion: http://haskell.org/haskellwiki/Image:Haskell-homepage-idea.png It answers the basic questions: - What's Haskell? - Where am I on the site? (Answered by a universally recognised tab menu) - What's it like? - How do I learn it? - Does it have an active community? - What's going on in the community? What are they making? - This language is weird. Are they human? -- Yes. The picture of a recent event can fade from one to another with jQuery. The colours aren't the most exciting, but someone who's a professional designer could do a proper design. But I like the idea of the site being like this; really busy but not scarily busy. Subsections of the site could use the header and footer and heading theme, but have a completely different primary-content layout. Probably sub-sections would need a left-nav. Keeping the design simple like this also makes it easy to theme the current Wiki to fit in with it seamlessly. Personally I don't have a problem with the existing site, functionally. It has all the stuff I want to look at. The only stuff that I had issue with as a newbie was finding The One Book I Should Read and The One Download I Should Get. The current site is starting to address this with a "Download Haskell" button. However, looking at it as a marketing site, it does look pretty lame and messy, and it gives you that impression of Haskell. So if people who own the site are going to redesign it, I thought I'd contribute a bit. Anyway, please contribute your ideas. (Again, I'm not a designer, so you don't need to pick at the aesthetics, a real designer can sort that out.) Cheers!

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Re: [Haskell-cafe] GSOC Haskell Project
by Mihai Maruseac 08 Apr '10

08 Apr '10

I have submitted my proposal to the gsoc site taking care to include all the feedback that was given either publicly or privately both from people on haskell-cafe, on #haskell or from my country. This is the last call for feedback before the submission period is ended. One last chance to improve the application.

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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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