- Haskell-Cafe - Haskell.org

SOC idea ticket: Rendering Engine
by Csaba Hruska 24 Mar '09

24 Mar '09

Hi! I've created a ticket for this idea: http://hackage.haskell.org/trac/summer-of-code/ticket/1572 Please write your opinion. I also put the source code here: http://code.google.com/p/lambdacube/ svn checkout *http*://lambdacube.googlecode.com/svn/trunk/lambdacube-read-only Cheers, Csaba Hruska

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Equations for `foo' have different numbers of arguments
by Manlio Perillo 24 Mar '09

24 Mar '09

Hi. There is a limitation, in Haskell, that I'm not sure to understand. Here is an example: module Main where divide :: Float -> Float -> Float divide _ 0 = error "division by 0" divide = (/) main = do print $ divide 1.0 0.0 print $ divide 4.0 2.0 With GHC I get: Equations for `divide' have different numbers of arguments With HUGS: Equations give different arities for "divide" However the two equations really have the same number of arguments. What's the problem? Thanks Manlio Perillo

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Something wrong with happs.org?
by Vimal 24 Mar '09

24 Mar '09

Hi, http://happs.org/ has some Javascript visible as plain text. It looks like some tags are missing in the page... I hope that's the right website, because it turned up first on my Google search "happs" with a nice description too. -- Vimal

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Help on using System.Win32.Com.Automation
by Wilkes Joiner 24 Mar '09

24 Mar '09

I'm playing around with the com package, but I'm having a hard time understanding how to map a COM call to the appropriate methodN or functionN call. Does anyone have any example code that uses the method1 or higher. Any help or pointers would be appreciated. Here's the code I have so far: import System.Win32.Com import System.Win32.Com.Automation dsn = "Provider=vfpoledb.1;Data Source=C:\\SomeDirectory\\" main = coInitialize >> openConnection >>= \con -> closeConnection con openDSN :: String -> IDispatch a -> IO () openDSN dsn con = method0 "Open" [inString dsn] con openConnection :: IO (IDispatch a) openConnection = createObject "ADODB.Connection" >>= \con -> openDSN dsn con >> return con closeConnection :: IDispatch a -> IO () closeConnection = method0 "Close" [] {- Wraps ADO Connection.Execute http://msdn.microsoft.com/en-us/library/ms675023(VS.85).aspx Set recordset = connection.Execute (CommandText, RecordsAffected, Options) execute :: String -> IDispatch a -> IO a execute cmd con = method1 "Execute" [inString cmd] (inEmpty,resWord64) con -} Thank You, Wilkes

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RE: [ANN] Safe Lazy IO in Haskell
by Wei Hu 24 Mar '09

24 Mar '09

Nicolas Pouillard <nicolas.pouillard <at> gmail.com> writes: > > Hi folks, > > We have good news (nevertheless we hope) for all the lazy guys standing there. > Since their birth, lazy IOs have been a great way to modularly leverage all the > good things we have with *pure*, *lazy*, *Haskell* functions to the real world > of files. > > We are happy to present the safe-lazy-io package [1] that does exactly this > and is going to be explained and motivated in the rest of this post. Hi, Please let me know if I understood your code correctly. So, the SIO module is used only to ensure that the file processing is finished before the finalizer closes the file, right? In System.IO.Lazy.Input, run is defined as > run :: NFData sa => LI sa -> IO sa > run = run' . fmap return' Can I change it to > run = run' . fmap return ? I think the semantics is the same because run' will strictly force the processing anyway?

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Don't forget to vote!
by Eelco Lempsink 24 Mar '09

24 Mar '09

In less than 4 hours the Haskell Logo poll will be closed. If you're not one of the 423 people that voted so far, please take 1-20 minutes (depending on your level of perfectionism) to support your favorite logo(s) :) -- Regards, Eelco Lempsink

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generalized shuffle
by Manlio Perillo 23 Mar '09

23 Mar '09

Hi. I have implemented a generalized shuffle function, for the random-shuffle package http://hackage.haskell.org/cgi-bin/hackage-scripts/package/random-shuffle I have not yet commited the change, before that I would like to receive some feedbacks (especially by the original author of the shuffle function, in Cc) The new function is defined in this example: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2808 Note that it make use of functions not exported by the System.Random.Shuffle module. I have generalized the original shuffle function in two ways: 1) It is now possible to obtain a random "sample" from a sequence, by doing, as an example: shuffles [1..10] [2, 3, 4, 2, 1] Note that this is equivalent at taking the first 5 elements of the full shuffled sequence, and the performance are pratically the same. 2) It is now possible to pass an infinite "r-sequence" to the shuffle function, as an example: shuffles [1..10] $ cycle [9, 8 .. 1] The result is an infinite list, with elements cyclically extracted from the "r-sequence". When the "r-sequence" contains random numbers, we get an infinite sequence containing all possible random "choices" of elements in the original sequence. Why I'm doing this? The reason is that in a project I need to extract random elements from a list (and I need to extract a lot of them), and using "normal" methods [1] seems to be rather inefficient. Using the `shuffles` function should be much more efficient, since it uses a tree, and this tree is built only once. [1] by normal method I mean: extract a random number, in the range [0, n] (where n is the length of the sequence), and get the element indexed by that number. Indexing for a list if very expensive. And it is not very fast, even using arrays (unless you use non portable unsafeRead). Thanks Manlio Perillo

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HSTringTemplate and syb-with-class
by Kemps-Benedix Torsten 23 Mar '09

23 Mar '09

Hello all, I'm trying to use the generic capabilities of HSTringTemplate. The documentation claims that the package is able to automatically generate instances of ToSElem if syb-with-class is installed but gives no further details. I installed syb-with-class and then installed HSTringTemplate with additional configure parameter syb-with-class=True. But when I import Text.StringTemplate.GenericWithClass and then try deriving ToSElem or $(derive ToSElem), I just get an error like "Can't make a derived instance of `ToSElem ...". Any suggestions or pointer to further docs? Kind regards Torsten

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Function to cast types
by Anonymous Anonymous 23 Mar '09

23 Mar '09

Hello, I'm new to haskell, I'm wondering how can you write a function that will do the following: fromIntToString :: Int -> String this is a cast function to cast an Int to a String. I know such function exist, however let's assume it didn't exist. How would I write such function? Cause I have no idea, because there are infinity possibilities if I do it like: fromIntToString x | x == 1 = "1" | x == 2 = "2" -- And so on... I've also thinked about defining the defining the data types Int and String (I know you cannot redefine them, at least I think so), however I've no succes. Thank you in advance for answering my question! PS: if possible please do not use any casting functions that are predefined.

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System.Random.Shuffle fix
by friggin friggin 23 Mar '09

23 Mar '09

I was looking for a shuffling algorithm to shuffle mp3-playlists so was very happy to see System.Random.Shuffle: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/random-shuffle-0… However I get errors,non-exhaustive patterns in function shufleTree or extractTree depending how I call it. Errors are at the bottom. I fixed it but I don't have the math skills to see if I perhaps broke it statistically ... Here is my fix, someone (don't remember who, helped me a little): http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2789#a2789 the shuffle at the end is with the fix. *Freet> S.shuffle [1..10] [1..3] Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package random-shuffle-0.0.2 ... linking ... done. [2,4,6*** Exception: src\System\Random\Shuffle.hs:(52,6)-(55,30): Non-exhaustive patterns in function shuffleTree *Freet> S.shuffle [1..3] [1..10] [2,*** Exception: src\System\Random\Shuffle.hs:(66,6)-(79,27): Non-exhaustive patterns in function extractTree *Freet> :load "c:/ghc/ghc-6.10.1/progs/Mp3Player/Shuffle.hs" [1 of 1] Compiling Shuffle ( C:\ghc\ghc-6.10.1\progs\Mp3Player\Shuffle.hs, interpreted ) Ok, modules loaded: Shuffle. *Shuffle> shuffle [1..3] [1..10] [2,1,3] *Shuffle> shuffle [1..10] [1..3] [2,4,6*** Exception: C:\ghc\ghc-6.10.1\progs\Mp3Player\Shuffle.hs:(27,13)-(31,30): Non-exhaustive patterns in function shuffle' *Shuffle> shuffle [1..10] [1..10] [2,4,6,8,10,3,9,7,5,1] *Shuffle>

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