March 2009 - Haskell-Cafe

Definition of "tail recursive" wrt Folds
by Mark Spezzano 25 Mar '09

25 Mar '09

Hi, Just looking at the definitions for foldr and foldl I see that foldl is (apparently) tail recursive while foldr is not. Why? Is it because foldl defers calling itself until last whereas foldr evaluates itself as it runs? What, strictly speaking, is the definition of ”tail recursive” as opposed to just “recursive”? Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.27/2021 - Release Date: 24/03/2009 4:00 PM

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Hugs on iPhone
by Kirk Martinez 24 Mar '09

24 Mar '09

I saw Miguel Mitrofanov ( http://www.nabble.com/Hugs-on-the-iphone-td19478992.html) successfully ported Hugs to the iPhone. I'm now wondering if anyone has tried to get Apple's blessing to put this in the App Store? It would be really great to be able to try out little Haskell ideas as the mood strikes. Of course, it's kind of essential to have an editor of some kind for more significant programs... Thanks, Kirk

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ACM Task for C++ and Java programmers in Haskell. How to make code faster?
by Vasyl Pasternak 24 Mar '09

24 Mar '09

Hi! Recently I've found interesting ACM research on C++ and Java efficiency. This task also was been solved on lisp/scheme and is described on http://www.flownet.com/ron/papers/lisp-java/ I tried to solve this task on Haskell but stuck with efficiency problems (approx in 1000 times slower and takes 20 times more memory). I hope the approach, I've used is correct and problem with data structures and implementation. It took more than 81% of time for garbage collection. I tried to use ByteStrings instead of Strings, but unfortunately it didn't help. The entire code I placed on http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2764 The original instructions are on: http://www.flownet.com/ron/papers/lisp-java/instructions.html Dictionary file could be downloaded from: http://www.flownet.com/ron/papers/lisp-java/dictionary. Input data: http://www.flownet.com/ron/papers/lisp-java/input.txt Expected output: http://www.flownet.com/ron/papers/lisp-java/output.txt Could someone help me to make this code faster? I'd like to see solution that will be elegant and fast, without heavy optimizations, that will make code unreadable. Also, if it possible, prepare the program to support SMP parallelism. Very thanks in advance. Hope you'll enjoy this task :) -- Best regards, Vasyl Pasternak

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GHC 6.10.2 - PCap package - affected by the change of Finalizers?
by Neil Davies 24 Mar '09

24 Mar '09

Hi Looks like pcap package needs a little tweek for 6.10.2 - programs compiled with it bomb out with ......error: a C finalizer called back into Haskell. use Foreign.Concurrent.newForeignPtr for Haskell finalizers. Is my interpretation of this error message correct? Cheers Neil

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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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