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Bit arithmetic in Haskell
by Artyom Shalkhakov 09 Dec '08

09 Dec '08

Hello, I'm trying to do some bit arithmetic. Here's the function: > import Data.Bits > import Data.Word > > g :: Word32 -> [Word32] > g x = [(x `shiftR` 24) .&. 0xFF, > (x `shiftR` 16) .&. 0xFF, > (x `shiftR` 8) .&. 0xFF, > x .&. 0xFF] This function should give bytes for the given number, like this: g 255 -> [0,0,0,255] g 256 -> [0,0,1,255] g 65535 -> [0,0,255,255] In reality, I get very strange answers. Can anybody help me please? Regards, Artyom Shalkhakov.

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Wrapping random
by Torsten Otto 09 Dec '08

09 Dec '08

Hi all, I teach a high school class in Computer Science. The current programming goal is to implement chat-bots, and we're using Haskell of course. Now one of my students had the seemingly easy idea of having the bot answer with a random sentence if it doesn't have "good" answer. Random in Haskell has its problems. I understand why you can't just call a function as you would in Java. I'm not firm enough with monads myself (and certainly don't want to go there in the class beyond I/O) so I'm calling for help here: Is there a way to wrap the generation of random numbers so that for the students it works like a function? We have this working: > import System.Random > main = > do randomNumber <- randomRIO (1::Int,2) > print (randomAnswer randomNumber) > randomAnswer r > | (r == 1) = "Nope!" > | (r == 2) = "Absolutely!" > | otherwise = "Error!" Now, how can we use it for something like this: >findAnswer [] = "h" >findAnswer (x:xs) > | (z == "unknown") = findAnswer xs > | otherwise = z > where z = findWord x lexikon where instead of getting "h" we'd like to call a function that would give us one of the strings out of randomAnswer. (findAnswer looks through a list [(keyword,response)]. I've looked at realworldhaskell and the wikibook among other sources, but I can't manage to piece anything useful together. How do I manage to get something of type IO to represent itself as a String? Any help would be greatly appreciated. Regards, Torsten Otto

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HDBC commit problem
by Moritz Tacke 09 Dec '08

09 Dec '08

Hi! I am running into problems while using HDBC with the sqlite3 backend. The HDBC "commit" statement frequently results in exceptions with the sqlite3 errorcode 1, "SQL logic error or no database". As far as I know, and as far as the HDBC docs tell me, "commit" simply writes all yet open transactions onto the disk. How can it fail? The HDBC documetation mentions the prepare- and execute-steps as the ones which might throw exceptions - what are the problems a call to "commit" might encounter? I do not suspect SQL errors to be the cause - those should lead to earlier exceptions - and the database is definitly present and opened, as select calls following the failing commit statments succeed. Greetings!

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RE: [Haskell-beginners] Profiling haskell code
by Sayali Kulkarni 08 Dec '08

08 Dec '08

Hey thanks Brent. This helped. I have one more question now. Consider I have two functions 1. gives me a range of numbers in an array. 2. has to get an array input for further process. Then how can I get the array generated by the first function tobe the input of the second function? Regards, Sayali -----Original Message----- From: Brent Yorgey [mailto:byorgey@seas.upenn.edu] Sent: Tuesday, November 18, 2008 5:47 PM To: Sayali Kulkarni Subject: Re: [Haskell-beginners] Profiling haskell code > I have just given it any random input array to be sorted. > The commands that I had sent earlier were tried on Cygwin... > ( > > > $ ghc --make Project.hs -prof -auto-all > > > > > > > > > $ Project +RTS -p > > > ) This ought to work fine. Just a note, to do any reasonable profiling you will need to give it a *much* larger list to sort. Otherwise it will execute so quickly that the timing data you get will be meaningless. > > Also can you tell me any other method for profiling the code that you > know? If you just want to see how long it takes to evaluate certain expressions, you can type ':set +s' in ghci; from then on after every expression you type it will tell you how long it took to evaluate and how much memory was used. -Brent

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[haskell] About CPS form funtions
by 甜瓜 08 Dec '08

08 Dec '08

Hi, I am confused by section 4.6 in Yet Another Haskell Tutorial written by Hal Daume III. The auther provides a CPS form of "fold" funtion, but I don't know: 1. How to transform a normal function into CPS function. I need some hint in writting "cfold". 2. Why "cfold" is CPS form? In my mind, if a CPS funtion calls to other CPS function, it must only feed that function with either direct variables or continuations. But in the tutorial, cfold defines: cfold f z l = cfold' (\x t g -> f x (g t)) z l Note that the lambda function body is f x (g t); which means call g(t) first, and take the result, then call f(x, r). It is illegal in CPS because g(t) will never return. This is not same with function f. Here, f can be treated as atomic operation (belong the CPS system). Anyone can help me? Thank you in advance. --- melon

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gtk2hs not rendering drawingArea
by Norbert Wojtowicz 06 Dec '08

06 Dec '08

Hello, I'm having problems getting a drawingArea to render, I've narrowed the program down to the following skeleton. Any suggestions on what I'm doing wrong? The label gets updated correctly, but the drawingArea just remains gray as if it was never rendered. I'm including an entire compilable skeleton in case someone wants to help me debug it. (I have a feeling I'm just missing something very obvious...) Thanks in advance, Norbert skeletonTest.hs: module Main where import Graphics.UI.Gtk -- hiding (fill) import Graphics.UI.Gtk.Glade import Graphics.Rendering.Cairo.SVG import Graphics.Rendering.Cairo import Control.Monad main = do initGUI let gFile = "brainSpin.glade" windowXmlM <- xmlNew gFile let windowXml = case windowXmlM of (Just windowXml) -> windowXml Nothing -> error "Can't find the glade file \"brainSpin.glade\" in the current directory" window <- xmlGetWidget windowXml castToWindow "brainSpinMain" onDestroy window mainQuit label <- xmlGetWidget windowXml castToLabel "label1" drawArea <- xmlGetWidget windowXml castToDrawingArea "drawArea" widgetShowAll window labelSetText label "foo" -- THIS is the offending code. Originally I was working with SVGs, but I simplified -- it to this, just to track down the problem. It seems any Render () does not -- get updated in the drawArea let r = do setSourceRGB 0 0 0 paint drawin <- widgetGetDrawWindow drawArea renderWithDrawable drawin r mainGUI brainSpin.glade: <?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE glade-interface SYSTEM "glade-2.0.dtd">  <glade-interface> <widget class="GtkWindow" id="brainSpinMain"> <child> <widget class="GtkVBox" id="vbox1"> <property name="visible">True</property> <child> <widget class="GtkLabel" id="label1"> <property name="visible">True</property> <property name="label" translatable="yes">label</property> </widget> </child> <child> <widget class="GtkDrawingArea" id="drawArea"> <property name="visible">True</property> </widget> <packing> <property name="position">1</property> </packing> </child> </widget> </child> </widget> </glade-interface>

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Useful set of -W switches for .ghci?
by Александър Л. Димитров 05 Dec '08

05 Dec '08

Hey list, I'm currently using :set -Wall in my .ghci to enforce some good coding practices on my side. Something like missing type signatures and the like. It has saved me quite a lot of hassle over the last couple of days as I experimented more with Haskell. But it seems, -Wall may be just a little too… overzealous: Prelude> 2^2 Warning: Defaulting the following constraint(s) to type `Integer' `Num t' arising from a use of `^' at <interactive>:1:0-2 Warning: Defaulting the following constraint(s) to type `Integer' `Integral t' arising from a use of `^' at <interactive>:1:0-2 Warning: Defaulting the following constraint(s) to type `Integer' `Num t' arising from a use of `^' at <interactive>:1:0-2 Warning: Defaulting the following constraint(s) to type `Integer' `Integral t' arising from a use of `^' at <interactive>:1:0-2 4 (I've shortened ghci's response a bit) So, do you have a good (recommended) set of -W switches for a newbie playing around in GHCi, so I can learn a good style? Forgive me if it's somewhere on page one of some book or tutorials - I've read so many of both, I'm beginning to forget things. Thanks, Aleks

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getRecursiveContents - example from `Real World Haskell'
by Johann Giwer 05 Dec '08

05 Dec '08

`Real World Haskell' is a great book. I really love it. When I tried an example from the 9th Chapter, I was a bit disappointed: *Main> f <- getRecursiveContents "/home/johann/" Heap exhausted; Current maximum heap size is 128000000 bytes (122 Mb); use `+RTS -M<size>' to increase it. The function lookes like this: getRecursiveContents :: FilePath -> IO [FilePath] getRecursiveContents topdir = do names <- getDirectoryContents topdir let properNames = filter (`notElem` [".", ".."]) names paths <- forM properNames $ \name -> do -- 1 let path = topdir </> name isDirectory <- doesDirectoryExist path if isDirectory then getRecursiveContents path else return [path] return (concat paths) -- 2 OK, I'm using a small machine and my home directory contains ~30,000 files. But that couldn't be the real problem. And even if this function is a small example it should work reliable. The programming language I know best (and this is meant relative -- I'm only a `would be programmer') is python. Python has good support for functional programming, but no builtin tail recursion. So my first idea about the bug in `getRecursiveContents' went in this direction. Two hours later I had worked out this solution: getRecursiveContents :: FilePath -> IO [FilePath] getRecursiveContents = getRecursiveContents' [] where getRecursiveContents' l p = E.handle (\_ -> return (p:l)) $ do -- 3 c <- getDirectoryContents p let c' = filter (`notElem` [".", ".."]) c x <- foldM (\l' p' -> getRecursiveContents' l' (p </> p')) l c' -- 4 return (x) Folding (4) and appending (3) would give less memory usage than mapping (1) and concatenation (2), I thought. This function worked well for small directory (for which the original one did, too). But tested with my home directory it went into an infinite loop. That led me to the actually problem: `doesDirectoryExist' also accepts symlinks to directories. Another hour later this was fixed: getRecursiveContents :: FilePath -> IO [FilePath] getRecursiveContents = getRecursiveContents' [] where getRecursiveContents' l p = do s <- getSymbolicLinkStatus p if isDirectory s then E.handle (\_ -> return (p:l)) $ do c <- getDirectoryContents p let c' = filter (`notElem` [".", ".."]) c x <- foldM (\l' p' -> getRecursiveContents' l' (p </> p')) l c' return (x) else return (p:l) Finally I fixed the original function (this only took about 30 min :-). The handle (5) catches errors caused by unreadable directories getRecursiveContents :: FilePath -> IO [FilePath] getRecursiveContents topdir = E.handle (\_ ->return [topdir]) $ do -- 5 names <- getDirectoryContents topdir let properNames = filter (`notElem` [".", ".."]) names paths <- forM properNames $ \name -> do let path = topdir </> name s <- getSymbolicLinkStatus path if isDirectory s then getRecursiveContents path else return [path] return (concat paths) The imports for all functions mentioned above are: import Control.Monad ( forM, filterM, foldM ) import qualified Control.Exception as E import System.Directory (doesDirectoryExist, getDirectoryContents) import System.FilePath ((</>)) import System.Posix (getSymbolicLinkStatus, isDirectory) Any suggestions about this solution are welcome. Johann

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Special session on Haskell language
by John Edward 03 Dec '08

03 Dec '08

Special session on Haskell language at MULTICONF-09 call for papers Special session on Haskell language will be held at the 2009 Multi Conference in Computer Science, Information Technology and Control systems and Computational Science and Computer Engineering (MULTICONF-09) (website: http://www.PromoteResearch.org) and it will be held during July 13-16 2009 in Orlando, FL, USA. We invite draft paper submissions. The event consists of the following conferences: · International Conference on Artificial Intelligence and Pattern Recognition (AIPR-09) · International Conference on Automation, Robotics and Control Systems (ARCS-09) · International Conference on Bioinformatics, Computational Biology, Genomics and Chemoinformatics (BCBGC-09) · International Conference on Enterprise Information Systems and Web Technologies (EISWT-09) · International Conference on High Performance Computing, Networking and Communication Systems (HPCNCS-09) · International Conference on Information Security and Privacy (ISP-09) · International Conference on Recent Advances in Information Technology and Applications (RAITA-09) · International Conference on Software Engineering Theory and Practice (SETP-09) · International Conference on Theory and Applications of Computational Science (TACS-09) · International Conference on Theoretical and Mathematical Foundations of Computer Science (TMFCS-09) The website http://www.PromoteResearch.org contains more details. Sincerely John Edward Publicity committee

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What causes <<loop>>?
by Martin Hofmann 03 Dec '08

03 Dec '08

I've already posted this mail on haskell-cafe, but apparently the subject suggested a too simple question, so I try it here again. I am picking up a discussion with the same topic from haskell-users on 8th November. Thunks with reference on themselves was mentioned as main reason for <<loop>>. > A safe recursive definition would be > let x = Foo (x+1) > However, if you leave out the constructor, > let x = x + 1 > you get a <<loop>> (or a deadlock). > Are there any other reasons? I am trying to debug monadic code which stores state information in a record maintaining several Data.Maps, but in vain so far. A state is modified/changed in several steps by a compound function i.e. changeA $ changeB $ changeC state Could this also lead to a deadlock? If so, can I prevent this using CPS? If the state transformation is the main purpose/computation of the monad at all, is it better to use CPS? Is code using mtl, records or Data.Map more prone to <<loop>> or does this not matter at all. How can I identify self-referencing thunks more easily? Is there any rule of thumb to prevent <<loop>>? Are there any less obvious newbie mistakes causing <<loop> then the one above? Sorry, a lot of questions at once, but I am kind of helpless because I can't find any reason of my <<loop>>, so any comments, experience, and tips are highly appreciated. Thanks, Martin

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