January 2010 - Beginners

HUnit or QuickCheck for test-driven developing a parser
by Adam Cigánek 20 Jan '10

20 Jan '10

Hello there, Let's say I'm writing a parser for a toy programming language, and I want to do it following a test-driven development philosophy. Which would be better testing tool for this task, HUnit or QuickCheck (or perhaps something else)? Cheers, adam.

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Defining of subset of constructors as valid values
by Lev Broido 18 Jan '10

18 Jan '10

Hi all I have the following code : data CmdTiming = RAS Time | CAS Time | PCH Time | ACT Time deriving (Ord,Show,Eq) data Command = RD_CMD {bank :: Int, col :: Int } | WR_CMD {bank :: Int, col :: Int} | ACT_CMD {bank :: Int, row :: Int} | PCH_CMD {bank :: Int} deriving Show I'd like to have something like : RD_CMD {bank :: Int, col :: Int,timing :: RAS Time } to limit possible values to RAS Time . The reason I use constructors is differentiate whether current value is applicable to some function , getting CmdTiming as parameter or not . I am not sure this is valid definition since RAS Time is not type . Regards, Lev

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MULTICONF-10 Call for papers
by John Edward 17 Jan '10

17 Jan '10

MULTICONF-10 Call for papers The 2010 multi-conference (MULTICONF-10) (website: http://www.promoteresearch.org) will be held during July 12-14, 2010 in Orlando, Florida, USA. The primary goal of MULTICONF is to promote research and developmental activities in computer science, information technology, control engineering, and related fields. Another goal is to promote the dissemination of research to a multidisciplinary audience and to facilitate communication among researchers, developers, practitioners in different fields.The following conferences are planned to be organized as part of MULTICONF-10. * International Conference on Artificial Intelligence and Pattern Recognition (AIPR-10) * International Conference on Automation, Robotics and Control Systems (ARCS-10) * International Conference on Bioinformatics, Computational Biology, Genomics and Chemoinformatics (BCBGC-10) * International Conference on Computer Networks (CN-10) * International Conference on Enterprise Information Systems and Web Technologies (EISWT-10) * International Conference on High Performance Computing Systems (HPCS-10) * International Conference on Information Security and Privacy (ISP-10) * International Conference on Image and Video Processing and Computer Vision (IVPCV-10) * International Conference on Software Engineering Theory and Practice (SETP-10) * International Conference on Theoretical and Mathematical Foundations of Computer Science (TMFCS-10) We invite draft paper submissions. Please see the website http://www.promoteresearch.org for more details. Sincerely John Edward

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Understanding Monadic I/O
by Markus Böhm 17 Jan '10

17 Jan '10

Hi, I'd like to understand in principle, how monadic I/O actions and combinators (>>=) are translated into imperative C--, to be executed sequentially. Does sequencing of IO actions mean nesting of C-- functions and passing values/state via additional function parameters? Is there any material with examples available? P.S.: Just to understand the magic better. I need it. BR -- Markus

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Re: [Haskell-beginners] Re: Code help requested
by Steve 17 Jan '10

17 Jan '10

On Sat, 2010-01-16 at 14:42 -0500, beginners-request(a)haskell.org wrote: > binary_find [1] 2 > ~> do_search [1] 2 0 1 > ~> mid = 0 + 1 `div` 2 = 0 > ~> [1] !! mid < 2 > ~> do_search [1] 2 (0+1) 1 > ~> mid = 1 + 0 `div` 2 = 1 > ~> [1] !! 1 => boom > > > where > > do_search list elem low high = > > if high < low then Nothing > > else > > if list !! mid > elem > > then do_search list elem low (mid - 1) > > else > > if list !! mid < elem > > then do_search list elem (mid + 1) high > > else Just mid > > where > > mid = low + (high - low) `div` 2 > > I'd prefer > mid = (low + high) `div` 2 > here. mid = (low + high) `div` 2 is buggy code. Java had this bug for 9 years until someone noticed. http://googleresearch.blogspot.com/2006/06/extra-extra-read-all-about-it-ne… mid = low + (high - low) `div` 2 is useful because it avoids Int overflow errors (when using very, very long lists). Steve

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MVar blocked or not?
by Adrian Adshead 17 Jan '10

17 Jan '10

Hi everyone, I have a beginer question on using MVar. In the simple case I am getting what I expect... ***CODE*** module Main ( main ) where import Control.Concurrent import System.IO main = do mv <- newEmptyMVar putMVar mv 'a' threadDelay (5 * 10^6) putMVar mv 'b' threadDelay (5 * 10^6) ***/CODE*** Output after the first 5 second delay is :- Demo.exe: thread blocked indefinitely It even works as expected when there is a reading thread ***CODE*** module Main ( main ) where import Control.Concurrent import Control.Monad import System.IO main = do mv <- newEmptyMVar forkIO (readMV mv) putMVar mv 'a' threadDelay (5 * 10^6) putMVar mv 'b' threadDelay (5 * 10^6) readMV mv = forever $ do ch <- takeMVar mv putChar ch >> hFlush stdout ***/CODE*** Output including expected delays :- ab Then it starts to act unexpectedly ***CODE*** module Main ( main ) where import Control.Concurrent import Control.Monad import System.IO main = do mv <- newEmptyMVar forkIO (readMV mv) putMVar mv 'a' threadDelay (5 * 10^6) putMVar mv 'b' threadDelay (5 * 10^6) readMV mv = forever $ do ch <- modifyMVar mv modMV putChar ch >> hFlush stdout threadDelay (9*10^5) modMV 'a' = return ('c','a') modMV 'c' = return ('a','c') modMV x = return (x,x) ***/CODE*** Output is :- acacac The problem is that the output stops after 5 seconds and the program exits after 10 seconds. But why? Questions:- 1. Why does the output stop? (Shouldn't the reader thread continue even if the main thread blocks on the second putMVar?) 2. Why does it exit? (Shouldn't it be blocked at the second putMVar? The readMV thread now only modifies the MVar so it should never be empty) 3. Is there a race condition using modifyMVar while there is another thread blocked in a putMVar? (I am assuming the second putMVar succeeded, but where did the 'b' go?) Many thanks for any help Adrian.

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Code help requested
by Joe Van Dyk 16 Jan '10

16 Jan '10

I've written two versions of the same program, one in ruby and one in haskell. Given words on stdin, find all the anagrams in those words. For nicer display, we're only going to display the top 3 results. I'm obviously new to haskell. The ruby version runs about 5x as fast on a large file. How can I improve the haskell version? http://gist.github.com/274774 # Ruby version input = STDIN.read.split("\n") result = Hash.new([]) input.each do |word| sorted_word = word.split('').sort.join result[sorted_word] += [word] end values = result.values.sort { |a, b| b.size <=> a.size } p values[0..3] # Haskell version import List import qualified Data.Map as Map -- Given as stdin -- presents -- serpents -- no -- on -- whatever -- Expected Output: -- [["serpents","presents"],["on","no"]] -- This version only displays words that have more than one -- match in the list, and sorts by the words that got the most matches. -- Can we do the map bit better? main = do input <- getContents print $ anagrams $ lines input anagrams words = sorted_anagrams where sorted_anagrams = sortBy sorter filtered_anagrams sorter a b = compare (length b) (length a) filtered_anagrams = Map.elems $ Map.filter filter_function all_anagrams filter_function words = length words > 1 all_anagrams = do_anagrams words Map.empty do_anagrams [] result = result do_anagrams words result = do_anagrams (tail words) (Map.unionWith (++) (Map.fromList [(sorted_current_word, [current_word])]) result) where current_word = head words sorted_current_word = sort current_word -- Joe Van Dyk http://fixieconsulting.com

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using "read"
by Luca Ciciriello 16 Jan '10

16 Jan '10

Thanks to All. reads solve my problem. Luca.

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Haskell, Ray Tracing, Parallel Computation
by Jon Harrop 16 Jan '10

16 Jan '10

Someone has tried to parallelize Lennart's Haskell from my ray tracer language comparison and they are claiming awesome performance results for Haskell, even beating HLVM: http://poorlytyped.blogspot.com/2010/01/haskell-ray-tracing-parallel.html Frankly, I don't believe their claims for a second. So I'd like to test their code for myself. However, apparently their code needs some third-party library that can be installed with: cabal install AC-Vector but, on my machine, that only seems to make the library accessible to GHC 6.10 and not to GHC 6.12 (which is needed to compile this benchmark). What is the easiest way for me to get this library working under GHC 6.12? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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using "read"
by Luca Ciciriello 15 Jan '10

15 Jan '10

Hi All. in the function: func :: String -> Int func str = read str An absolute beginner as i am, want to use "read" to convert a String in a Int, but I don't know how to protect my function func from an exception when str is not a number like "12A". How can I do? Thanks in advance for any answer to this my simply question. Luca.

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