- Beginners - Haskell.org

Call stack
by Nicolas Couture-Grenier 08 Mar '10

08 Mar '10

Is there an easy way to print the call stack of the evaluation of an expression in ghci without setting breakpoints or changing the code? That would be very useful.

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Speed performance problem on Windows?
by Travis Erdman 08 Mar '10

08 Mar '10

I'm working through one of Don Stewart's many excellent articles ... http://cgi.cse.unsw.edu.au/~dons/blog/2008/06/04#fast-fusion I faithfully re-created the source of his initial GHC reference implementation as: import System.Environment import Text.Printf mean :: Double -> Double -> Double mean n m = go 0 0 n where go :: Double -> Int -> Double -> Double go s l x | x>m = s / fromIntegral l | otherwise = go (s+x) (l+1) (x+1) main = do [d] <- map read `fmap` getArgs printf "%f\n" (mean 1 d) Then, compiled and executed like this: C:\Documents and Settings\Travis\My Documents\Haskell Code>ghc -O2 biglistmean.hs -optc-O2 -fvia-C --make -fforce-recomp [1 of 1] Compiling Main ( biglistmean.hs, biglistmean.o ) Linking biglistmean.exe ... C:\Documents and Settings\Travis\My Documents\Haskell Code>biglistmean 1000000 500000.5 C:\Documents and Settings\Travis\My Documents\Haskell Code>biglistmean 10000000 5000000.5 C:\Documents and Settings\Travis\My Documents\Haskell Code>biglistmean 100000000 50000000.5 C:\Documents and Settings\Travis\My Documents\Haskell Code>biglistmean 1000000000 500000000.067109 On the final test of 10^9, Don reports that it took 1.76 secs on his machine. In contrast, just 10^8 takes 12.63 secs on my machine (sophisticatedly timed with handheld stopwatch) and on the coup de grace 10^9 test, it takes 2min:04secs. Yikes! My hardware is a little old (Win XP on Pentium 4 3.06GHz w 2 GB RAM) but not THAT old. I'm using the latest Haskell Platform which includes ghc v 6.10.4. Primary question: What gives here? Incidental questions: Is there a nice way to time executed code in Windows ala the "time" command Don shows under Linux? Also, does the ordering of the compiler flags have any impact (I hope not, but I don't want to be surprised ...) Thanks, Travis Erdman

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Translating a while
by Nicolas Couture-Grenier 08 Mar '10

08 Mar '10

I'm learning Haskell and I'm trying to translate a pseudocode algorithm to generate the next permutation from a previous permutation. A permutation is a list of n numbers (let's call it a) in {1 .. n} appearing once in arbitrary order. The first step is to find the largest index j in the list for which a[j] < a[j+1]. The pseudocode is simple: j:= n-1 while a[j] > a[j+1] j:=j-1 I've coded a haskell function to do this, but it is much uglier than the pseudocode : j :: Integral a => [a] -> Int j [] = 0 j xs = if (head (tail (reverse xs)) < last xs) then (length xs)-2 else j (take (length xs - 1) xs) Does anyone has a more elegant solution for this first step?

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Help in pattern matching
by Joe Fox 08 Mar '10

08 Mar '10

Hi Guys , I am having a problem with patter matching . I have declared following data type data TestData = DataConst1 Int String | DataConst2 Int Int | DataConst3 Sting String | DataConst4 String Int and now i have function *"test"* which behaves similarly for (DataConst1 and DataConst2 ) and behaves similarly from DataConst3 and DataConst4. for ex test (DataConst1 x y) = x test (DataConst2 x y )= x test (DataConst3 x y) = y test(DataConst4 x y )= y no my problem here is , is there any way i can merge these 4 statements in two statements .... some way i can match DataConst1 and DataConst2 in the same line it would save me a lot of rewriting of code

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Call for papers: MULTICONF-10, USA, July 2010
by John Edward 05 Mar '10

05 Mar '10

It would be highly appreciated if you could share this announcement with your colleagues, students and individuals whose research is in computer science, computer engineering, information science and related areas. Call for papers: MULTICONF-10, USA, July 2010 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 Communications and Networks (CCN-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) MULTICONF-10 will be held at Imperial Swan Hotel and Suites. It is a full-service resort that puts you in the middle of the fun! Located 1/2 block south of the famed International Drive, the hotel is just minutes from great entertainment like Walt Disney World® Resort, Universal Studios and Sea World Orlando. Guests can enjoy free scheduled transportation to these theme parks, as well as spacious accommodations, outdoor pools and on-site dining — all situated on 10 tropically landscaped acres. Here, guests can experience a full-service resort with discount hotel pricing in Orlando. Please see the website http://www.PromoteResearch.org for more details. Sincerely John Edward

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Simple IO problem
by Ali Razavi 05 Mar '10

05 Mar '10

Why doesn't this work the way it's supposed to, or the way it's intuitively apparent from the code, that is, showing the prompt first, getting the line next, and printing the result finally? main = do putStr "Please Enter Your Name: " name <- getLine putStrLn ("Hello " ++ name) changing putStr with putStrLn rectifies it to the expected behavior, but I wonder why this version misbehaves. FWIW, I use ghc in cygwin. Ali

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Cabal modules and directories
by Rehno Lindeque 04 Mar '10

04 Mar '10

I'm sorry because this is a very elementary question but despite my attempts I can't seem to get cabal working for my project structure. I have two main source directories: src\haskell src\semanticdb Inside src\haskell I have main.hs with the module Main. Inside src\semanticdb I have semanticdb.hs with module OSIX.SemanticDB My cabal file contains the following 3 lines: main-is: main.hs hs-source-dirs: src/haskell src/semanticdb other-modules: OSIX.SemanticDB Then when I build with 'cabal build' I get the following error: cabal: can't find source for OSIX.SemanticDB in src/haskell, src/semanticdb, dist/build/autogen cabal --version gives: cabal-install version 0.6.4 using version 1.6.0.3 of the Cabal library I'd prefer to keep my directory structure as is if possible, but it doesn't seem to be working for me... Is there anything I can do or should I just restructure the directories? (NOTE: I'm using leksah as my IDE, so the cabal file is mostly generated automatically) Thanks, -Rehno

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Re: [Haskell-cafe] how to send a string to a tuple
by Magnus Therning 03 Mar '10

03 Mar '10

On 03/03/10 19:51, Pradeep Wickramanayake wrote: > Hi > > I have taken a string output from a file > > But I want to send that string to another function which will convert in > to a tuple > > Can someone teach me please May I suggest that you move this over to haskell-beginners instead? Also, include any code you have so far. You're more likely to get answers if you include the code you are having problems with. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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substitution
by John Moore 02 Mar '10

02 Mar '10

Hi, Can anyone explain what I'm doing wrong in the subtitution of the Let statement. If you look at test three. It wont substitute the second var "x". I enclose the file because this program is getting too big to copy and paste. John

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Using monad with arrow
by Jarno Antikainen 02 Mar '10

02 Mar '10

Hello, I have a function that returns its value in a monad, and I'd like to use that value in a function that returns an arrow, but I cannot figure out how to do that. The monad in this case is CouchMonad and the arrow is ArrowXml, but I guess that doesn't really matter. For example, the function that returns its value in a monad: toXNode :: String -> CouchMonad XNode and a function that returns an arrow: processReplacement :: (ArrowXml a) => String -> String -> a XmlTree XmlTree In processReplacement I'd like to get a value from toXNode function, take it out from the monad and use the plain XNode value. Were it all simply CouchMonad, it would be something like this: processReplacement elementId docId = do node <- toXNode docId setNode node `when` idMatches elementId But this is an arrow, so I need something different. I've read quite a few arrow articles and tutorials, but couldn't yet grasp how to do this and what is involved. Any ideas or pointers to examples? Thanks! BR, Jarno

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