September 2009 - Beginners

Circular Linked Lists
by Matthew J. Williams 04 Feb '10

04 Feb '10

Dear All How would one mimic, in Haskell, a C++ circular linked list i.e., where the last element precedes (points to) the first? Sincerely Matthew J. Williams

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edit-compile-test loop
by Tom Doris 19 Oct '09

19 Oct '09

Hi I'd like to know what the typical edit-compile-test loop looks like with the Haskell platform; that is, in C++ this would be edit, run make to compile everything in the project into libraries and executables, then run an executable test suite. I'm confused as to how people work on larger projects in Haskell - do you work on a single module and load it into ghci to test as you develop, then compile the entire package and run a test suite? Or do you generally only use ghci for prototyping and not when in the middle of proper development? Or do you compile the package and load that into ghci? I'd like to know as I'm starting to work on patches for some hackage packages which have proper cabal builds etc., and want to follow the correct (and efficient!) convention. Thanks Tom

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map question
by Joost Kremers 18 Oct '09

18 Oct '09

Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude> map (+ 1) [1,2,3,4] [2,3,4,5] Prelude> map (* 2) [1,2,3,4] [2,4,6,8] Prelude> map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude> map (2 /) [1,2,3,4] [2.0,1.0,0.6666666666666666,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude> map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany

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$= operator
by Nathan Holden 03 Oct '09

03 Oct '09

I was reading Eleara examples out of Hackage, and I kept noticing lines like this: > t <- get GLFW.time > GLFW.time $= 0 Most of the rest of it made more-or-less sense (I mean, there IS a reason I'm still asking questions to beginners!) but I couldn't seem to find anything about it on my usual reference sources, and it's uses don't make that much sense to me.

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subtle dissimilarity between fromInteger and fromIntegral
by Hong Yang 30 Sep '09

30 Sep '09

Hi, Can someone explain the subtle dissimilarity between fromInteger and fromIntegral? Thanks, Hong

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Re: [Haskell-beginners] remove XML tags using Text.Regex.Posix
by Jan Jakubuv 30 Sep '09

30 Sep '09

Hi Robert, On Tue, Sep 29, 2009 at 12:25:07PM -0700, Robert Ziemba wrote: > I have been working with the regular expression package (Text.Regex.Posix). > My hope was to find a simple way to remove a pair of XML tags from a short > string. > > I have something like this "<tag>Data</tag>" and would like to extract > 'Data'. There is only one tag pair, no nesting, and I know exactly what the > tag is. > This is so simple that I would not recommend anything other than regular expressions. Use the following pattern: pat = "<tag>(.*)</tag>" It creates a group withing the matched string containing the data (it is done using parenthesis). Use `[[String]]` as a result type and you receive a list of matches where each match is described by a list of strings whose first member is the whole matched string (including <tag> and </tag>) and it is followed by values of groups (in our case we have just one group). Thus: *Main> "text<tag>data</tag>text" =~ pat :: [[String]] [["<tag>data</tag>","data"]] It is easy extract the data using `(!!)` and `head`: *Main> (!! 1) . head $ ("text<tag>7</tag>text" =~ pat :: [[String]]) "7" > My first attempt was this: > > "<tag>123</tag>" =~ "[^<tag>].+[^</tag>]"::String > > result: "123" > The problem with your pattern is that `[^<tag>]` doesn't mean what you think it does. Its meaning is “one character which is not `<`, `t`, `a`, or `>`” as Patrick already described in his mail. > Upon further experimenting I realized that it only works with more than 2 > digits in 'Data'. I occured to me that my thinking on how this regular > expression works was not correct - but I don't understand why it works at > all for 3 or more digits. > It doesn't work for all 3 or more digits: *Main> "<tag>tag</tag>" =~ "[^<tag>].+[^</tag>]" :: String "" Briefly, it doesn't work when the data contains one of characters `<`, `t`, `a`, `g`, `>`. Finally, consider using pat = "<tag>([^<]*)</tag>" which works with more tags in the same line as well. Sincerely, jan. -- Heriot-Watt University is a Scottish charity registered under charity number SC000278.

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remove XML tags using Text.Regex.Posix
by Robert Ziemba 30 Sep '09

30 Sep '09

I have been working with the regular expression package (Text.Regex.Posix). My hope was to find a simple way to remove a pair of XML tags from a short string. I have something like this "<tag>Data</tag>" and would like to extract 'Data'. There is only one tag pair, no nesting, and I know exactly what the tag is. My first attempt was this: "<tag>123</tag>" =~ "[^<tag>].+[^</tag>]"::String result: "123" Upon further experimenting I realized that it only works with more than 2 digits in 'Data'. I occured to me that my thinking on how this regular expression works was not correct - but I don't understand why it works at all for 3 or more digits. Can anyone help me understand this result and perhaps suggest another strategy? Thank you.

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Can you identify this recursion pattern please?
by Colin Paul Adams 29 Sep '09

29 Sep '09

This looks like some kind of unfold to me. Can you help? I have the following code that works for the first two levels: -- get the list of all (recursive) child gallery names -- first, just the immediate children childNames <- childGalleries db (name gallery) -- now let's try the next level grandchildren <- mapM (childGalleries db) childNames mapM putStrLn (childNames ++ concat grandchildren) The database query childGalleries gives me a list of all the immediate children. So what I want is recursively, the names of all the descendants, as well as the original name, concatenated to a flat list. I'm struggling to identify the higher-order recursion function I need. -- Colin Adams Preston Lancashire

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Distributing executables
by Joe Van Dyk 29 Sep '09

29 Sep '09

How hard is it to distribute a haskell binary that contains a bunch of libraries embedded in the executable? Is it pretty easy to make a large statically-linked executable? -- Joe Van Dyk http://fixieconsulting.com

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Finance.Quote.Yahoo
by Hong Yang 28 Sep '09

28 Sep '09

Hi All, I was using a modified version of Finance.Quote.Yahoo module (I only added "setproxy"), and got unwanted print on the screen. My questions are as follows: 1) How can I get rid of this screen print? (I guess HTTP package called by the Finance.Quote.Yahoo module internally prints those info. Should I temporarily turn off STDOUT then turn it on? How to do this?) 2) "ghc-pkg list" shows HTTP-4000.0.8. How come the output shows "HTTP-4000.0.5"? Thanks, Hong P.S. The following is the screen output: Sending: GET http://download.finance.yahoo.com/d/quotes.csv?s=BAX&f=l1 HTTP/1.1 Content-Length: 0 User-Agent: hs-HTTP-4000.0.5 Host: download.finance.yahoo.com proxy uri host: xxxproxy, port: :74 Creating new connection to xxxproxy:74 Received: HTTP/1.1 200 OK Proxy-Connection: Keep-Alive Connection: Keep-Alive Transfer-Encoding: chunked Via: 1.1 SAUSISA02 Date: Mon, 28 Sep 2009 18:49:43 GMT Content-Type: application/octet-stream P3P: policyref="http://p3p.yahoo.com/w3c/p3p.xml", CP="CAO DSP COR CUR ADM DEV TAI PSA PSD IVAi IVDi CONi TELo OTPi OUR DELi SAMi OTRi UNRi PUBi IND PHY ONL UNI PUR FIN COM NAV INT DEM CNT STA POL HEA PRE GOV" Cache-Control: private Content-Length: 7

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