October 2008 - Beginners

variables in haskell
by Matthew J. Williams 15 Oct '08

15 Oct '08

Hello listers, would one be correct in thinking that 'bound variables' such as those used in haskell were in fact constants? Sincerely Matthew Williams

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Can we specify the module of the function being referred
by Amit Kumar Dhar 14 Oct '08

14 Oct '08

Dear listers, I was writing a program which defines a function named "show". But at each of the recursive calls to itself "ghci" is showing the following message: ' Ambiguous occurrence `show' It could refer to either `show', defined at temp.hs:5:0 or `show', imported from Prelude ' I don't want to import the 'show' function from prelude. Is there any way out or I have to declare instance of the 'Show'? Can we specify the module of a function from which it will be referred? -- Amit kumar Dhar

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How to explain this behaviour of ghc ?
by sylvain 10 Oct '08

10 Oct '08

Hi, when loaded in ghci (version 6.8.2) the following code snippet data Test = A Int instance Show Test main = print (A 1) leads to: *Main> main *** Exception: stack overflow How to explain that ? Thanks, Sylvain

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Call for Contributions - Haskell Communities and Activities Report, November 2008 edition
by Janis Voigtlaender 10 Oct '08

10 Oct '08

Dear Haskellers, so much has happened in the Haskell world in the past months. Therefore, I would very much like to collect contributions for the 15th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 31 October 2008 (please send your contributions to hcar(a)haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in any project related to Haskell that has not previously been mentioned in the HC&A Report, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the May 2008 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The best format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2008/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2008/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should have received your old entry as a template already (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Do not worry about writing correct LaTeX, I will be able to handle your file. Please do not use HTML or even DOC. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or ``historic'' overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep entries up. There is no minimum length of an entry! The report aims at being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports over distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write ``wishlist'' items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Cryptography
by Jeffrey Drake 08 Oct '08

08 Oct '08

I have an application where I would like to use AES to have a public/private key pair and digitally sign/verify documents. I have found documentation for a library that does the encryption: http://www.haskell.org/crypto/doc/html/Codec-Encryption-AES.html However, it seems to have only two methods: encrypt :: AESKey a => a -> Word128 -> Word128 decrypt :: AESKey a => a -> Word128 -> Word128 A problem for me, I don't know where AESKey is supposed to come from, or how to use this to sign things. My ignorance of this topic does not help. Would it be correct to say that signing a document is similar to an MD5 hash on a document? In addition, this project requires a matching python component (that works together with the haskell). I am in a similar situation, I have found this: http://bitconjurer.org/rijndael.py that provides a similar functionality. But it seems to provide insights (and thus portability to haskell) on how to generate the public/private keys by providing a single key and performing a lot of math on it to generate the two pairs. But it still does not help me sign anything. Any help whatsoever on this topic is welcome. With regards, Jeffrey.

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Perfect numbers
by Matthew Williams 03 Oct '08

03 Oct '08

Hi Guys, I'm new to Haskell and I was wondering if you can help me: One of the first program's I tend to write when I'm looking at a new language is a program to generate a list of perfect numbers: --My First Perfect Number Generator factors :: Integer -> [Integer] factors x = [z | z <- [1..x-1], x `mod` z == 0] is_perfect :: Integer -> Bool is_perfect x = if sum(factors x) == x then True else False do_perfect :: [Integer] -> [Integer] do_perfect x = [z |z <- x, is_perfect z ] Then to run it: > do_perfect [1..9000] I'm using GHC to run it. My problem / question is this: It's running quite a lot slower than equivalent programs in erlang and python. I suspect it's down to the way I've written it. Any thoughts (or comments in general) Many thanks Matt ______________________________________________________________________ This email may contain privileged or confidential information, which should only be used for the purpose for which it was sent by Xyratex. No further rights or licenses are granted to use such information. If you are not the intended recipient of this message, please notify the sender by return and delete it. You may not use, copy, disclose or rely on the information contained in it. Internet email is susceptible to data corruption, interception and unauthorised amendment for which Xyratex does not accept liability. While we have taken reasonable precautions to ensure that this email is free of viruses, Xyratex does not accept liability for the presence of any computer viruses in this email, nor for any losses caused as a result of viruses. Xyratex Technology Limited (03134912), Registered in England & Wales, Registered Office, Langstone Road, Havant, Hampshire, PO9 1SA. The Xyratex group of companies also includes, Xyratex Ltd, registered in Bermuda, Xyratex International Inc, registered in California, Xyratex (Malaysia) Sdn Bhd registered in Malaysia, Xyratex Technology (Wuxi) Co Ltd registered in The People's Republic of China and Xyratex Japan Limited registered in Japan. ______________________________________________________________________

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