- Beginners - Haskell.org

More Deserialization Woes
by Tom Hobbs 01 Jul '10

01 Jul '10

Hi guys, Thanks for all the previous help I'm having with serialization in Haskell, I couldn't have gotten as far as I have without the help from this group. Many thanks! Now I've put you in a good mood, maybe you can help me with my latest problem... :-) I have the following code which reads strings from a handle; import qualified Data.ByteString.Lazy.UTF8 as UTF readNames 0 _ = [] readNames n h = do length <- fmap (fromIntegral . runGet getWord32be) $ L.hGet h 4 name <- L.hGet h length (UTF.toString name) : readNames (n-1) h Where n is the number of names remaining to read and h is the handle to the stream that I'm reading from. It is my intention that this function would return a [String] of all the names. I'm sure further explainations probably aren't necessary, but just in case; - "length" becomes equal to some number (represented by four bytes) which describes the length of the string to read - "name" becomes the string I'm trying to read - Then I add that String to the list and go again But the problem I'm getting is this; Couldn't match expected type `[a]' against inferred type `IO b' In a stmt of a 'do' expression: length <- fmap (fromIntegral . runGet getWord32be) $ L.hGet h 4 In the expression: do { length <- fmap (fromIntegral . runGet getWord32be) $ L.hGet h 4; name <- L.hGet h length; (UTF.toString name) : readNames (n - 1) h } In the definition of `readNames': readNames n h = do { length <- fmap (fromIntegral . runGet getWord32be) $ L.hGet h 4; name <- L.hGet h length; (UTF.toString name) : readNames (n - 1) h } I (think I) understand what the message is saying, but not why it's occurring. I read the documentation as "UTF8.tostring" essentially rips the [Char] out of the ByteString and returns that, so where does the "IO b" come into it? Is it complaining because the reads from the handle are IO operations and it's expecting me to use the IO monad somewhere? What i want to be able to do is do the IO stuff inside the monad, but end up with a [String] which I can then use in a pure function. RWH and Learn You A Good Haskell all cover some stream reading, but their examples are to different from what I'm trying to do that I'm getting stuck. Any pointers are very gratefully accepted. Many thanks, Tom

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sqrt root issues
by Bryce Verdier 01 Jul '10

01 Jul '10

Hi all, I'm having a real hard time wrapping my head around a problem; Here is a basic, no frills, function. prime divisor divided | divided == 1 = True | divisor== divided= True | mod divided divisor== 0 = False | otherwise = next_prime where next_prime = prime (divisor + 2) divided and the calling function: is_prime input = prime 3 input I'm trying to get the square root of input as an Integer. In GHCI: :t (toInteger $ round $ sqrt 25) (toInteger $ round $ sqrt 25) :: Integer but if I change is_prime input = prime 3 (toInteger $ round $ sqrt input) I get this error: problem3.hs:19:33: No instance for (RealFrac Integer) arising from a use of `round' at problem3.hs:19:33-37 Possible fix: add an instance declaration for (RealFrac Integer) In the first argument of `($)', namely `round' In the second argument of `($)', namely `round $ sqrt input' In the expression: toInteger $ round $ sqrt input problem3.hs:19:41: No instance for (Floating Integer) arising from a use of `sqrt' at problem3.hs:19:41-50 Possible fix: add an instance declaration for (Floating Integer) In the second argument of `($)', namely `sqrt input' In the second argument of `($)', namely `round $ sqrt input' In the expression: toInteger $ round $ sqrt input Failed, modules loaded: none. Can someone please help me out and tell me what I'm missing? Thanks in advance, Bryce

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Renato Leal has invited you to Dropbox
by Dropbox 30 Jun '10

30 Jun '10

We're excited to let you know that Renato Leal has invited you to Dropbox! Renato Leal has been using Dropbox to sync and share files online and across computers, and thought you might want it too. Visit http://www.dropbox.com/link/20.R3oEQnNe2Y/NjIzNjAwNTgxNw to get started. - The Dropbox Team ____________________________________________________ To stop receiving invites from Dropbox, please go to http://www.dropbox.com/bl/e9a1876312ac/beginners%40haskell.org

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Working with System.Timeout in Windows
by Adolfo Builes 30 Jun '10

30 Jun '10

Hi Guys, I have been trying to do something with timeout of the System.Timeout but It hasn't work in Windows. When I first built, it didn't work, so I went to #haskell, and they told me that I should use -threaded, then after recompiling with -threaded it didn't work either. Does someone one what could probably be going on ? I'm using it with simpleHTTP of Network.HTTP, basically what I want to do is trying to get a source, but stop trying if I don't get an answer between a certain amount of time, something like: r <- timeout timeOutTime $ simpleHTTP req With curl for example I could use CURLOPT_TIMEOUT ( in Linux ), but I haven't found a way to say something like that using simpleHTTP. Cheers, Adolfo

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shortening code
by rizwan hudda 30 Jun '10

30 Jun '10

I have recently started learning haskell, and was trying to write a code for this problem http://www.spoj.pl/problems/KAMIL in haskell. The aim is to write a shortest possible code for the given task. I have previously solved this in c,c++,perl,python.My best was 57 characters using perl. So here's my code in HASKELL: main = interact $ unlines. map (show.foldl (\a x-> if null $ filter (==x) "TDLF" then a else a+a) 1) .lines It is 107 characters [ non white space and newline ]. I was interested in knowing how i could further optimize the size of this code. Thanks in advance. -- Thanks and regards Rizwan A Hudda http://sites.google.com/site/rizwanhudda

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Re: 答复: [Haskell-beginners] Cksum for StartProcess.exe
by Tim Perry 30 Jun '10

30 Jun '10

Thank you. I'm in the clear.... Sent from my i(Wish-It-Worked-As-A)Phone On Jun 30, 2010, at 1:08, 赵欣 <zhaoxin(a)7500.com.cn> wrote: StartProcess.exe size 8192Byte MD5 88d4a76b2917f29d734912719d99521c *StartProcess.exe -----邮件原件----- 发件人: beginners-bounces(a)haskell.org [mailto:beginners-bounces@haskell.org] 代表 Tim Perry 发送时间: 2010年6月30日 15:26 收件人: beginners(a)haskell.org 主题: [Haskell-beginners] Cksum for StartProcess.exe My virus scanner thinks StartProcess.exe is infected. I was hoping someone could post a valid cksum for it so I could compare. Thanks - Tim Ps: "Haskell Platform\2010.1.0.0\winghci\StartProcess.exe" Sent from my i(Wish-It-Worked-As-A)Phone _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Cksum for StartProcess.exe
by Tim Perry 30 Jun '10

30 Jun '10

My virus scanner thinks StartProcess.exe is infected. I was hoping someone could post a valid cksum for it so I could compare. Thanks - Tim Ps: "Haskell Platform\2010.1.0.0\winghci\StartProcess.exe" Sent from my i(Wish-It-Worked-As-A)Phone

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Functional Programming in Finance
by Syed Imran 29 Jun '10

29 Jun '10

Hi, I hope you are well. I am a Head Hunter in the City of London, I have recently joined the Haskell-Cafe forum. I am currently looking for someone with a strong computer science or mathematical background to work with some of the leading financial institutions in London and the USA. Functional Programming is seeing a significant amount of investment with a few of our clients, who are using Haskell as well as F# (and some also use OCaml). I appreciate you may not be actively looking to work in the investment banking arena at present however I would be keen to touch base with you should you be interest now or in the future. Please could you let me know your current situation and also forward me a copy of your CV and contact number so as I can call. Look forward to hearing from you. Kindest Regards Syed Imran Consultant Carrington Fox UK Ltd. 11 Tokenhouse Yard London EC2R 7AS Tel: +44 207 073 2626 Fax: +44 207 073 2627 s.imran(a)carringtonfox.com<mailto:s.imran@carringtonfox.com> www.carringtonfox.com<http://www.carringtonfox.com> ________________________________ Carrington Fox UK Ltd. 11 Tokenhouse Yard London EC2R 7AS Tel: +44 207 073 2626 Fax: +44 207 073 2627 www.carringtonfox.com Privileged/Confidential Information may be contained in this message. If you are not the addressee indicated in this message (or responsible for delivery of the message to such person), you may not copy or deliver this message to anyone. Please advise immediately if you or your employer do not consent to Internet email for messages of this kind. Opinions, conclusions and other information in this message that do not relate to the official business of my firm shall be understood as neither given nor endorsed by it. Carrington Fox reserves the right to share information between companies within our group. Carrington Fox UK Limited. Registered in England: 04373672 Registered Office: 11 Tokenhouse Yard, London, EC2R 7AS. VAT Number: 798-4652-61 Terms and Conditions of Business available on our web-site.

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docstring equivalent
by prad 29 Jun '10

29 Jun '10

in python you can get info about a function or class because they are docstringed. is there some equivalent mechanism for haskell usable in ghci? -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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Hint on exercise from Real World Haskell
by Chris Saunders 29 Jun '10

29 Jun '10

I have been working on the exercises from chapter 1 of Real World Haskell. I seem to be stuck on a solution to: 4.Modify the WC.hs example again, to print the number of characters in a file. I'm hoping to get a hint for this one. I may request a solution later but for now I would prefer just a hint. Regards Chris Saunders

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