June 2010 - Beginners

Learning about channels
by Benjamin Edwards 17 Jul '10

17 Jul '10

NB: This was posted in fa.haskell first, I guess it was the wrong forum for this kind of question as it was left unanswered :) Hi, I'm having a few issues getting some toy programs to work whilst I try to get a better understanding of how to model processes and channels. I am just trying to use the real base blocks and failing miserably. Here is an example (yes this is utterly contrived and sill, but I lack imagination... sue me): I want my main thread to do the following: 1. make a channel 2. spawn a thread (producer) that will write a series of lists of integers to the the channel, then exit. 3. spawn another thread that will read from the channel and sum all of the input. It should exit when both the channel is empty and and the producer thread has finished writing to it. 4. Main thread should print the sum. My current code should uses a trick I have seen else where which is to have the result of "task" running in the thread put into an MVar. So my condition for the reading thread exiting is to check if the MVar of the producer thread is not empty and if the channel is empty. If those two things are true, exit the thread. Unfortunately if somehow seems able to to get to a stage where the produce thread has finished and the channel is empty, but is blocking on a read. I have the following code, but it always blocks indefinitely on a read. I am sure there is something obviously deficient with it, but I can't work out what it is. Any help would be greatly appreciated. Of course, if I'm doing it all wrong, please tell me that too :) module Main where import Control.Concurrent import Control.Concurrent.STM import Control.Monad (forever) import Data.Map as M main :: IO () main = do oc <- newChan counter <- newTVarIO (0 :: Integer) p <- forkJoin $ produce oc [1..1000] c <- forkJoin $ loop oc p counter takeMVar c >>= print produce :: Chan [Integer] -> [Integer] -> IO () produce ch [] = return () produce ch xs = do let (hs,ts) = splitAt 100 xs writeChan ch hs produce ch ts loop :: Chan [Integer] -> MVar () -> TVar Integer -> IO Integer loop ch p n = do f <- isEmptyMVar p e <- isEmptyChan ch if e && (not f) then atomically (readTVar n) else do xs <- readChan ch atomically $ do x <- readTVar n writeTVar n (x + sum xs) loop ch p n forkJoin :: IO a -> IO (MVar a) forkJoin task = do mv <- newEmptyMVar forkIO (task >>= putMVar mv) return mv

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problem with System.Directory.Tree
by Anand Mitra 06 Jul '10

06 Jul '10

Hello All, I want to build a program which will recursively scan a directory and build md5sum for all the files. The intent is to do something similar to unison but more specific to my requirements. I am having trouble in the initial part of building the md5sums. I did some digging around and found that "System.Directory.Tree" is a very close match for what I want to do. In fact after a little poking around I could do exactly what I wanted. ,---- | import Monad | import System.Directory.Tree | import System.Directory | import Data.Digest.Pure.MD5 | import qualified Data.ByteString.Lazy.Char8 as L | | calcMD5 = | readDirectoryWith (\x-> liftM md5 (L.readFile x)) `---- This work perfectly for small directories. readDirectoryWith is already defined in the library and exactly what we want ,---- | *Main> calcMD5 "/home/mitra/Desktop/" | | "/home/mitra" :/ Dir {name = "Desktop", contents = [File {name = | "060_LocalMirror_Workflow.t.10.2.62.9.log", file = | f687ad04bc64674134e55c9d2a06902a},File {name = "cmd_run", file = | 6f334f302b5c0d2028adeff81bf2a0d9},File {name = "cmd_run~", `---- However when ever I give it something more challenging it gets into trouble. ,---- | *Main> calcMD5 "/home/mitra/laptop/" | *** Exception: /home/mitra/laptop/ell/calc-2.02f/calc.info-27: | openFile: resource exhausted (Too many open files) | *Main> 29~ `---- If I understand what is happening it seems to be doing all the opens before consuming them via md5. This works fine for small directories but for any practical setup this could potentially be very large. I tried forcing the md5 evaluation in the hope that the file descriptor will be freed once the entire file is read. That did not help, either because I could not get it right or there is some more subtle I am missing. I also had a look at the code in module "System.Directory.Tree" and although it gave me some understanding of how it works I am no closer to a solution. regards -- Anand Mitra

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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 30 Jun '10

30 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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