July 2011 - Haskell-Cafe

Stack space overflow in HaskellNet
by Manfred Lotz 27 Jul '11

27 Jul '11

Hi there, If I take example imap.hs import System.IO import Network.HaskellNet.IMAP import Text.Mime import qualified Data.ByteString.Char8 as BS import Control.Monad -- the next lines were changed to fit to my local imap server imapServer = "imap.mail.org" user = "" pass = "" main = do con <- connectIMAP imapServer login con user pass mboxes <- list con mapM print mboxes select con "INBOX" msgs <- search con [ALLs] mapM_ (\x -> print x) (take 4 msgs) forM_ (take 4msgs) (\x -> fetch con x >>= print) and change the last line (in order to print all messages of the mailbox) into: forM_ msgs (\x -> fetch con x >>= print) I get Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize -RTS' to increase it. Is this something to be fixed in the HaskellNet code, or in the example code? -- Manfred

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Question re SMTPClient
by Jayakumar Ramanathan 27 Jul '11

27 Jul '11

How does one pass authentication info to the SMTP server when using the package STMPClient? Thanks! Jay

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Typeclasses, data, constructors
by Sergiy Nazarenko 27 Jul '11

27 Jul '11

Hi everyone! I have data declaration like this: data MyTable = MyTableOne Int String | MyTableTwo Int String String and function that insert new row in tables: newRow :: MyTable -> IO Int newRow (MyTableOne fld1 fld2 ) = ..... newRow (MyTableTwo fld1 fld2 fld3 ) = ...... That work perfectly, I enjoy it. But now I want to get rows, and I don't know what to do :( I want func something like this: getRow :: Int -> [MyTable] .... - what should I write here ? Possibly use this function: getRow 23 :: MyTableTwo (like haskell function Read for example: read "7" :: Int) I try to use typeclasses, class GetRow a where hasID :: Int -> IO a instance GetRow MyTableOne where hasID myid = return [(MyTableOne 1 "name")] instance GetRow MyTableTwo where hasID myid = return [(MyTableTwo 1 "name" "path")] But I have error: Not in scope: type constructor or class `MyTableOne' make: *** [Foo.so] Error 1 Not in scope: type constructor or class `MyTableTwo' make: *** [Foo.so] Error 1 I ask you to suggest me right way in my efforts Cheers!

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file splitter with enumerator package
by Eric Rasmussen 26 Jul '11

26 Jul '11

Hi everyone, A friend of mine recently asked if I knew of a utility to split a large file (4gb in his case) into arbitrarily-sized files on Windows. Although there are a number of file-splitting utilities, the catch was it couldn't break in the middle of a line. When the standard "why don't you use Linux?" response proved unhelpful, I took this as an opportunity to write my first program using the enumerator package. If anyone has time, I'm really interested in knowing if there's a better way to take the incoming stream and output it directly to a file. The basic steps I'm taking are: 1) Data.Enumerator.Binary.take -- grabs the user-specified number of bytes, then (because it returns a lazy ByteString) I use Data.ByteString.Lazy.hPut to output the chunk 2) Data.Enumerator.Binary.head -- after using take for the big chunk, it inspects and outputs individual characters and stops after it outputs the next newline character 3) I close the handle that steps 1&2 used to output the data and then repeat 1&2 with the next handle (an infinite lazy list of filepaths like part1.csv, part2.csv, and so on) The full code is pasted here: http://hpaste.org/49366, and while I'd like to get any other feedback on how to make it better, I want to note that I'm not planning to release this as a utility so I wouldn't want anyone to spend extra time performing a full code review. Thanks! Eric

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Library for Sparse Vectors?
by dokondr 26 Jul '11

26 Jul '11

Hi, Can't find on hackage any sparse vector library. Does such thing exist? I need efficient storage and dot product calculation for very sparse vectors with about 10 out of 40 000 non-zero components. One solution would be to represent Sparse Vector as Data.Map with (component_index, component_value) pairs to store non-zero components of the vector. In this case, for example, calculating cosine similarity ( http://en.wikipedia.org/wiki/Cosine_similarity) for for every pair of 10 000 vectors, would not be very nice and efficient, I am afraid. Thanks!

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Effects of Monads in Parallelisation
by Burak Ekici 26 Jul '11

26 Jul '11

Dear List, I am currently trying to understand the effects of monads in the sense of parallelisation in Haskell. Could somebody please explain the difference between 'rpar' and 'par'? I mean, what has been changed after the encapsulation of 'par' function by Eval monad? If you asked to compare the parallelisation via monads with non-monadic manner of it, what could you say? Many thanks in advance. All the best, Burak.

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Fwd: Re: How to select last inserted record from Table Using Database.HSQL.MySQL
by Steffen Schuldenzucker 26 Jul '11

26 Jul '11

Forwarding to list -------- Original Message -------- Subject: Re: [Haskell-cafe] How to select last inserted record from Table Using Database.HSQL.MySQL Date: Tue, 26 Jul 2011 14:27:56 +0300 From: Sergiy Nazarenko <nazarenko.sergiy(a)gmail.com> To: Steffen Schuldenzucker <sschuldenzucker(a)uni-bonn.de> Thanx a lot! I could solve that problem in this way: cmd = " INSERT INTO mytable (bar,foo) VALUES ("val1","val2"); SELECT LAST_INSERT_ID() as id;" lst <- query connection cmd >>= collectRows (\st -> getFieldValue st "id") lst has required value Cheers!

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Why the reluctance to introduce the Functor requirement on Monad?
by Arlen Cuss 26 Jul '11

26 Jul '11

Hi cafe! I feel a bit like I'm speaking out of turn for bringing this up -- and I'm sure it must have been brought up many times before -- but I hope there can be something fruitful had from a discussion. In my travels I've read several people with much better grasp of Haskell than I have mention -- with a sad sigh of resignation -- that functions like liftM and return abound because some Monads don't state their fulfillment of Functor (or Applicative, but that's even more recent), and thus we can't use fmap/<$> or pure. I understand a motivation might be that code would break if the former lot were removed, but surely they could shifted to the latter (and the former simply be defined as the latter). It might be a very large effort, I suppose, to comb through the standard libraries and make everything compile again, but is there something else I'm surely missing? Cheers, A

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ICFP 2011: Call for participation
by Wouter Swierstra 26 Jul '11

26 Jul '11

===================================================================== Call for Participation The 16th ACM SIGPLAN International Conference on Functional Programming (ICFP 2011) http://www.icfpconference.org/icfp2011/ Tokyo, Japan September 19-21, 2011 ===================================================================== ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. * Program: http://www.icfpconference.org/icfp2011/program.html * Registration link: https://regmaster3.com/2011conf/ICFP11/register.php * Local arrangements (including travel and accommodation): http://www.biglab.org/icfp11local/index.html Schedule including related events: September 18 Workshop on Generic Programming (WGP) Workshop on High-Level Parallel Programming and Applications (HLPP) Workshop on ML September 19-21 ICFP September 22 Commercial Users of Functional Programming – Day 1 (CUFP Tutorials) Haskell Symposium September 23 Commercial Users of Functional Programming – Day 2 (CUFP Tutorials) Erlang Workshop Haskell Implementors' Workshop September 24 Commercial Users of Functional Programming – Day 3 (CUFP Talks) Continuation Workshop Conference organizers: * General Co-Chairs: Manuel Chakravarty, University of New South Wales Zhenjiang Hu, National Institute of Informatics * Program Chair: Olivier Danvy, Aarhus University * Local Arrangements Chair: Soichiro Hidaka, National Institute of Informatics * Workshop Co-Chairs: Gabriele Keller, University of New South Wales Derek Dreyer, MPI-SWS * Programming Contest Chair: Eijiro Sumii, Tohoku University * Publicity Chair: Wouter Swierstra, Radboud Universiteit Nijmegen =====================================================================

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More QuickCheck Questions
by Mark Spezzano 25 Jul '11

25 Jul '11

Hi all, I still don't understand exactly how the QuickCheck "sized" function is meant to work, and what it's useful for. Please explain! Also, I think that class CoArbitrary can help to generate random *functions *(as opposed to values; but tell me if I'm wrong) but I don't see how this could be useful. Also what does shrink do? Example code of these would be tremendously helpful understanding these concepts. Thanks in advance, Mark Spezzano

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