February 2010 - Haskell-Cafe

online Real World Haskell - tool used?
by Sophie 05 Feb '10

05 Feb '10

Not a Haskell question, pardon me. I believe the online Real World Haskell book with commenting system was generated with an open-source tool (written by one of the authors?). Any idea where I might find this? [I think I had gotten the link long ago, lost it]. I am also curious what authoring tools were used to write the book. Thanks! Sophie

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could we get a Data instance for Data.Text.Text?
by Jeremy Shaw 05 Feb '10

05 Feb '10

Hello, Would it be possible to get a Data instance for Data.Text.Text? This would allow us to create a Serialize instance of Text for use with happstack -- which would be extremely useful. We (at seereason) are currently using this patch: http://src.seereason.com/haskell-text-debian/debian/patches/add_Data_instan… which basically adds: +textType = mkStringType "Data.Text" + +instance Data Text where + toConstr x = mkStringConstr textType (unpack x) + gunfold _k z c = case constrRep c of + (CharConstr x) -> z (pack [x]) + _ -> error "gunfold for Data.Text" + dataTypeOf _ = textType + This particular implementation avoids exposing the internals of the Data.Text type by casting it to a String in toConstr and gunfold. That is similar to how Data is implemented for some numeric types. However, the space usage of casting in Float to a Double is far less than casting a Text to a String, so maybe that is not a good idea? Alternatively, Data.ByteString just does 'deriving Data'. However, bytestring also exports Data.ByteString.Internal, wheres Data.Text.Internal is not exported. Any thoughts? I would like to get this handled upstream so that all happstack users can benefit from it. - jeremy

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Stack ADT?
by michael rice 05 Feb '10

05 Feb '10

Can't find a Stack datatype on Hoogle? Where should I look? Michael

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programmatic DB interface?
by Johannes Waldmann 05 Feb '10

05 Feb '10

Hello. What is currently the recommended higher level data base interface for Haskell? I want to construct queries in a programmatic/algebraic way, and I want typed queries and results. - I guess I want LINQ. The examples in http://book.realworldhaskell.org/read/using-databases.html seem to construct queries as strings. That's horrible! Looks exactly like PHP! Well, no, but it's a lower abstraction level than that I want to work at. I'm sure I'm just missing some package/framework here. Would takusen help? Where's its home page? this does not work: http://darcs.haskell.org/takusen/ Best, Johannes.

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multMM :: Matrix -> Matrix -> Matrix --multiplies two matrices question (Homework)
by 조광래 05 Feb '10

05 Feb '10

hi i was trying to solve it but All i got is type Matrix=[[Double]] multMM :: Matrix -> Matrix -> Matrix --multiplies two matrices multMM m t = [[sum (zipWith (*) (head m)(a)) ] ]where a = [head a | a<- t] Main> multMM [[2,1,-6],[1,-3,2]] [[1,0,-3],[0,4,20],[-2,1,1]] [[14.0]] from this i could get only the first multiplication 14 that is by 2*1 + 1*0 + -6*-2 = 14 how can i make this go throw all rows and colums to get multiple of matrixs? Thank you~

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Dutch HUG: meeting Monday 8th in Utrecht
by Tom Lokhorst 05 Feb '10

05 Feb '10

I'd like to invite functional programmers in The Netherlands to the Dutch Haskell User Group [1, 2] meeting next Monday at 19:00. Two talks are scheduled: - Sebastiaan Visser and Tom Lokhorst will give a talk entitled "AwesomePrelude, Liberating Haskell from data types!" - Martijn van Steenbergen will talk about serializing monadic computations. In particular, he will describe a monadic EDSL to describe compass-and-straightedge constructions and how to serialize and deserialize such values. The meeting will be in the Booth Hall [3] of the Utrecht University Library. There's free car parking [4] at the library, and it's easily accessible by bike and bus [5]. After the talks we will move to The Basket [6] for drinks. See you next Monday! - Tom Lokhorst [1]: http://www.haskell.org/haskellwiki/Dutch_HUG [2]: http://groups.google.com/group/dutch-hug [3]: http://www.uu.nl/EN/library/contact/university_library/zaalverhuur/Pages/de… [4]: http://www.uu.nl/EN/library/contact/university_library/Parkeren/Pages/defau… [5]: http://www.uu.nl/EN/library/contact/university_library/plattegrondenrou/Pag… [6]: http://utrecht.thebasket.nl/

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safe lazy IO or Iteratee?
by David Leimbach 05 Feb '10

05 Feb '10

Hi everyone, This is not an attempt to start a flame war. I'm just trying to get a good feel for the advantages and disadvantages of the newer safe lazy io lib available on Hackage vs using Iteratee. It does appear to me that using something like Itereatee gives a bit of room to really tweak the low level enumerator/iteratee relationship, and, if you asked my old boss, I'm just a big fan of folds too, I use them a lot in my Erlang :-), so Iteratee really does appeal to me on many levels. Yet at the same time, I'm quite enamored with the beauty of "interact" and functions of that sort. I realize mixing the effects of the lazy IO and pure code may not be the clearest way to write code for everyone, but there is something about being able to get linewise data as interact (unlines . fmap someLineWiseFunction . lines) that is just kind of cool. Dave

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Sparks created, but not used (par/pseq, ST monad)
by Michael Lesniak 05 Feb '10

05 Feb '10

Hello haskell-cafe, I currently have a problem with running par/pseq in the ST monad. The function testST is the minimal counterexample that works -- or, to be more clear, does not work as expected for me. As a remark, the tasks/function calls in my "real application" are much more computational expensive, but the code is too long to post here. > -- File ST.lhs > module Main where > import Control.Monad.ST > import Control.Parallel > main = testST > > testST :: IO () > testST = do > putStrLn "Starting" > (runST $ f 10) `pseq` putStrLn "Stopping" > where > f :: forall s. Int -> ST s () > f n = do > p $ "\nTask:" ++ show n > if n < 0 > then return () > else do > (n1,n2) <- return (n-1,n-2) > q n1 > q n2 > a <- f n1 > b <- f n2 > (a `par` b) `pseq` return a > > -- Some helper functions > p x = unsafeIOToST (putStrLn x) > q x = unsafeIOToST (print x) As far as I understand, compiling with ghc --make -O2 -threaded ST.lhs -o st -XRankNTypes and running with ./st +RTS -N -s should create sparks that could be run in parallel. The problem I have now, is that sparks are created but not converted, according to -s output: ... a lot of (uninteresting) stuff SPARKS: 232 (0 converted, 0 pruned) ... I have not found any information on this type of behaviour on the net and would be glad if someone could give me points or hints what's happening and how I can improve this. Cheers, Michael -- Dipl.-Inf. Michael C. Lesniak University of Kassel Programming Languages / Methodologies Research Group Department of Computer Science and Electrical Engineering Wilhelmshöher Allee 73 34121 Kassel Phone: +49-(0)561-804-6269

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Re: [Haskell-cafe] safe lazy IO or Iteratee?
by John Lato 04 Feb '10

04 Feb '10

Hello, > From: David Leimbach <leimy2k(a)gmail.com> > > Hi everyone, > > Yet at the same time, I'm quite enamored with the beauty of "interact" and > functions of that sort. I realize mixing the effects of the lazy IO and > pure code may not be the clearest way to write code for everyone, but there > is something about being able to get linewise data as > > interact (unlines . fmap someLineWiseFunction . lines) > > that is just kind of cool. I think so too. I really like the "interact" style of code; I find it supremely elegant. However I also find it insufficient for many interesting cases. One general problem is the difficulty of embedding monadic code into interact functions. Usually "interact" requires a pure function. Although in many cases I think it would be possible to have interactM :: (String -> IO String) -> IO () in practice these functions don't usually exist, perhaps because they can have some surprising behavior. In my opinion IO is a weakness of Haskell, and many Haskell users either subconciously or actively seek out alternative approaches. I offer as evidence the frequency with which beginners ask the cafe about "handle closed before data could be read" errors, and also the proliferation of IO libraries. I know which I approach I think represents the best way forward, but I would be surprised if there was a community consensus at this point. In any case, I'm glad there are a multitude of options and each probably has a place and purpose. Cheers, John Lato

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setNonBlockingFD?
by Magnus Therning 04 Feb '10

04 Feb '10

network-fancy fails to build because "Not in scope: 'setNonBlockingFD'". Any pointers to what should be used in 6.12? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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