June 2009 - Haskell-Cafe

Problem with Data.Map
by michael rice 08 Jun '09

08 Jun '09

I'm trying to understand Map type for possible use in another problem I'm working on, but am stymied right off the bat. ==========Here's my source: import Data.Map (Map) import qualified Data.Map as Map l1 = "abc" l2 = [1,2,3] ==========Here's my error: Prelude> :l maptest [1 of 1] Compiling Main ( maptest.hs, interpreted ) Ok, modules loaded: Main. *Main> l1 Loading package syb ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.0 ... linking ... done. "abc" *Main> l2 [1,2,3] *Main> zip l1 l2 [('a',1),('b',2),('c',3)] *Main> fromList $ zip l1 l2 <interactive>:1:0: Not in scope: `fromList' *Main> =========== Why isn't this working? Michael

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purely functional lazy non-deterministic programming
by Sebastian Fischer 08 Jun '09

08 Jun '09

[crosspost from Haskell-libraries and Curry mailing list] Dear Haskell and Curry programmers, there is now a Haskell library that supports lazy functional-logic programming in Haskell. It is available from http://sebfisch.github.com/explicit-sharing and can be obtained from Hackage using cabal-install. The project page links to tutorials that explain how to use the library and to an ICFP'09 paper (joint work with Oleg Kiselyov and Chung-chieh Shan) that explains the implemented ideas in depth. Have fun! Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

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ANN: alloy-1.0.0 (generic programming)
by Neil Brown 08 Jun '09

08 Jun '09

Hi all, I've just put the first release of the Alloy generic programming library on Hackage [1]. Alloy (née Polyplate) is intended to be a fairly fast blend of several other generics approaches, such as SYB (but without the dynamic typing) and Uniplate (but allowing an arbitrary number of target types), for performing transformations on specific types in large tree structures. There is a tutorial explaining how to use the library [2], the API documentation should be live on Hackage shortly [1], and there is also a recent draft of a paper with benchmarks and details of the inner workings for those who are interested [3]. [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/alloy [2] http://twistedsquare.com/Alloy-Tutorial.pdf [3] http://twistedsquare.com/Alloy.pdf Thanks, Neil.

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Maintaining laziness: another example
by Martijn van Steenbergen 08 Jun '09

08 Jun '09

Hello, While grading a Haskell student's work I ran into an example of a program not being lazy enough. Since it's such a basic and nice example I thought I'd share it with you: One part of the assignment was to define append :: [a] -> [a] -> [a], another to define cycle2 :: [a] -> [a]. This was her (the majority of the students in this course is female!) solution: > append :: [a] -> [a] -> [a] > append [] ys = ys > append xs [] = xs > append (x:xs) ys = x : (append xs ys) > > cycle2 :: [a] -> [a] > cycle2 [] = error "empty list" > cycle2 xs = append xs (cycle2 xs) This definition of append works fine on any non-bottom input (empty, finite, infinite), but due to the unnecessary second append case, cycle2 never produces any result. Martijn.

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Re: [Haskell] ANNOUNCE: testrunner-0.9
by Max Bolingbroke 08 Jun '09

08 Jun '09

Your feature list sounds like an almost exact duplicate of that for my test-framework package, which has been available on Hackage for months (although it's almost totally unadvertised!): https://github.com/batterseapower/test-framework/tree http://hackage.haskell.org/cgi-bin/hackage-scripts/package/test-framework There is also John's testpack: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/testpack Seems like we need to advertise these better :-) Cheers, Max 2009/6/8 Reinier Lamers <tux_rocker(a)reinier.de>: > Dear all, > > testrunner is a new framework to run unit tests. It has the following > distinguishing features: > > * It can run unit tests in parallel. > * It can run QuickCheck and HUnit tests as well as simple boolean expressions. > * It comes with a ready-made main function for your unit test executable. > * This main function recognizes command-line arguments to select tests by name > and replay QuickCheck tests. > > testrunner was conceived as a part of the darcs project. > > The home page is http://projects.haskell.org/testrunner/. > > testrunner can be downloaded from > http://projects.haskell.org/testrunner/releases/testrunner-0.9.tar.gz, or with > darcs with a "darcs get http://code.haskell.org/testrunner/". > > Regards, > Reinier Lamers (tux_rocker) > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > >

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Hsmagick crash
by Ron de Bruijn 08 Jun '09

08 Jun '09

Hi, I am trying to extract the image data from various file formats and it appeared that hsmagick would be the right package to use. However, it doesn't actually work or I use it incorrectly. If you have installed hsmagick and change the value of some_png_file to some existing png file, you should see that it crashes at some random pixel. For the particular 256*256 image I had, it crashed on pixel_nr `elem` [54,56,57]. I am open to suggestions for better ways to get a Array (Int,Int) RGB from e.g. a png file. import Graphics.Transform.Magick.Images import Graphics.Transform.Magick.Types import Foreign.Storable import Control.Monad image_file_name_to_2d_array file = do himage <- readImage file let ptr_to_image = image himage himage_ <- peekElemOff ptr_to_image 0 let bounds@(_rows, _cols) = (rows himage_,columns himage_) number_of_pixels = fromIntegral _rows * fromIntegral _cols mapM (\pixel_nr -> do putStrLn ("Pixel: " ++ show pixel_nr) pixel_packet <- liftM background_color_ $ peekElemOff ptr_to_image pixel_nr let red_component = red pixel_packet putStrLn ("Pixel packet: " ++ show red_component) return red_component) [0.. number_of_pixels - 1] some_png_file = "foo.png" t = do initialize_image_library image_file_name_to_2d_array some_png_file initialize_image_library = initializeMagick Best regards, Ron de Bruijn

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Monad transformer responsibilities
by Martijn van Steenbergen 08 Jun '09

08 Jun '09

Hello, Suppose I have two projects: 1) one that defines a monad transformer and an accompanying type class that captures my monad-specific operations and 2) one that uses the other project, combining the monad transformer with, say, Parsec. Now while writing my Parsec parser I want to use my monad transformer operations without using lift: I need an instance MyMonadT Parsec. Where should this instance go? I can think of three answers, all unsatisfactory: 1) For obvious reasons it shouldn't go in the Parsec package. 2) For pretty much the same reasons it shouldn't go in my monad transformer package, either. Also, it is undesirable to add a dependency on Parsec just for this instance, and the package should not have to know about the projects that are going to use it. 3) If I put it in the second project it is an orphan instance, which is undesirable for well-known reasons. What is the best solution? Thank you, Martijn.

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Cabal "addressibility" problem
by Vasili I. Galchin 08 Jun '09

08 Jun '09

Hello, The following is a fragment in my cabal file: Executable GraphPartitionTest Main-Is: Swish.HaskellRDF.GraphPartitionTest.hs Other-modules: Swish.HaskellRDF.GraphPartition Swish.HaskellRDF.GraphClass Swish.HaskellUtils.ListHelpers Swish.HaskellUtils.TestHelpers When I try to do a build I get: Setup: can't find source for Swish in . Cabal/Setup seem to be looking in the current directory for source but as you can see I gave "full" paths, i.e. Swish.HaskellRDF* or Swish.HaskellUtils*. What I doing wrong? Regards, Vasili

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Haddock : parse error on input `{-# UNPACK'
by Erik de Castro Lopo 08 Jun '09

08 Jun '09

Hi all, I'm trying to compile the binary-strict module from hackage and I'm getting the exact same error as in the hackage build log: http://hackage.haskell.org/packages/archive/binary-strict/0.4.2/logs/failur… src/Data/Binary/Strict/IncrementalGet.hs:106:11: parse error on input `{-# UNPACK' Is this a bug? Is there any way to work around it? TIA, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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How to convert DiffTime to a number?
by Magicloud Magiclouds 08 Jun '09

08 Jun '09

Hi, Documents said that DiffTime could treat as "how many seconds". So how to convert it to a number of seconds? Thanks. -- 竹密岂妨流水过山高哪阻野云飞

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