- Beginners - Haskell.org

haskell testing framework - missing htfpp
by Ovidiu D 11 Apr '13

11 Apr '13

I'm trying to use HTF by following this tutorial: http://hackage.haskell.org/packages/archive/HTF/0.9.0.0/doc/html/Test-Frame… and when I build the project I get the error: ghc: could not execute: htfpp I'm using: $ dpkg -l | grep haskell-platform ii haskell-platform 2012.2.0.0ubuntu1 all Standard Haskell libraries and tools What am I doing wrong? Thanks, ovidiu

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More than one way to skin a cat question - repeat
by Angus Comber 11 Apr '13

11 Apr '13

LYAH beginner on page 55. What I try to do is see the heading of a section, eg repeat function and then try to come up with my own version before looking at books implementation. Here is my implementation: repeat' :: a -> [a] repeat' x = [x] ++ repeat' x Here is books: repeat' :: a -> [a] repeat' x = x:repeat' x Mine appears to work. Is mine just as good? Are there problems with my way? Is books way more idiomatic Haskell? I want to learn the best approaches hence my question.

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doesFileExist cannot get ~/blah_blah right
by Hong Yang 10 Apr '13

10 Apr '13

Hi, I got "not exists ..." from fileExists <- doesFileExist "~/blah_blah" if fileExists then print "exists ..." else print "not exists ..." using the directory-1.1.0.2 package, when ~/blah_blah does exist. I do not want to change "~" to a specific home directory such as "/home/a_userid" for the sake of portability. What else can I do before the bug (suppose this is a bug) is fixed? Thanks, Hong

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Re: [Haskell-beginners] Problems with parsing in attoparsec
by Michel Kuhlmann 10 Apr '13

10 Apr '13

Hi Mukesh, thanks for the hint. I will try to combine `take 6` with `number`, to avoid `read . T.unpack`. I post, if that succeeds. Greetings, Michel

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Conduit composition
by Ovidiu D 10 Apr '13

10 Apr '13

Given the following works as expected (i.e. prints the value twice): main = Conduit.sourceList [1..14] $= Conduit.map show $= Conduit.iterM putStrLn $= Conduit.iterM putStrLn $$ Conduit.sinkNull I would expect the following to work as well: main = Conduit.sourceList [1..14] $= Conduit.map show $= display $$ Conduit.sinkNull display = Conduit.iterM putStrLn $= Conduit.iterM putStrLn ...but I get the compilation error: Couldn't match expected type `String' with actual type `()' Expected type: Conduit.Conduit String m0 a0 Actual type: Conduit.Source IO () In the second argument of `($=)', namely `display' In the first argument of `($$)', namely `Conduit.sourceList [1 .. 14] $= Conduit.map show $= display' I don't understand why the type of display is inferred to a Conduit.Source. Can somebody please explain? What I want is to have readable names for certain segments in my pipe. Is that possible? Thanks, ovidiu

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Problems with parsing in attoparsec
by Michel Kuhlmann 09 Apr '13

09 Apr '13

Hi, I couldn't figure out, why I'm failing with attoparsec. I have the follwing code (https://github.com/michelk/devsurf/tree/readNet) import Data.Attoparsec.Text import qualified Data.Text as T import Language.DevSurf.Readers.SpringNet import Prelude hiding (takeWhile, take) data IndexedFace = IndexedFaceTriangle Int Int Int | IndexedFaceQuad Int Int Int Int deriving (Show) -- | block of faces-lines faces :: Parser [[(Int, IndexedFace)]] faces = faceLine `sepBy` endOfLine -- | Line containing up to two indexed faces faceLine :: Parser [(Int, IndexedFace)] faceLine = face `sepBy` (take 6) -- | single indexed-face face :: Parser (Int, IndexedFace) face = do i <- index v1 <- index v2 <- index v3 <- index v4 <- index case v4 of 0 -> return (i, IndexedFaceTriangle v1 v2 v3) _ -> return (i, IndexedFaceQuad v1 v2 v3 v4) -- | node or face index 6 wide index :: Parser Int index = do i <- take 6 return . read . T.unpack $ i When testing, the follwing is working: testEleL2 = do let ele = T.pack " 1 10 1 9 0 2 1 2 9 0\n" r = parse faceLine ele case r of Partial _ -> print $ feed r (T.pack "") _ -> print r testEles = do let eles = T.pack $ concat [" 1 10 1 9 0 2 1 2 9 0\n" --," 3 11 9 2 0\n" ," 4 12 8 1 0\n" ] r = parse faces eles case r of Partial _ -> print $ feed r (T.pack "") _ -> print r testEles = do let eles = T.pack $ concat [" 1 10 1 9 0 2 1 2 9 0\n" ," 3 11 9 2 0\n" --," 4 12 8 1 0\n" ] r = parse faces eles case r of Partial _ -> print $ feed r (T.pack "") _ -> print r But this is not working testEles = do let eles = T.pack $ concat [" 1 10 1 9 0 2 1 2 9 0\n" ," 3 11 9 2 0\n" ," 4 12 8 1 0\n" ] r = parse faces eles case r of Partial _ -> print $ feed r (T.pack "") _ -> print r testEles = do let eles = T.pack $ concat [" 1 10 1 9 0 2 1 2 9 0\n" ," 3 11 9 2 0 4 12 8 1 0\n" ] r = parse faces eles case r of Partial _ -> print $ feed r (T.pack "") _ -> print r Please help. Thanks, Michel

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wx lists on holidays? DnD Question..
by luc taesch 08 Apr '13

08 Apr '13

It looks like I cannot push any messges to the wx lists for 28/4. is there only moderator , that would be out in holidays ? Your mail to 'wxhaskell-users' with the subject ... Is being held until the list moderator can review it for approval. Is ther only one moderator possible ? (Otherwise, I may volonteer, as I may need it for the next month ;-)- are there any other channel in the meantime ? any IRC ? I had a Drag and Drop question I documented here : http://stackoverflow.com/questions/15867654/wx-haskell-drag-and-drop-example if anyone could help..

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code review
by Chris Bolton 08 Apr '13

08 Apr '13

Just a little code review if anyone has the time :) http://codereview.stackexchange.com/questions/24741/simple-in-memory-db

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operations on cyclic data structures
by Josh Stratton 07 Apr '13

07 Apr '13

I'm new to haskell and functional programming in general. One of the things I've tried to wrap my brain around is dealing with cyclic immutable data structures like the half-edge data structure. First, how would an experienced haskeller design their data structures to perform an operation on this structure? I've implemented this structure with mutable structures in c++ but when when I've asked about cyclic structures I'm pointed to research papers which overwhelm my experience in immutable data structures like those in haskell. Also how efficient could this structure be if only a small portion of the mesh is updated? Would the entire mesh need to be rebuilt? Thanks.

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showing user defined data types
by Dan Lior 05 Apr '13

05 Apr '13

Hello, I defined a type Pixel : data Pixel = Black | White deriving (Eq, Show) and would like to "show" it as show :: Pixel -> String show Black = "#" show White = "." Apparently, ghc finds this ambiguous. That seems strange to me since Pixel is my own custom type and I don't imagine that ghc has another definition for showing it. Isn't this what "overloading" is supposed to enable? I can get around the problem by : import Prelude hiding(show) but that seems to drastic since it hides all the various overloaded incarnations of "show". I only need it to hide one of them (the one for Pixel). I'm obviously misunderstanding something. Is there a simple way to define my own show function for my own user defined data type? Thanks in advance. dan

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