March 2008 - Haskell-Cafe

separate input calculation output
by Thomas Engel 25 Mar '08

25 Mar '08

Hello, I'm new to haskell and want to write a small program for some winder calcucations. I want to input 10-15 values and based on this input doing some calculations. After this calculations I want to print out the results. My problem is: How can I get the input values into the calculations and back the result to the output. In an other language I would use global variables for this. So what should I use in haskell? An other point is that I want to separate the input from the calculations and the output. How can I achieve this with haskell? Modules? Data types? See some pseudo code below. inputvalues :: IO() inputvalues = do hSetBuffering stdout NoBuffering putStr "Berechnung Bremsgenerator \n\n" ve <- readNum "Bitte Maschinengeschwindigkeit (m/min) angeben: " de <- readNum "Bitte Durchmesser der vollen Rolle eingeben (mm): " dewalze <- readNum "Bitte Durchmesser der Walze eingeben (mm): " dewand <- readNum "Bitte Wandstaerke der Walze eingeben (mm): " and so on .............. readNum :: String -> IO Float readNum text = do putStrLn text readLn -- calculations nvr :: Float-> Float-> Float nvr ve de = ve / de /pi *1000 pzugmin :: Float-> Float-> Float->Float pzugmin ve femin be = 1.666 * ve * femin * be * 0.00000001 pzugmax :: Float-> Float-> Float->Float pzugmax ve femax be = 1.666 * ve * femax * be * 0.00000001 and so on .............. outputvalues = do putStr "\n\nErgebnisse Bremsgenerator \n\n" putStr "Drehzahl leere Trommel: " putStr(show(round(nvr))) putStr " U/min \n" putStr "Drehzahl volle Trommel: " putStr(show(round(drehbgmax))) putStr " U/min \n" and so on ...................... Thanks Thomas

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getChar
by Cetin Sert 25 Mar '08

25 Mar '08

Hi, is there a version of getChar that doesn't buffer keyboard input until enter is pressed? specialReadln :: IO String specialReadln = do c ← getChar if c == '#' then do return [] else do cs ← specialReadln return (c:cs) I want the input process to terminate when '#' or any other specific key has been pressed. Best Regards, Cetin Sert

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Problem with OpenAL
by George Giorgidze 25 Mar '08

25 Mar '08

I tried OpenAL binding (the one which is on the Hackage), but with no luck. I can not hear anything from speakers and also according to generated output on console it seems that "AL.play" never completes playback of a buffer as buffer remains registered as "unprocessed" in OpenAL context. Here is the piece of code. I am not getting any error messages from OpenAL library functions. playOpenAL :: Int -> IO () playOpenAL sr = do mDevice <- AL.openDevice Nothing when (isNothing mDevice) $ error "opening OpenAL device" let device = fromJust mDevice mContext <- AL.createContext device [ AL.Frequency (fromIntegral sr) , AL.Refresh (fromIntegral sr) ] when (isNothing mContext) $ error "creating OpenAL context" let context = fromJust mContext AL.currentContext AL.$= (Just context) let sampleNumber = 256 bufSize = sampleNumber * (sizeOf (undefined :: CShort)) buf2 <- mallocBytes bufSize -- here I am filling buf2 with noise .... [source] <- AL.genObjectNames 1 [buffer] <- AL.genObjectNames 1 let region = AL.MemoryRegion buf2 (fromIntegral bufSize) AL.bufferData buffer AL.$= (AL.BufferData region AL.Mono16 (fromIntegral sr)) AL.queueBuffers source [buffer] AL.loopingMode source AL.$= AL.OneShot let waitForSource = do putStrLn "waiting" s <- AL.get (AL.buffersProcessed source) putStrLn $ show s s <- AL.get (AL.buffersQueued source) putStrLn $ show s state <- AL.get (AL.sourceState source) case state of AL.Playing -> do threadDelay 1024 waitForSource _ -> return () putStrLn "Start Playing ... " AL.play [source] waitForSource AL.currentContext AL.$= Nothing AL.destroyContext context b <- AL.closeDevice device when (not b) $ error "closing device" Is this library still maintained? Best, George

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How to program with sqlite?
by Deng Chao 25 Mar '08

25 Mar '08

Hi all, I'm learning sqlite, and as I know haskell has some libraries like HDBC or HSQL can access sqlite DB. Can anybody give me a small example to show how to use it? It will be very appreciate? Thanks! Best Regards, Deng Chao

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Random Monad
by Matthew Pocock 24 Mar '08

24 Mar '08

Hi, Who currently maintains the Random monad code? I have some patches to contribute. Matthew

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deconstruction of the list/backtracking applicative functor?
by Conal Elliott 24 Mar '08

24 Mar '08

Is there a known deconstruction of the list/backtracking applicative functor (AF)? If I decompose the list type into pieces (Maybe, product, composition), I think I can see where the ZipList AF comes from, but not the list/backtracking AF. Is there some construction simpler than lists (non-recursive) that introduces cross products? Thanks, - Conal

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ghc warn-unused-imports
by Evan Laforge 24 Mar '08

24 Mar '08

So it appears that GHC will warn about "unused" imports when you import qualified if you could have gotten the symbol from somewhere else. For instance, if you write: import qualified Control.Monad.Trans as Trans import qualified Control.Monad.Writer as Writer and use "Trans.lift", it will complain because you could have used "Writer.lift". Then if you go import State, it will want you to use State.liftIO instead of Trans.liftIO (Writer doesn't have liftIO). Now, one question is why these modules are exporting random stuff from Trans, but in general the practice of warning when you use qualified imports seems not only misleading (since if you comment out the import your code breaks, and it's not obvious where it wanted you to get the symbol), but brittle since a messy export list on some module you import can suddenly trigger a flood of warnings. So I'd suggest that if you say "import qualified M" and then use "M.something", then "M is unused" msgs shouldn't appear. warn-unused-imports is genuinely useful most of the time, so I don't like to put on -fno-warn-unused-imports. Is there a better way to suppress the "false positives" here?

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BUG: genObjectNames dies on Win32
by Jefferson Heard 24 Mar '08

24 Mar '08

Could this get forwarded on to another more appropriate maling list? Confirmed on GHC and GHCi 6.6 and 6.8, Graphics.Rendering.OpenGL.GL.genObjectNames n is dying if I ask it to return an IO :: [DisplayList] For an example, just open GHCI and change context to Graphics.Rendering.OpenGL.GL and do genObjectNames 4 :: IO [DisplayList] GHCi will merely die. Compiled with GHC, I'm simply running out of memory. It eats RAM indefinitely. -- Jeff

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HXT and types in Control.Arrow.ArrowTree
by Robert Vollmert 24 Mar '08

24 Mar '08

Hello, I'm having some trouble with HXT, the types of some functions in particular. This may well be caused by a lack of understanding for arrow programming -- I'd appreciate any hints. In short, I'm constantly running into what appear to be artificial type restrictions in Control.Arrow.ArrowTree. For example, the signature of "deep" is deep :: (Tree t, ArrowTree a) => a (t b) (t b) -> a (t b) (t b) instead of the more general deep :: (Tree t, ArrowTree a) => a (t b) c -> a (t b) c Say I have an arrow getLink :: (ArrowXml a) => a XmlTree Link that converts an HTML link (<a href="url">text</a>) into some appropriate data type (code below). To collect all links in a document, I'd like to use deep getLink instead of deep (hasName "a") >>> getLink Is this the wrong approach to converting XML into Haskell data types? Cheers Robert module Links where import Text.XML.HXT.Arrow data Link = Link { url, text :: String } deriving Show getLink :: (ArrowXml a) => a XmlTree Link getLink = isElem >>> hasName "a" >>> (getAttrValue0 "href" &&& getAllText) >>> arr (uncurry Link) getAllText :: (ArrowXml a) => a XmlTree String getAllText = listA (deep isText >>> getText) >>> arr concat

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more on FFI build error
by Galchin Vasili 24 Mar '08

24 Mar '08

line #102 ... allocaBytes (#const sizeof(struct mq_attr)) $ \ p_attrs -> do definition of struct mq_attr on Linux ... struct mq_attr { long int mq_flags; /* Message queue flags. */ long int mq_maxmsg; /* Maximum number of messages. */ long int mq_msgsize; /* Maximum message size. */ long int mq_curmsgs; /* Number of messages currently queued. */ long int __pad[4]; }; build errors received .... vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ runhaskell Setup.hs build Preprocessing library unix-2.2.0.0... MQueue.hsc: In function 'main': MQueue.hsc:102:0: error: invalid application of 'sizeof' to incomplete type 'struct mq_attr' MQueue.hsc:102:0: error: invalid application of 'sizeof' to incomplete type 'struct mq_attr' MQueue.hsc:102:0: error: invalid application of 'sizeof' to incomplete type 'struct mq_attr' compiling dist/build/System/Posix/MQueue_hsc_make.c failed comma ??? Kind regards, vasili

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