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Re: [Haskell-beginners] [Haskell-cafe] Haskell seems setup for iterative numerics; i.e. a standard example is Newton's method where lazy evaluation ...
by KC 05 Sep '12

05 Sep '12

The REPA package/library doesn't have LU factorization, eigenvalues, etc. On Wed, Sep 5, 2012 at 12:59 PM, Carter Schonwald <carter.schonwald(a)gmail.com> wrote: > Hello KC, > you should check out the Repa library then and see how it works for you. > Cheers > -Carter > > On Wed, Sep 5, 2012 at 12:46 PM, KC <kc1956(a)gmail.com> wrote: >> >> separates control from computation. >> >> It seems as if Haskell would be better for iterative matrix methods >> rather than direct calculation. >> >> -- >> -- >> Regards, >> KC >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- -- Regards, KC

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Haskell seems setup for iterative numerics; i.e. a standard example is Newton's method where lazy evaluation ...
by KC 05 Sep '12

05 Sep '12

separates control from computation. It seems as if Haskell would be better for iterative matrix methods rather than direct calculation. -- -- Regards, KC

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unfamiliar syntax in class definition
by Christopher Howard 05 Sep '12

05 Sep '12

The MonadState class is defined as: quote: -------- class Monad m => MonadState s m | m -> s where ...etc... -------- Please explain the part "| m -> s", that is, the use of the pipe symbol and the right arrow symbol which follow "MonadState s m". -- frigidcode.com indicium.us

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XML output
by Torsten Otto 05 Sep '12

05 Sep '12

Hi, all! I'm struggling with a strange phenomenon. If I read a file containing __ a b c __ the program below works as expected. If I read a svg file __ <?xml version="1.0"?> <svg xmlns="http://www.w3.org/2000/svg"> <rect x="0" y="0" width="180" height="180" fill="white"/> </svg> ___ the first two lines are not processed. Program: __ module SVG() where import IO main :: IO () main = do s <- readFile "sierpinski1.svg" putStrLn s writeFile "sierpinski1_.svg" s __ Results of putStrLn: __ *SVG> main a b c *SVG> main <rect x="0" y="0" width="180" height="180" fill="white"/> </svg> *SVG> __ File Output lists the whole file, though. I don't understand this, why is GHCi stripping the output this way? Thanks, regards, Torsten

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Thread Blocking
by mukesh tiwari 04 Sep '12

04 Sep '12

Hello All I was going trough Real World Haskell and it says "If we try to put a value into an MVar that is already full, our thread is put to sleep until another thread takes the value out". I wrote a simple code to block main import Data.List import Control.Concurrent fun m = do putMVar m 10 return () main = do m <- newEmptyMVar forkIO $ fun m putMVar m 10 return () What I am expecting that main should be blocked at least couple of times but its behaving more deterministically. [mukesh.tiwari@ Programming]$ ghc-7.4.1 -threaded -fforce-recomp Concurrent.hs [1 of 1] Compiling Main ( Concurrent.hs, Concurrent.o ) Linking Concurrent ... [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ ./Concurrent +RTS -N2 [mukesh.tiwari@ Programming]$ I am expecting to get thread blocked indefinitely on MVar at least half the time. Could some one please tell me why this deterministic behavior ? Regards Mukesh Tiwari

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monads / do syntax
by Christopher Howard 04 Sep '12

04 Sep '12

What does the following do expression translate into? (I.e., using >>= operator and lambda functions.) code: -------- h = do a <- (return 1 :: IO Integer) b <- (return 2) return (a + b + 1) -------- -- frigidcode.com indicium.us

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space leak processing multiple compressed files
by Ian Knopke 04 Sep '12

04 Sep '12

Hi everyone, I have a collection of bzipped files. Each file has a different number of items per line, with a separator between them. What I want to do is count the items in each file. I'm trying to read the files lazily but I seem to be running out of memory. I'm assuming I'm holding onto resources longer than I need to. Does anyone have any advice on how to improve this? Here's the basic program, slightly sanitized: main = do -- get a list of file names filelist <- getFileList "testsetdir" -- process each compressed file files <- mapM (\x -> do thisfile <- B.readFile x return (Z.decompress thisfile) ) filelist display $ processEntries files putStrLn "finished" -- processEntries -- processEntries is defined elsewhere, but basically does some string processing per line, -- counts the number of resulting elements and sums them per file processEntries :: [B.ByteString] -> Int processEntries xs = foldl' (\x y -> x + processEntries (B.lines y)) 0 xs -- display a field that returns a number display :: Int -> IO () display = putStrLn . show

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programming with Error monad
by Dennis Raddle 04 Sep '12

04 Sep '12

I recently completed a large project and started another one. The first project was about processing music, in particular taking MusicXML and/or Sibelius files (Sibelius is a music notation program) and producing MIDI files according to the specifications I wanted. The second project is about algorithmic music composition. To handle errors in the first project, I used "throw" to throw exceptions. Now I am using the Error monad... actually a combination of ErrorT and State. (In the state I hold a random generator for use in algorithms that require pseudo-randomness.) I notice two things now. The first thing is that I can catch and rethrow errors to provide annotation about the context. It's like getting a stack trace automatically every time there is an error. I realize now this makes it easier to debug, and during my first project I spent a lot of time debugging that might have been avoidable. (I confess I never could figure out the ghci debugger, which might have helped.) The second thing is that there seems to be less laziness. Every statement in a do statement seems to execute even if I don't use result. I think. I would have to do some more experiments, but that is what it looks like right now. That is another good thing for debugging... less laziness (and maybe guaranteed order of execution too?) means it's easier to understand what the program is attempting to do. Does this seem correct? Dennis

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TagLib
by linuxlinux2006 03 Sep '12

03 Sep '12

Hello Please tell me why this program does not work: http://hpaste.org/74171 Tags in audio file are recorded correctly, other programs see them. Thanks for the help

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(no subject)
by Dennis Raddle 02 Sep '12

02 Sep '12

I wanted to run the map function from Data.Map, let's call it M.map, but inside a monad transformer stack including the Error monad. M.map has this type: M.map :: (Ord k) => (a -> b) -> Map k a -> Map k b However, I want to use a mapping function that has type (Monad m) => a -> m b (i.e. errors could be thrown during the computation, a log could be written, etc) I wrote the following. Any comments on this way of doing things? mapMapM :: (Monad m, Ord k) => (a -> m b) -> Map k a -> m (Map k b) mapMapM g mapIn = do let h (k,a) = do b <- g a return (k,b) y <- mapM h (M.toAscList mapIn) return $ M.fromAscList y

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