September 2006 - Haskell-Cafe

System threads?
by Lyle Kopnicky 25 Sep '06

25 Sep '06

Hi folks, I'm working on a project for which the solution is highly parallelizable. I've been writing it so far for GHC as a single-threaded app. I'd like to be able to split the job into multiple pieces, and spawn different system threads for each piece, so they will run on separate CPUs. Either each thread needs to write to the same IO handle, or else the results need to be collected when the threads are merged, so the results can all be printed. I'm working on Linux. What's the easiest way to accomplish this? Glasgow Parallel Haskell? Is there a Posix.Threads library? I understand that Haskell threads (Control.Concurrent) all run in one system thread, so would not take advantage of SMP. I suppose in the worst case I can write a shell program that divides up the problem, runs several worker processes to output intermediate results to files, then combines the results to the desired filehandle. Thanks, Lyle

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Unable to profile program using Data.ByteString.Lazy
by Lyle Kopnicky 24 Sep '06

24 Sep '06

Hi folks, I wrote a program that uses some of the Data.ByteString libraries. I'm using GHC 6.4.1 and FPS 0.7. The program compiles and works just fine. But when I try to profile it, by compiling with -prof, I get: Failed to load interface for `Data.ByteString.Lazy': Could not find module `Data.ByteString.Lazy': locations searched: Data/ByteString/Lazy.hi Data/ByteString/Lazy.hi-boot /f/g/lib/fps-0.7/Data/ByteString/Lazy.p_hi Why can it find the module when it's compiling without -prof, but not when it's compiling with it? I would really like to get profiling to work. Thanks, Lyle

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FFI and LD_PRELOAD -> segfault
by Stephan Walter 24 Sep '06

24 Sep '06

Hi, I'm trying to use a shared lib written in Haskell to overload C functions via LD_PRELOAD. You might think this is a bit silly, but hey, why not? I want to overload connect() from sys/socket.h. I'll document what I've written so far; unfortunately there doesn't seem to be a lot of documentation about this topic. This is my haskell code (in testffi.hs): > module Socks where > > import Foreign.C.Types > import Foreign.Ptr > > newtype S_sockaddr = S_sockaddr () > > foreign export ccall "connect" connect :: > CInt -> Ptr (S_sockaddr) -> CUInt -> IO CInt > > connect :: CInt -> Ptr (S_sockaddr) -> CUInt -> IO CInt > connect _ _ _ = return (-1::CInt) Here's how I compile it: ghc -Wall -c -fffi testffi.hs ghc -Wall -optl "-shared" -optl "-Wl,-soname,libtestffi.so" \ -o libtestffi.so testffi.o testffi_stub.o I'm not at all sure about these compiler and linker options, but this is the best I could come up with (i.e., no errors or warnings) Then I run a test program which calls connect(). I won't include the C source of that program, but it basically connects to the IP address given as the first argument (at port given by third argument) and sends a string. Basic error checking is done, i.e. the program does test the return value of connect(): LD_PRELOAD=./libtestffi.so ./conntest 127.0.0.1 "HELO" 1234 This aborts with a segfault in scheduleWaitThread() from ./libtestffi.so The test program doesn't use threads, so I'm wondering what I did wrong? Any help is appreciated. Greetings, Stephan Walter

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haskell.org down
by Pasqualino 'Titto' Assini 24 Sep '06

24 Sep '06

Haskell.org has been down for over 7 hours (see attached log). The Web master might use host-tracker.com (or a similar free tracking service) to keep an eye on it. Regards, titto _____ From: HostTracker Notifier [mailto:noreply@host-tracker.com] Sent: 24 September 2006 10:41 To: tittoassini(a)gmail.com Subject: Operation Restored Alert Hello, http://haskell.org is now back to normal. Operation restored at 2006-09-24 10:36:55. Url was down as a resultat of: Http error:Http_client.No_reply Error was detected at 2006-09-24 03:36:16 Downtime total 7 hour(s) 39 sec(s). Check failures total: 14. Your login:titto -- Best regards, http://host-tracker.com/ support team uid:957058 tid:188350 eid:188350-61-0-0-669df3829d00102989dd000c7651f68b ffeid:188350-47-0-0-a2bced5e9cc5102989dd000c7651f68b

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[newbie] How to test this function?
by Bruno Martínez 23 Sep '06

23 Sep '06

I've written a function that looks similar to this one getList = find 5 where find 0 = return [] find n = do ch <- getChar if ch `elem` ['a'..'e'] then do tl <- find (n-1) return (ch : tl) else find n First, how do I fix the identation of the if then else? Second, I want to test this function, without hitting the filesystem. In C++ I would use a istringstream. I couldn't find a function that returns a Handle from a String. The closer thing that may work that I could find was making a pipe and convertind the file descriptor. Can I simplify that function to take it out of the IO monad? How? I thought about getContents, but that eats all input. Thanks in advance, Bruno

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Haskell Program
by Tim Newsham 23 Sep '06

23 Sep '06

The company I work for was gracious enough to allow me to release a haskell program I wrote for them: http://www.isecpartners.com/file_fuzzers.html There's a python version and a rewrite in haskell. I used some looping constructs discussed earlier on this list to keep the haskell code similar to its original python code. Don Stewart was kind enough to help me optimize some of the code using ByteString and a faster PRNG (the System.Random class seems to be a bit on the slow side!). Prior to using ByteString and optimizing the code it was a bit slower than the python version, but with optimizations it is quite a bit faster. (In practice, the external program being invoked is usually the performance bottleneck). Tim Newsham http://www.thenewsh.com/~newsham/

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Polymorphism/monomorphism (and the restriction)
by Aaron McDaid 22 Sep '06

22 Sep '06

Hi, I think the following might help a little in understanding the monomorphic restriction (which I don't fully understand myself). I'm a bit of a newbie so apologies in advance if I've made a mistake or if my description isn't as useful to others as it seems to me. I've been following a thread on haskell(a)haskell.org and I think the below might help. I used GHCi, version 6.2.2 (it fails in hugs but that seems to be because of hugs non-compliance with the standard in this case). First off, I'm guessing that I'm getting Haskell98 behaviour here and not some GHCi extension. Please tell me if this is not the case. Run the code listing at the bottom of this email and you should get the output which I've also listed below. This code experiments with "Int", "Float" and "(Num a) => a", and I tried to print x*2 and x/2 for each. (4::Int)/2 isn't allowed because / isn't defined for Ints. You can see that kN :: (Num a) => a took two different types depending on what method ( / or * ) was applied to it. kN / 2 = 2.0 kN * 2 = 8 kN/2 is a Float (it can't use Int as / isn't defined for Int, so it uses Float, for which / is defined). kN*2 is an Int. The above outputs demonstrates polymorphism, doesn't it? i.e. Not only has the compiler got a variety of types to choose from, but a variety of types can be used at runtime? The output for kI and kF is obvious. The interesting thing is that k behaves as a Float in both cases. This is monomorphism isn't it? i.e. the compiler may have a variety of types to choose from, but it picks one and sticks to it for every usage. In summary, k didn't give the same outputs as kN. And the monomorphism restriction is a rule which means that sometimes things are forced to a monomorphic type (like k as Float here) when it could have given it a polymorphic type like "kN :: (Num a) => a" I'm fairly new to these lists, so apologies if I'm covering old ground again. My first aim is to understand exactly what polymorphism and monomorphism is and demonstrate corresponding results, before thinking about the restriction. Thanks, Aaron -- The code kI :: Int kI = 4 kF :: Float kF = 4 kN :: (Num a) => a kN = 4 k = 4 main = do p "kI * 2" $ kI * 2 p "kF / 2" $ kF / 2 p "kF * 2" $ kF * 2 p "kN / 2" $ kN / 2 p "kN * 2" $ kN * 2 p "k / 2" $ k / 2 p "k * 2" $ k * 2 p :: (Show a) => String -> a -> IO () p s = putStrLn.(s++).(" = "++).show -- the output - remember kI / 2 is not possible. kI * 2 = 8 kF / 2 = 2.0 kF * 2 = 8.0 kN / 2 = 2.0 kN * 2 = 8 k / 2 = 2.0 k * 2 = 8.0 -- PS: If you delete the k / 2 line from the program, then k * 2 becomes simply 8 (not 8.0). It uses Int if possible, and Float if that's not available.

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Why am I not allowed to use CStringLen in foreign export?
by Andreas Marth 22 Sep '06

22 Sep '06

Hi! I try to export a Haskell function to VBA. (At the moment without COM.) Because VBA seems to expect a String with length information I tried to return a CStringLen as defined in Foreign.C.String. But if I try to compile it I get an "unacceptable argument type in foreign declaration: CStringLen". My reduced test program now is a strange "Hallo world!" program: >module Test where >import Foreign.C.String > >foreign export stdcall hello :: IO CStringLen >hello :: IO CStringLen >hello = do newCStringLen ("Hallo world!") Do I do some thing wrong here? (If I use CString instead of CStringLen and accordingly newCString instead of newCStringLen everything compiles fine but VBA crashes.) Any help is appreciated, Andreas

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Trying to write a TCP proxy
by John Ky 22 Sep '06

22 Sep '06

Hello, I'm trying to use haskell to put together a TCP proxy I can put between my browser and my webserver. This is as far as I got. The webserver isn't returning my request: > listen = withSocketsDo $ do > putStrLn "Listening..." > socket <- listenOn $ PortNumber 8082 > (handleToClient, hostName, portNumber) <- accept socket > putStrLn "Connected to client" > contents <- hGetContents handleToClient > putStrLn "Received from client:" > putStrLn contents > handleToServer <- connectTo "127.0.0.1" (PortNumber 8080) > putStrLn "Connected to server" > hPutStrLn handleToServer contents > putStrLn "Waiting for server to respond" > hPutStrLn handleToServer "\r\n\r\n" > putStrLn "Waiting for server to respond" > result <- hGetContents handleToServer > putStrLn result > hClose handleToServer > hClose handleToClient > sClose socket Am I doing something wrong? Thanks, -John

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Re: Haskel DLL crashes Excel
by Cyril Schmidt 22 Sep '06

22 Sep '06

Andreas Marth wrote: > As you might now already know I try to let VBA call Haskell via a DLL. I had to do this a while ago; the best I could come up with was: 1. Make a Haskell DLL that exports plain C-style interface. 2. With Visual Studio, make a COM DLL that exports the interfaces to be used by VBA and passes the calls to the Haskell DLL. This COM DLL, among other things, handles the conversion between C-style strings and BSTR (there are examples on msdn.microsoft.com of doing this). Hope this helps. Cheers, Cyril

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