- Haskell-Cafe - Haskell.org

How to use arrays efficiently?
by Lauri Oksanen 16 May '08

16 May '08

Hi, I have a list of index-value pairs and I want to get a list which is sorted by the index and which contains index-wise sums of all the values. Here is a function that does this (and a bit more). --- pixelHistogram samples = let index s t = let x = round $ (fromIntegral imageWidth) * (1-s) y = round $ (fromIntegral imageHeight) * (1-t) in y*imageWidth + x indexedSamples = [(index s t, color) | (s, t, color) <- samples] pixelArray :: Array Int Color pixelArray = accumArray (\+) black (0, imageWidth*imageHeight) indexedSamples in elems pixelArray --- The problem is that this function is very inefficient. It seems that the whole indexedSamples list is stored in memory when creating pixelArray. When I do some profiling I see that the heap consumption of this function grows linearly. If I just write the samples list to a file instead of first summing them, I get a nice and flat heap profile. So how to do this efficiently? Thanks, Lauri

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fgl Data.Graph examples
by Anatoly Yakovenko 16 May '08

16 May '08

Can someone post some simple examples of using the Data.Graph library? So I can define a simple graph let g = buildG (1,2) [(1,2), (2,1)] but how do i label my edges and nodes? i want to be able to name my nodes, and add weights to the edges and be able to update those weights as well. Thanks, Anatoly

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ChessLibrary project
by Andrew Wagner 16 May '08

16 May '08

All, I've made my first code drop for a new project I'm working on (though it's really an extension of work I've been doing for about 5 years). Information and code can be found at http://code.haskell.org/ChessLibrary/ . Comments and suggestions warmly welcomed. Also, I'm looking for an experienced haskeller to serve as a mentor for this project - just to provide regular code reviews and architectural advice, mainly. Thanks for looking!

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elem of infinite set of tuple
by leledumbo 16 May '08

16 May '08

I don't know how Haskell should behave on this. Consider this function: elemOf (x,y) = (x,y) `elem` [ (a,b) | a <- [0..], b <- [0..] ] If I try to query elemOf (1,1), the program keeps searching and searching but it never makes it. But if I query elemOf (0,1) (or anything as long as the first element is 0), it can find it easily. I wonder how it's handled. >From my point of view, instead of starting from (1,0), the program starts from (0,0), which will never finish since the limit of the second element is infinite. -- View this message in context: http://www.nabble.com/elem-of-infinite-set-of-tuple-tp17272802p17272802.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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runInteractiveCommand: program ends before writing or reading all the output
by Olivier Boudry 16 May '08

16 May '08

Hi all, It's the first time I use the runInteractiveCommand and I was probably bitten by laziness. When I run the following program and send its output to a file using '>' redirection I get the full output of the called process. But if I run it in the console I get only half of the output. As console is slower than disk I assume the called process terminates before all data has been read from it or the main process terminates before data has been written to stdout. I thought using waitForProcess, closing called process output and flushing stdout would solve the problem but it doesn't. > -- Compile with -threaded option > module Main where > > import Control.Concurrent (forkIO) > import System.Environment (getArgs) > import System.FilePath (dropExtension, takeFileName) > import System.IO (Handle, hClose, hFlush, hGetContents, stdout) > import System.Process (runInteractiveCommand, waitForProcess) > > main :: IO () > main = do > (file:_) <- getArgs > (_, out, _, pid) <- runInteractiveCommand $ "dumpbin /EXPORTS " ++ file > forkIO (createDefFile file out) > waitForProcess pid > hClose out > hFlush stdout > > createDefFile :: String -> Handle -> IO () > createDefFile file inp = do > putStrLn $ "LIBRARY " ++ (dropExtension . takeFileName) file ++ ".dll" > putStrLn "EXPORTS" > text <- hGetContents inp > mapM_ writeExport $ keepExports $ map words $ lines text > where > keepExports :: [[String]] -> [String] > keepExports = map head > . filter (not . null) > . takeWhile (["Summary"]/=) > . drop 1 > . dropWhile (["ordinal","name"]/=) > writeExport ('_':xs) = putStrLn xs > writeExport xs = putStrLn xs Any idea regarding the cause of this problem? Thanks, Olivier.

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bottomless Top?
by Conor McBride 16 May '08

16 May '08

Folks [statutory warning: functors, terminal objects, unsafeCoerce] I had a peculiar notion this morning to wonder whether I could get away with shape :: Functor f => f a -> f () shape fa = unsafeCoerce fa Sure enough, ghci gives me *Top> shape "moo" [(),(),()] *Top> shape [undefined] [*** Exception: Prelude.undefined which shows that pattern matching does just enough work to check that an element of () isn't bottom. I'm just wondering if that's really what's happening, what other implementations do, and how the garbage collector would react: would all those a's hang around for as long as the shape? I'm also wondering whether it makes sense to have a "bottomless Top" type, with constructor _ and lazy pattern _ (with (undefined :: Top) equal to _). Then the constant-time shape operator makes the same sort of sense as the constant-time inflate :: Functor f => f Zero -> f a Any thoughts on either or both? Cheers Conor

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getting output from shell commands
by Philip Müller 15 May '08

15 May '08

Hi, I would like to be able to run a shell command like curl and process the output in my Haskell program, but I couldn't find anything helpful about it. Something like main = do args <- getArgs inp <- exec "curl" args putStrLn (processInput inp) would be very helpful. Regards Philip

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GHC predictability
by Jeff Polakow 15 May '08

15 May '08

Hello, One frequent criticism of Haskell (and by extension GHC) is that it has unpredictable performance and memory consumption. I personally do not find this to be the case. I suspect that most programmer confusion is rooted in shaky knowledge of lazy evaluation; and I have been able to fix, with relative ease, the various performance problems I've run into. However I am not doing any sort of performance critical computing (I care about minutes or seconds, but not about milliseconds). I would like to know what others think about this. Is GHC predictable? Is a thorough knowledge of lazy evaluation good enough to write efficient (whatever that means to you) code? Or is intimate knowledge of GHC's innards necessary? thanks, Jeff PS I am conflating Haskell and GHC because I use GHC (with its extensions) and it produces (to my knowledge) the fastest code. --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Re: [Haskell] Re: [Haskell-cafe] ANN: Haddock version 2.1.0
by David Waern 15 May '08

15 May '08

2008/5/2 Simon Marlow <marlowsd(a)gmail.com>: > David Waern wrote: > > > No it doesn't, but it's on the TODO list. It needs a fix in GHC. > > > > By the way, I'm going to experiment with doing the parsing of comments > > on the Haddock side instead of in GHC. > > If that works out, we won't have to fix these things in GHC anymore. > > > > Sounds great - along the lines that we discussed on cvs-ghc a while back? Yes, something along the lines of separately parsing the comments and recording their source locations, and then trying to match them with the source locations of the AST nodes. I don't think there are any lexical rules for where Haddock comments can exist, so it should work. David

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FFI: newbie linking problem
by Olivier Boudry 15 May '08

15 May '08

Hi all, I'm trying to call the following C function. RFC_RC _stdcall RfcUTF8ToSAPUC(const RFC_BYTE *utf8, unsigned utf8Length, SAP_UC *sapuc, unsigned *sapucSize, unsigned *resultLength, RFC_ERROR_INFO *info) I wrote a foreign import for this function: foreign import ccall unsafe "sapnwrfc.h RfcUTF8ToSAPUC" f_RfcUTF8ToSAPUC :: RfcBytePtr -> CUInt -> SapUCPtr -> Ptr CUInt -> Ptr CUInt -> RfcErrorInfoPtr -> IO RfcRetCodeType But when compiling I keep getting this error: C:\Temp\Haskell\HSAP>ghc --make -LC:/Temp/nwrfcsdk/lib -lsapnwrfc SAP/Types.hs Test.hs [1 of 2] Compiling SAP.Types ( SAP/Types.hs, SAP/Types.o ) Linking Test.exe ... SAP/Types.o(.text+0x91c):fake: undefined reference to `RfcUTF8ToSAPUC@24' collect2: ld returned 1 exit status I ran nm.exe on the sapnwrfc.lib to see the exports and got this list: SAPNWRFC.dll: 00000000 I .idata$4 00000000 I .idata$5 00000000 I .idata$6 00000000 T .text 00000000 T _RfcUTF8ToSAPUC@24 U __IMPORT_DESCRIPTOR_SAPNWRFC 00000000 I __imp__RfcUTF8ToSAPUC@24 It looks like RfcUTF8ToSAPUC@24 exists. I tried prepending a _ but it doesn't change anything. GHC tries to link directly to the .dll file and not to the .lib file. Is this a problem? Should I convert the .lib to a .a and link the .a? Is there a way to list the exports directly from the dll file? Thanks, Olivier.

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