January 2011 - Beginners

question about list comprehension or creating array
by Ching-Chao Chang 18 Jan '11

18 Jan '11

Hi, I want to create an immutable array like this In .hs file e = [1,2,3] a = array [1,3] [(i,v) | i<-[1..3], v<-e] I expect the output to be "array (1,3) [(1,1),(2,2),(3,3)]" But I got "array (1,3) [(1,3),(2,3),(3,3)]" Why is that? Can anyone tell me how to get the output that I expect? Thanks a lot, cchang

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How to structure a "speaking" fuzzy clock?
by Kamil Stachowski 17 Jan '11

17 Jan '11

Hello, I am writing a program, the main purpose of which is to print fuzzy time in a casual manner, e.g. "ten to eleven" rather than "10:52". Considering my (lack of) programming experience, I got pretty far with it with the help of D. Fischer and B. Yorgey; you can take a look at the results at https://github.com/caminoix/fuzzytime. However, I also have another purpose, and that is to better learn Haskell. I can't help the feeling that my solution might perhaps work, but is nevertheless ugly, naive or just un-Haskell. Therefore, I would like to ask you how you would structure such a program. I'm not asking for an implementation, just a general idea, such as "a datatype to keep the fuzzy time, deriving Show with a set of separate functions for different languages". Thanks in advance!

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Cleaning up after the Close button is pressed
by Colin Hume 16 Jan '11

16 Jan '11

Hi everyone, I have to perform cleanup when my application terminates. GHC.ConsoleHandler handles cleanup from Ctrl-C and Ctrl-Break very nicely under Windows. Unfortunately, the RTS does not support delivery of the Close button event. The reason given in rts/win32/ConsoleHandler.c is that an improperly-written Haskell handler will prevent the user from being able to kill the application. I have a few questions about this. 1) Is there a different way of cleaning up on Windows after the Close button is pressed? 2) It seems strange that GHC intentionally prevents the application from cleaning up in order to guarantee that it will terminate as the user expects. Why is it not the application developer's responsibility to ensure that it can be killed? Thanks, Colin

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question about comprehension or array creation
by cchang 16 Jan '11

16 Jan '11

Hi, I tried to create an array like the following. "array (1,3) [(1,1), (2,2), (3,3)]" through code in .hs e = [1,2,3] array (1,3) [(i,v) | i<-[1..3], v<-e] but I got "array (1,3) [(1,3), (2,3), (3,3)]" why v is always 3 in this case? Can anyone shed some light on this? Thanks,

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How to fold a map
by Tim Baumgartner 16 Jan '11

16 Jan '11

For all those still having problems with their folds (like myself), the following might be enlightning: http://www.youtube.com/watch?v=3063VGrrAdg Tim

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IO String inside a CmdArgs data
by Kamil Stachowski 15 Jan '11

15 Jan '11

Hello, I am trying to use N. Mitchell's CmdArgs and I would like one of the fields to store the current time. A minimal example below: {-# LANGUAGE DeriveDataTypeable #-} import System.Console.CmdArgs import System.Time data FuzzyTimeConf = FuzzyTimeConf { time :: String } deriving (Show, Data, Typeable) ftConf = FuzzyTimeConf { time = confDefaultTime } confDefaultTime = "it is here that I would like to get the current time" main :: IO () main = do print =<< cmdArgs ftConf I want confDefaultTime to store the current time as HH:MM, so I guess I will also need to extract it in some way. I've tried many different versions and can't come up with anything working. confDefaultTime can't be an IO String because than it can't derive Show and Data which is required for cmdArgs to work in FuzzyTimeConf. However, I don't seem to be able to find a way to make it a String and hold the actual current time. Can you help me, please?

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Network.Curl.Opts problem (may show up on haskell-cafe as well)
by Michael Litchard 15 Jan '11

15 Jan '11

Reposting this, as it seems my Haskell Cafe posts for the past few days aren't making it to the list. I'm having trouble passing header strings properly, and I'd like some advice on how to proceed. Below is a capture of what is being sent, versus what I am trying to send. I won't include all code, only what I think is necessary. If I have omitted something important, please let me know. How could I discover what the cause of the discrepancy is? Thanks again for any feedback. Here's a snippet from the header, what is being sent. > GET /resourceList.do?form=webForwardsForm&readOnly=false&policyLaunching=true&resourcePrefix=webForwards&path=%2FshowWebForwards.do&messageResourcesKey=webForwards&actionPath=%2FresourceList.do HTTP/1.1 User-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0) Host: 172.16.1.18 Accept: */* Accept-Encoding: gzip,deflate Referer: https://172.16.1.18/showWebForwards.do Cookie: domainLogonTicket=SLXa10225c6e8389b3eb181e3df5dcf08de; logonTicket=SLXa10225c6e8389b3eb181e3df5dcf08de; lbTrack=OAIAGHMWQDOLYYTJEXQHXBYPXVALXNREKIHAYYRZSOGYJLUYNNCJ--------; SSLX_SSESHID=bvgx4mggmy6v ^ compare this to CurlHttpHeaders Here's the part of the source I think is relevant > launch :: String -> String -> IO (Either String String) > launch user pass = do > -- Initialize Curl > curl <- initCurl > -- Sequence of steps > let steps = do > curlResp curl urlInitial method_GET > curlResp curl urlLogin $ loginOpts user pass > curlResp curl urlFlash1 method_GET > curlResp curl urlFlash2 method_GET > curlResp curl urlGetResource resourceOpts <---- here's where the problem is revealed > runErrorT steps > main :: IO () > main = do > -- username and password > user:pass:_ <- getArgs > -- Launch webpage > resp <- launch user pass > -- Response comes as Either String String > -- You have to handle each case > case resp of > Left err -> print err > Right body -> putStrLn body > resourceOpts :: [CurlOption] > resourceOpts = > [ CurlHttpHeaders > [ "Accept text/javascript, text/html, application/xml, text/xml, */*" > , "Accept-Language en-us,en;q=0.5" > , "Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7" > , "Keep-Alive 115" > , "Connection keep-alive" > , "X-Requested-With XMLHttpRequest" > , "X-Prototype-Version 1.6.0.3" > ] > , CurlEncoding "gzip,deflate" > , CurlReferer "https://172.16.1.18/showWebForwards.do" > ]

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Parser as an instance of the monad class
by Paul Higham 14 Jan '11

14 Jan '11

I am working my way through Graham Hutton's book and find that his approach to introducing monads is rather nice. Trouble is that the code there does not work "out of the book". Specifically, if you define the Parser type as follows: type Parser a :: String -> [(a,String)] you can then define the return and bind functions as return v = \x -> [(v,x)] p >>= f = \x -> case p x of [] -> [] [(v,y)] -> (f v) y but you get name conflicts with the functions of the same names in the Prelude. Fair enough. There are a number of ugly things that you can do at this point that are just plain wrong, so I thought that the right thing to do would be to get the Parser type to be manifested as an instance of the Monad class. Ok, now what? Since Parser is only a type synonym it cannot be used directly in the following way: instance Monad Parser where return v = . . . p >>= f = . . . but since Parser is not an algebraic type as it is defined that won't work either. So how do you do it? help . ::paul

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Question in "Monads for Functional Programming"
by Aggelos Mpimpoudis 14 Jan '11

14 Jan '11

Hi all, I am referring to P. Wadler, "Monads for functional programming," Advanced Functional Programming, pp. 24-52, 1995. I cannot understand in chapter 2.6, why a*k = k a. I cannot ask anyone else this time of night (3am here J), thus I posted here! Best regards! -- Aggelos Mpimpoudis Doctoral Researcher, Pervasive Computing [p-comp.di.uoa.gr] SDE, Nessos IT S.A. [www.nessos.gr] (m): +306942075153 <http://gr.linkedin.com/in/aggelosmp> Description: Linkedin <http://www.facebook.com/aggelosmp> Description: Facebook <http://www.studentguru.gr/blogs/grnemo/> Description: Blog RSS <http://twitter.com/aggelosmp> Description: Twitter

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filesystem verification utility
by Anand Mitra 12 Jan '11

12 Jan '11

Hi, I had a requirement to generate various kinds of I/O patterns on a filesystem and subsequently verify this. The initial version of the program below implements a random I/O pattern with multiple threads. Even when the number of I/O is as small as 200 and 10 concurrent theads, the amount of memory used is huge. When I run the program it consumes close to 1to2GB memory. Moreover the rate at which it generates the I/O is very low which is not good for testing a filesystem. I have used System.POSIX.IO but I tried System.IO and did not see much difference either. I would appreciate help in identifying ways to improve this. {-# OPTIONS -fglasgow-exts #-} import System.Random import Data.List import Monad import System.Posix.IO import System.Posix.Types import Data.Time.Clock import System.Posix.Files import Data.Maybe import GHC.IO.Device (SeekMode(..)) import Control.Exception import Data.Typeable import System import Control.Concurrent --myrandomRlist :: (Num t, Random t) => t -> IO [t] --myrandomRlist x = liftM (randomRs (0,x)) newStdGen myrandomRlist :: (Num t, Random t) => t ->t -> StdGen -> [t] myrandomRlist min max seed = randomRs (min,max) seed data IoLoc = IoLoc {offset :: FileOffset, size::ByteCount, num::Int } deriving (Show, Typeable) instance Exception IoLoc data Corrupt = Corrupt String IoLoc deriving (Show, Typeable) instance Exception Corrupt data FileHdr = FileHdr {fileName::FilePath, seed::StdGen, minIoSize::Int, maxIoSize::Int, ioCount::Int} deriving (Show) data FileIO = FileIO {fd::Fd, params::FileHdr, fileData::[IoLoc]} genPattern :: FilePath -> IoLoc -> String genPattern f l = take (read $ show $size l) (cycle $ "(" ++ f ++ ")" ++ "[" ++ (show (offset l)) ++ ":" ++ (show (size l)) ++ (show (num l)) ++ "]") doHdrRead :: Fd -> IO [Char] doHdrRead fd = do (str, count) <- fdRead fd 100 return (takeWhile (\x-> not (x == '\n')) str) doGetHdr :: Fd -> IO FileHdr doGetHdr fd = do file <- doHdrRead fd seed <- doHdrRead fd minIoSize <- doHdrRead fd maxIoSize <- doHdrRead fd ioCount <- doHdrRead fd return (FileHdr file (read seed) (read minIoSize) (read maxIoSize) (read ioCount)) doHdrFromFile :: FilePath -> IO FileIO doHdrFromFile name = do fd <- openFd name ReadWrite (Just ownerModes) (OpenFileFlags {append=False, exclusive=False, noctty=True, nonBlock=False, trunc=False}) hdr <- doGetHdr fd return (FileIO fd hdr []) doHdrWrite :: Fd -> [Char] -> IO ByteCount doHdrWrite fd str = do fdWrite fd (take 100 (str ++ "\n" ++ (cycle ['\0']))) doWriteHdr :: Fd -> FileHdr -> IO ByteCount doWriteHdr fd hdr = do doHdrWrite fd (show (fileName hdr)) doHdrWrite fd (show (seed hdr)) doHdrWrite fd (show (minIoSize hdr)) doHdrWrite fd (show (maxIoSize hdr)) doHdrWrite fd (show (ioCount hdr)) doWriteFile :: FilePath -> IO FileHdr doWriteFile name = do fd <- openFd name ReadWrite (Just ownerModes) (OpenFileFlags {append=False, exclusive=False, noctty=True, nonBlock=False, trunc=True}) seed <- newStdGen hdr <- return (FileHdr name seed 4096 (2*8096) 200) doWriteHdr fd hdr return hdr overLap (IoLoc off1' sz1' num1) (IoLoc off2' sz2' num2) = ((off1 > off2) && (off1 < off2 +sz2)) ||((off1+sz1 >off2) &&(off1+sz1 < off2 +sz2)) where off1 = read (show off1') sz1 = read (show sz1') off2 = read (show off2') sz2 = read (show sz2') genIoList1 :: [IoLoc] -> [Int] -> [Int] -> Int -> Int -> [IoLoc] genIoList1 list offset size _ 0 = list genIoList1 list offset size 0 _ = list genIoList1 list (offset:os) (size:ss) count bound = if isNothing $ find (overLap x) list then genIoList1 ([x] ++ list) os ss (count - 1) (bound -1) else genIoList1 list os ss count (bound -1) where x = IoLoc (read (show offset)) (read(show size)) count -- offset = (read (show offset1)) -- size = (read(show size1)) genIoList :: FileHdr -> [IoLoc] genIoList (FileHdr name seed min max count) = genIoList1 [] (myrandomRlist 4000 1099511627776 seed) (myrandomRlist min max seed) count (count*2) doActualIo :: Fd -> IoLoc -> String -> IO () doActualIo fd (IoLoc off sz num) str = do off <- fdSeek fd AbsoluteSeek off fdWrite fd str return () doVerifyIo :: Fd -> IoLoc -> String -> IO () doVerifyIo fd (IoLoc off sz num) str = do off <- fdSeek fd AbsoluteSeek off (filedata, count) <- fdRead fd sz if str == filedata then return () else throwIO (Corrupt ("Data corruption in #" ++ (take 200 str) ++ "#" ++ (take 200 filedata))(IoLoc off sz num)) mainWrite file = do hdr <- doWriteFile file hdrIO <- doHdrFromFile file iolst <- return $ genIoList hdr app <- return $ zip iolst (map (genPattern (fileName hdr)) iolst) putStrLn (fileName hdr) mapM (\(x,y)-> doActualIo (fd hdrIO) x y) app mapM (\(x,y)-> doVerifyIo (fd hdrIO) x y) app return hdr mainVerify file = do hdrIO <- doHdrFromFile file hdr <- return (params hdrIO) iolst <- return $ genIoList hdr file <- return $ filter (\x->not ( x=='"')) (fileName hdr) app <- return $ zip iolst (map (genPattern file) iolst) putStrLn $ filter (\x->not ( x=='"')) (fileName hdr) mapM (\(x,y)-> doVerifyIo (fd hdrIO) x y) app return hdr main = do x <- getArgs if (length x) == 3 then do main1 else do putStrLn "USAGE:\nfile-io write/verify <full-path file name> <number of threads> \ \\n\n\ \Simple IO load generator with write verification. This utility is designed\n\ \to generate multi-threaded IO load which will write a pattern to the file. \n\ \When this is invoked with the same parameters with the verify option \n\ \the data written will be verified.\n\ \ " main1 = do [op, name, numProcs] <- getArgs m <- newEmptyMVar n <- return $read numProcs case op of "write" -> do mapM forkIO [(fillfile (name ++ (show i)) ) m|i<-[1..n]] "verify" -> do mapM forkIO [(verifyfile (name ++ (show i)) ) m|i<-[1..n]] x <- mapM takeMVar $ take (read numProcs) $ repeat m putStrLn $ show $ and x fillfile filename m = do mainWrite filename putMVar m True verifyfile filename m = do mainVerify filename putMVar m True

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