October 2009 - Haskell-Cafe

Stupid error, probably
by b1g3ar5 12 Oct '09

12 Oct '09

I can't get the following to work in Leksah - but it works OK in GHC. Can anyone spot the error? I wondered if it was becasue the libraries loaded are different - but I'm just a Haskell beginner ... I have: myGroupBy :: Int→ [a]→ [[a]] myGroupBy = takeWhile not . null . (unfoldr (Just . (splitAt 3))) and I am getting the error: Couldn't match expected type `Int' against inferred type `[a]' In the second argument of `(.)', namely `(splitAt 3)' In the first argument of `unfoldr', namely `(Just . (splitAt 3))' In the second argument of `(.)', namely `(unfoldr (Just . (splitAt 3)))' But I think that splitAt :: Int->[a]->([a],[a]) so: splitAt 3 :: [a]->([a], [a]) and: Just.(splitAt 3) :: [a]->Maybe ([a], [a]) which seems OK as the first argument of unfoldr: unfoldr :: (b-> Maybe(a,b))->b->[a] with the a and b of unfoldr being [a] and [a]. The mention of Int in the error makes me wonder if I've got the wrong splitAt - with the arguments reversed maybe. Any ideas? Thanks. N

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Need help with ghc static compile for cgi using mysql
by Austin King 12 Oct '09

12 Oct '09

I'm trying to host a cgi I've written. It uses Database.HDBC, Database.HDBC.MySQL, Network.CGI, and Text.XHtml.Transitional Here is the command I'm using to compile ghc --make -optl-static -optl-pthread -static -o test.cgi -package cgi -package xhtml -package HDBC-mysql -package HDBC -optl -lz Main.hs and I get "(.text+0x120): undefined reference to `compress'" Searching around I found that mysql recommended passing "lz" to the linker... so I added -optl -lz Complete command and output: ghc --make -optl-static -optl-pthread -static -o test.cgi -package cgi -package xhtml -package HDBC-mysql -package HDBC -optl -lz Main.hs [1 of 5] Compiling Config ( Config.hs, Config.o ) [2 of 5] Compiling Model ( Model.hs, Model.o ) [3 of 5] Compiling View ( View.hs, View.o ) [4 of 5] Compiling Controller ( Controller.hs, Controller.o ) [5 of 5] Compiling Main ( Main.hs, Main.o ) Linking test.cgi ... /opt/ghc-6.10.4/lib/ghc-6.10.4/network-2.2.1.2/libHSnetwork-2.2.1.2.a(HsNet.o): In function `my_inet_ntoa': HsNet.c:(.text+0x20): multiple definition of `my_inet_ntoa' /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(my_net.o):(.text+0x0): first defined here /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(mf_pack.o): In function `unpack_dirname': (.text+0x75b): warning: Using 'getpwnam' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(libmysql.o): In function `read_user_name': (.text+0x6081): warning: Using 'getpwuid' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(mf_pack.o): In function `unpack_dirname': (.text+0x76a): warning: Using 'endpwent' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /opt/ghc-6.10.4/lib/ghc-6.10.4/network-2.2.1.2/libHSnetwork-2.2.1.2.a(HsNet.o): In function `hsnet_getaddrinfo': HsNet.c:(.text+0x15): warning: Using 'getaddrinfo' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(ssl.o): In function `yaSSL::read_file(yaSSL::SSL_CTX*, char const*, int, yaSSL::CertType)': (.text+0x29f3): warning: memset used with constant zero length parameter; this could be due to transposed parameters /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(my_gethostbyname.o): In function `my_gethostbyname_r': (.text+0x3c): warning: Using 'gethostbyname_r' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /opt/ghc-6.10.4/lib/ghc-6.10.4/network-2.2.1.2/libHSnetwork-2.2.1.2.a(BSD__213.o): In function `s9bc_info': (.text+0x55): warning: Using 'getprotobyname' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(libmysql.o): In function `mysql_server_init': (.text+0x6b22): warning: Using 'getservbyname' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(my_compress.o): In function `my_uncompress': (.text+0x6b): undefined reference to `uncompress' /usr/lib/gcc/i486-linux-gnu/4.3.3/../../../../lib/libmysqlclient.a(my_compress.o): In function `my_compress_alloc': (.text+0x120): undefined reference to `compress' collect2: ld returned 1 exit status I'm using ghc 6.10.4 on Ubuntu 9.04 i386 I have libmysqlclient15-dev installed.... Thanks, Austin

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Simple program. Simple problem?
by michael rice 12 Oct '09

12 Oct '09

What is going wrong here? Michael ======= import System.Random coinToss :: StdGen -> IO () coinToss gen = putStrLn "What's your guess, heads or tails ('h' or 't')?" >> getChar >>= \c -> let (randInt, _) = randomR(0,1) gen :: (Int, StdGen) in if c == ((!!) "ht" randInt) then putStrLn "You win!" else putStrLn "You lose!" main = do gen <- getStdGen coinToss gen gen <- newStdGen main ======= [michael@localhost ~]$ runhaskell cointoss.hs What's your guess, heads or tails ('h' or 't')? h You win! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? h You lose! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? ^Ccointoss.hs: cointoss.hs: interrupted [michael@localhost ~]$

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Can type be changed along a >>= chain?
by michael rice 12 Oct '09

12 Oct '09

The first of these works, but not the second. It would seem that the type cannot change along a >>= chain, but I may be missing something in the code. Is the second example illegal? If so, is there a different way to change a String to an Int along the >>= chain? Michael =========== import Data.Char {- transform :: IO () transform = putStrLn "What is your word?" >> getLine >>= \str -> return ('Q':str) >>= \str -> return ('Z':str) >>= \str -> putStrLn $ "Transformed word is " ++ show str -} transform :: IO () transform = putStrLn "What is your digit string?" >> getLine >>= \str -> return ('9':str) >>= \str -> return (read str :: Int) >>= \i -> putStrLn $ "The number is " ++ show i

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statistics package and randomness
by Michael Mossey 12 Oct '09

12 Oct '09

I'm trying to learn how to use randomness in Haskell and it seems very non-straightforward and complex. I could do a lot of things using 'split' from System.Random, but apparently it's broken. There is the statistics package here: http://hackage.haskell.org/package/statistics Is this a better solution? It uses the ST monad in the RandomVariate module. Can someone point me to a tutorial explaining ST, and/or a tutorial in the RandomVariate module? Pseudorandomness seems like one case where it would just be a hell of a lot simpler to have a global generator--never split the state. Is the ST monad some way to accomplish this? Thanks, Mike

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"safe" way to use Rand?
by Michael Mossey 12 Oct '09

12 Oct '09

I'm looking at Control.Monad.Random which provides the Rand monad. I would like to know how to use this for generating multiple infinite series, while trusting that the implementation never uses "split" behind the scenes. (Note: I'm on Windows XP, and there appears to be a bug in getStdGen. It does NOT return an arbitrary generator, but rather the same one every time I run the program. However, newStdGen DOES return an arbitrary generator. So I'm using that, even though I know it accesses split behind the scenes. My thinking is that this only happens once so it is okay.) For example, is this code split-free? simple :: Rand StdGen [Int] simple = getRandomRs (0::Int, 10) main1 = do gen <- newStdGen let answer = (flip evalRand) gen $ do xs <- simple ys <- simple return $ (take 5 xs) ++ (take 3 ys) print answer Then, to elaborate on my specific problem, I need to create special types of infinite series. For example, I might need to create one that looks like this: 0 0 5 0 0 0 2 0 0 0 0 0 5 0 9 0 0 8 ... The pattern here is that there is some random number of zeros followed by a single non-zero value, followed again by a random number of zeros, etc. forever. This is one way to implement this. Does all look well here? makeSeries :: [Int] -> [a] -> a -> [a] makeSeries (i:is) (f:fs) zero = replicate i zero ++ [f] ++ makeSeries is fs zero lessSimple :: Rand StdGen [Int] lessSimple = do counts <- getRandomRs (1::Int , 5 ) values <- getRandomRs (1::Int , 9 ) return $ makeSeries counts values 0 main2 = do gen <- newStdGen let answer = evalRand lessSimple gen print . take 20 $ answer We could even have several of these series zipped together. Is this split-free? main3 = do gen <- newStdGen let fs = (flip evalRand) gen $ do s1 <- lessSimple s2 <- lessSimple return $ zip s1 s2 print . take 20 $ fs

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SDL build problem
by Henk-Jan van Tuyl 11 Oct '09

11 Oct '09

L.S., I am trying to install SDL, but I get error messages about multiple definition of `main'; before I spend many hours of debugging, does anybody know why this happens? See the log below. Regards, Henk-Jan van Tuyl C:\Programs\Haskell\Libs\SDL-0.5.5>runhaskell Setup configure --extra-include-dirs=C:\Programs\MSYS\1.0\include\SDL Setup.lhs:2:2: Warning: In the use of `defaultUserHooks' (imported from Distribution.Simple): Deprecated: "Use simpleUserHooks or autoconfUserHooks, unless you need Cabal-1.2 compatibility in which case you must stick with defaultUserHooks" Warning: defaultUserHooks in Setup script is deprecated. Configuring SDL-0.5.5... checking for sdl-config... /usr/bin/sdl-config configure: creating ./config.status config.status: creating config.mk config.status: creating SDL.buildinfo config.status: creating includes/HsSDLConfig.h config.status: includes/HsSDLConfig.h is unchanged C:\Programs\Haskell\Libs\SDL-0.5.5>runhaskell Setup build Setup.lhs:2:2: Warning: In the use of `defaultUserHooks' (imported from Distribution.Simple): Deprecated: "Use simpleUserHooks or autoconfUserHooks, unless you need Cabal-1.2 compatibility in which case you must stick with defaultUserHooks" Preprocessing library SDL-0.5.5... dist\build\Graphics\UI\SDL\General_hsc_make.o:General_hsc_make.c:(.text+0x0): multiple definition of `main' C:\Programs\ghc\ghc-6.10.4/gcc-lib/libmingw32.a(main.o):main.c:(.text+0x0): first defined here C:/Programs/MinGW/lib/libSDLmain.a(SDL_win32_main.o): In function `console_main': /Users/hercules/trunk/SDL-1.2/./src/main/win32/SDL_win32_main.c:246: undefined reference to `SDL_main' collect2: ld returned 1 exit status linking dist\build\Graphics\UI\SDL\General_hsc_make.o failed command was: C:\Programs\ghc\ghc-6.10.4\gcc.exe -BC:\Programs\ghc\ghc-6.10.4\gcc-lib -IC:\Programs\ghc\ghc-6.10.4\include\mingw -lSDL -L/usr/lib -lmingw32 -lSDLmain -lSDL -mwindows -LC:\Programs\ghc\ghc-6.10.4\base-4.1.0.0 -lwsock32 -luser32 -lshell32 -LC:\Programs\ghc\ghc-6.10.4\integer-0.1.0.1 -LC :\Programs\ghc\ghc-6.10.4\ghc-prim-0.1.0.0 -LC:\Programs\ghc\ghc-6.10.4 -LC:\Programs\ghc\ghc-6.10.4/gcc-lib -lm -lffi -lgmp -lwsock32 dist\build\Graphics\UI\SDL\General_hsc_make.o -o dist\build\Graphics\UI\SDL\General_hsc_make.exe --

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The Monad.Reader (14) - Call for copy
by Brent Yorgey 11 Oct '09

11 Oct '09

Call for Copy: The Monad.Reader - Issue 14 ------------------------------------------ Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 14 will be: **Friday, January 8, 2010** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki-page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll sort something out.

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ANN: text 0.5, a major revision of the Unicode text library
by Bryan O'Sullivan 11 Oct '09

11 Oct '09

Get it while it's fresh on Hackage, folks! Details of the changes here: http://bit.ly/1u4UOT

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How do I get this done in constant mem?
by mf-hcafe-15c311f0c＠etc-network.de 11 Oct '09

11 Oct '09

Hi all, I think there is something about my use of the IO monad that bites me, but I am bored of staring at the code, so here you g. The code goes through a list of records and collects the maximum in each record position. -- test.hs import Random import System.Environment (getArgs) import System.IO (putStr) samples :: Int -> Int -> IO [[Double]] samples i j = sequence . replicate i . sequence . replicate j $ randomRIO (0, 1000 ** 3) maxima :: [[Double]] -> [Double] maxima samples@(_:_) = foldr (\ x y -> map (uncurry max) $ zip x y) (head samples) (tail samples) main = do args <- getArgs x <- samples (read (head args)) 5 putStr . (++ "\n") . show $ maxima x I would expect this to take constant memory (foldr as well as foldl), but this is what happens: $ ghc -prof --make -O9 -o test test.hs [1 of 1] Compiling Main ( test.hs, test.o ) Linking test ... $ ./test 100 +RTS -p [9.881155955344708e8,9.910336352165401e8,9.71000686630374e8,9.968532576451201e8,9.996200333115692e8] $ grep 'total alloc' test.prof total alloc = 744,180 bytes (excludes profiling overheads) $ ./test 10000 +RTS -p [9.996199711457872e8,9.998928358545277e8,9.99960283632381e8,9.999707142123885e8,9.998952151508758e8] $ grep 'total alloc' test.prof total alloc = 64,777,692 bytes (excludes profiling overheads) $ ./test 1000000 +RTS -p Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. $ so... does sequence somehow force the entire list of monads into evaluation before the head of the result list can be used? what can i do to implement this in constant memory? thanks! matthias

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