August 2011 - Beginners

Strange behavior
by Luca Ciciriello 29 Aug '11

29 Aug '11

I found a strange behavior in my code Here a (very) simplified version of my code: {-# LANGUAGE PArr #-} {-# OPTIONS -fdph-seq #-} module Main where import Control.Parallel import GHC.PArr parArr :: [:String:] parArr = [: "1", "2", "3", "4" :] isElement :: String -> Bool isElement x = x `elemP` parArr main :: IO () main = do putStrLn $ (show . isElement) "5" The aspected result is "False" ("5" isn't a member of parArr), but the result I get is "True". I've compiled this code with the command: ghc --make -Odph -threaded Test.hs -rtsopts -o test and launched with: test +RTS -N2 My system is MacOS X 10.7.1 with Xcode 4.1 and GHC 7.0.4 (I've found the same behavior on Windows). Its obvious I'm missing something. Could someone help me? Thanks. Luca.

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problem using hsc2hs
by Graham Gill 28 Aug '11

28 Aug '11

I've been trying to build package unix-compat-0.2.2.1 on a Windows XP machine which has the latest Haskell package installed (2011.2.0.1). I use "cabal install unix-compat". (I've updated my package list.) I run into a problem when the file Extensions.hs is generated from Extensions.hsc using hsc2hs. Extensions.hs is created, but empty. When I run the cabal install with verbose on, I'm able to see the hsc2hs command which generates Extensions.hs. Running that hsc2hs command with verbose on, I see the steps where it creates intermediate files .c, .o and .exe, then runs the .exe, which sends its output to stdout, and that stdout is redirected into Extentions.hs. The intermediate files are deleted. Extensions.hs is empty. However, if I manually run each command which hsc2hs runs, Extensions.hs has the intended contents. I've reproduced the details below. Could this be a case of a problem with how lazy IO is used? Here are the details. This command (I've cut out the paths from Extensions* files, so they're all created in/read from the current directory), "C:\Program Files\Haskell Platform\2011.2.0.1\bin\hsc2hs.exe" --cc="C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" --ld="C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" --cflag=-fno-stack-protector --lflag=-fno-stack-protector --cflag=-D__GLASGOW_HASKELL__=700 --cflag=-Dmingw32_BUILD_OS --cflag=-Dmingw32_HOST_OS --cflag=-Di386_BUILD_ARCH --cflag=-Di386_HOST_ARCH --cflag=-DMISSING_POSIX_TYPES --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib/include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib/include" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-locale-1.0.0.2" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\filepath-1.2.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1" --lflag=-luser32 --lflag=-lgdi32 --lflag=-lwinmm --lflag=-ladvapi32 --lflag=-lshell32 --lflag=-lshfolder --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0" --lflag=-lwsock32 --lflag=-luser32 --lflag=-lshell32 --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\integer-gmp-0.2.0.3" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\ghc-prim-0.2.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib" --lflag=-lm --lflag=-lwsock32 --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib" --verbose -o Extensions.hs Extensions.hsc run in a Windows command shell, generates Extensions_hsc_make.c, Extensions_hsc_make.o, Extensions_hsc_make.exe, and then runs .\Extensions_hsc_make.exe >Extensions.hs After this the intermediate files Extensions_hsc_make.* are deleted. Extensions.hs has no contents. Since I ran with verbose on, I could see each step. Running each step individually: 1) create Extensions_hsc_make.c from Extensions.hsc, using --no-compile flag to hsc2hs: "C:\Program Files\Haskell Platform\2011.2.0.1\bin\hsc2hs.exe" --cc="C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" --ld="C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" --cflag=-fno-stack-protector --lflag=-fno-stack-protector --cflag=-D__GLASGOW_HASKELL__=700 --cflag=-Dmingw32_BUILD_OS --cflag=-Dmingw32_HOST_OS --cflag=-Di386_BUILD_ARCH --cflag=-Di386_HOST_ARCH --cflag=-DMISSING_POSIX_TYPES --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib/include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib/include" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-locale-1.0.0.2" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\filepath-1.2.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1" --lflag=-luser32 --lflag=-lgdi32 --lflag=-lwinmm --lflag=-ladvapi32 --lflag=-lshell32 --lflag=-lshfolder --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0" --lflag=-lwsock32 --lflag=-luser32 --lflag=-lshell32 --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\integer-gmp-0.2.0.3" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\ghc-prim-0.2.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib" --lflag=-lm --lflag=-lwsock32 --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib" --no-compile -o Extensions.hs Extensions.hsc 2) create Extensions_hsc_make.o from Extensions_hsc_make.c: "C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" -c Extensions_hsc_make.c -o Extensions_hsc_make.o -fno-stack-protector -D__GLASGOW_HASKELL__=700 -Dmingw32_BUILD_OS -Dmingw32_HOST_OS -Di386_BUILD_ARCH -Di386_HOST_ARCH -DMISSING_POSIX_TYPES -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0\include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6\include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1\include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10\include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0\include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib/include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib/include" -I"C:/Program Files/Haskell Platform/2011.2.0.1/lib/include/" 3) create Extensions_hsc_make.exe from Extensions_hsc_make.o: "C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" Extensions_hsc_make.o -o Extensions_hsc_make.exe -fno-stack-protector -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\old-locale-1.0.0.2" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\filepath-1.2.0.0" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1" -luser32 -lgdi32 -lwinmm -ladvapi32 -lshell32 -lshfolder -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0" -lwsock32 -luser32 -lshell32 -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\integer-gmp-0.2.0.3" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\ghc-prim-0.2.0.0" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib" -lm -lwsock32 -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib" 4) and creating Extensions.hs using Extensions_hsc_make.exe: Extensions_hsc_make.exe >Extensions.hs This generates a file with the intended contents. There are two other files in the unix-compat package, Files.hsc and User.hsc, which run into the same problem. So what's the difference between running each step individually, which works, and running the combination of all steps, which gives me an empty file? cheers! Graham

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cabal-dev ghci issues and reinstalling platform with profiling on windows
by MH 28 Aug '11

28 Aug '11

I have windows 7 with mingw/msys environment. I just installed the latest haskell platform and cabal-dev. For some reason, cabal-dev ghci fails with no much information. I tried to add -v and message is not helpfull at all. Here is what I did. $ cd haskell_dev $ cabal-dev install cabal.exe: No cabal file found. Please create a package description file <pkgname>.cabal $cabal init .... $ cabal-dev install $ cabal-dev ghci FAILED: Failed to extract GHC build arguments -------------------------------------------------- Run ghci configured as per the specified cabal file. Usage: cabal-dev.exe <command> ................... $ cabal-dev ghci -v c:\build\Haskell\2011.2.0.1\lib\extralibs\bin\cabal.exe --numeric-version c:\build\Haskell\2011.2.0.1\bin\ghc-pkg.exe --version Using ghc-pkg 7.0.3 c:\build\Haskell\2011.2.0.1\lib\extralibs\bin\cabal.exe --version c:\build\Haskell\2011.2.0.1\lib\extralibs\bin\cabal.exe --config-file=c:\build\Projects\eclipseworkspace\Test\cabal-dev\cabal.config build --verbose=2 --with-ghc=fake-ghc-cabal-dev FAILED: Failed to extract GHC build arguments The second question relates to installing haskell platform on windows 7 with profiling enabled. I tried cabal install haskell-platform --reinstall -p (and some other options) but it didn't work as haskell-platform is not on hackage. Is reinstalling each library one by one the only option available for windows users? Thanks, Malik

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Laziness, automatic memoization and arrays
by Oscar Picasso 27 Aug '11

27 Aug '11

Hi, I have a very simple question about laziness, memoization and arrays. If I have: import Data.Array a = listArray (1,10) $ map (\x -> (x*x)) [1..10] main = do print c ! 4 print c ! 4 print c ! 4 Can we assume that the operation 4 * 4 is evaluated only once, when first calling print c ! 4 in the main method? Oscar

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control codes in output of haskell mode
by Viktor Winschel 24 Aug '11

24 Aug '11

Hi, I get as output from ghci in aquamacs (so on a mac box) haskell mode control codes, like ^[[?25lDatabase: Running the code in the console everything is ok. Can I switch that off such that standard packages get their output right? thanx Vik

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why is the :type different in ghci?
by Ovidiu Deac 23 Aug '11

23 Aug '11

Can somebody explain the following situation? $ ghci GHCi, version 7.0.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :type map show map show :: Show a => [a] -> [String] Prelude> map show [1,2,3] ["1","2","3"] Prelude> let f = map show Prelude> :type f f :: [()] -> [String] Prelude> f [1,2,3] <interactive>:1:8: No instance for (Num ()) arising from the literal `3' Possible fix: add an instance declaration for (Num ()) In the expression: 3 In the first argument of `f', namely `[1, 2, 3]' In the expression: f [1, 2, 3]

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combining two monads
by Ovidiu Deac 23 Aug '11

23 Aug '11

Hi, I wrote some code and I want to refactor it to use the Error monad combined with IO operations. I found some examples of monad transformers but I didn't really get them. So I have a function in IO monad which does several operations which might fail. These operations work with files so they have to be in IO monad but on the other hand I would like to use Error monad such that I can throw errors. So far I came up with something like this. data Config = Config { root :: String } data ParsedData = ParsedData {...} data MyError = ConfigError | ParseError | OtherError String instance Error MyError where strMsg = OtherError loadConfig :: String -> Either MyError (IO Config) loadConfig fn -> ...might throwError ConfigError parseFile :: String -> Either MyError (IO ParsedData) parseFile fn -> .. might throwError ParseError main -> do config <- loadConfig "mycfg.cfg" parsedData <- parseFile (root config) ... Can somebody please enlighten me about how to apply monad transformers in this situation? Thanks, Ovidiu

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bugs or lack thereof
by Dennis Raddle 22 Aug '11

22 Aug '11

I love Haskell for the fact that my programs have relatively few bugs other than typos or obvious mistakes in types that the compiler catches. I just wrote about 100 lines of code to do a fairly complex task, which is to make a map of changing loudness in a musical document (including slopes, not just flat level changes) and after I got the typos out and it compiled, it just worked. I have been a professional programmer in imperative languages for 20 years, so I can think pretty accurately about what I am doing, but I'm new to Haskell, and it's a marked difference in the ease of getting something to work right. Dennis

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main: <<loop>> ....?
by Sunil S Nandihalli 22 Aug '11

22 Aug '11

Hello everybody, main: <<loop>> -> where did this come from. I know I am not printing it anywhere .. and it is not printing some of things I am tracing using Debug.Trace module. Can somebody help? Thanks, Sunil.

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Re: [Haskell-beginners] main: <<loop>> ....?
by David Virebayre 22 Aug '11

22 Aug '11

2011/8/22 Sunil S Nandihalli <sunil.nandihalli(a)gmail.com>: > Is there a way I can find out where it is in infinite loop? I don't know of any other way to find it than looking at the code, I hope someone with more experience will be able to explain a better way. After looking at the source code you posted, does it print anything before it prints <<loop>> ?

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