- Beginners - Haskell.org

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 28 Aug '11

28 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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Re: [Haskell-beginners] main: <<loop>> ....?
by Sunil S Nandihalli 22 Aug '11

22 Aug '11

Is there a way to disable haskell automatic infinite loop detection .. that might make it simpler to figure out where it is happening by just looking at the trace-output in stdout.. Thanks On Mon, Aug 22, 2011 at 1:06 PM, David Virebayre <dav.vire+haskell(a)gmail.com> wrote: > 2011/8/22 Sunil S Nandihalli <sunil.nandihalli(a)gmail.com>: >> Hello everybody, >> main: <<loop>> -> where did this come from. I know I am not printing > > Your program has an endless loop that was detected by ghc. > > David. >

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How to convert a Data.Set to a Descending List?
by Sunil S Nandihalli 21 Aug '11

21 Aug '11

Hello everybody, Is there a more efficient way to convert a Set to a descending list? I was looking for a function similar to Set.toAscList something like Set.toDecList . I feel first converting to a ascending list and then reversing may be in-efficient given that I actually don't need all the elements only a last couple of elements.. Thanks, Sunil.

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