December 2008 - Haskell-Cafe

intercalate and (byte)strings
by wman 23 Dec '08

23 Dec '08

I encountered the following code : -- B == Data.ByteString ; L == Data.ByteString.Lazy contents' = B.intercalate B.empty $ L.toChunks contents with a previously unencountered function intercalate. A quick google query later i knew that it's just intersperse & concat nicely bundled and started wondering why anybody would do this, as simple contents' = B.concat $ L.toChunks contents would do (probably nearly) the same. The only thing I am able to come up with is that it somehow helps streamline the memory usage (if it has some meaning). Is there some reason to use intercalate <empty> <list> instead of concat <list> (probably when dealing with non-lazy bytestrings) ? Thx, wman.

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Detecting system endianness
by Mauricio 23 Dec '08

23 Dec '08

Hi, Is there some way I can check the endianness of the machine my haskell code is running in? Thanks, Maurício

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Is this related to monomorphism restriction?
by Maurício 23 Dec '08

23 Dec '08

Hi, Why isn't the last line of this code allowed? f :: (TestClass a) => a -> Integer f = const 1 a = (f,f) g = fst a The only thing I can think about is monomorphism restriction, but it's allowed (or even the third line would not be accepted). Is there something I could read to understand that? Thanks, Maurício

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Parsec question
by Erik de Castro Lopo 23 Dec '08

23 Dec '08

Hi all, I'm rather new to Haskell and I'm diving right into the deep end writing a parser using Parsec. In particular I'm using Text.ParserCombinators.Parsec.Language to do some of the heavy lifting and have this: import qualified Text.ParserCombinators.Parsec.Language as L import qualified Text.ParserCombinators.Parsec.Token as T lexer = T.makeTokenParser L.emptyDef { L.identStart = letter <|> char '_', L.identLetter = alphaNum <|> char '_', .... identifier :: CharParser st String identifier = T.identifier lexer and now I need to parse things "this.that.the.other". I'd like to have a function with the following signature: qualifiedIdentifier :: CharParser st [ String ] which should return [ "this", "that", "the", "other" ] and write it in terms of identifier and thats where I'm stuck. Anyone care to whack me with the cluestick? Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- "If you don't have freedom as a principle, you can never see a reason not to make an exception." -- Richard Stallman.

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Re: [Haskell-cafe] Re: [Haskell] ANN: HLint 1.0
by Claus Reinke 23 Dec '08

23 Dec '08

>>> Well, sort of. Ok, we can parse that. Let's assume a variable x holds >>> the output of :show modules as a String. We call lines on it, then map >>> words on it, do a !! 2 on it, and we get ["Util.hs,", "Recorder.hs,", >>> "Game.hs,", "Monadius.hs,", "Demo.hs,"]. Chuck in a map (filter (\= >>> ',')), and we get a good list. We can turn the list into a string >>> suitable for hlint with a quick unwords. >>> >>> So our long sought after command becomes ':def hoogle (\_ -> return $ >>> ":! " ++ (unwords $ map (filter (\= ',')) $ (map words $ lines x) !! >>> 2))'. But wait, how do we get 'x'? How do we call :show modules inside >>> a Haskell expression? >> The first url includes a link to a .ghci mini-tutorial (section 4) that, >> among other things, implements >> :redir <var> <cmd> -- execute <cmd>, redirecting stdout to <var> > Perhaps my cold has fogged my head too much, but I'm not sure how > :redir would help. I could do :redir var "hlint .", but that's as > unsatisfactory as :! "hlint ." You were asking about getting the output of ':show modules' into a variable 'x', so that you can process it further. ':redir x :show modules' should do just that. There is another example command for implementing ':edit' this way (by now a native ghci command). Claus

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example editrc
by brian 23 Dec '08

23 Dec '08

This is making ghci with editline nicer for me. It says to use the vi key mapping, tab for completion, C-l to clear the screen, and jj to go into vi command mode. $ cat ~/.editrc bind -v bind \\t rl_complete bind ^L ed-clear-screen bind jj vi-command-mode

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Initializing GHC from Python
by Ron de Bruijn 23 Dec '08

23 Dec '08

Hi, We have just published a small article on how one can initialize GHC from Python, with only optional use of C. You can read it at http://gamr7.com/blog/?p=65 . Best regards, Ron de Bruijn

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New Haskell logo contest
by Fritz Ruehr 23 Dec '08

23 Dec '08

I think this logo contest is a great idea. I submitted my "classy Haskell" logo from the merch. page, but I have to admit I like some of the other ones on the submission page a whole lot. Hey, *here's* an idea: maybe whoever wins the logo contest has to take over management of the Haskell merchandise page on CafePress! :) -- Fritz Ruehr

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cabal, multi-file applications
by Cetin Sert 23 Dec '08

23 Dec '08

Hi, I get the following error message: cetin@unique:~/lab/test/qths/hnm$ make configure runhaskell Setup.hs configure Configuring HNM-0.2... cetin@unique:~/lab/test/qths/hnm$ make build runhaskell Setup.hs build Preprocessing library HNM-0.2... Preprocessing executables for HNM-0.2... Building HNM-0.2... demo3.hs:4:7: Could not find module `HNM.WLAN': Use -v to see a list of the files searched for. make: *** [build] Error 1 when I try to build the following program: http://sert.homedns.org/hs/hnm/ http://sert.homedns.org/hs/hnm/hnm.cabal How can I tell in my cabal file that wlan.hs should be built first than settings.hs than demo3.hs? Best Regards, Cetin Sert

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[Byte8] <-> ByteString
by Galchin, Vasili 23 Dec '08

23 Dec '08

Hello, I have been reading through Data->ByteString. What is the is most elegant and efficient way to map/unmap [Byte8] <-> ByteString? Thanks, Vasili

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