January 2011 - Haskell-Cafe

Misleading MVar documentation
by Edward Z. Yang 16 Jan '11

16 Jan '11

Merry Christmas all! Is it just me, or does the Control.Concurrent.MVar documentation seem a bit misleading? In particular, we should explicitly note the race conditions for not just swapMVar but also readMVar, withMVar, modifyMVar_ and modifyMVar, and clarify that the safety guarantees of the latter three pertain to their handling of asynchronous exceptions. It might also be good to tell people that if they need race-free operations of this style, STM is a good alternative to look at, even if only one variable is being synchronized over. Cheers, Edward

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hoogle command line program on Linux
by Erik de Castro Lopo 15 Jan '11

15 Jan '11

Hi all, I'm testing out hoogle 4.1.2 on Debian Linux and getting the following when trying to update the local hoogle databases: erik > sudo hoogle data Starting default Starting keyword hoogle: keyword.txt: commitAndReleaseBuffer: invalid argument (Invalid or incomplete multibyte or wide character) My LANG related envorinment variables are set as follows: erik > env | grep LANG LANG=en_AU.UTF-8 GDM_LANG=en_AU.UTF-8 LANGUAGE=en_AU.UTF-8 LC_LANG=en_AU.UTF-8 When faced with a similar problem while working on Ben Lippmeier's compiler, I fixed this by replacing all instances of readFile with readUtf8File which is defined as: readUtf8File :: FilePath -> IO String readUtf8File filePath = do h <- openFile filePath ReadMode hSetEncoding h utf8 hSetEncoding stdout utf8 hGetContents h Is it possible to fix this without hacking the hoogles sources? I'd like to package hoogle for Debian, but first I need to get it working. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Question concerning Network.Curl.Opts
by Michael Litchard 14 Jan '11

14 Jan '11

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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Monad transformer: apply StateT to List monad
by michael rice 14 Jan '11

14 Jan '11

{- From: http://en.wikibooks.org/wiki/Haskell/Monad_transformers if for instance we apply StateT to the List monad, a function that returns a list (i.e., a computation in the List monad) can be lifted into StateT s [], where it becomes a function that returns a StateT (s -> [(a,s)]). That is, the lifted computation produces multiple (value,state) pairs from its input state. -} import Control.Monad.Trans.State.Lazy type GeneratorState = StateT Int -- a function in the list monad f :: Int -> [Int] f n = [0..n] Will someone please demonstrate the above comment from the wiki page. Michael

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questions about network.curl/ web client programming
by Michael Litchard 13 Jan '11

13 Jan '11

I'm learning both haskell and web programming as I go here, this question entails both. I'm writing a screen scraping program, and I'm at the point where I need to send certain data in the header. My question is, is that what method_HEADER is for? If so, could I see an example? If not, how does one go about shaping header data?

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Re: [Haskell-cafe] parsec2 vs. parsec3... again
by Permjacov Evgeniy 13 Jan '11

13 Jan '11

On 12/23/2010 06:01 AM, Evan Laforge wrote: > This is not very encouraging! Especially strange is how Text > generates *more* allocation... I'd expect less since it doesn't unpack > all the Texts. Errgh. To check against predicate, library HAS to unpack checked character. There is no way around it. > There's an obvious problem where I get the digits as a String and then > parse that with list functions, but I can't see any way to get parsec > to return a chunk of Text. This is roughly how parsec itself parses > numbers, in Text.Parsec.Token. > > Any ideas or experience? > If you wish performance so desperatley, you can try hand-coded parsing. What I mean is, that if library has to unpack characters to check them against isDigit predicate, why not to use it in building numeral value immidiatley? This will eliminate intermidiate list. However, every back-tracking parser is slow by definition. If you wish maximum possible speed, consider hand-written lexer (this is not too hard) and possibly Happy to generate parser. BTW, how much utf16 text is around? I never found any in wild web.

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Writer monad corresponds to synthesized attribute?
by Sean Leather 13 Jan '11

13 Jan '11

I have been attempting to translate something I did using UUAG into monadic code. It involved inherited, synthesized, and chained attributes. It has been said that such attributes correspond to the Reader, Writer, and State monads, respectively [1]. The former and latter are straightforward, but I'm somehow missing the correlation between the Writer monad and synthesized attributes. I couldn't figure out the appropriate combination of tell/listen/pass/censor to do what I was doing with a synthesized Map. I eventually gave up and passed the synthesized values along with the monadic result. Has anybody done this before? Regards, Sean [1] http://www.haskell.org/haskellwiki/The_Monad.Reader/Issue4/Why_Attribute_Gr…

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Haskell for Gingerbread
by Chris Kuklewicz 13 Jan '11

13 Jan '11

Hi folks, I have been looking at developing for my Android phone which is running Gingerbread (Android version 2.3). The important thing about the official development kit is this: The new native development kit (C/C++ cross compiler for ARM) is that the you can create android applications from pure C without using the Dalvik/Java virtual machine at all. The thinking behind this was probably for game developers to be able to avoid their VM. So all that might be needed is a Haskell compiler with a C-backend that emits ARM-compatible code and an initially minimal android runtime. Implementing to the new "native_activity.h" allow for the usual application life-cycle: onStart, onPause, onResume, onStop... Some options I have not had a chance to look into: 1) GHC arm port and -via-C 2) jhc (and lhc) generated C 3) port nhc98 4) port yhc bytecode runtime Does anyone know who else is thinking along any of these lines? Are there 5th or 6th routes to take? Cheers, Chris Kuklewicz

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ghci seg faulting, have no idea why.
by Michael Litchard 12 Jan '11

12 Jan '11

this just started happening, don't know why. Could anyone offer suggestions, troubleshooting methods? ghci GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Segmentation fault

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The "type & data" constructor confused me !
by z_axis 12 Jan '11

12 Jan '11

data Layout a = forall l. (LayoutClass l a, Read (l a)) => Layout (l a) readsLayout :: Layout a -> String -> [(Layout a, String)] readsLayout (Layout l) s = [(Layout (asTypeOf x l), rs) | (x, rs) <- reads s] Why is the first parameter of readsLayout "(Layout l)" not "Layout (l a)" ? Sincerely! ----- e^(π.i) + 1 = 0 -- View this message in context: http://haskell.1045720.n5.nabble.com/The-type-data-constructor-confused-me-… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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