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"Cyclic" instance of Enum
by Patrick LeBoutillier 01 Feb '11

01 Feb '11

Hi, I've created an instance of Enum that repeats the same elements when it gets to the end: module Music.Note.Name where data Name = C | D | E | F | G | A | B deriving (Show, Eq) instance Enum Name where toEnum 0 = C toEnum 1 = D toEnum 2 = E toEnum 3 = F toEnum 4 = G toEnum 5 = A toEnum 6 = B toEnum i = toEnum $ i `mod` 7 fromEnum C = 0 fromEnum D = 1 fromEnum E = 2 fromEnum F = 3 fromEnum G = 4 fromEnum A = 5 fromEnum B = 6 enumFromTo x y = map toEnum [a .. b'] where a = fromEnum x b = fromEnum y b' = if a <= b then b else b + 7 enumFromThen x1 x2 = error "enumFromThen not supported for Music.Note.Name" enumFromThenTo x1 x2 y = error "enumFromThenTo not supported for Music.Note.Name" Does this violate any implicit rules that an Enum instance should follow? Is there a better way to do this? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Almost there. Bizzaro problem with libcurl forces kludge that I would like to be rid of.
by Michael Litchard 31 Jan '11

31 Jan '11

So, I've been writing a http client designed to simulate a session with one of my company's products. The crucial thing we need from this program is a cookie jar. When I began I was doing exploratory code to figure things out about http. I managed to make a cookie jar with my toy code, along with the intermediate steps needed to generate the cookies that represent a session. Then it was time to integrate my experimental toy code into something closer to what production code should look like. This is where bizarro behavior starts to happen. All the intermediate steps appear to be working, as later steps are dependent on earlier steps succeeding. Yet in the end, no cookie jar. We did an strace on the binary and it appears that there seems to be a problem with the reading/writing of the file to be the cookie jar, and we don't know why. The kludge solution is to write a wrapper that calls the program in such a way that I can capture standard error and parse out the cookies. I would hate for things to remain this way. It disturbs me. So, my toy code can create a cookie jar. But what will one day be production code does everything correctly except produce the cookie jar. Included is the entire program, standard error, and strace output. Also, my toy program which succeeds in producing a cookie jar. If anyone wants to take a stab at this that would be peachy. I'm looking for avenues to explore and good questions to ask, as I am out of both. P.S. Thanks to everyone who got me this far. If this is what we end up having to use, it does get the job done. I'm just trying to get rid of my kludge work-around. > {-# LANGUAGE FlexibleContexts #-} > module Main where ------------------------------------------------------------------------------ -- Imports ------------------------------------------------------------------------------ > import Control.Monad > import Control.Monad.Error > import Network.Curl > import System.Environment (getArgs) > import HtmlParsing > import SessionCreator > main :: IO () > main = do > curl <- initCurl > user:pass:_ <- getArgs > resp <- generateResourceHtml curl user pass > > case resp of > Left err -> print err > Right body -> obtainCookies curl body > {-# LANGUAGE FlexibleContexts #-} > module SessionCreator > ( > initCurl, > curlResp, > urlBase, > urlInitial, > urlLogin, > urlLogOut, > urlFlash1, > urlFlash2, > urlLaunch, > urlLaunchTest, > urlQuickCreate, > urlGetResource, > urlShowWebForwards, > respBody, > -- login > resourceOpts, > loginOpts > ) where ------------------------------------------------------------------------------ -- Imports ------------------------------------------------------------------------------ --import qualified Data.ByteString as B > import Network.Curl > import Control.Monad > import Control.Monad.Error > initCurl :: IO Curl > initCurl = do > curl <- initialize > setopts curl curlOpts > return curl > curlOpts :: [CurlOption] > curlOpts = > [ CurlCookieSession True > , CurlCookieJar "cookies.txt" > , CurlCookieFile "cookies2.txt" > , CurlFollowLocation True > , CurlUserAgent "Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)" > , CurlVerbose True > , CurlHeader True > , CurlAutoReferer True > , CurlFailOnError True > ] > --- loginOpts looks like this for now > loginOpts :: String -> String -> [CurlOption] > loginOpts user pass = > [ CurlFailOnError True > , CurlPost True > , CurlNoBody False > , CurlPostFields > [ "_charset_=UTF-8" > , "javaScript=true" > , "username=" ++ user > , "password=" ++ pass > ] > ] > 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" > ] > curlResp :: (Error e, MonadError e m, MonadIO m) => > Curl -> URLString -> [CurlOption] -> m String --CurlResponse > curlResp curl url opts = do > resp <- liftIO $ (do_curl_ curl url opts :: IO CurlResponse) > let code = respCurlCode resp > status = respStatus resp > if code /= CurlOK || status /= 200 > then throwError $ strMsg $ "Error: " ++ show code ++ " -- " ++ show status > else return $ respBody resp ------------------------------------------------------------------------------ URLs ------------------------------------------------------------------------------ > (//) :: String -> String -> String > a // b = a ++ "/" ++ b > urlBase :: URLString > urlBase = "https://172.16.1.18" > urlInitial = urlBase // "showLogon.do" > urlLogin = urlBase // "default/showLogon.do" > urlFlash1 = urlBase // "showFlashCheck.do" > urlFlash2 = urlBase // "logon.do?flashVersion=10.1.102" > urlShowWebForwards = urlBase // "showWebForwards.do" > urlGetResource = urlBase // "resourceList.do?form=webForwardsForm&readOnly=false&policyLaunching=true&resourcePrefix=webForwards&path=%2FshowWebForwards.do&messageResourcesKey=webForwards&actionPath=%2FresourceList.do" > urlCreateWebForward = urlBase // "quickCreateWebForward.do?actionTarget=create&actionPath=%2FquickCreateWebForward.do%3FactionTarget%3Dcreate&quickCreate=true&evalJS=true&evalScripts=true" > urlLogOut = urlBase // "logoff.do" > urlQuickCreate = urlBase // "quickCreateWebForward.do?actionTarget=create&actionPath=%2FquickCreateWebForward.do%3FactionTarget%3Dcreate&quickCreate=true&evalJS=true&evalScripts=true" > module HtmlParsing > ( > obtainCookies > ,generateResourceHtml > ) where > import Text.HTML.TagSoup > import Data.List.Split > import Data.List > import Data.Maybe > import Numeric > import Network.Curl > import Control.Monad > import Control.Monad.Error > import SessionCreator > processHTML :: String -> [[String]] > processHTML htmlFILE = > let parsedHTML = parseTags htmlFILE > allTagOpens = sections (~== TagOpen "a" [("href","")]) parsedHTML > taggedTEXT = head $ map (filter isTagOpen) allTagOpens > allHREFS = map (fromAttrib "href") taggedTEXT > allPotentials = map (dropWhile (/= '?')) allHREFS > removedNulls = filter (not . null) allPotentials > removedQs = map (drop 1) removedNulls > in map (splitOn "&") removedQs > createIDList :: [[String]] -> [String] > createIDList strungPairs = > let nvList = map (map (splitOn "=")) strungPairs > nvTuple = map (map (list2Tuple)) nvList > netofResources = map (lookup "resourceId") nvTuple > removedNothings = catMaybes netofResources > sortedIds = (map head . group . sort) $ map (\x -> read x :: Int) removedNothings > in map ("ResourceId" %%) $ map show sortedIds > list2Tuple :: [a] -> (a,a) > list2Tuple [x,y] = (x,y) > (%%) :: String -> String -> String > a %% b = a ++ "=" ++ b > generateResourceHtml :: Curl -> String -> String -> IO (Either String String) > generateResourceHtml curl user pass = do > let makeIDPage = do > curlResp curl urlInitial method_GET > curlResp curl urlLogin $ loginOpts user pass > curlResp curl urlFlash1 resourceOpts > curlResp curl urlFlash2 resourceOpts > curlResp curl urlShowWebForwards resourceOpts > curlResp curl urlQuickCreate resourceOpts > curlResp curl urlGetResource resourceOpts > runErrorT makeIDPage > obtainCookies :: Curl -> String -> IO () > obtainCookies curl responseBody = do > mapM_ (flip (curlResp curl) resourceOpts) $ > screenScraping responseBody > screenScraping :: String -> [String] > screenScraping responseBody = > let collectedStrings = processHTML responseBody > collectedIDLists = createIDList collectedStrings > in constructedResourceURIs urlBase collectedIDLists > constructedResourceURIs :: String -> [String] -> [String] > constructedResourceURIs url resourceIDs = > let frontURI = url ++ "/launchWebForward.do?" > midURI = map (frontURI ++) resourceIDs > in map (++ "&policy=0&returnTo=%2FshowWebForwards.do") midURI Here is the toy (produces a cookiejar) program. Attached are all the output files {-# LANGUAGE FlexibleContexts #-} module Main where ------------------------------------------------------------------------------ -- Imports ------------------------------------------------------------------------------ import Control.Monad import Control.Monad.Error import Network.Curl import System.Environment (getArgs) ------------------------------------------------------------------------------ -- Curl ------------------------------------------------------------------------------ initCurl :: IO Curl initCurl = do curl <- initialize setopts curl curlOpts return curl curlOpts :: [CurlOption] curlOpts = [ CurlCookieSession True , CurlCookieJar "cookies.txt" , CurlCookieFile "cookies.txt" , CurlFollowLocation True , CurlUserAgent "Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)" , CurlVerbose True ] loginOpts :: String -> String -> [CurlOption] loginOpts user pass = [ CurlFailOnError True , CurlPost True , CurlNoBody False , CurlPostFields [ "_charset_=UTF-8" , "javaScript=true" , "username=" ++ user , "password=" ++ pass ] ] curlResp :: (MonadError String m, MonadIO m) => Curl -> URLString -> [CurlOption] -> m String --CurlResponse curlResp curl url opts = do resp <- liftIO $ (do_curl_ curl url opts :: IO CurlResponse) let code = respCurlCode resp status = respStatus resp if code /= CurlOK || status /= 200 then throwError $ "Error: " ++ show code ++ " -- " ++ show status else return $ respBody resp ------------------------------------------------------------------------------ -- URLs ------------------------------------------------------------------------------ (//) :: String -> String -> String a // b = a ++ "/" ++ b urlBase :: URLString urlBase = "https://172.16.1.18" urlInitial = urlBase // "showLogon.do" urlLogin = urlBase // "default/showLogon.do" urlFlash1 = urlBase // "showFlashCheck.do" urlFlash2 = urlBase // "logon.do?flashVersion=9.0.100" urlLaunch = urlBase // "launchWebForward.do?resourceId=4&policy=0&returnTo=%2FshowWebForwards.do" urlLaunchTest = urlBase // "launchWebForward.do?resourceId=5&policy=0&returnTo=%2FshowWebForwards.do" curlResp :: (MonadError String m, MonadIO m) => Curl -> URLString -> [CurlOption] -> m String --CurlResponse curlResp curl url opts = do resp <- liftIO $ (do_curl_ curl url opts :: IO CurlResponse) let code = respCurlCode resp status = respStatus resp if code /= CurlOK || status /= 200 then throwError $ "Error: " ++ show code ++ " -- " ++ show status else return $ respBody resp ------------------------------------------------------------------------------ -- URLs ------------------------------------------------------------------------------ (//) :: String -> String -> String a // b = a ++ "/" ++ b urlBase :: URLString urlBase = "https://172.16.1.18" urlInitial = urlBase // "showLogon.do" urlLogin = urlBase // "default/showLogon.do" urlFlash1 = urlBase // "showFlashCheck.do" urlFlash2 = urlBase // "logon.do?flashVersion=9.0.100" urlLaunch = urlBase // "launchWebForward.do?resourceId=4&policy=0&returnTo=%2FshowWebForwards.do" urlLaunchTest = urlBase // "launchWebForward.do?resourceId=5&policy=0&returnTo=%2FshowWebForwards.do" ------------------------------------------------------------------------------ -- IO ------------------------------------------------------------------------------ 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 urlLaunch method_GET -- curlResp curl urlLaunchTest method_GET -- You'd use this to catch the exception right away -- `catchError` \e-> ... 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 So to sum up again, I'm trying to figure out why the toy program generates a cookie jar and the other program does everything it should *except* produce a cookie jar. Thanks to anyone who wants to slog through this.

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example of compostion
by Rainer Grimm 31 Jan '11

31 Jan '11

Hello, I'm looking for a good example of function composition. It's very impressive, how concise you can extract information out of a file in Haskell. So I write a short programm, which gets all title words out of a given file sorted by their length. How could the following programm be improved? import Data.Char (isUpper,isLower) import Data.List (sortBy) import Data.Ord (comparing) isTitle word= isFirstUpperCase ( word) && isTailLowerCase ( word ) where isFirstUpperCase= isUpper . head isTailLowerCase= all isLower . tail main = do putStrLn "The name of the input file:" file <- getLine inputString <- readFile file let allTitles= reverse . sortBy (comparing length ) . filter isTitle $ words inputString putStrLn $ show $ take 15 allTitles putStrLn $ show (length allTitles) ++ " titles in " ++ file Thanks in advance, Rainer -- Rainer Grimm rainer(a)grimm-jaud.de http://www.grimm-jaud.de <javascript:void(0);>

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Any advice on installing gtk2hs
by patricklynch 31 Jan '11

31 Jan '11

Good afternoon, Any advice on installing gtk2hs. I used the instructions at: http://www.haskell.org/haskellwiki/Gtk2Hs and get a bunch of error messages. Thanks.

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Re: [Haskell-beginners] Any advice on installing gtk2hs
by patricklynch 31 Jan '11

31 Jan '11

PS, this was for the Windows installation. -----Original Message----- From: patricklynch [mailto:kmandpjlynch@verizon.net] Sent: Monday, January 31, 2011 1:54 PM To: 'beginners(a)haskell.org' Subject: Any advice on installing gtk2hs Good afternoon, Any advice on installing gtk2hs. I used the instructions at: http://www.haskell.org/haskellwiki/Gtk2Hs and get a bunch of error messages. Thanks.

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Developing dependant libraries simultaneously
by John Smith 30 Jan '11

30 Jan '11

I am working on libraries A, B, and C simultaneously. B and C both depend on A. Whenever A changes, I have to rebuild an reinstall it for the changes to be picked up by B and C. Is there any way to tell cabal or GHC that B and C should import A, and check for changes when building? I'm using EclipseFP, but adding a project-reference does not appear to have any effect.

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Trouble with liftIO in ReaderT
by John Smith 30 Jan '11

30 Jan '11

I'm trying to attach a handler to a Gtk2Hs event, and get the error: No instance for (Control.Monad.IO.Class.MonadIO (mtl-1.1.0.2:Control.Monad.Reader.ReaderT (GHC.Ptr.Ptr EAny) IO)) arising from a use of `liftIO' Possible fix: add an instance declaration for (Control.Monad.IO.Class.MonadIO (mtl-1.1.0.2:Control.Monad.Reader.ReaderT (GHC.Ptr.Ptr EAny) IO)) This occurs with the sample at http://www.haskell.org/haskellwiki/Gtk2Hs/Tutorials/Intro import Graphics.UI.Gtk import Control.Monad.Trans(liftIO) main = do initGUI window <- windowNew window `on` deleteEvent $ liftIO mainQuit >> return False -- i.e., on window deleteEvent (liftIO mainQuit >> return False) widgetShow window mainGUI Why can't GHC find the instance MonadIO m => MonadIO (ReaderT r m)?

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[Haskell-Beginners] Laziness & referential transparency
by Tim Baumgartner 29 Jan '11

29 Jan '11

Hi everybody, after reading a post by Don Stewart on http://cgi.cse.unsw.edu.au/~dons/blog/2008/05/16#fast, I was not really satisfied with the necessity to rewrite mean xs = sum xs / fromIntegral (length xs) to an explicitly recursive function. I investigated a bit further and got really confused after the following GHCi session: Prelude> length [1..10^8] + length [1..10^8] 200000000 Prelude> let xs = [1..10^8] Prelude> length xs + length xs <interactive>: out of memory Can someone explain this and give a simple, reasonable rule by which I can predict such things in the future? Isn't it a pitfall? Tim

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A very counterintuitive behaviour of Haskell
by Renzo Orsini 28 Jan '11

28 Jan '11

In studying Haskell, I produced the following output from GHC: xxx-3:~ xxx$ 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. Prelude> let f 7 = "ok" Prelude> let f x = "no" Prelude> f 3 "no" Prelude> f 7 "no" I suppose it is correct. However, for someone who is interested in the language, it seems very counterintuitive... Somebody would be so kind to explain to a neophyte this "feature" of the language? Thank you very much. Renzo

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Re: [Haskell-beginners] A very counterintuitive behaviour of Haskell
by Renzo Orsini 27 Jan '11

27 Jan '11

Thank you very much to everybody. Your explanation is perfectly clear! I was reading the tutorial at http://learnyouahaskell.com/syntax-in-functions . Initially the tutorial cited GHCi, so I mistakenly wrote the functions on different lines. Thank very much again. I regained the desire of continuing my study... :-) Renzo On Jan 27, 2011, at 16:09 , jean verdier wrote: > You have defined 2 functions that are called f and only the last > definition is used (f x = "no"). > The definition you want should be written > let f 7 = "ok"; f x = "no" > so the function is defined once. > The problem comes from using the interpreter and not from haskell. > > > On Thu, 2011-01-27 at 15:55 +0100, Renzo Orsini wrote: >> In studying Haskell, I produced the following output from GHC: >> >> xxx-3:~ xxx$ 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. >> Prelude> let f 7 = "ok" >> Prelude> let f x = "no" >> Prelude> f 3 >> "no" >> Prelude> f 7 >> "no" >> >> >> I suppose it is correct. However, for someone who is interested in the language, it seems very counterintuitive... Somebody would be so kind to explain to a neophyte this "feature" of the language? >> >> Thank you very much. >> >> Renzo >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners > >

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