January 2011 - Beginners

haskell for FPGAS
by David Blubaugh 09 May '11

09 May '11

To All, Has anyone developed HASKELL FOR FPGA development ?? Thank You, Daid Blubaugh

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dealing with chained functions
by Alec Benzer 20 Apr '11

20 Apr '11

Is there a generally accepted "better" way of dealing with the following: say I have three functions that all rely on output from each other. Ie, we get raw input, which one function processes. another function then requires the output from this function and produces some other output, which a third function then requires. would it be better to implement the functions like this: f1 :: a -> b f2 :: b -> c f3 :: c -> d final :: a -> d or like this: f1 :: a -> b f2 :: a -> c f3 :: a -> d Ie, in the latter set, f2 calls f1 on its input, and then proceeds to process it. f3 calls f2 on its input, etc. In the former case, final would just chain all three functions together itself (or I might just do it in main or somewhere else that's appropriate) Are one of these methods generally considered a better practice than the other, specifically in a functional context? My gut tells me that the former method looks more appropriate, but I'm not sure if it'd really be the accepted practice regardless of context, or if one might use either of these methods depending on the specifics of what they're doing. Or if it's just a matter of taste/opinion?

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Informal proof of bijective mapping between Naturals and Natural pairs
by Lyndon Maydwell 03 Feb '11

03 Feb '11

Hi Beginners. I was musing over weather Naturals were isomorphic to Natural pairs and wrote the following "proof". I don't have any formal mathematical training beyond calculus, and was wondering if this was an okay approach to take, if there were any better techniques available, if my terminology makes sense, and if my use of Haskell could be improved. Any pointers would be greatly appreciated! The code follows, and is also available as a gist: https://gist.github.com/804248 ---------- import Control.Monad.Logic import Data.Maybe import Data.List import Data.Numbers.Primes import Test.QuickCheck -- This program demonstrates a mapping between the pairs of natural numbers and a subset of co-primes. -- The property should hold for all sized lists, not just pairs. Ordering is preserved. -- Question: Are we able to compress the range to create a bijection? -- Answer: Yes! We can use the breadth-wise indces of the products, rather than the products themselves. -- Pair to Number x_p = (evens !!) y_p = (odds !!) xy_p x y = x_p x * y_p y xy_c x = ns_nc . xy_p x -- Number to Pair p_xy n = (x,y) where x = fromJust $ findIndex (`divides` n) evens y = fromJust $ findIndex (`divides` n) odds c_xy = p_xy . nc_ns -- Sparse Number to Compact Number numbers = odds >>- \x -> evens >>- \y -> return (x*y) -- I don't really understand LogicT... :-( ns_nc n = fromJust $ findIndex (==n) numbers -- Compact Numner to Sparse Number nc_ns = (numbers !!) -- Helpers evens = map (primes !!) [0,2..] odds = map (primes !!) [1,3..] x `divides` y = y `mod` x == 0 -- Id referencing properties prop_p1 = forAll (elements [(x,y) | x <- [1..6], y <- [1..7]]) f where f (x,y) = (x,y) == c_xy (xy_c x y) prop_p2 = forAll (elements [1..100]) f where f c = c == uncurry xy_c (c_xy c)

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Installing HDBC
by Kathleen Lynch 02 Feb '11

02 Feb '11

Good morning, I'm trying to install Sqlite on a Windows PC... I'm using "Real World Haskell" as a guide in regard to this... It indicates that HDBC must be installed first and points to http://hackage.haskell.org/. I displayed this url and see the following in regard to downloading HDBC: Downloads a.. HDBC-2.2.6.1.tar.gz (Cabal source package) b.. package description (included in the package) If I'm correct, tar is a Unixy thing... Can someone tell me how to download this to a Windows Vista PC? Thank you

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Re: [Haskell-beginners] Any advice on installing gtk2hs
by patricklynch 01 Feb '11

01 Feb '11

To Aditya Siram ...thanks for writing the procedure in the link in the email below...... ...I got stuck on the command: 'cabal install gtk' ...it gives the following message: Linking: C:\Users\User\AppData\Local\cairo-0.12.043\cairo-0.12\dist\setup\setup.exe ... Cannot find gtk2hsC2hs Please install 'gtk2hs-buildtools' first and check that the install Directory is in your PATH (e.g.. HOME/.cabal/bin) My PATH is C:/Users/User/AppData/Roaming/cabal/bin Should I change my PATH? Good day -----Original Message----- From: patricklynch [mailto:kmandpjlynch@verizon.net] Sent: Monday, January 31, 2011 1:55 PM To: 'patricklynch'; beginners(a)haskell.org Subject: RE: Any advice on installing gtk2hs 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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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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