- Beginners - Haskell.org

Problems with Data types a la carte
by Rodrigo Geraldo 23 Apr '10

23 Apr '10

Hi ! I've tried to implement the expression interpreter present in Data types a la carte, putting the new "cases" of my data type in a separate module. When I'm trying to create a simple expression like this: x :: Expr (Val :+: Add :+: Mult) x = (val 2000 `mult` val 399) `plus` (val 3) I'm getting the following type error that I can't solve: src/Main.hs:25:4: No instance for (Add :<: Mult) arising from a use of `plus' at src/Main.hs:25:4-43 Possible fix: add an instance declaration for (Add :<: Mult) In the expression: (val 2000 `mult` val 399) `plus` (val 3) In the definition of `x': x = (val 2000 `mult` val 399) `plus` (val 3) src/Main.hs:25:5: No instance for (Val :<: Mult) arising from a use of `val' at src/Main.hs:25:5-12 Possible fix: add an instance declaration for (Val :<: Mult) In the first argument of `mult', namely `val 2000' In the first argument of `plus', namely `(val 2000 `mult` val 399)' In the expression: (val 2000 `mult` val 399) `plus` (val 3) I've attached the source code... Regards Rodrigo

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Generating Random Float
by Giorgio Stefanoni 23 Apr '10

23 Apr '10

Hi guys, I'm working with Haskell for a little project at the university and I'm stuck with a problem. I'm trying to generate a random number in the interval [0.0,1.0]. It is straightforward for me to implement the function on ints: rand :: IO Int rand = do r <- newStdGen let (x1, r2) = randomR (0,1) r print x1 return x1 However, if I try to generate a Float instead, I always get an Arithmetic Overflow error that I can not understand. Code: rand :: IO Float rand = do r <- newStdGen let (x1, r2) = randomR (0.0::Float,1.0::Float) r print x1 return x1 Do you have any clue why this happens? Thanks in advance, -- Giorgio

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eve / did = 0.talktalktalk...
by Alexander.Vladislav.Popov 22 Apr '10

22 Apr '10

Dear Everebody. Hepl me please to parallelize (parallel computing of evedidtalk function) the rebus: -- | eve / did = 0.talktalktalk... ten :: Integral a => [a] ten = [0..9] infixr 7 /: (/:) :: (Integral a) => [a] -> [a] -> [a] (/:) [] _ = [0] (/:) _ [] = [] (/:) x y = coldiv (getInteger x) (getInteger y) getInteger :: (Num a) => [a] -> a getInteger = foldl ((+) . (*10)) 0 coldiv :: (Integral a) => a -> a -> [a] coldiv a b = q : if r == 0 then [] else coldiv (r * 10) b where (q, r) = a `quotRem` b evedidtalk = [ ([e, v, e], [d, i, d], [t, a, l, k]) | e <- ten, v <- ten, v /= e, d <- ten, d /= e, d /= v, i <- ten, i /= e, i /= v, i /= d, t <- ten, t /= e, t /= v, t /= d, t /= i, a <- ten, a /= e, a /= v, a /= d, a /= i, a /= t, l <- ten, l /= e, l /= v, l /= d, l /= i, l /= t, l /= a, k <- ten, k /= e, k /= v, k /= d, k /= i, k /= t, k /= a, k /= l, take 9 ([e, v, e] /: [d, i, d]) == [0, t, a, l, k, t, a, l, k] ] Sincerely, Alexander

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sunrise sunset
by Markus Böhm 22 Apr '10

22 Apr '10

Is there any Haskell package/impl already that calculates sunrise and sunset for a given location (via lat/lon)? -- Mike

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cabal re-configure not recognized
by Ashish Agarwal 21 Apr '10

21 Apr '10

When changing a cabal file, if one runs 'cabal build' immediately, a message states that you must re-configure. So I run 'cabal configure' and then again try 'cabal build', but I get the same message again. The problem goes away if I wait about a minute. So I suspect there is an issue with the file modification time being ahead of what cabal thinks the time is. Has anyone experienced this problem, and know what might be the fix? Thank you. The issue occurs only on my Ubuntu machine. I do not have this problem with the same code on Mac OS X.

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Creating an exe file
by legajid 21 Apr '10

21 Apr '10

Hi, After running interpreted programs, i want to create an exe file; in a first time, just for my local machine. Later shall i try to install on other machines ... To do this, i wrote a batch file : d: cd d:\prog\lang\haskell set shs=%1 ghc %shs% -o%shs:hs=exe% del %shs:hs=hi% del %shs:hs=o% pause where %1 is my hs file full path. Example of a run : D:\>cd d:\prog\lang\haskell D:\PROG\lang\haskell>set shs=E:\didier\haskell\cases.hs D:\PROG\lang\haskell>ghc E:\didier\haskell\cases.hs -oE:\didier\haskell\cases.exe D:\PROG\lang\haskell>del E:\didier\haskell\cases.hi D:\PROG\lang\haskell>del E:\didier\haskell\cases.o D:\PROG\lang\haskell>pause Appuyez sur une touche pour continuer... It's ok with some modules i have to import, like import Data.List import Data.Maybe But, when using wxhaskell, i get a lot of errors, although i include import Graphics.UI.WXCore import Graphics.UI.WX here is a small list of errors : E:\didier\haskell\graphe.o:fake:(.data+0x2f0): undefined reference to `wxcorezm0zi11zi1zi2_GraphicsziUIziWXCoreziDialogs_infoDialog_closure' E:\didier\haskell\graphe.o:fake:(.data+0x300): undefined reference to `wxzm0zi11zi1zi2_GraphicsziUIziWXziDraw_zdf4_closure' E:\didier\haskell\graphe.o:fake:(.data+0x314): undefined reference to `wxzm0zi11zi1zi2_GraphicsziUIziWXziDraw_zdf4_closure' what should the correct ghc command line be, in order to get a successful compilation ? Thanks for help, Didier Running ghc 6.10.4 on Windows XP SP2, with wxhaskell-0.11.1.2

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ANN: scan-0.1.0.3, a Haskell style scanner
by Christian Maeder 19 Apr '10

19 Apr '10

Dear Haskell friends, I like to announce a Haskell style scanner at http://hackage.haskell.org/package/scan documented under http://projects.haskell.org/style-scanner/ It's best used in conjunction with hlint http://community.haskell.org/~ndm/hlint/ and gives many suggestions regarding the file format. Feedback is welcome. Thanks Christian (Discussions should only be posted to either haskell-cafe@ or beginners@)

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Re: [Haskell-beginners] Beginner trying to install Haskell support in Eclipse and failing
by Alan Shaw 17 Apr '10

17 Apr '10

I installed scion from hackage ok, but then I remembered this, from http://eclipsefp.sourceforge.net/ : You cannot use the version from Hackage (0.1.0.2) since commands have been added for eclipsefp. You'll need to build Scion from source (git clone git://github.com/JPMoresmau/scion.git, runhaskell Setup.hs configure/build/install) So I did the other thing: cabal install -f-cabal_1_8 And now I have got updates in Eclipse from http://eclipsefp.sf.net/updates, and I'm about to "Create a Haskell Project." so if that process doesn't mystify me I'm good to go for now. And I plan to update ghc "soon." Thanks, all! -A On Sat, Apr 17, 2010 at 7:43 PM, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: > > Am Sonntag 18 April 2010 01:07:17 schrieb Alan Shaw: > > Hi, > > I'm using cabal for the first time, and I'm confused. > > > > I'm trying to build scion as instructed at > > http://eclipsefp.sourceforge.net/build.html . > > I have done: > > > > git clone git://github.com/JPMoresmau/scion.git > > cd scion > > cabal install > > > > > > I'm now at the point where this tells me the following: > > > > Resolving dependencies... > > cabal.exe: dependencies conflict: ghc-6.10.4 requires Cabal ==1.6.0.3 > > however > > Cabal-1.6.0.3 was excluded because scion-0.1.0.2 requires Cabal ==1.8.* > > && ==1.8.* > > That's odd, the .cabal file says > > if flag(cabal_1_8) > build-depends: Cabal == 1.8.* > cpp-options: -DCABAL_VERSION=108 > else > build-depends: Cabal == 1.6.* > cpp-options: -DCABAL_VERSION=106 > > and since you don't have Cabal-1.8.*, it should set the flag to false and > try again. > > Anyway, try setting the flag to false on the command line: > > cabal install -f-cabal_1_8 > > or install the hackage package: > > cabal install scion > > > > > Some more information: > > > > $ cabal -V > > cabal-install version 0.6.2 > > using version 1.6.0.3 of the Cabal library > > $ cabal update > > Downloading the latest package list from hackage.haskell.org > > Note: there is a new version of cabal-install available. > > To upgrade, run: cabal install cabal-install > > $ cabal install cabal-install > > (lots and lots of output, ending with:) > > Linking dist\build\cabal\cabal.exe ... > > Installing executable(s) in C:\Program Files (x86)\Haskell\bin > > Is that directory in your path (and before the directory cabal- > install-0.6.2 is in)? > > > $ cabal -V > > cabal-install version 0.6.2 > > using version 1.6.0.3 of the Cabal library > > perhaps > > cabal install cabal-install-0.8.0 (or whatever the latest version working > on windows is) > > > > > > > Help? > > > > Thanks, > > -A >

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Beginner trying to install Haskell support in Eclipse and failing
by Alan Shaw 17 Apr '10

17 Apr '10

Hi, I'm using cabal for the first time, and I'm confused. I'm trying to build scion as instructed at http://eclipsefp.sourceforge.net/build.html . I have done: git clone git://github.com/JPMoresmau/scion.git cd scion cabal install I'm now at the point where this tells me the following: Resolving dependencies... cabal.exe: dependencies conflict: ghc-6.10.4 requires Cabal ==1.6.0.3 however Cabal-1.6.0.3 was excluded because scion-0.1.0.2 requires Cabal ==1.8.* && ==1.8.* Some more information: $ cabal -V cabal-install version 0.6.2 using version 1.6.0.3 of the Cabal library $ cabal update Downloading the latest package list from hackage.haskell.org Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install $ cabal install cabal-install (lots and lots of output, ending with:) Linking dist\build\cabal\cabal.exe ... Installing executable(s) in C:\Program Files (x86)\Haskell\bin $ cabal -V cabal-install version 0.6.2 using version 1.6.0.3 of the Cabal library Help? Thanks, -A

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calling polymorphic function in Selenium question
by MH 16 Apr '10

16 Apr '10

I am running the following code that is using Selenium. If you look at the function "start", you will see that the return type of the function is polymorphic. Now in main function, I call start function to get selenium IO monad and sequentially call selenium commands (open, doCommand etc...). The problem that I have here is, while I can call all Selenium commands with signature (String -> Selenium String), I can't call commands with signature (String -> Selenium Bool). As I understand it, even though "start" function shall return IO (Selenium a -> IO (Either String a)), it actually return IO (Selenium String -> IO (Either String String)). How shall go about fixing this problem? I need to be able to call both types of Selenium commands 1. doCommand :: SCommand -> [String] -> Selenium String OR selectFrame :: String -> Selenium String AND 2. isTextPresent :: String -> Selenium Bool Thanks. ========================================================= module SeleniumTest where import Control.Monad.Error import Data.Maybe import Network.BSD import Network.URI import Test.Selenium.Server import Test.Selenium.Syntax infixr 0 $$ -- | Starts up a session and returns a wrapper function that will run -- commands. Gives common defaults for browser and host. start :: String -> IO (Selenium a -> IO (Either String a)) start url = do -- host <- getHostName start' Firefox "localhost" url -- | Starts up a session and returns a wrapper function that will run -- commands. start' :: Browser -> HostName -> String -> IO (Selenium a -> IO (Either String a)) start' browser host url = do let uri = fromJust (parseURI url) sel = mkSeleniumRCSession host browser uri result <- runSelenium sel startSelenium return $ runSelenium (either (\msg -> error msg) id result) ($$) :: Show t => (Selenium () -> r) -> Selenium t -> r ($$) s c = s $ do r <- c; liftIO (putStrLn $ "Result: " ++ show r); return () -- | Stops a session (in the wrapper returned by start) stop :: Selenium () stop = stopSelenium --main::IO() main = do selenium <- start "http://www.google.com" selenium $ open "/" selenium $ doCommand SWindowMaximize [] selenium $ typeText (Name "q") "stuff" selenium $ clickAndWait (Name "btnG") return selenium ===============================================================

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