October 2014 - Beginners

"Delegating" class instance definition
by Nicolaas du Preez 12 Oct '14

12 Oct '14

data Cycle = Cycle { name :: String, size :: Double } deriving (Eq, Show) instance Ord Cycle where (<) a b = (<) (size a) (size b) -- problem statement! … How can I specify that, on the right-hand side of the problem statement, I refer to (<) :: Double -> Double -> Bool instead of (<) :: Cycle -> Cycle -> Bool? Is there another way to state that "Cycle is an instance of Ord based on size"?

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Connecting telnet connection to a wx widget
by Tilmann 04 Oct '14

04 Oct '14

Hi, I am stuck at this problem. I want to read from a telnet connection and display the result in a wxHaskell widget (textCtrl). I got the widget and I got the connection piped to stdout, but I don´t know how to connect the two. Any help is greatly appreciated! Versions: conduit 1.2.0.2, wx 0.91.0.0 Thanks a lot, Tilmann This is the code I got so far: module Main where import Control.Concurrent (forkIO, killThread) import Control.Monad.IO.Class (MonadIO, liftIO) import Control.Monad.Trans.Resource import Data.Conduit import Data.Conduit.Binary import Network (connectTo, PortID (..)) import System.IO import Graphics.UI.WX main = start gui gui = do f <- frame [] t <- textCtrl f [] e <- entry f [on click := (onClick t "...")] set f [layout := boxed "console" (grid 5 5 [[floatLeft $ expand $ hstretch $ widget t] ,[expand $ hstretch $ widget e]])] onClick t val pt = set t [ text :~ \x -> val] -- http://www.mega-nerd.com/erikd/Blog/CodeHacking/Haskell/telnet-conduit.html telnet :: String -> Int -> IO () telnet host port = runResourceT $ do (releaseSock, hsock) <- allocate (connectTo host $ PortNumber $ fromIntegral port) hClose (releaseThread, _) <- allocate (forkIO $ runResourceT $ sourceHandle stdin $$ sinkHandle hsock) killThread sourceHandle hsock $$ sinkHandle stdout

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Aeson: parsing json with 'data' field
by Miro Karpis 04 Oct '14

04 Oct '14

Hi, please can you help me with this.......I have a json file which contains a field with name "data". Problem is that I can not create a data type with "data", (or can I)? How else can I handle this? I know I can convert all json to Object and then search for the field....but I was hoping for some friendly/easier option. json example: { "data" : { "foo" : "bar" } } below definition returns: parse error on input ‘data’ data Foo = Foo { data :: String } Cheers, Miro

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Error message: "Couldn't match expected type"
by S. H. Aegis 02 Oct '14

02 Oct '14

Hello. I'm learning Haskell on OSX. How can I fix this error message? (Code from The Haskell School of Expression, P40) Sorry for border you... Sok. import Graphics.SOE main() = runGraphics ( do w <- openWindow "My First Graphics Program" (300, 300) drawInWindow w (text(100, 200) "HelloFraphicsWorld") spaceClose w ) spaceClose :: Window -> IO() spaceClose w = do k <- getKey w if k == ' ' then closeWindow w else spaceClose w [1 of 1] Compiling Main ( firstGraphics.hs, firstGraphics.o ) firstGraphics.hs:3:1: Couldn't match expected type `IO t0' with actual type `() -> IO ()' In the expression: main When checking the type of the function `main'

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Different behaviors between GHC 7.6.3 and 7.8.3
by Emilio De Camargo Francesquini 01 Oct '14

01 Oct '14

Hello everyone, I'm quite intrigued about these different behaviors between GHCs 7.6.3 and 7.8.3: GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> let x = 3 + 4 Prelude> :print x x = (_t1::Integer) Prelude> :force x x = 7 Prelude> print _t1 7 Prelude> GHCi, version 7.8.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> let x = 3 + 4 Prelude> :print x x = (_t1::Num a => a) Prelude> :force x x = _ Prelude> print _t1 ghc: panic! (the 'impossible' happened) (GHC version 7.8.3 for x86_64-unknown-linux): tcTyVarDetails a{tv atm} [tv] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Is it really a bug or is it something I really shouldn't be doing? I've found this (fixed) bug (https://ghc.haskell.org/trac/ghc/ticket/8557) with a similar error output, but it does not seem to be the same case. Thanks Emilio

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