Re: [Haskell-beginners] Error Handling and case statements

2 Nov 2009

      Am Montag 02 November 2009 05:57:41 schrieb iæfai:
...
I have been trying to work out a problem for the last few hours with  
little success.
In the following code, using ConfigFile, I obtain the results of the  
configuration file, but in the main function I am trying to get the  
Config type out of the case statement. I need to be able to generate  
that error, but it means the two branches of the case are not the same  
type.
I am not particularly attached to this direction, I am quite willing  
to do any way that works. I might be adding more configuration in the  
future.
Any ideas?
iæfai
--
import Network.Shed.Httpd
import Network.URI
import Data.Either
import Data.ConfigFile as C
import Control.Monad.Error
import Control.Applicative
import ChessBoard
data Config = Config { documentRoot :: String } deriving (Read, Show)
main :: IO ()
main = do
     opt <- getConf "./config"
     config <- case opt of
         Left (_, err) -> ioError (userError err)
         Right (config) ->  config
     docPath <- documentRoot config
Wrong type here, documentRoot config :: String
...
     putStrLn "Starting up httpd."
     server <- initServer 6666 request
     return ()
main = do
    opt <- getConf "./config"
    case opt of
        Left (_,err) -> ioError (userError err)
        Right config -> do
            let docPath = documentRoot config
            putStrLn "Starting up httpd."
            server <- initServer 6666 request
            return ()

-- though if you don't use the server later, it would be better to replace the last two 
lines with just "initServer 6666 request"

Perhaps better to separate getting the config from using it:

main = do
    opt <- getConf "./config"
    case opt of
        Left (_,err) -> ioError (userError err)
        Right config -> workWith config

workWith config = do
    let docPath = documentRoot config
    putStrLn ...
...

request :: Request -> IO Response
request req = do
     putStrLn $ "Recieved " ++ (show $ uriPath $ reqURI req)
     return $ Response 404 [] "Not found."
-- Mostly from Chris Done's Blog
getConf :: FilePath -> IO (Either (C.CPErrorData, String) Config)
getConf filePath = runErrorT $ do
        let cp = C.emptyCP { optionxform = id }
        contents <- liftIO $ readFile filePath
        config <- C.readstring cp contents
        let get = C.get config "DEFAULT"
        Config <$> get "Document-Root"

Re: [Haskell-beginners] Error Handling and case statements

Daniel Fischer