[Haskell-beginners] Error Handling and case statements

[Daniel Fischer]

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"