Re: [Haskell-beginners] Trying to use map

On Thursday 27 January 2011 00:36:57, Michael Litchard wrote:

google hosed my indentation

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obtainCookies :: IO Curl -> String -> IO () obtainCookies curl responseBody = do curl' <- curl mapM_ (flip (curlResp curl') resourceOpts) $ screenScraping responseBody

On Wed, Jan 26, 2011 at 3:32 PM, Michael Litchard wrote:

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Thank you.

Here is where things stand now

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obtainCookies :: IO Curl -> String -> IO () obtainCookies curl responseBody = do curl' <- curl mapM_ (flip (curlResp curl') resourceOpts) $

screenScraping responseBody

and the error

htmlParsing.lhs:78:2:

Couldn't match expected type `[Char]' against inferred type `GHC.IO.Exception.IOException' Expected type: String

Inferred type: IOError When using functional dependencies to combine MonadError IOError IO, arising from the dependency `m -> e' in the instance declaration at <no location info> MonadError String IO,

arising from a use of `curlResp' at HtmlParsing.lhs:80:29-42 When generalising the type(s) for `obtainCookies'

You have real problems now. The MonadError Class has a functional dependency, and there's the instance MonadError IOException IO where ... In curlResp, you give the constraint (MonadError String m, MonadIO m), so by the functional dependency, m can't be IO. I see two possibilities a) modify curlResp, 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 IOException is an instance of Error, as is String, so this is more general than the previous. You can't directly throw the error string, you have to pass it to strMsg before throwing. That's not much of a change, if possible, I'd do that. b) make a newtype wrapper IOS around IO, provide a MonadIO instance for that and an instance MonadError String IOS, then change obtainCookies (and what needs to be changed thereafter, such changes tend to propagate): obtainCookies :: IO Curl -> String -> IOS () obtainCookies curl responseBody = do curl' <- liftIO curl mapM_ (flip (curlResp curl') resourceOpts) $ screenScraping responseBody

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I'm way beyond my ken here, trying to grow. I have to do error handling (this will be production code when it grows up), got some suggestions on how to do so, but am flying blind in new territory.

Any suggestions on how to proceed would be much appreciated.