[Haskell-beginners] Catching Network Connection Error

22 May 2009

      Hi all,
I am trying to Network.HTTP to connect to a website and catch the error if
the network is down. Here's what I have so far:

data Errors = DocumentParseError
            | WebsiteUnreachableError
            | ISBNNotFoundError

respHTML' :: ISBN -> IO (Either Errors (IO String))
respHTML' isbn =
    do
      rsp <- simpleHTTP (getRequest $ "
http://isbndb.com/api/books.xml?results=details&access_key="
                                        ++ key
                                        ++ "&index1=isbn&value1="
                                        ++ isbn :: Request_String)
      return $ Right $ getResponseBody rsp
                  `E.catch` (\(e :: E.SomeException ) ->
                                 return $ Left $ WebsiteUnreachableError)

I get the following error when I load it into GHCI:
    Couldn't match expected type `[Char]'
           against inferred type `Either Errors b'
      Expected type: IO String
      Inferred type: IO (Either Errors b)
    In the expression: return $ Left $ WebsiteUnreachableError
    In the second argument of `E.catch', namely
        `(\ (e :: E.SomeException)
              -> return $ Left $ WebsiteUnreachableError)'
Failed, modules loaded: none.

Control.Exception.catch has the following signature :
catch :: forall a. IO a -> (IOError -> IO a) -> IO a

I would expect that the handler function needs to return an IO (Either
Errors (IO String)). Why is it trying to return an IO String?

[Haskell-beginners] Catching Network Connection Error

aditya siram