On Fri, May 22, 2009 at 01:20:39AM -0500, aditya siram wrote:
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?
Because the call to 'catch' only applies to 'getResponseBody rsp', which is already inside the 'return $ Right $'. Perhaps you want to put parentheses around (return $ Right $ getResponseBody rsp) ? -Brent