Better is this:
data MalformedAddressException = MalformedAddressException String
deriving (Show, Typeable)
throwDynIO x = throwIO (DynException $ toDyn x)
-- in inet_error
... throwDynIO (MalformedAddressException "blah blah") ...
-- in HAppS-Server
... Exception.catchDyn (inet_addr uri) (\(MalformedAddressException s) -> ...)
-- ryan
On Wed, Sep 17, 2008 at 5:51 PM, Marc Weber
On Sun, Jul 27, 2008 at 07:23:14PM +0200, Adrian Neumann wrote:
Hello,
I think it'd be nice if the compiler could warn me if there are any exceptions which I'm not catching, similar to checked exceptions in Java. Does anyone know of a possibility to do that in Haskell?
He, I have found a use case for your request: from network
inet_addr :: String -> IO HostAddress inet_addr ipstr = do withCString ipstr $ \str -> do had <- c_inet_addr str if had == -1 then ioError (userError ("inet_addr: Malformed address: " ++ ipstr)) else return had -- network byte order
from HAppS-Server:
host <- Exception.catch (inet_addr uri) -- handles ascii IP numbers (\_ -> getHostByName uri >>= \host -> case hostAddresses host of [] -> return (error "no addresses in host entry") (h:_) -> return h)
Very bad because this catches Exceptions thrown by trowTo as well, doesn't it?
On the other hand just catching the UserError can be useless if the maintainers decide to throw a custom Exception in the future (which can and should be done in the future when extensible exceptions are standard?)
In this case I would miss this update and miss to update the code. If we could only catch exceptions. Using Either would be another choice here. But it would lead to much more code.
Anyway It think using Either is better because it can't lead to code as shown above.
Another nice use case for Exceptions are timouts as implemented by HAppS as well. However I must conclude that a function call including the code above can just absorb my exception and rethrow another one (or in a worse case continue?) So maybe I have to change the TimOut code to do a forever (throwTo threadId TimOutException) to make sure it quits as fast as possible? This could lead to different trouble.
So I think using Either is the best option although there is some more code to write.
Marc Weber _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe