John Goerzen wrote:
* Catch doesn't work in many common cases anymore. In particular, the example right there in haddock is broken:
catch (openFile f ReadMode) (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
That's broken, of course, because the type of e isn't fixed. But it's also broken because openFile has a different return type than hPutStr.
Which means it has been broken for quite some time.
* I like the new system overall, but it is annoying that we can't do the sort of catch that is contemplated in the example above anymore. This is especially annoying for me in several cases where I want to catch *any* exception and do things with it, especially when doing unit testing.
You can actually do that - all you have to do is catch SomeException, say openFile f ReadMode `catch` \e -> do hPutStrLn stderr $ "Couldn't open " ++ f ++ ": " ++ show (e :: SomeException) throw e This works because SomeException is an instance of the Exception class with the obvious trivial implementations for toException and fromException. It should be documented, of course. onException does basically that - so it works for any exceptions: onException :: IO a -> IO b -> IO a onException io what = io `catch` \e -> do what throw (e :: SomeException) HTH, Bertram