Excerpts from Immanuel Litzroth's message of Wed Jan 07 16:53:30 -0600 2009:
I'm trying to use the new (for me at least) extensible exceptions and I am little amazed that I cannot get catch, try or mapException to work without telling them which exceptions I want to catch. What is the rationale behind this?
The rational is that it's normally not a good idea to simply gobble all exceptions; although the library makes it possible to do this anyway. You can either use the ScopedTypeVariables extension and do: ... `catch` \(e::SomeException) -> ... Or without an extension you can do: ... `catch` handler where handler :: SomeException -> IO a handler e = ... (It's really a matter of taste if you want to use a non-haskell98 extension, although considering that the new extensible exceptions library uses deriving data/typeable and existentials anyway, I think ScopedTypeVariables are the way to go.)
How does bracket manage to catch all exceptions? What should onException do?
onException takes an IO action and what to do if it fails - if the IO action fails, it is caught and your 'failure action' is run, followed by onException re-throwing the error.
Is there some example code that uses these exceptions, or better documentation?
The GHC docs don't have source-code links (don't know why,) but luckily in order to aid in using the new extensions system with older GHCs there has been a hackage package uploaded that provides the identical functionality: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/extensible-except... The source is here: http://hackage.haskell.org/packages/archive/extensible-exceptions/0.1.1.0/do... As for documentation e.g. haddock stuff, this is currently a bug as there is none: http://hackage.haskell.org/trac/ghc/ticket/2655 I recommend this paper for info, it's very easy to follow: http://www.haskell.org/~simonmar/papers/ext-exceptions.pdf Austin