[Haskell-cafe] Taking Exception to Exceptions

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? How does bracket manage to catch all exceptions? What should onException do? Is there some example code that uses these exceptions, or better documentation? Thanks in advance, Immanuel