RE: Exceptions

Attached is another variant of the extensible exceptions idea, it improves on the previous designs in a couple of ways: there's only one catch & throw, regardless of what type you're throwing or catching. There is an extensible hierarchy of exceptions, and you can catch and re-throw subclasses of exceptions. So this design contains a dynamically-typed extensible hierarchy, but it's fairly lightweight. Adding a new leaf exception type requires 3 lines + 1 line for each superclass (just 1 line for a top level leaf, as before). Adding a new node requires about 10 lines + 1 line for each superclass, msotly boilerplate. Perhaps the type class hackers can do better than this! Cheers, Simon