Fourth Grant Report: Structured Error Messages

Progress on my grant for error message is slow but steady. Since my last report, I've done the following things:

• Merged the nom-exceptions Rakudo branch, so now you can reliably throw Perl 6 objects as exceptions.
• Implemented several error classes and roles in Rakudo
• Started to throw typed errors both from runtime libraries and from inside the compiler
• Hacked the default exception printer Rakudo to be much more flexible, for example you can now write exception classes that supress backtraces from the standard handler.
• Wrote tests for typed run time and compile time errors, and at the same time developed a test function that makes it easy to write such tests.

It's time for a quick review of how far I am along the various deliverables in the original grant proposal.

• D1: Specification. I think the hard work here is done already, what remains to do is finding good default and how to manipulate them (for example, how to generally switch on/off printing of backtraces?).
• D2: Error catalogue, tests: I've not worked on this one too much. The error classes and roles so far mostly served to exercise the implementation; going through the existing errors from the various compilers and formalizing them will be quite a bit of work, but only moderately complicated.
• D3: Implementation, documentation. Like D1, the hard part is mostly done. We can now throw errors from within the compiler actions and from the setting, next up will be the grammar. Then all places where errors are thrown need to be changed to use the new error classes. Again that'll be much work, but easy to do. Documentation is still missing.

All in all I feel I'm well on the way, and most complex decisions have been made.

For a more user oriented view of the new exception system I'd like to point you to my Perl 6 advent calendar post on exceptions.