Craig Ringer <craig@2ndquadrant.com> writes: > The other area where there's room for extension without throwing out the > whole thing and rebuilding is handling of new top-level statements. We can > probably dispatch the statement text to a sub-parser provided by an > extension that registers interest in that statement name when we attempt to > parse it and fail. Even then I'm pretty sure it won't be possible to do so > while still allowing multi-statements. I wish we didn't support > multi-statements, but we're fairly stuck with them.
Well, as I said, I've been there and done that. Things get sticky when you notice that those "new top-level statements" would like to contain sub-clauses (e.g. arithmetic expressions) that should be defined by the core grammar. And maybe the extension would also like to define additions to the expression grammar, requiring a recursive callback into the extension. It gets very messy very fast.
Yuck. You'd ping-pong between two parsers, and have to try to exchange sensible starting states. Point taken.
So even that seemingly not-that-bad restricted option turns out to be far from it, which just goes to show what a pit of snakes parser extensibility is...