[antlr-interest] double NOT removal during tree parsing?

[Tomas Potrusil]

I've found that the grammar produces correct "non-debugging" parser.
But when the code is generated with the -debug flag it behaves as
described below. So there must be a bug somewhere inside the
debugger...

Regards,
Tomas

2009/5/29 Tomas Potrusil <potrto at centrum.cz>:
> Halo,
>
> I'm writing a SQL grammar. I've decided to create a basic AST and then
> to optimize it a little bit during the tree parsing. One of the
> optimizations is to remove two consequent NOT nodes in a condition
> subtree. My naive implementation is this:
>
> condition
>        :       ^(OR condition condition)
>        |       ^(AND condition condition)
>        |       (^(NOT ^(NOT .))) => ^(NOT ^(NOT condition)) -> condition
>        |       ^(NOT condition)
>        |       predicate ;
>
> predicate
>        :       ^(comparisonOperator expression expression)
>
> This works perfectly fine for simple conditions (NOT NOT pred). But
> when the double NOT subtree is a part of a more complex condition (NOT
> NOT pred1 AND pred2) the processing goes wrong. When debugging I see
> the decisioning for the pred2 predicate is somehow sick. And even more
> the incoming tokens are not those what I expect should come in (when
> looking at the LT events).
>
> To be more explaning, the condition is this: NOT NOT col1 < col2 AND
> col3 > col4.
> So the pred2 predicate states for col3 > col4. By the first parsing it
> is transformed into: ^('>' col3 col4). When this is parsed by the tree
> parser (after the double not optimization of NOT NOT col1 < col2) the
> token '>' is consumed directly in the condition rule! And consecutive
> token (LT event) is not "col3" but some completely different, let's
> say X. With this X token the parser goes to predicate rule where it
> fails, of course (NoViableAltException).
>
> Do you see anything wrong in the stated tree grammar?
>
> Regards,
> Tomas
>