[antlr-interest] Tree Rewrite Caveats

[Terence Parr]

Thanks...i'llLook at these before we go out.
Ter
On May 23, 2008, at 8:21 PM, Robin Hamilton-Pennell wrote:

> Hi all,
>
> Been playing around with the tree rewriting stuff in 3.1b1. It's much
> better than the previous test build was. I'm not through with my
> experimentation and likely will find more things, but I thought I'd
> make some notes on a few troubles I ran into, in case it helps anyone
> else.
>
> Firstly (this is probably true of tree parsers in general), if you use
> a custom class as your AST node and the Java target, you will probably
> need to add the method errorNode() in your TreeAdaptor class. This
> will also entail either adding error node utility methods to your AST
> node, or creating a custom subclass of CommonErrorNode. I could be
> completely misguided on this; please say so.
>
> Second, every (sub)rule that you are rewriting must match a tree node,
> I think. This seems obvious in retrospect, but it took me a while to
> figure out. That means that you can't do this:
>
> stmt:
>    expr   ->   expr
>    ;
>
> However, you can do either of the following (or change your AST
> generation code):
>
> stmt:
>    expr  // omit rewrite rule
>    ;
>
> stmt:
>    // (after changing your AST generation)
>    ^(EXPR expr)   ->   ^(EXPR expr)
>    ;
>
> I should mention that both of these issues involves the rewrite=true
> option. I haven't tested the effects of turning that off, actually.
>
> Again, I could simply be completely misguided on this, in which case,
> please enlighten me.
>
> Thanks,
> Robin