[antlr-interest] Skipping or ignoring whole subtrees in a tree grammar

[Frank Smith]

Use the wild card.  That causes a tree grammar to skip a token e.g.:

...
| ^(DELETED_FOO .)
...

you can still access bar if you want by using DELETED_FOO.start.getChild(0);
if you want to use it in an action somehow, but not be stepped through
automatically by the walker.  I used this a lot- works great.

Best regards,
     Frank

On 8/13/07, Hardy, Stephen <Stephen_Hardy at rabbit.com> wrote:
>
> Hi all,
>
> supposing I have a tree grammar which includes something like
>
> foo
>    : ^(FOO          bar  { some action... } )
>    | ^(DELETED_FOO  bar)
>    ;
>
> Suppose bar is a complex, deeply recursive, subtree; and I wish to avoid
> as much as possible the overhead of parsing it in the DELETED_FOO case.
> (Imagine that DELETED_FOO was once a FOO, but the parser or application
> decided after the fact that it was an irrelevant part of the input, and
> changed the token to mark it as deleted, but without actually removing
> the subtree.  Why not delete the tree?  Well, a different tree grammar
> may still want to see the contents, just not this one.)
>
> Is there a way of telling ANTLR to basically 'skip' any analysis of the
> subtree under DELETED_FOO?  For example, a routine which simply consumes
> tokens from the DOWN which immediately follows DELETED_FOO until the
> matching (balanced) UP token is seen?
>
> I seem to recall a mention of something like this in TDAR, but darned if
> I can locate it.
>
> Regards,
> SJH
>
>
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