[antlr-interest] Problems with semantic predicates

[Thomas Brandon]

http://fisheye2.cenqua.com/changelog/antlr?cs=4048 It's not in 3.0.1
but is in the latest 3.1 daily builds (I can only assume it is fully
implemented now, I haven't tried it).
But if as your example suggests you just want to alter the AST
construction not the actual parsing then I think you want predicated
rewrite rules not semantic predicates (as I believe Jim was suggesting
though I think he gave the wrong syntax). Like:
r2[boolean flag]
 : Token1
   -> {$flag}? ^(NODE Token1)
   -> Token1
 ;
I imagine these won't be hoisted and should only be run after any
backtracking so you shoudln't have any of your other troubles.

Tom.
On Fri, Apr 18, 2008 at 10:12 PM, Haralambi Haralambiev
<hharalambiev at gmail.com> wrote:
> Hello,
>
> Is there a way to lure Ter's interest to answer whether this braces (sorry
> for the mistake ;-) ) functionality is in place?
>
> I am using the latest version, 3.0.1, that is downloadable from the site, so
> I guess it's not implemented :-(
>
> Hari
>
>
>
> On 4/18/08, Johannes Luber <jaluber at gmx.de> wrote:
> > Haralambi Haralambiev schrieb:
> >
> > > Hi Johannes,
> > >
> > > I don't understand what's the benefit of the two brackets.
> > >
> > > When I write {{$r::flag = false;}} the code that ANTLR produces is
> "{((r_scope)r_stack.peek()).flag = false;}", i.e. the brackets are added
> also, but they are still in "if (backtracking == 0)" statement, which is the
> check I want to skip somehow.
> > >
> > > Hari
> > >
> >
> > The two braces (brackets are "[]") should be the syntax which enables an
> action to be executed always, even during backtracking. But either you are
> using an too old version of ANTLR or Ter didn't implement this variant after
> all.
> >
> > Johannes
> >
> >
> > >
> > > On 4/17/08, *Johannes Luber* <jaluber at gmx.de <mailto:jaluber at gmx.de>>
> wrote:
> > >
> > >    Haralambi Haralambiev schrieb:
> > >
> > >        Hello Johannes,
> > >
> > >        So, if I understand correctly, you suggest to have the following
> > >        (using a scope):
> > >
> > >        r2
> > >        : {$r::flag}? Token1 -> ^(NODE Token1)
> > >        | Token1
> > >        ;
> > >
> > >        If the above is a correct assumption, then how can I set the
> > >        scope variable, as I could only think of the following way:
> > >
> > >        r
> > >        scope {
> > >         boolean flag;
> > >        }
> > >        : Token1^ Token2!
> > >           {$r::flag = true;}    r2
> > >        | Token1^
> > >         {$r::flag = false;}
> > >         r2
> > >        ;
> > >
> > >        This solution, however, will fail, as the action for setting the
> > >        scope variable will not be executed until the backtracking is
> > >        over. But I need this variable set, as it will be used as a
> > >        predicate for the parsing. Thus, we have a catch-22 situation.
> > >
> > >        A solution I could think of is not to have two alternatives for
> > >        r2, but rather construct the CommonTree manually with java code
> > >        and within to use the knowledge of the boolean flag. But I
> > >        dislike this solution and am open to suggestions.
> > >
> > >
> > >    Try using {{$r::flag = true;}}.
> > >
> > >    Johannes
> > >
> > >
> > >
> >
> >
>
>