[antlr-interest] Rewrite action causing error in parser?

[Ross Bamford]

Further investigation confirms that this does seem to crop up quite a bit,
which suggests I'm definitely doing something wrong. I'm just hoping that
someone might be able to suggest a different way to do what I need to do
(i.e. insert an imaginary token in place of the optional one if it's not
specified)?

Thanks,
Ross


On Sun, Oct 16, 2011 at 11:53 PM, Maximilien Colange <macolange at gmail.com>wrote:

> It appears that this "bug" is frequently reported.
> It would be nice if ANTLR raised an error (or a warning) when a token is
> given a reference in a syntactic predicate.
>
> However, I do not know whether it is easy to detect. I already
> encountered this problem, and it occured in a "hidden" ANTLR-generated
> syntactic predicate. I am afraid the error is difficult to detect in
> such cases.
>
> And just for curiosity, why is not it possible to reference local
> variables or to assign from token in a syntactic predicate ?
>
> --
> Maximilien
>
> Le 10/15/11 11:34 PM, Jim Idle a écrit :
> > Your problem does not look to be the rewrite rule, but the fact that you
> > are referencing a local variable in a predicate, or have tried to assign
> > from a token in a predicate.
> >
> > Look for something like this
> >
> > ((id=IDENTIFIER)=>  id=IDENTIFIER)? ....
> >
> >
> > But regardless, this is the rewrite rule that is the problem as far as I
> > can see. Try commenting it out for instance.
> >
> > Jim
> >
> >> -----Original Message-----
> >> From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> >> bounces at antlr.org] On Behalf Of Ross Bamford
> >> Sent: Saturday, October 15, 2011 5:40 AM
> >> To: antlr-interest at antlr.org
> >> Subject: [antlr-interest] Rewrite action causing error in parser?
> >>
> >> Hi all,
> >>
> >> I have a grammar I'm currently working on (posted in another thread the
> >> other day), which has the following rule:
> >>
> >> meth_call_expr
> >>    :   (id = IDENTIFIER DOT)? func_call_expr ->  ^(METHOD_CALL {
> >> ($id==null) ?
> >> adaptor.create(SELF, "SELF") : adaptor.create(IDENTIFIER,
> >> $id.getText()) }
> >> func_call_expr)
> >>    ;
> >>
> >> As you can see, I'm using an action in the rewrite rule to insert
> >> either the
> >> (optional) IDENTIFIER, or an imaginary SELF node if IDENTIFIER is not
> >> specified. The problem I'm having is that this generates a parser that
> >> won't compile. Specifically, it generates the following bit of code
> >> (edited by hand for brevity and to highlight the error):
> >>
> >> /* **** [ CODE ] **** */
> >>      // $ANTLR start synpred6_BasicLang
> >>      public final void synpred6_BasicLang_fragment() throws
> >> RecognitionException {
> >>
> >>          Token =null; //<-- ERROR HERE
> >>
> >>          /* ... later on ... */
> >>
> >>          switch (alt23) {
> >>              case 1 :
> >>                  //
> >> C:\\Users\\chantelle\\workspace\\basiclang\\src\\com\\roscopeco\\basicl
> >> ang\\parser\\BasicLang.g:99:8:
> >> id= IDENTIFIER DOT
> >>                  {
> >>
> >> id=(Token)match(input,IDENTIFIER,FOLLOW_IDENTIFIER_in_synpred6_BasicLan
> >> g232);
> >> if (state.failed) return ; //<-- AND HERE
> >>
> >>                  match(input,DOT,FOLLOW_DOT_in_synpred6_BasicLang234);
> >> if
> >> (state.failed) return ;
> >>
> >>                  }
> >>                  break;
> >>
> >>          }
> >> /* **** [ END ] **** */
> >>
> >> Obviously the problem is the "Token =null" line, which should be "Token
> >> id = null". Changing it by hand fixes the errors and makes the parser
> >> work as expected.
> >>
> >> So I have two questions - is this the right way to go about inserting
> >> an imaginary token if an optional token isn't in the input, and if so,
> >> what am I doing wrong to cause the error above?
> >>
> >> Thanks in advance,
> >> Ross
> >>
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