[antlr-interest] Bug in Generated Parser?

[Johannes Luber]

Terence Parr schrieb:
> 
> On Jul 17, 2008, at 2:05 PM, Gavin Lambert wrote:
> 
>> At 00:51 18/07/2008, Johannes Luber wrote:
>> >> I'm seeing different behavior running my parser with a syntax
>> >> error the second time run. I've tracked it down to where I
>> >> believe the problem is, located in BaseRecognizer.cs. The
>> >> "follow.Remove( Token.EOR_TOKEN_TYPE )" logic is being
>> >> called, which modifies the state of the static readonly
>> >> BitSet object found on the parser. The problem with that
>> >> is that even though I am creating a completely different
>> >> parser instance, the static variable has been altered for
>> >> as long as the assembly is in memory. I've copied a simple
>> >> program to illustrate the problem at the bottom of this mail.
>> >
>> >I haven't heard of it yet, but it's a Java target problem, too,
>> >so Ter has to say, if removing the static modifier for the
>> >BitSet variables in the parser is enough and doesn't affect
>> >anything else.
>>
>> I don't think that's sufficient -- after all, a single instance might 
>> be re-used, or the rule-path followed might query that structure again 
>> (expecting it to be in its unmodified state).
> 
> I think i saw this...seems I forced it to dup after I found this...
> 
>> Read-only variables should never be modified.  Any code that is doing 
>> so is clearly erroneous.  (Perhaps it should be modifying a clone of 
>> the object instead of the original?)
>>
> 
> yes, a clone.  Ok, looks like mine is ok:
> 
>     protected BitSet combineFollows(boolean exact) {
>         int top = state._fsp;
>         BitSet followSet = new BitSet();
>         for (int i=top; i>=0; i--) {
>             BitSet localFollowSet = (BitSet)state.following[i];
>             /*
>             System.out.println("local follow depth "+i+"="+
>                                
> localFollowSet.toString(getTokenNames())+")");
>              */
>             followSet.orInPlace(localFollowSet);
>             if ( exact ) {
>                 // can we see end of rule?
>                 if ( localFollowSet.member(Token.EOR_TOKEN_TYPE) ) {
>                     // Only leave EOR in set if at top (start rule); 
> this lets
>                     // us know if have to include follow(start rule); 
> i.e., EOF
>                     if ( i>0 ) {
>                         followSet.remove(Token.EOR_TOKEN_TYPE);
>                     }
>                 }
>                 else { // can't see end of rule, quit
>                     break;
>                 }
>             }
>         }
>         return followSet;
>     }
> 
> Note that it is creating a new BitSet at the start...it is removing only 
> from that set.
> 
> Ter
> 
BaseRecognizer.cs has the same function. Whatever the reason is, it's 
not there.

Johannes