[antlr-interest] Again Cobol:

[Mark Taylor]

Dave: Yes, Cobol is that evil.  The period closes all previous scopes that
don't require explicit scope terminators (some constructs like PERFORM do,
some don't (COMPUTE)).  Thanks, that worked.  I replaced the PERFORM rule
with:

perform:     'perform' ((stmt)=>stmt)+ 'end-perform' ;

and I no longer get the error.

Now some rhetorical questions: why does it work?  Or, why is it not
obvious?  Where are syntactic predicates explained?  Is this in the online
docs somewhere I can't find?  Clearly I have some studying to do.  I haven't
picked up T.P.s book yet but I aim too.



On Thu, Jun 11, 2009 at 8:42 AM, Dave Dutcher <dave at tridecap.com> wrote:

>  I'm new to Antlr myself, so maybe you already know everything I'm going
> to suggest.
>
> So your grammar has trouble with input like
>
> if A == B then move Y to Z move X to Y.
>
> And you're suggesting that the parser should just consume the move X to Y
> as part of the if statement, even though it could also be interpreted as
> another seperate statement?  This reminds me of the classic IF IF ELSE
> problem, but I don't have my Antlr book with me to lookup how that is
> usually solved.
>
> One method might be to just turn on backtracking.  Otherwise I would think
> you could add syntatic predicates like ((stmt)=>stmt)+ which, as I
> understand it, would make Antlr consume all the statements it can as that
> part of the rule.  I haven't tested this though.
>
> Dave
>
>
>
>  ------------------------------
> *From:* antlr-interest-bounces at antlr.org [mailto:
> antlr-interest-bounces at antlr.org] *On Behalf Of *Mark Taylor
> *Sent:* Thursday, June 11, 2009 7:39 AM
> *To:* antlr-interest at antlr.org
> *Subject:* [antlr-interest] Again Cobol:
>
> I'm working on a Cobol grammar (oh, the foolishness of youth...  wait I'm
> not THAT young...) and I need some advice about the ambiguities.  In
> particular I'm getting the famous: "error(211): CobolTest.g:11:30: [fatal]
> rule if has non-LL(*) decision due to recursive rule invocations reachable
> from alts 1,2.  Resolve by left-factoring or using syntactic predicates or
> using backtrack=true option."  Yes, this has come up before, but there was
> no clear answer.  This time I have a specific example (see below).
>
> Below is the smallest grammar which exhibits the problem.  You can see I
> have stmt+ in both the IF rule and the PERFORM rule.  The problem is the
> 'END-IF'?.  Since END-IF is optional in Cobol, there is no clear scope
> terminator.   I've tried left refactoring the (stmt+ ....) into a separate
> rule for both IF and PERFORM but that doesn't seem to work either.  I don't
> see how a syntactic predicate could be applied to this either.
>
> If I were writing this as a recursive descent parser by hand (I'm trying
> Antlr so I don't have to do this) I would write a statementlist() method
> that would simply loop on all statement beginnings keywords.  Then when an
> END-IF, ELSE, END-PERFORM, or some other arbitrary scope terminator appeared
> in the input the loop would simple exit and return the list of valid
> statements.  The question is: how to get Antlr to behave like this?
>
> Any advice is appreciated.
>
> <pre>
>
> grammar CobolTest;
>
> program:     sentence+ EOF;
>
> sentence:    stmt+ '.' ;
>
> stmt:        if
>         |    move
>         |   perform ;
>
> if:            'if' condition 'then'? stmt+ ('else' stmt+)? 'end-if'? ;
>
> move:        'move' ID 'to' ID ;
>
> perform:     'perform' stmt+ 'end-perform' ;
>
> condition:     ID '==' ID ;
>
> ID  :       ('a'..'z'|'A'..'Z')+ ;
> INT :       '0'..'9'+ ;
> NEWLINE:    '\r'? '\n' {skip();} ;
> WS  :       (' '|'\t')+ {skip();} ;
>
> </pre>
>
>
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