[antlr-interest] Again Cobol:

[Mark Taylor]

Thanks Dave and Jim, that helps a lot!

On Thu, Jun 11, 2009 at 10:10 AM, Dave Dutcher <dave at tridecap.com> wrote:

>  I read about syntatic predicates in TP's book.  And looking over the
> online docs, I can't really find where they are explained either.  A google
> search did turn up this article
> http://www.antlr.org/wiki/display/ANTLR3/How+to+remove+global+backtracking+from+your+grammar
>
>
> which looks like it might answer some of your questions.
>
> Dave
>
>
>  ------------------------------
> *From:* Mark Taylor [mailto:mttdgf at gmail.com]
> *Sent:* Thursday, June 11, 2009 9:34 AM
> *To:* Dave Dutcher
> *Cc:* antlr-interest at antlr.org
> *Subject:* Re: [antlr-interest] Again Cobol:
>
> 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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