[antlr-interest] Again Cobol:
Dave Dutcher
dave at tridecap.com
Thu Jun 11 08:10:36 PDT 2009
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+f
rom+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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