[antlr-interest] Bug in Antlr - confusion on ANTLR grammar syntax???

[Sebastian Kaliszewski]

John MccLain wrote:
> I am not understanding this at all. I am building a grammar
> to parse powerbuilder. Here is all of the parser grammar - I reduced it down
> to localize
> the confusion
> 
> expression: simpleexp ((LE|GE|NE|EQ|LT|GT) simpleexp )?
> ;
> 
> simpleexp:term
> ;
> 
> term: factor
> ;
> 
> factor: NUM
> ;
> and the lexer grammar (k = 4) for NUM is:
> 
> NUM: ('0'..'9')+
> {System.out.println("2" + getText());}
> ;
> 
> What I dont understand is the ANTLR grammar syntax (I think).
> Given an input of "1" (without the quotes), The rule 'simpleexp
> ((LE|GE|NE|EQ|LT|GT) simpleexp )?'
> causes the parser to process then hang inside the parser, and not exit.
> However, when I change the rule to
> just 'simpleexp', the parser does not hang, and I exit gracefully out of
> main. The parser start rule is 'expression'.

I suspect id dos not hang it simply just waits for more input probably. 
Does entering one more symbol (or sending the eof) still yeild no 
result? (I think it will return then).

> My question is - why does 'simpleexp ((LE|GE|NE|EQ|LT|GT) simpleexp )?' not
> work? the parenthetic stmt is OPTIONAL - I wouldn't think it should cause a
> different behavior than just 'simpleexp'.

Of course it should. Parser must check if input does not fit the 
(LE|GE|NE|EQ|LT|GT) part before deciding whether to return or to 
continue processing.


> If I use 'simpleexp | simpleexp
> ((LE|GE|NE|EQ|LT|GT) simpleexp )?', all is well,

Probalby you have just 1 token lookahead (if you didn't set the 
lookahead in your parser the default is 1). Then the rule is ambigous 
and Antlr just chooses the first one which terminates immediately.

> but I don't understand why
> I should have to do this???

Your start rule should have have EOF checking in it.
something like:

expression: simpleexp ((LE|GE|NE|EQ|LT|GT) simpleexp )? EOF


EOF checking is not implicit since many languages allow arbitrary 
garbage afterthe program.

rgds
-- 
Sebastian Kaliszewski


 
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