[antlr-interest] Help controlling parser decisions

[Thomas Brandon]

On 7/25/07, Ted Villalba <ted.villalba at gmail.com> wrote:
> Thanks for the responses.
> Seems straight forward enough to create the disambiguating semantic
> predicate, but perhaps Im not starting out with the right assumptions.
>
> If I want to accept near as a term if it begins( or ends) a sentence, then I
> thought I could do something like this:
>
> value   :  value_ -> ^(VALUE value_) ;
>
> value_  :  keyBOOL terms* (operator^ value)*
>             | LPAREN! value RPAREN! ( operator^ value)*
>             ;
>
> keyBOOL : {input.LT(1).getText().equals("NEAR")}? terms;
>
> terms   : WCHAR+  -> ^(TERMS WCHAR+ )
>            ;
>
> But when I try to enter SO=(NEAR apples oranges), the parser no likey.
> Still getting:
>      line 1:5 no viable alternative at input 'NEAR'.
>
> Am I missing an obvious puzzle piece ?
> I tried instead to assume all booleans were terms and then tested each of
> the terms in a similar approach, but wasn't successful yet at
> differentiating, on demand, the operators from the terms.
The problem in the above rules isn't obvious to me. Looks like it
should work, though you seem to have some uneeded + and *'s given that
term is already WCHAR+, but that shouldn't break it as far as I can
see.
Were you running it under the ANTLRWorks interpreter? That won't do
actions so it won't work there. You need to use the debugger in this
case.
I got the following grammar to parse your example fine as well as to
parse "near=(near apples oranges)", correctly handling the first
"near" as a tag and the second as a keyBool:
grammar WQL;

options{
    output=AST;
    ASTLabelType=CommonTree;
}

tokens{ TAG; VALUE; TERMS;} //imaginary token types

@header{
import java.util.HashMap ;
}

@members {
HashMap fieldMap = new HashMap();
}

start
   :(	query
   		{System.out.println("AST:\n"+$query.tree.toStringTree());}
   	)+
	;

query
	:	field
    ;

field
	:	tag '=' LPAREN value RPAREN -> ^('=' tag value)
    |	tag '=' terms -> ^('=' tag terms)
    |	qid
	;

value
	:	keyBOOL terms?
    ;

keyBOOL
	:	near
	|	far
	;

terms
	:	WCHAR+  -> ^(TERMS WCHAR+ )
    ;

tag	:	WCHAR
    ;

qid	: '#'! DIGIT
    ;

near:	{input.LT(1).getText().toLowerCase().equals("near")}? WCHAR
	;

far	:	{input.LT(1).getText().toLowerCase().equals("near")}? WCHAR
	;

DIGIT   : ('0'..'9');
WS      : (' '|'\t'|'\r'|'\n')+ {skip();};
LPAREN    : '(' ;
RPAREN    : ')' ;
QUOTE    : '"';
WCHAR   : ~('='|'('| ')'|'"'|' '|'\t'|'\n'|'\r'|'#')+;

AFAICT there are no changes to the basic method you gave. I removed
some stuff to simplify putting together the grammar and I cleaned up
some of the unneccessary +s and *s. But no major changes.

Tom.
>
> Thank you for the help,
> Ted
>
>
>