Hello,

I'm building a grammar that supports boolean expressions. I'm using the usual expression grammar pattern, however in my case there's a slight complication: operator OR is implied when there's no operator between the two adjacent terms.

I've pulled sufficient amount of hair trying to generate the expected AST fragment for the or_expr rule. I just can't figure out how to use rewrite rule syntax to achieve what I need, namely "if there's only one and_expr then just output it, don't create new nodes; otherwise, create new subtree with OR token as root and the two and_expr as children.

Any advise will be appreciated.
I tried using multiple rewrites as mentioned in the "Definitive ANTLR Reference" book (bottom of page 168), but oddly enough I'm getting syntax errors...


Here's the grammar:

			

			grammar SearchQuery;
			

			options {
			

			output = AST;
			

			backtrack = true;
			

			}
			

			query     : or_expr+ EOF! ;
			

			or_expr   : and_expr (OR? and_expr)* ;
			

			and_expr  : atom (AND^ atom)* ;
			

			atom      : TERM | NOT^ TERM | LEFT_PAREN! or_expr RIGHT_PAREN! ;
			

			

			// lexer
			

			AND   : 'and' ;
			

			OR    : 'or' ;
			

			NOT   : 'not' ;
			

			LEFT_PAREN  : '(' ;
			

			RIGHT_PAREN : ')' ;
			

			TERM    : (LETTER | NUMBER)+ ;
			

			WS     : (~(LETTER | NUMBER | SYMBOL)+) { skip(); } ;
			

			fragment LETTER : 'a'..'z' | 'A'..'Z' ;
			

			fragment NUMBER : '0'..'9' ;
			

			fragment SYMBOL : LEFT_PAREN | RIGHT_PAREN ;