Hi,

I'm trying to get the hang of grammars, parsers etc.

I started by reading the excellent introduction to Antlr by Ashley Mills .

Starting with the simple expression evaluation example,
I found the "implementation" of nested expressions (chap. 9.1) a bit awkward, as it requires brackets around all expressions.

So I tried to change the grammar to this:

			

			class ExpressionParser extends Parser;
			

			

			options { buildAST=true; }
			

			

			expr      : sumExpr SEMI;
			

			sumExpr   : prodExpr ((PLUS^|MINUS^) prodExpr)*;
			

			prodExpr  : powExpr ((MUL^|DIV^|MOD^) powExpr)* ;
			

			powExpr   : atom (POW^ atom)? ;
			

			atom      : INT
			

			| LPAREN^ expr RPAREN! ;
			

			

			class ExpressionLexer extends Lexer;
			

			

			PLUS  : '+' ;
			

			MINUS : '-' ;
			

			MUL   : '*' ;
			

			DIV   : '/' ;
			

			MOD   : '%' ;
			

			POW   : '^' ;
			

			SEMI  : ';' ;
			

			LPAREN: '(' ;
			

			RPAREN: ')' ;
			

			protected DIGIT : '0'..'9' ;
			

			INT   : (DIGIT)+ ;
			

			

			{import java.lang.Math;}
			

			class ExpressionTreeWalker extends TreeParser;
			

			

			expr returns [double r]
			

			{ double a,b; r=0; }
			

			

			: #(LPAREN a=expr)       { r=a; }
			

			| #(PLUS a=expr b=expr)  { r=a+b; }
			

			| #(MINUS a=expr b=expr) { r=a-b; }
			

			| #(MUL  a=expr b=expr)  { r=a*b; }
			

			| #(DIV  a=expr b=expr)  { r=a/b; }
			

			| #(MOD  a=expr b=expr)  { r=a%b; }
			

			| #(POW  a=expr b=expr)  { r=Math.pow(a,b); }
			

			| i:INT { r=(double)Integer.parseInt(i.getText()); }
			

			;
			

		

(3+5);

I tried to understand the problem by utilizing the article Debugging and Testing Grammars [...] by Mr. Parr and got the following list of derivations:

			

			derivation:
			

			

			=>
			EOF
			

			=>
			EOF
			

			=>
			EOF
			

			=>
			EOF
			

			=> (
			EOF EOF
			

			=> (
			; EOF EOF
			

			=> (
			+
			; EOF EOF
			

			=> (
			+
			; EOF EOF
			

			=> (
			+
			; EOF EOF
			

			=> ( 3 +
			; EOF EOF
			

			=> ( 3 +
			; EOF EOF
			

			=> ( 3 +
			; EOF EOF
			

			=> ( 3 + 5 ; EOF EOF
			

		

Could somebody give me a hint, please?

Thanks in advance,

Andreas