[antlr-interest] Re: Newbie question reg. expression grammar

[andreaszielke21]

Stupid mistake!!! (or rather stupid me :( )

I defined expr as
expr      : sumExpr SEMI;

and atom as 
atom      : INT 
          | LPAREN^ expr RPAREN! ;

A nested expression with this grammar needs to be terminated by a 
semicolon -- not what I intended...




--- In antlr-interest at yahoogroups.com, "andreaszielke21" 
<andreas.zielke at g...> wrote:
> Hi,
> I'm trying to get the hang of grammars, parsers etc.
> 
> I started by reading the excellent introduction to Antlr by Ashley 
> Mills. 
> 
(http://supportweb.cs.bham.ac.uk/documentation/tutorials/docsystem/bui
> ld/tutorials/antlr/antlrhome.html)
> 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()); }
>   ;
> 
> Now simple expression without brackets are parsed correctly, but 
even 
> (3+5);
> yields a unexpected token error, which I don't understand.
> I tried to understand the problem by utilizing the 
article "Debugging 
> and Testing Grammars [...]" 
> (http://www.antlr.org/article/parse.trees/index.tml) by Mr. Parr 
and 
> got the following list of 
> derivations: 
> 
> derivation:
>     <expr>
>  => <sumExpr> EOF
>  => <prodExpr> EOF
>  => <powExpr> EOF
>  => <atom> EOF
>  => ( <expr> EOF EOF
>  => ( <sumExpr> ; EOF EOF
>  => ( <prodExpr> + <prodExpr> ; EOF EOF
>  => ( <powExpr> + <prodExpr> ; EOF EOF
>  => ( <atom> + <prodExpr> ; EOF EOF
>  => ( 3 + <prodExpr> ; EOF EOF
>  => ( 3 + <powExpr> ; EOF EOF
>  => ( 3 + <atom> ; EOF EOF
>  => ( 3 + 5 ; EOF EOF
> 
> I don't understand why there's a second(!) EOF and why the 
derivation 
> from atom to (expr) doesn't work... :(
> Could somebody give me a hint, please?
> 
> Thanks in advance,
> Andreas



 
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