[antlr-interest] Noob needs help with: warning "Decision can match input such as X... using multiple alternatives"

[Norman Dunbar]

Evening Adam,

as one noob to another then, here's what I think. I noticed you haven't 
had any replies yet, so idf I put something obviously incorrect, I'm 
sure we'll get jumped on by those who know better than I do! Let's hope 
so anyway!


> I'm learning antlr in hopes of creating a DSL.  To this end I need to
> define a rule for nested "expressions" like:
>
>         (1+2)*((3-4)/5)+7
>

Does this help? It's not mine, it's one of the ANTLR examples:

grammar SimpleCalc;

tokens {
	PLUS 	= '+' ;
	MINUS	= '-' ;
	MULT	= '*' ;
	DIV	= '/' ;
}


/*------------------------------------------------------------------
  * PARSER RULES
  *------------------------------------------------------------------*/

expr	: term ( ( PLUS | MINUS )  term )* ;

term	: factor ( ( MULT | DIV ) factor )* ;

factor	: NUMBER ;


/*------------------------------------------------------------------
  * LEXER RULES
  *------------------------------------------------------------------*/

NUMBER	: (DIGIT)+ ;

WHITESPACE : ( '\t' | ' ' | '\r' | '\n'| '\u000C' )+ 	{ $channel = 
HIDDEN; } ;

fragment DIGIT	: '0'..'9' ;


Now, I'm not convinced that the above is complete, yet, but as a simple 
grammar it's fine. I'd be thinking about a FACTOR being also able to be 
a '(' followed by an expr followed by a ')'. Something like:


factor	: NUMBER |
	LPAREN expr RPAREN;

Where LPAREN and RPAREN are defined in the tokens table.


HTH


Cheers,
Norm.


-- 
Norman Dunbar
Dunbar IT Consultants Ltd

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