[antlr-interest] Checking for missing optional token in rule

[Jim Idle]

paren_delimited_numeric_unit_list
     : LPAREN number unit1=unit
       (COMMA number
         (unit2=unit
            {
                C code Action when unit2 is present;
            }
         )?
       )* RPAREN
         -> (number unit)+
     ;

If you want to perform a custom rewrite, then set a flag (@declarations
and @init) and use the conditional rewrite syntax.

If you use unit2= though, then unit2 will only ever see the last one.

Are you sure that you are not just trying to do something like this:

paren_delimited_numeric_unit_list
     : LPAREN n+=number u+=unit (COMMA n+=number (u+=unit)? )* RPAREN
         -> ^(LPAREN ($n $u)+)
     ;


Jim

> -----Original Message-----
> From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> bounces at antlr.org] On Behalf Of Burton Samograd
> Sent: Monday, July 23, 2012 2:55 PM
> To: antlr-interest at antlr.org
> Subject: [antlr-interest] Checking for missing optional token in rule
>
> See the comment below for an explanation:
>
> paren_delimited_numeric_unit_list
>     : LPAREN number unit1=unit (COMMA number unit2=unit? {
>             // I would like to check if unit2 was present in the input
> tokens
>             // and do something if it was not.
>         })* RPAREN
>         -> (number unit)+
>         ;
>
> In the tree walker we can say ($unit2 == NULL) to check for presence,
> but that technique does not work in the parser.  Is there a way to do
> what I would like to do?
>
> Thanks.
>
> --
> Burton Samograd
>
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