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

[Burton Samograd]

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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