Handling ANTLR return values

Question

I have a grammar rule like this:

a returns [string expr] : (b | c) {$expr = "Build expression from b OR c here";}

b returns [string expr] : 'B' {$expr = "From b";}

c returns [string expr] : 'C' {$expr = "From c";}

I would like to replace

$expr = "Build expression from b OR c here"; 

with an instruction that puts in the $expr variable whatever was returned from b OR c. I know there is a solution to this by performing this assignment like this:

a returns [string expr] : b {$expr = $b.expr;} | c {$expr = $c.expr;}

but was wondering whether there is a much simpler way like naming the whole group with a variable and using that instead:

a returns [string expr] : group = (b | c) {$expr = $group.expr;}

I've tried this and it doesn't work in ANTLR, even though the group variable is used to get the value returned by "b".

Answer 1

You cannot use the same label for multiple non-terminals unless they reference the same rule in the grammar. This means syntax like group=(A | B) only works for token references ( A and B are terminals). The following syntax can be used for this.

a returns [string expr]
    :   b {$expr = $b.expr;}
    |   c {$expr = $c.expr;}
    ;