>  I intend in the BNF interface to introduce parentheses, which will
> indicate that the semantics do not see the enclosed symbols.  This can be
> seen a generalization of the current keep/discard separators issue in
> sequences.  I need this generalization is needed for some other features I
> am working on.

> What I intend is that, for example, in the rule

> sum ::= term (op_add) term  action => do_addition

> the semantics would see the two 'term' values, but would not see the
> op_add.  You'd write the semantics like this:

> do_addition {
>    my (undef, $a, $b) = @_;
>    return $a + $b;
> }

> On the other hand if the rule were written as follows

> sum ::= term op_add term  action => do_addition

> the traditional semantics would be written

> do_addition {
>    my (undef, $a, undef, $b) = @_;
>    return $a + $b;
> }

> Reasons for the discard-if-parenthesized convention:  Parenthesis
> traditionally suggest hidden or optional.  Also, I may allow parentheses
> for grouping, in which case the semantics of the parenthesized part would
> become quite complex.  If an alternative to discarding a complex
> parenthesized group was allowed, it would need special additional notation
> anyway.

> Possible reasons to dislike it:  In Perl 5 regexes, parentheses by default
> mean capture and, for regexes, capture is roughly analogous to semantic
> visibility.  I regret breaking from the Perl convention, but parentheses in
> Perl regexes *are* ugly and semantically overloaded, and I think the
> notation suggested above will be more graceful.

> Feedback from this list has changed my mind in the past, and I'd like to
> hear from you.

> -- jeffrey