This and other RFCs are available on the web at http://dev.perl.org/rfc/ =head1 TITLE C<||> and C<&&> should propagate result context to both sides =head1 VERSION Maintainer: Peter Scott <[EMAIL PROTECTED]> Date: 5 Aug 2000 Last-Modified: 26 Aug 2000 Mailing List: [EMAIL PROTECTED] Version: 2 Number: 45 =head1 ABSTRACT Currently the expressions lvalue = expr_A || expr_B lvalue = expr_A && expr_B evaluate C<expr_A> in scalar context, regardless of the type of C<lvalue>, only propagating list or scalar context to C<expr_B>. This proposal is that the context of C<lvalue> should be propagated to C<expr_A> as well. =head1 DESCRIPTION It would be nice to be able to say @a = @b || @c instead of having to resort to @a = @b ? @b : @c The reason that it is not currently possible is that C<@b> (or the list expression in its place) has to be evaluated in scalar context to determine whether to evaluate C<@c>, and that propagating context to C<@b> would require reevaluating it, which might have undesirable side effects (instead of C<@b>, it might be C<decrement_balance()>). Tom Christiansen pointed out that for consistency, both C<||> and C<&&> need to be changed, since in the latter case, if C<@b> is empty, then C<@a> will currently get the single element 0. We want it to get an empty list. =head1 IMPLEMENTATION It seems that it ought to be possible to evaluate something in a list context and test whether there are any entries in the resulting list without having to reevaluate the expression in a scalar context. The work-around with the trinary operator also evaluates C<@b> twice (which H.Merijn Brand pointed out could even be tied and hence evaluation not idempotent). It's true that we are evaluating something in list context and then applying a boolean interpretation to the result (empty list is false, otherwise true); in this case we are trading a lesser consistency (likely only to be appreciated by someone who's been thinking for a long time about contexts) for a greater one. =head1 IMPACTS L<RFC 82> would require a different interpretation, namely that the result would be the list formed by applying C<&&> or C<||> to each successive pair of elements in C<@b> and C<@c>. This author likes the idea of certain component-wise operators, and appreciates the importance of consistency, but just can't see a component-wise interpretation of the logical operators being either useful or intuitive. =head1 REFERENCES L<perlop/"C-style Logical Or"> L<RFC 21>, "Replace C<wantarray> with a generic C<want> function" L<RFC 82>, "Apply operators component-wise in a list context"
