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=head1 TITLE
Elements of @_ should be read-only by default
=head1 VERSION
Maintainer: John Tobey <jtobey@john-edwin-tobey.org>
Date: 28 Sep 2000
Last Modified: 1 Oct 2000
Mailing List: perl6-language@perl.org
Number: 344
Version: 2
Status: Frozen
=head1 ABSTRACT
Unprototyped subs should not be allowed to modify their callers' data
simply by assigning to elements of the arg array.
=head1 COMMENTS ON FREEZE
This RFC generated no discussion in 3 days.
=head1 DESCRIPTION
In Perl 5, you can modify a caller's value by assigning directly to
elements of C<@_>, like this:
sub perp {
$_[0] = 'ha!';
}
sub victim {
my $num = 42;
perp($num);
print $num; # prints ha!
}
This form of passing arguments by reference is obsolete now that Perl
has hard references and C<\$> in prototypes.
The feature is surprising. At least, I was surprised when I first
learned you could modify a caller's value by assigning to $_[0].
The feature is confusing, since it means that the recommended
convention for naming parameters (by assigning C<@_> to a C<my> list)
alters semantics. For example, the following subs do I<not> have the
same effect as above:
sub perp1 {
my $arg = shift;
$arg = 'ha!';
}
sub perp2 {
my ($arg) = @_;
$arg = 'ha!';
}
The Perl 5 (and older) behavior may preclude optimizations in compiled
code. If a compiler knows that arguments are passed by value, it may
generate code that places the value directly in a register or on the
stack. With the Perl 5 semantics, it would normally have to pass a
pointer to each scalar and dereference the pointer to obtain the
argument's value.
I think this change should not affect subs with a prototype, so the
examples in L<perlsub/Prototypes> would still work. People who bother
to use prototypes and sub attributes should know what they are getting
into. The prototype and attribute system will, I think, give plenty
of opportunities to specify compiler optimizations. Just the default
case should be changed.
=head1 IMPLEMENTATION
A fascist implementation would emit a compile-time error any time
C<@_> or one of its elements were assigned to, taken a refernce to,
etc. A friendlier version would automatically copy the arg value to a
new temporary.
This change would mean a moderate Perl 5 compatibility breakage. The
Perl-5-to-Perl-6 converter could insert whatever trick is used to
obtain the Perl 5 behavior (perhaps a C<(@)> prototype) when it
detects (or suspects) argument modification.
=head1 REFERENCES
RFC 154: Simple assignment lvalue subs should be on by default
L<perlsub>
The C-- homepage - http://www.cminusminus.org/