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=head1 TITLE
lvalue subs: parameters, explicit assignment, and wantarray() changes
=head1 VERSION
Maintainer: Nathan Torkington <[EMAIL PROTECTED]>
Date: Aug 16, 2000
Version: 1
Mailing List: [EMAIL PROTECTED]
Number: 118
=head1 ABSTRACT
RFC 107 proposed that lvalue subs should receive their rvalues as
subroutine arguments. I counter-propose that lvalue subs should
receive their rvalues as lexical variables named in a prototype
associated with the :lvalue modifier. The lvalue-ness of a given
subroutine call should be discoverable with the wantarray() function
(or its replacement), and the assignment behaviour should be written
in the subroutine rather than implicitly done by Perl.
=head1 DESCRIPTION
Subroutines may be called with multiple values. Lvalue subroutines
may be assigned multiple values. Simple passing on the argument
list conflates the two:
foo(@args) = @rvalues;
would be identical to:
foo(@args, @rvalues);
Perl's list flattening would prevent the subroutine from knowing where
one began and the other ended. Better would be if the rvalue were
passed as a last or first argument, making it equivalent to:
foo(\@rvalues, @args);
or
foo(@args, \@rvalues);
I don't think this is appropriate, either. Perl's OO tried implicit
arguments, and now we're rebelling against them. Better would be to
have the values magically appear as lexicals in the subroutine.
=head2 Prototypes
To the language it would appear as a prototype associated with the
:lvalue modifier:
sub foo (@) : lvalue ($first, $second, @rest) {
print "My first rvalue is $first\n";
print "My second rvalue is $second\n";
print "The rest of my rvalues are: @rest\n";
print "My arguments are: @_\n";
}
=head2 Explicit assignment
This would change the perl5 meaning of lvalue subroutines. Currently
you must have the lvalue as the last value before the return, and the
assignment is implicitly done by Perl. I advocate making it explicit:
# this is perl5
sub foo :lvalue {
$variable; # am I rvalue or lvalue sub? I don't know
}
foo = 5; # implicitly $variable = 5
I would now have this as:
# this is perl6
sub foo :lvalue ($new) {
$variable = $new;
}
=head2 Lvalue context discoverable
Then the wantarray() replacement, whatever it may be, can be used
to say whether you're lvalue or rvalue and what to do. That makes
an lvalue sub more like the STORE method of a tied variable, where
you can decide whether the storage is acceptable:
# still perl6
sub foo :lvalue ($new) {
if (want("lvalue")) {
if ($new >= 0) {
$variable = $new;
} else {
croak "Attempt to store negative number in foo";
}
} else {
return $variable;
}
}
=head1 IMPLEMENTATION
Straightforward. Requires changes to:
=over 4
=item prototypes
Permit named function prototypes. On the lvalue modifier.
=item wantarray
Identify lvalue context.
=back
=head1 REFERENCES
RFC 107: lvalue subs should receive the rvalue as an argument
RFC 57: Subroutine prototypes and parameters
RFC 21: Replace wantarray with a generic want function
