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=head1 TITLE
The AUTOLOAD subroutine should be able to decline a request
=head1 VERSION
Maintainer: Leon Brocard <[EMAIL PROTECTED]>
Date: 10 Aug 2000
Version: 2
Mailing List: [EMAIL PROTECTED]
Number: 8
=head1 ABSTRACT
In Perl 5, the first AUTOLOAD subroutine found in an object's hierarchy
is used and any other AUTOLOAD subroutines higher up are ignored. I
propose that in Perl 6 we allow AUTOLOAD subroutines to decline a
request, which allows other AUTOLOADs to cope with it.
=head1 DESCRIPTION
AUTOLOAD is a very useful and handy way to handle cool and exciting
things, but as has been pointed out many times before, only one
AUTOLOAD can be called in the object's hierarchy, even if multiple ones
exist. This is due to the fact that AUTOLOAD subroutines must do
something - they can not currently decline a request. I propose one of
the methods which has previously been discussed on p5p.
Instead of calling the right thing or actually doing the right thing,
AUTOLOAD subroutines should return a coderef which will be run as if
it were the method called. If an AUTOLOAD subroutine does not wish to
cope with a method call, it should return undef. Perl would then walk
the OO hierarchy and find the next AUTOLOAD to call, eventually failing
with an error if no AUTOLOAD method is found which will accept the
call.
=head2 EXAMPLE
The following Perl 5 code (an extension of the AUTOLOAD example in the
perlsub manpage):
sub AUTOLOAD {
my $program = $AUTOLOAD;
my @args = @_;
$program =~ s/.*:://;
if ($program =~ /^[aeiou]/) {
system($program, @args);
}
}
... would be rendered as the following in Perl 6:
sub AUTOLOAD {
my $program = $AUTOLOAD;
my @args = @_;
$program =~ s/.*:://;
if ($program =~ /^[aeiou]/) {
return sub { system($program, @args) }
} else {
return undef;
}
}
=head2 $AUTOLOAD
While we're at it, it *may* be a good idea to remove the global
$AUTOLOAD variable and instead pass it as the first parameter of the
AUTOLOAD subroutine call. For: general global drought, the fact that
perlsub's argument "because, er, well, just because, that's why..." is
a bit weak. Against: makes AUTOLOAD more complicated, breaking the
"subroutine parameters end up as @_" paradigm (apparently).
=head2 UNIVERSAL->can
This proposal has the added bonus that the UNIVERSAL->can method (or
whatever replaces it) will now work with AUTOLOAD-ed methods, whereas
in Perl 5 it used to fail.
=head1 IMPLEMENTATION
This strikes me as being a fairly easy thing to do, but then again
internals ain't my thing, baby.
=head1 REFERENCES
Mike Guy's "AUTOLOADREF" idea on p5p:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2000-03/msg01317.html
Ilya Zakharevich "sub autoload" idea on p5p:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2000-01/msg01381.html
