Re: RFC 193 (v1) Objects : Core support for method delegation

Michael G Schwern <[email protected]> writes:

> On Mon, Sep 04, 2000 at 09:53:39PM -0000, Perl6 RFC Librarian wrote:
> > Objects : Core support for method delegation
> 
> I like it!  One gripe (of course)...
> 
> 
> > The proposed delegation mechanism would work via a pragma:
> > 
> > 	use delegation
> > 		attr1 => [qw( method1 method2 method3 )],
> > 		attr2 => [qw( method4 method5 )],
> > 		attr3 => [],
> > 		attr4 => [],
> 
> I will often use a more complicated data structure for my objects,
> often organizing all sub-objects into a hash of hashes...
> 
>     $obj->{locks}{MacOSX} = $macosx_obj;
>     $obj->{locks}{Mac}    = $mac_obj;
>     $obj->{locks}{BSD}    = $bsd_obj;
> 
> which is nice when you stuff alot of things into an object.  If I
> wanted to deligate to those objects in $obj->{locks}, how would I
> under your proposal?

Flatten the hierarchy? Make your aggregations into classes themselves
and set up delegation rules there?

> 
> In a similar vein, I can see a use for wanting to deligate a set of
> methods to an entire list of objects.  Consider...
> 
>     $obj->{locks} = [$macosx_obj, $mac_obj, $bsd_obj];
> 
> it would be nice to be able to state that "method1" should deligate to
> each object in the $obj->{locks} list until it is found.

    package ListOfObjects;

    use strict;
    use Symbol qw/gensym/;

    sub new {
        my($class) = shift;
        my $self   = bless {}, ref($class) || $class;
        $self->push(@_);
    }

    sub push {
        my $self = shift;
        while (shift) {
            my $attr = gensym;
            $self->{$attr} = $_;
            use delegate $attr => [];
        }
    }


> Also, what happens when a deligated attribute does not contain an
> object when Perl checks?  Should it produce a warning?  I'd say no.  I
> can easily see cases where you'd like to be able to deligate to
> objects which may or may not be instanciated at run-time.  If a
> warning was issued, it would be difficult to circumvent.  You'd have
> to place a dummy object in that slot.

You know, this may be a case for Mister Fowler's RFC about auto
instantiated objects. Except C<Dog $self->{attr}> isn't actually valid
syntax is it? There's certainly a case for just using the (singleton)
null object as a placeholder until a real object comes along, and it
can solve a host of other problems.

--
Piers