Michael Fowler <[EMAIL PROTECTED]> writes:
> On Fri, Sep 01, 2000 at 10:22:49AM +0100, Piers Cawley wrote:
> > And then there's:
> >
> > - Makes factory methods impossible.
>
> my Dog $spot;
> my $pub = $spot->procreate;
>
> Sure looks like a factory method to me. Just because you don't get to say
> my Dog $pup for some optimization doesn't mean they're impossible.
Given that C<my Dog $pup> would appear to be (currently) the only way
to get any optimization at compile time then I submit that the ability
to optimize all method calls at compile time by declaring typed
variables for stuff is *far* more important than the questionable
gains to be had from implicit instantiation. I can think of *far* more
occasions where I want the optimization than occasions where I don't.
Maybe a compromise along the lines of:
my Dog $spot = LIST; # $spot = Dog->new(LIST)
my Dog $patches; # $patches is undefined but we assert that
# it'll be a Dog. (Whether you can do
# $patches->new is left as an exercise)
Where the first element of LIST is not a Dog. If you want to
autocreate a null object then you can do C<my Dog $spot = ()>
>
>
> > - Conveniently forgets all about polymorphism.
>
> package Mammal;
>
> sub hair_color { ... }
>
>
> package Dog;
> use base qw(Mammal);
>
> sub CREATE { bless({}, "Dog") }
>
>
> package main;
>
> my Dog $spot;
> $spot->hair_color("blue"); # works just fine
That's not polymorphism, that's inheritance.
Polymorphism is:
try {
....
}
catch Exception::SelfHandler {
$!->handle_self
}
Note that in this context C<Exception::SelfHandler> is an abstract
superclass. The precise behaviour of the handler is defined by its
subclasses.
Another example:
package MailorderPetshop;
sub make_ad {
my MailorderPetshop $self = shift;
$self->stock_iterator->reset;
try {
STOCKLOOP: while (my Pet $pet = $self->stock_iterator->next) {
print $pet->describe, "\t", $pet->price;
}
}
catch Exception::WrongType {
# Oops, we're not looking for cages etc, skip that
next STOCKLOOP;
}
}
Again, the various C<$pet>s are instances of classes which are
subclasses of Pet, but in the context of the C<make_ad> all we care
about is that they are pets.
> I'm not proposing revamping the entire OO system, and I'd like to know what
> made you think I was.
No, but you are proposing the removal of a behaviour which has the
potential to be *very* useful when it gets better optimized, in favour
of something of questionable benefit. However, see my suggestion at
the top of this reply for something that may prove to be a useful
compromise.
Also, if you're really proposing this as a way of saving typing,
consider the idea that got floated on p5p a while back:
use shortcut;
with Long::Package::Base;
my ::Dog $spot = ::Dog->new('Fido');
Which has the advantage of being useful all over the place...
--
Piers
