What if Perl OO was a Core Feature?

2013-09-20

Over on Reddit /r/perl, there’s a rather blatant troll complaining about the lack of OO as a core feature in Perl. The tone there is clearly not constructive and not worth responding further, but I feel compelled to answer a question: what would be improved if OO was a core feature, rather than built out of existing components of the language?

Personally, I think the fact that OO can be built this way is a demonstration of flexibility. It also allows you to build your own OO system that suits your needs. If the standard kind of blessed data structure doesn’t work for you, try Inside-Out Objects. This also had hidden benefits later on; when roles/traits/mixins became all the rage, Perl just added them via CPAN modules. There was no long, drawn out, design-by-committee bikeshedding discussion like there was for Java.

If you really wanted to, you could even build an object system without bless() or packages. The constructor would return a hash, which is filled with sub references:

sub new_object
{
    my %object = (
        foo => sub { 'foo' },
        bar => sub { 'bar' },
        baz => sub { 'baz' },
    );
    return %object;
}

my %obj = new_object;
say $obj{foo}->(); # Prints 'foo'

Inheritance is a matter of a different constructor calling the original constructor, then filling in different subrefs:

sub new_inherit_object
{
    my %object = new_object();
    $object{foo} = sub { 'override foo' };
    $object{qux} = sub { 'qux' };
    return %object;
}

my %obj = new_inherit_object;
say $obj{foo}->(); # Prints 'override foo'

(No, I don’t really suggest doing this. It’s just an interesting exercise in building an object system using limited language components. Although this isn’t too far from how JavaScript builds objects.)

Getting back to the usual bless() system, there is a drawback: the learning curve. Think of all the things you have to grasp before you can understand Perl OO. There’s subroutines, passing arguments to subroutines, and putting those subroutines into packages. Any bless‘d object is going to be a complex data structure, and before you can figure those out, you need to grasp references. Even for somebody experienced in another programming language, that’s a lot to ask before you can get them up to speed on object programming. It’s even harder for somebody with no programming background at all.

Moose somewhat alleviates this. It can hide a lot of the details of the complex data structure, though obviously you’re going to need to dig into that at some point. The constructors in Moose can also do sensible things for you. It’s a lot better than Java, where the constructors tend to have a lot of this.foo = foo; statements to set private member variables from the arguments.

What Moose can’t fix is subroutines. Sub signatures are a sore spot for Perl. I think it really is an embarrassment that we still have this problem in 2013. And we need signatures down pat before we can even start thinking about multi-method dispatch.