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Why is Perl not putting Moose in the core?

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As you may know, we’re designing a new OO system for the Perl language. A couple of people keep asking why we don’t just put Moose in the Perl core.

Please note that no one is taking away bless or Moose or anything like that. If you prefer these tools, that’s fine. After all, many people strongly objected to Moose when it first came out, echoing the arguments I hear against Corinna (except that Moose was/is slow. Corinna is not, despite the fact that no optimization has been done on the Object::Pad prototype). It took years before Moose (and later Moo) won the hearts and minds of Perl OOP developers.

So the following explanation is not for the those dead-set against Corinna. It’s for everyone else who is curious about why Perl might adopt the Corinna proposal for native object-oriented programming (hereinafter referred to as OOP), even when the Perl 5 Porters (P5P) have rejected Moose.

What’s an Object in Perl?

If you know Perl, the answer to the above question is “nothing.” Perl knows nothing about OOP. It doesn’t know what objects are. Instead, there are a few features introduced in Perl 5 to give one a basic toolkit for emulating some OOP behaviors.

  1. An object is a data structure that knows to which class it belongs.
  2. A class is the same thing as a package.
  3. A method is subroutine that expects a reference to an object (or a package name, for class methods) as the first argument.
  4. You inherit from a class by adding its name to your namespace’s @ISA array.

Does that sound like a kludge? It is, but it’s worked well enough that developers without much experience in OOP have accepted this.

It’s actually kinda rubbish because it’s more or less an assembler language for OOP behavior, but it works. In fact, it’s modeled after Python’s original implementation of objects, but with changes to take into account that we are not, in fact, Python.

So imagine a Person class written in core Perl. You have a read/write name and that’s all. We’ll keep it simple. This is what you used to see in OOP Perl.

package Person;

use strict;
use warnings;

sub new {
    my ( $class, $name ) = @_;
    return bless { name => $name } => $class; # this is the instance

sub name {
    my $self = shift;
    if (@_) {
        $self->{name} = shift;
    return $self->{name};


So you want to subclass that and create a Person::Employee and need to keep their salary private:

package Person::Employee;

use strict;
use warnings;
our @ISA = 'Person';

sub new {
    my ( $class, $name, $salary ) = @_;
    my $self = $class->SUPER::new($name);
    $self->{salary} = $salary;
    return $self;

sub _salary { # the _ says "private, stay out"
    my $self = shift;
    return $self->{salary};


OK, kinda clumsy, but it works. However, if you want to see the salary:

use Data::Dumper;
print Dumper($employee);

Admittedly, salary is often very secret and the above code is rubbish, but there’s plenty of other data that isn’t really secret, but you don’t want to expose it because it’s not part of the interface and people shouldn’t rely on it. But what’s going on with that leading underscore?

In Python, you won’t get a method for that, but if you know how the method names are mangled under the hood, you can jump through hoops and call them. For Perl, you just have that method. Developers know they shouldn’t call them, but I investigated three code bases from our various clients and found:

  • Codebase 1: very good Perl, lots of tests, tremendous discipline, and 72 calls to private methods outside the class (or subclass) they were defined in)
  • Codebase 2: very sloppy Perl, few tests, and an organizational mess, but only 59 calls to private methods (to be fair, it was a smaller codebase, but not by much)
  • Codebase 3: a combination of miserable and wonderful Perl, almost no tests, and the largest code base we have ever worked with. About 20,000 calls to private methods.

“Private” methods in Perl are not private. At all. You have a deadline, you have a very hit-or-miss codebase, and you put something together as fast as you can. All of a sudden, the “private” code isn’t very private any more. Work with a large group of programmers with varying levels of ability? They’re going to call those private methods, they’re going to call $customer->{name} instead of $customer->name, and do all sorts of other naughty things.

What this means is that your private methods are now part of the public interface whether you like that or not. You might think you can change them, but you do so at the risk of wide-spread carnage.

If there is one things I’ve learned in decades of programming for multiple clients, it’s this:

Relying on developers to “do the right thing” is a disaster if the wrong thing is easy to do.

Corinna is designed to scale.

Types and Objects

Before we get to the discussion of why Moose isn’t going into the core, let’s consider types (we’ll skip type theory).

Types have names, allowed values, and allowed operators.

For many programmers, when they think of types, they think about int, char, and so on. Those are the names of the types.

Types have a set of values they can contain. For example, unsigned integers can hold zero and positive values up to a certain limit (determined by the amount of memory assigned to that type). You cannot assign -2 to an unsigned integer.

You also have a set of allowed operations for those types. For example, for many programming languages, multiplying the string “foo” by a number is a fatal error, often at compile time. However, Perl allows this:

$ perl -E 'say "Foo" * 3'

Of course, you should almost always enable warnings, in which case you’ll see a warning like this:

Argument "Foo" isn't numeric in multiplication (*) at -e line 1.

Or if you prefer, you can make these kinds of errors fatal:

use warnings FATAL => "numeric";

Types such as int, char, bool, and so on, are largely there for the computer. They usually map directly to things the CPU can understand.

But what does this have to do with objects?

Objects have names (the class), a set of values (often complex), and a set of allowed operations (methods).

These map to problem domains the programmer is concerned with. In other words, they’re complex types written to satisfy developer needs, not computer needs. They’re not just a grab-bag of miscellaneous features that have been cobbled together ... well, they currently are for Perl.

Now let’s look at Moose.

Why not Moose?

First, Moose isn’t going into core because P5P said “no.” It pulls in a ton of non-core modules that P5P has said they don’t want to maintain. That should end the argument, but it hasn’t.

A moose with with bloody antlers as the velvet is falling away. (I would credit this if I could find the source)
Truly a majestic beast.

Some argue for Moo in core, but Moo has this in its documentation:


my $meta = Foo::Bar->meta;
my @methods = $meta->get_method_list;

Returns an object that will behave as if it is a Moose metaclass object for the class. If you call anything other than make_immutable on it, the object will be transparently upgraded to a genuine Moose::Meta::Class instance, loading Moose in the process if required.

So if we bundle Moo we have to bundle Moose, or break backwards-compatibility on a hugely popular module.

But there’s more ...

As stated earlier, objects in Perl are:

  1. An object is a data structure that knows to which class it belongs.
  2. A class is the same thing as a package.
  3. A method is subroutine that expects a reference to an object (or a package name, for class methods) as the first argument.
  4. You inherit from a class by adding its name to your namespace’s @ISA array.

Moose is simply a sugar layer on top of that. Native objects in Perl have no understanding of state or encapsulation. You have to figure out how to cobble that together yourself.

Here’s the person class (using only core Moose and no other helper modules):

package Person;
use Moose;

has name => (
    is       => 'rw',    # it's read-write
    isa      => 'Str',   # it must be a string
    required => 1,       # it must be passed to the constructor


Right off the bat, for this simplest of classes, it’s shorter, it’s entirely declarative, and it’s arguably more correct. For example, you can’t do $person->name(DateTime->now) as you could have with the core OOP example. You could fix that with the core example with a touch more code, but you’d have to do that with every method you wrote.

You have to change construction a bit, too. Either of the following works:

my $person  = Person->new( name => 'Bob' );
my $person2 = Person->new({ name => 'Bob2' });

Why two different ways? Who knows? Live with it.

But what about the subclass?

package Person::Employee;
use Moose;
BEGIN { extends 'Person' }

has _salary => (
    is       => 'ro',       # read-only
    isa      => 'Num',      # must be a number
    init_arg => 'salary',   # pass 'salary' to the constructor
    required => 1,          # you must pass it to the constructor


And to create an instance:

my $employee = Person::Employee->new(
    name   => 'Bob',
    salary => 50000,

And we don’t have a ->salary method, but we can still access ->_salary. Hmm, not good. That’s because Moose manages state, but doesn’t make it easy to provide encapsulation. But at least it protects the set of allowed values.

try {
    $employee->name( DateTime->now );
    say $employee->name;
catch ($error) {
    warn "Naughty, naughty: $error";

And that prints something like:

Naughty, naughty: Attribute (name) does not pass the type constraint because: Validation failed for 'Str' with value DateTime=HASH(0x7ffb518206d8) at accessor Person::name (defined at moose.pl line 11) line 10
        Person::name('Person::Employee=HASH(0x7ffb318d47b8)', 'DateTime=HASH(0x7ffb518206d8)') called at moose.pl line 42

Oh, but it actually doesn’t fully protect that set of values:

try {
    $employee->{name} = DateTime->now;
    say $employee->name;
catch ($error) {
    warn "Naughty, naughty: $error";

Note that we’ve reached inside the object and set the value directly. The above code cheerfully prints the current date and time.

Naturally, you can set the salary the same way. You don’t want people messing with private data.

And finally, Corinna.

class Person {
    slot $name :reader :writer :param;

Hmm, that looks pretty easy, but what about that ugly salary problem?

class Person::Employee :isa(Person) {
    slot $salary :param;

Now, internally, everything has access to $salary, but nothing outside the class does. It’s no longer part of the public API and that’s a huge win. You literally cannot set it from outside the class.

It’s also great that $salary is just a local variable and we don’t have to keep doing method lookups to get it. With Paul Evan’s Object::Pad test bed for Corinna, that’s been a huge performance gain, despite little work being done on optimization.

But sadly, we don’t yet have full safety on the domain of values. There’s been some discussion, but to make that work, we need to consider types across all of the Perl language. That means variable declarations, slot declarations, and signatures. We’re not there yet, but already we have something better. This, admittedly, is the biggest downside of Corinna, but we have a more solid foundation for OOP.

Why Corinna?

To be honest, encapsulation isn’t very compelling to many Perl developers. In fact, many of the best things about OOP software isn’t compelling to Perl developers because Perl doesn’t seem to have many OOP programmers with OOP experience outside of Perl, so it’s hard for them to appreciate what is missing.

As a case in point, here was a complaint from someone on the Perl 5 Porter’s mailing list that echos complaints made elsewhere:

Rather than peeling the OOP “onion” back layer by layer, build it out from what exists now. Starting with what’s needed to augment “bless”, prototypes, and overload.pm.

The problem, at its core, is that this misunderstands the problem space and current attempts to fix this.

The following is an edit of the response written by Chris Prather which nicely sums up some of the problems.

Without trying to sound offensive, this list kinda suggests you’ve not really done any extensive thought about what an object system is and should be. Most people don’t and shouldn’t ever need to. A list of things that in my opinion would need enhancement:

  • Classes: Perl just gives you packages with a special @ISA variable for inheritance. Packages are just a bag of subroutines, they have no idea of state.
  • Attributes: bless associates a package with a data structure to provide “attributes”, except it doesn’t actually provide attributes, it just provides a place to store data and leaves you to figure out what attributes are and what that means. This also means that all instance data is public by default. While we pretend that it doesn’t because Larry told us not to play with shotguns, it hasn’t stopped a lot of people putting shotgun-like things onto CPAN (or into Perl Best Practices).
  • Metamodel: The way you interrogate and manipulate a package is ... not obvious. Package::Stash exists on CPAN simply to provide an API for this manipulation because it’s fraught with edge cases and weird syntax.
  • Methods: Perl’s concept of a method is a subroutine called in a funky way. Combined with the public nature of the data, this means you can call any method on any object ... and the only thing that can prevent this is the method itself. I’ve never seen anyone write enough validation code at the beginning of their methods to deal with what is actually possible to throw at a method.
  • Class composition: Design Patterns: Elements of Reusable Object-Oriented Software, published literally 4 days after Perl 5.000 says to prefer composition to inheritance. Perl’s only solution to reusable behavior is inheritance. Worse, Perl supports multiple inheritance using a default algorithm that can cause weird, non-obvious bugs.
  • Object Construction Protocol: Ensuring that all of the attributes are initialized properly in the correct data structure during construction is left entirely as a lemma for the programmer.
  • Object Destruction Protocol: See above, but because Perl has universal destruction where we can’t even guarantee the order in which things are destroyed.

The fact that Perl’s built in object system just gives you a bag of primitives and leaves you to build a robust object system for every application you write is kinda the reason things like Moose exist. Moose’s choices to solve many of these problems is the reason Corinna exists. Let’s take Classes, attributes, and methods for example (because this is the most obvious change in Corinna). Classes are supposed to be a template for creating objects with initial definitions of state and implementations of behavior. Perl’s native system only provides the second half of that.

Ultimately, by using Corinna, we can have Perl know what an object type is, not just the current hodge-podge of SVs, AVs, HVs, and so on.

Subroutines and methods will no longer be the same thing, so namespace::clean and friends become a thing of the past.

We can eventually write my $object = class { ... } and have anonymous classes.

Corinna (via the Object::Pad test bed) already gets the tremendous benefits of compile-time failures if $name is missing, instead of the run-time failure of $self->name not existing.

We can have compile-time failures of trying to call instance methods from class methods, something we’ve never had before. In fact, there are many possibilities opened up by Corinna that you will never have with Moo/se.

That’s why Moose isn’t going into core. It also gives insight into why we’re not trying to gradually improve core OOP but are instead jumping right in: you can’t get there from here because they’re fundamentally different beasts. You can’t take a handful of discrete primitives and suddenly transform them into a holistic approach to OOP. Maybe someone could map out a way of doing that in an incremental fashion and by the year 2037, the last three Perl programmers will have a perfectly reasonable OOP system. I am tired of waiting.

I’m not saying Corinna is going to make it into core, but the prospects for it look very good right now. Corinna opens up a world of possibilities that Moo/se can’t give us because Moo/se is trapped within the constraints of Perl. Corinna is removing those constraints.

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