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Classes Should Not Override Parent Attributes

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I’ve been spending a lot of time designing a new OO syntax for the Perl language called Corinna. This syntax is not designed to be another object-oriented module. Instead, it’s designed to be a be a core part of the language. It’s received a lot of support (and a few detractors), but I have a problem with it.

Lately I’ve been worried about the slot variable :builder attribute and I’ve been concerned that it’s problematic. I’ve nattered on about this on IRC and just generally let the concerns simmer in my mind. If we’re going to get to propose something for the core, it’s better that we avoid mistakes before that time rather than after. Then Damian Conway shared some thoughts with me about his concerns for :builder and that was the final straw. Something has to change.


As part of our proposal, we have this:

class SomeClass {
    has $x :builder;
    method _build_x () {
        return rand;

If you call SomeClass->new, you’ll get a random numeric value assigned to the $x slot variable. Note that $x has no reader or writer, so it’s designed to be completely encapsulated. But it’s not and that’s a problem.

That’s been bugging me for quite a while, but to explain that, I need to go back in time.

In 1967 (my birth year, but I swear it’s a coincidence), the first truly class-based OO language was released, Simula 67 and it introduced classes, polymorphism, encapsulation, and inheritance (and coroutines (!), but let’s not go there).

Classes, polymorphism, and encapsulation are relatively uncontroversial, but the debate over inheritance has raged for decades.

Alan Kay, the man who invented the term “Object Oriented” in Utah “sometime after 1966” and who’s been doing OO programming before most of us were born, has made it clear over the years that inheritance in OO is very problematic. He’s even left it out of the implementation of some OO languages he’s created. And then I found a Quora question entitled “What does Alan Kay think about inheritance in object-oriented programming?” and someone purporting to be Alan Kay responded with a deeply thoughtful answer. His first paragraph is fantastic (emphasis mine):

Simula I didn’t have inheritance (paper ca 1966) and Simula 67 did (paper ca 1968 or so). I initially liked the idea — it could be useful — but soon realized that something that would be “mathematically binding” was really needed because the mechanism itself let too many semantically different things to be “done” (aka “kluged”) by the programmer. For example, there is no restriction of any kind to have a subclass resemble a superclass, be a refinement of a superclass, etc. All relies on the cleanliness of mind of programmers (and even the most clean of these often just do things they need when in the throes of debugging).

In other words, if I create a class named Vehicle::Motorized, there is nothing to stop your Order::Customer class from inheriting from it. Would that make sense? Probably not. But there we are.

Next, let’s take a look at a Python object:

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def inverse(self):
        return Point(self.y,self.x)

point = Point(7,3.2).inverse()
point.x = "foo"

And for those who would like to see that in our current proposal:

class Point {
    has ($x,$y) :reader :writer :new;

    method inverse {
        return $class->new( x => $y, y => $x );
my $point = Point->new( x => y => 3.2 )->inverse;
say $point->x;

As you can see, we’ve assigned “foo” to the x value of a Point object. We’ve done this in both Python and our new OO syntax for Perl. Sadly, due to the need to harmonize types across the language, we can’t have type constraints in v0.1.0. This is a serious limitation, but we’re taking baby steps.

But what about the Python version? Surely they must validate their data, right? No. Pythonistas have assured me that validating your data is very unpythonic .

(I envision spit-takes from tons of Moo/se fans out there right now).

Python 3.7 contains data classes and those will help, but I don’t know how widespread the usage is.

Variables and Types

But what does that have to do with inheritance?

For many languages, you can try something like (example in the Go programming language) var myvar float64 = "foo" and it will throw an exception. It might be compile-time. It might be runtime. But it will die a horrible death because the variable, myvar, has a type associated with it and if you assign something that doesn’t match that type (or that the language can’t/won’t coerce), then things go boom.

Why is that? Because on line 372 of your whopping huge function, it’s easy to have something like myvar := stuff[0:2] and not be sure what kind of data stuff[0:2] contains.

So we can say that—for a small subset of types the programming language can understand—variables in those kinds of languages are the “experts” about what they can do and if you try to put that variable in an invalid state, it blows up.

That’s great. But what about var celsius float64 = -1300.15. Is that valid?

Absolute zero is -273.15 on the Celsius scale, so we can make a reasonable argument that the above is not valid. In languages like Raku , we easily fix that:

subset Celsius of Num where  * >= -273.15;
my Celsius $temp = -1300.15;

The above will fail at compile-time. Most languages that allow types, however, aren’t quite as flexible, but you can use classes instead.

Classes are Types

my $temp = Celsius->new( temperature => -1300.15 );

Assuming your class is sane, the above should throw an exception because you’re trying to create an object with an invalid state.

If you think that’s similar to how built-in types work, it is. We can think of a class as a user-defined type. Types have a set of allowed values and operators you can apply to those values. Classes have (often complex) sets of allowed values and methods (operators) you can apply to those values.

Just as an int is often an expert in values -32,768 to 32,767, or -2,147,483,648 to 2,147,483,647 if you’re using 4 bytes (larger values for more bytes, of course), so is your Celsius class an expert about allowed Celsius temperatures. And your Personal Shopper knows exactly how much money it can spend, and even if it knows you have enough money to buy cocaine, it’s still going to throw an exception if you request that because it’s an illegal operation.

The Problem with :builder

And this gets us to :builder. In Perl‘s Moo/se family of OO, you can do something similar to this (but using our new syntax):

class Parent {
    has $x :builder;
    method _build_x () {
        return rand;

class Child isa Parent {
    method _build_x () { return "not a number" }

This has exactly the same issue with see with Python’s point.x = "foo". It’s hardly necessary to belabor the point, but I will: $x is supposed to be private, but we’ve grossly violated encapsulation.

Ah, you reply. But Moo/se has types. So let’s look at Moo/se, using a better example (not great, but enough to get the point across):

package Collection {
    use Moose;

    # this is a personal module which gives me a "saner" Perl
    # environment
    use Less::Boilerplate;
    use Types::Standard qw(Int ArrayRef);
    has _index => (
        is      => 'rw',
        isa     => Int->where('$_ >= 0'),
        builder => '_build__index',

    # default, but maybe someone wants a different default
    sub _build__index { 0 }

    has items => (
        is       => 'ro',
        isa      => ArrayRef,
        required => 1,

    sub BUILD ( $self, @ ) {
        my @items = $self->items->@*;
        my $type = ref $items[0];
        foreach my $item (@items) {
            if ( not defined $item ) {
                croak("items() does not allow undefined values");
            if ( ref $item ne $type ) {
                croak("All items in collection must be of the same type");

    sub num_items ($self) {
        return scalar $self->items->@*;

    sub next ($self) {
        my $i = $self->_index;
        return if $i >= $self->num_items;
        $self->_index( $i + 1 );
        return $self->items->[$i];

    sub reset ($self) {

It goes without saying that writing an iterator like the above can be rather problematic for a number of reasons, and yes, the _build__index method is kind of silly, but it shows the issue. For example, what would a negative index do? In the above, it would throw an exception because of the Int->where( '$_ >= 0' ) constraint. But what would happen if we set the index to a value larger than the size of the items array reference (and ignoring that the above allows $coll->_index($integer)).

package My::Collection {
    use Moose;
    extends 'Collection';

    sub _build__index { 5 }

my $collection = My::Collection->new( items => [qw/foo bar baz/] );

while ( defined( my $item = $collection->next ) ) {
    say $item;

What do you think the above does? The constraint passes, but it prints nothing. That’s because the allowed value of the index is tightly coupled to something else in that class, the number of items in the collection.

Oh, but Ovid, I wouldn’t write something like that!

Yes, you would. I do it too. We all do it when we’re quickly hacking on that 883 line class from our 1,000 KLOC codebase and we’re rushing to beat our sprint deadline. Programmers do this all the time because we have a language which allows this, but OO modules which encourage this. You can inspect tons of OO modules and find attributes (a.k.a “slot variables” in our new system) which are coupled with other values. We like to minimize that, but sometimes we can’t and classes are supposed to encapsulate this problem and handle it internally.

My suggestion is that, if you want to allow this internally, you have to do it manually with the ADJUST phaser .

class Collection {
    has $index;
    has $items :new;

    ADJUST (%args) {
        # other checks here

        $index = $self->_default_index;
        if ( $index > $items->@* - 1 ) {
            # don't let subclasses break our code

    method _default_index () { 0 }

    # other methods here

Do I recommend that? No. Can you do it? Yes. And it has some advantages.

First, you are being explicit about which slots are can access in your subclasses. Second, you have fine-grained control over slot initialization ordering.

By default, with Corinna, slots are initialized in the order declared (until you hit :new, in which case all :new slots are initialized at once). In Moo/se it’s harder to predict the order of initialization. That’s why you often see a bunch of attributes declared as lazy to ensure that non-lazy attributes are set first. If you have circular lazy dependencies, it can be hard to work out and you fall back to BUILD or BUILDARGS as needed.

Yes, this does mean a touch more work on the off chance you need to override a slot in a subclass. I’ve been writing Moo/se for years and I can count on the fingers of one hand the number of times I’ve needed to do this. However, I can’t count how many times I’ve unnecesssarily littered my namespace with sub _build... methods because this is largely what Moo/se prefers and what developers will call you out on in code reviews if you do it some other way.

Oh, and we also hit issues like this. Which of these is correct?

class Position {
    slot $x :name(ordinate) :builder;

    # and many lines later...

    method _build_ordinate { return rand }   # Does the builder follow the public slot name?

class Position {
    slot $x :name(ordinate) :builder;

    # and many lines later...

    method _build_x { return rand }          # Or does it follow the private slot variable name?

If we were to allow builders, we should, at least, mandate the syntax :builder($method_name) and not allow default builder names.

Builder Guidelines

So here are a few guidelines we should follow for assigning default values to slots.

  • Every slot should have a default value.

It is often a code smell to have an undefined slot.

  • Use the = default syntax

In the example below, :new says we can pass the value to the constructor, but if we don’t, it will get the value 42.

has $x :new = 42;

  • YAGNI (You Ain’t Gonna Need It).

Don’t allow overriding a private slot value unless you have a clear need for it. This also prevents littering the namespace with a _build_... methods.

  • Liskov is your friend

Remember the Liskov Substitution Principle : if you have a subclass and it must be allowed to override some of this data, remember that a subclass should be a more specialized version of a parent class and must be usable anywhere the parent can be.

  • Check for coupling

If the slot data is tightly coupled to some other slot data, consider breaking the coupling or ask yourself if delegation is more appropriate. Have checks in BUILD (Moo/se) or ADJUST (Corinna) to verify the integrity of your class.

If all of the above seems like too much to remember, just don’t allow child classes to set parent slot data. Corinna should follow one of the early principles guiding its design: you should be allowed to do bad things, but the language design shouldn’t encourage bad things.

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