Introduction: The current state of play

Perl has very minimal” support for object-​oriented (OO) programming out of the box by its own admission. It’s class-​based but classes are just packages used differently. Objects are just data structures blessed into a class, methods are just subroutines whose first argument is an object or class name, and attributes/​properties are often just the key-​value pair of a hash stored in the object. (This last is a feature shared with JavaScript, whose prototype-​based objects are just collections of key-​value pairs with the keys addressed as properties.) You’ve got polymorphism, inheritance, and it’s up to you to enforce encapsulation.

This can take a lot of work to use effectively. To help address that, several systems have been developed over the years to reduce boilerplate and provide modern (or postmodern”) OO features that developers from other languages expect. My favorite for a while has been Moo: it’s got the features I need 90% of the time like built-​in constructors, roles (an alternative to composition through inheritance), attributes, type validation, and method modifiers for enhanced polymorphism. And if I need to dig around in the guts of classes, attributes, and the like I can always upgrade to Moo’s big brother Moose and its meta-​object protocol with minimal effort.

Corinna, Object::Pad, and porting dbcritic

But there’s a new kid on the block. Curtis Ovid” Poe has been spearheading Corinna, an effort to bring effective OO to the Perl core and leapfrog [emphasis his] the capabilities of many OO languages today.” No CPAN modules, no chain of dependencies; just solid OO features and syntax built-​in. And while Corinna is a ways off from shipping, Paul LeoNerd” Evans (maybe I should get a cool nickname too?) has been implementing some of these ideas as new Perl keyword syntax in his Object::Pad module.

Both Ovid and LeoNerd have been asking developers to try out Object::Pad, not just as a new toy, but to get feedback on what works and what needs to be added. So I thought I’d try porting an older small Moo-​based project named dbcritic to this new reality. In the process, I learned some of the advantages and disadvantages of working with Object::Pad. Hopefully, this can inform both it and Corinna’s evolution as well as other curious developers’ evaluations. You can follow my coding efforts in this GitHub branch.

First, the marquee result: the code for App::DBCritic (the class I started with) is cleaner and shorter, with 33 lines shaved off so far. Mainly this is due to Object::Pad’s more concise attribute syntax (called slots” in its documentation) and lack of explicit support for Moo’s attribute coercion. I only used the latter for one attribute in the Moo version and I’m not sure it worked particularly well, so it wasn’t hard to jettison. But if your code supports coercions extensively, you’ll have to look into Object::Pad’s BUILD or ADJUST phase blocks for now.

Before, a Moo attribute with various options:

has schema => (
    is        => 'ro',
    coerce    => 1,
    lazy      => 1,
    default   => \&_build_schema,
    coerce    => \&_coerce_schema,
    predicate => 1,

After, an Object::Pad slot. No coercion and builder code is handled in a later ADJUST block:

has $schema :reader :param = undef;

Speaking of ADJUST blocks, it took a little bit of insight from the #perl IRC channel to realize that they were the appropriate place for setting slot defaults that are computed from other slots. Previously I was using a maze of dependencies mixing Moo lazy attributes and builder methods. Clarifying the main set of optional constructor arguments into a single ADJUST block helped untangle things, so this might be an indication that lazy attributes are an antipattern when trying to write clean code. It’s also worth noting that Object::Pad ADJUST blocks run on object construction, whereas Moo lazy attributes are only built when needed. This tends to matter for database access.

The ADJUST block for the $schema slot:

    my @connect_info = ( $dsn, $username, $password );
    if ($class_name and eval "require $class_name") {
        $schema = $class_name->connect(@connect_info);
    elsif ( not ( blessed($schema) and $schema->isa('DBIx::Class::Schema') ) ) {
        local $SIG{__WARN__} = sub {
            if ( $_[0] !~ / has no primary key at /ms ) {
                print {*STDERR} $_[0];
        $schema = App::DBCritic::Loader->connect(@connect_info);
    croak 'No schema defined' if not $schema;

Object::Pad’s slots have one great advantage over Moo and Moose attributes: they directly support Perl array and hash data structures, while the latter only supports scalars and references contained in scalars. This means methods in your class can eliminate a dereferencing step, again leading to cleaner code. I used this specifically in the @violations array and %elements hash slots and was very pleased with the results.

The @violations and %elements slots and their ADJUST blocks:

has %elements;

    %elements = (
        Schema       => [$schema],
        ResultSource => [ map { $schema->source($_) } $schema->sources ],
        ResultSet    => [ map { $schema->resultset($_) } $schema->sources ],

has @violations;

    @violations = map { $self->_policy_loop( $_, $elements{$_} ) }
        keys %elements;

method violations { wantarray ? @violations : \@violations }


I did have some development lifecycle issues with Object::Pad, but they’re mainly a result of its future-​facing syntax. I had to give up using perltidy and perlcritic in my build and test phases, respectively: perltidy doesn’t understand slot attributes like :reader and :param and will emit an error file (but code still compiles), and several of the perlcritic policies I use report problems because its PPI parser doesn’t recognize the new syntax. I could add exceptions in the perlcriticrc file and litter my code with more ## no critic annotations than it already had, but at this point, it was easier to just disable it entirely.

Another thing I had to disable for now was my Dist::Zilla::Plugin::Test::UnusedVars-generated Test::Vars test for detecting unused variables, as it reports multiple failures for the hidden @(Object::Pad/slots) variable. It does have options for ignoring certain variables, though, so I can explore using those and possibly file a pull request to ignore that variable by default.

Conclusion: The future looks bright

Overall I’m satisfied with Object::Pad and by extension some of the syntax that Corinna will introduce. I’m going to try porting the rest of dbcritic and see if I can work around the issues I listed above without giving up the kwalitee improvement tools I’m used to. I’ll post my findings if I feel it merits another blog.

What do you think? Is this the future of object-​oriented Perl? Let me know in the comments below.

circus theme party

Last week’s article got a great response on Hacker News, and this particular comment caught my eye:

I think this is the real point about Perl code readability: it gives you enough flexibility to do things however you like, and as a result many programmers are faced with a mirror that reflects their own bad practices back at them.

orev, Hacker News

This is why Damian Conway’s Perl Best Practices (2005) is one of my favorite books and perlcritic, the code analyzer is one of my favorite tools. (Though the former could do with an update and the latter includes policies that contradict Conway.) Point perlcritic at your code, maybe add some other policies that agree with your house style, and gradually ratchet up the severity level from gentle” to brutal.” All kinds of bad juju will come to light, from wastefully using grep to having too many subroutine arguments to catching private variable use from other packages. perlcritic offers a useful baseline of conduct and you can always customize its configuration to your own tastes.

The other conformance tool in a Perl developer’s belt is perltidy, and it too has a Conway-​compatible configuration as well as its default Perl Style Guide settings. I’ve found that more than anything else, perltidy helps settle arguments both between developers and between their code in helping to avoid excessive merge conflicts.

But apart from extra tools, Perl the language itself can be bent and even broken to suit just about anyone’s agenda. Those used to more bondage-​and-​discipline languages (hi, Java!) might feel revulsion at the lengths to which this has sometimes been taken, but per the quote above this is less an indictment of the language and more of its less methodical programmers.

Some of this behavior can be rehabilitated with perlcritic and perltidy, but what about other sins attributed to Perl? Here are a few perennial favorites”:

Objects and Object-​Oriented Programming

Perl has a minimalist object system based on earlier-​available language concepts like data structures (often hashes, which it has in common with JavaScript), packages, and subroutines. Since Perl 5’s release in 1994 much verbose OO code has been written using these tools.

The good news is that since 2007 we’ve had a sophisticated metaobject-​protocol-​based layer on top of them called Moose, since 2010 a lightweight but forward-​compatible system called Moo, and a couple of even tinier options as described in the Perl OO Tutorial. Waiting in the wings is Corinna, an effort to bring next-​generation object capabilities into the Perl core itself, and Object::Pad, a testbed for some of the ideas in Corinna that you can use today in current code. (Really, please try it—the author needs feedback!)

All this is to say that 99% of the time you never need trouble yourself with bless, constructors, or writing accessors for class or object attributes. Smarter people than me have done the work for you, and you might even find a concept or three that you wish other languages had.


There are two major ones: list and scalar. Another way to think of it is plural” vs. singular” in English, which is hopefully a thing you’re familiar with as you’re reading this blog.

Some functions in Perl act differently depending on whether the expected return value is a list or a scalar, and a function will provide a list or scalar context to its arguments. Mostly these act just as you would expect or would like them to, and you can find out how a function behaves by reading its documentation. Your own functions can behave like this too, but there’s usually no need as both scalars and lists are automatically interpreted into lists.” Again, Perl’s DWIMmery at work.

Subroutine and Method Arguments

I’ve already written about this. Twice. And presented about it. Twice. The short version: Perl has signatures, but they’ve been considered experimental for a while. In the meantime, there are alternatives on CPAN. You can even have type constraints if you want.

I’ll leave you with this: Over the past month, Neil Bowers of the Perl Steering Council has been collecting quirks like these from Perl developers. The PSC is reviewing this collection for potential documentation fixes, bug fixes, further discussion, etc. I wouldn’t expect to see any fundamental changes to the language out of this effort, but it’s a good sign that potentially confusing features are being addressed.